Frequently Asked Questions List for comp.periphs.scsi
Copyright Gary Field, 1994-2001, all rights reserved, permission granted for non-commercial distribution in un-modified form.
The official resource for finding out those things you always wanted to know about SCSI, but were afraid to ask.
Current Editor: Gary Field (scsifaq@engineer.com)
(Where you see reference to [Editor(GF)] that means me.)
Last updated: September 18, 2001
Note: Please allow the whole file to load before clicking on any links. If you click on a link before the portion of the page it points to has loaded, you just get sent to the top of the document.
What is the SCSI FAQ:
The SCSI FAQ is a living document that attempts to serve as a reference for people who are trying to learn about SCSI
and/or troubleshooting a SCSI system. I don't look at the FAQ as a standards document. There are many topics for which I could simply regurgitate the SCSI standards documents, but instead, I generally choose to describe a useful, "practical" answer instead of an obtuse, detailed, answer which would only further confuse a newcomer to SCSI. I feel more people are served by this method. I will NEVER knowingly include wrong information in the interest of simplicity however. Believe me, when I put something in that's incorrect (it has happened on rare occasions), I get plenty of email pointing out the error to me. Sometimes my opinion even plays into an answer (Oh well, you get what you pay for).
Occasionally, something I have written strikes controversy, usually either due to a gray area in one of the standards or due to a misintrepretation of what I wrote or included. When I get notified of the disagreement, I research the topic, listen to reasonable arguments, and make the corrections or not as I see fit.
Articles get into the FAQ in several ways:
1) I see a question appear a number of times and write an
answer for that topic and put it in.
2) Someone else writes up an answer for a question they feel
needs answering and sends it to me. I edit it for accuracy
if necessary,
correct the grammar/spelling somewhat, edit the text into a consistent
format, and put it in.
3) I see what I feel is a well written response to a question
posted in comp.periphs.scsi and ask the author if I can include it.
Submitting articles for the FAQ:
If you feel the urge to write up an article for the SCSI FAQ that you
feel qualified to answer, please format it the way the existing articles
are and send it to me, (preferrably in HTML format, but .txt or .doc is
OK too).
FAQ history: Created by Johnathan Vail (vail@prepress.pps.com) from articles submitted to him by comp.periph.scsi readers.
Maintained by Johnathan Vail until November 1993.
Maintained by Gary Field from November 1993 to the present.
About the editor:
My credentials in SCSI technology are pretty substantial. I've been
working with this stuff on a daily basis, continuously since 1985 on both
PCs and various UNIX platforms. I write and enhance SCSI device drivers
for a living. All of my own computers use all SCSI I/O.
I also wrote/re-wrote "The Book of SCSI: I/O
for the New Millennium", and wrote the UNIX chapter in Brian Sawert's
book "The Programmer's Guide to SCSI".
There are areas of SCSI which I am not expert on, and when a question of fact comes up in one of those areas, I research the issue using the SCSI standards documents, or, ask my colleagues who are expert in that area, about it.
In general I get nothing but compliments about the FAQ. The most common complaint is that it's not always up to date on certain topics. I try my best to keep it updated, but SCSI marches on...
"Who pays for the "SCSI Info Central" Web site where the FAQ is distributed from"? you ask.
SCSI Info Central
http://www.scsifaq.org/
is my own personal web site that I've registered as scsifaq.org, connected
to the Internet via AT&T(formerly MediaOne) cable modem. I spend a
considerable amount of my own time and money maintaining the site, and
I hope people benefit from it.
Acknowledgements
Thanks to David Sanderson and Roland Bauer for helping me improve the quality and compatibility of the HTML in this document. I would also like to thank the denizens of comp.periphs.scsi for their stimulating banter which triggered the writing of many of the articles in the FAQ.
The Eternal Question
Should I buy SCSI or just go with EIDE/ATA?
This has to be the most commonly asked question regarding SCSI!
I hope this will summarize my thoughts on that issue:
For someone to who doesn't need a real multi-tasking workstation or server, the only reason for paying the extra money for SCSI is flexibility. EIDE/ATA is strictly for "inside the case" peripherals. SCSI allows you to attach a large collection of add-ons like scanners, CD recorders, tape drives (or even devices not conceived of yet), either inside or outside the CPU case in whatever manner suits your needs or wishes.
If you like non-technical analogies:
SCSI is like a palace, with an architecture that was well thought out from the beginning and built upon over a period of time to make it even greater than originally envisioned.
IDE/ATA is like a log cabin, with a dirt floor, built from whatever was found lying around in late Fall just before the snow came. It can't be expanded because it has no foundation and would collapse under its own weight.
Both provide shelter. SCSI costs more (but not as much as a palace :-)).
Take your pick.
If automobile analogies are more to your liking:
A Ford Escort will get you to work just as fast as a Volvo station wagon. Which would you rather go on vacation in? Which would you rather be in if an accident occurs?
If your computer is nothing more than a machine that's only purpose is to perform a certain set of tasks, and you don't expect to want any more out of it, IDE is probably for you.
On the other hand, if you enjoy computing and are always looking for more things your computer can do for you, SCSI will help enhance the experience for you. You won't regret the investment.
Just as with a palace however, you need to learn your way around. That's
where this FAQ comes in!
Where to get the latest copy of this FAQ:
SCSIFAQ.ORG - Now at a browser near you!
- Pointers to the comp.periphs.scsi FAQ are posted to Usenet during the first week of each month (and usually other times too).
- via World Wide Web (WWW): (I'm looking for mirrors in So. America, Africa, Asia, and Australia)
SCSI Info Central where you will also find The SCSI Game Rules
Attention SCSI vendors: There are a few articles in this FAQ where vendor contact information, and in a few cases, part numbers, are listed. This is not an attempt to steer business to any particular vendor but only to provide possible sources of certain "hard to find" SCSI accessories (particularly special cables, adapters and terminators). If you want to be listed in one or more articles please send your contact info and which items you can provide to the FAQ editor.
I will not include pointers for devices like hard disks, tapes, CDROMs
etc., which I consider readily available.
Categories:
- Generic SCSI Questions
- SCSI Documentation and Books
- SCSI Manufacturer Contact Information
- Manufacturer Specific Questions
- SCSI Performance Determination and Enhancement
- Platform Specific Questions
- Host Adapter Model Specific Questions
- The Future of SCSI and Storage in General
- What is SCSI?
- SCSI sounds interesting. What do I need to get started?
- What information should I provide when asking a question in the comp.periphs.scsi newsgroup?
- What do all these SCSI buzzwords mean?
- What is the history of SCSI (What is SASI)?
- Can I access a SASI drive with a SCSI controller?
- How Should I Choose a SCSI Host Adapter?
- What is meant by PIO, DMA, and Bus Master DMA?
- How should I lay out my SCSI bus? What should I avoid?
- Where do I put the terminators?
- What is a SCSI terminator? Why do I need them?
- What is terminator power (TERMPWR)? Why do I need it? Where does it come from?
- Where Should I place the SCSI host adapter on the SCSI bus?
- Is the spacing of connectors on a SCSI cable important?
- How long can my SCSI bus be?
- How should I set the IDs of my devices?
- How do I connect up the device ID switch in an external case?
- What are the pros and cons regarding SCSI vs IDE/ATA ?
- Should I spend the extra money on SCSI or just get IDE?
- Why do SCSI disks cost so much more than IDE/ATA disks?
- Can I have both IDE/ATA drives and SCSI in the same system?
- Is it possible for two computers to access/share the same SCSI devices?
- What is target mode and what host adapters and drivers support it?
- What is FAST SCSI?
- How can I check a passive SCSI terminator ?
- Can someone explain to me the difference between 'normal' SCSI and differential SCSI?
- What are the pinouts for differential SCSI?
- How can I tell if I have a single ended or differential drive ?
- What are the pinouts for SCSI connectors?
- Where can I find pictures of the various SCSI connectors?
- I've got a SCSI disk with an 80 pin connector. Someone called it an SCA drive. Can I connect this to my SCSI bus?
- What is the difference between SCSI-1 and SCSI-2?
- What is the difference between SCSI-2 and SCSI-3?
- Are SCSI-3 hard drives and/or controllers available yet?
- After perusing the latest issue of Computer Shopper, I came away with the impression that companies are calling F&W SCSI-2 HD's SCSI-3. Is this an incorrect assumption, or is F&W SCSI-2 known as SCSI-3?
- Is SYNCHRONOUS faster than ASYNCHRONOUS?
- Is the NCR 53C90 Faster than spec?
- What is ASPI?
- What is CAM?
- What is FPT (Termination)?
- What is Active Termination?
- Why Is Active Termination Better?
- How can I tell whether an unmarked terminator is active or passive?
- Where can I buy terminators ?
- What is Plug and Play SCSI?
- Will attaching a SCSI-1 device to my SCSI-2 bus hurt its performance?
- Can I connect a SCSI-3 disk to my SCSI-1 host adapter?
- Can I connect a SCSI-2 CDROM to a SCSI-3 host adapter?
- Can I connect a Narrow SCSI2 disk to a WIDE SCSI3 host adapter?
- Can I connect a WIDE device to my narrow SCSI host adapter?
- Can I connect a narrow device to my WIDE SCSI host adapter?
- How does device ID numbering work with WIDE vs NARROW devices?
- What is spindle-sync and why would I want it?
- What if I have a SCSI drive larger than a gigabyte (1024MB)?
- My SCSI bus works, but is not reliable. What should I look at?
- Where can I find information about programming using the ASPI interface from DOS and Windows?
- What kinds of Optical Drives are available?
- I connected an old narrow device to my WIDE bus with a 68 pin to 50 pin adapter. Now my TERMPWR seems to be shorted out. What is happening?
- What is LVD (Low Voltage Differential)?
- My drive stopped working. Is it dead?
- What issues are involved in connecting an Ultra160 (Ultra3) drive to an Ultra2 host adapter?
- Where can I get various SCSI documentation?
- How can I find out about the emerging SCSI standards?
- Where can I get official ANSI SCSI documents?
- What SCSI books and tutorials are available?
- Where can I find SCSI info on the Web?
- Where can I get information on various disk drives and controllers?
SCSI Performance Determination and Enhancement:
- What benchmark tools are available for SCSI testing and where can I get them?
- Why doesn't my Ultra WIDE drive perform at 40 MB/sec like the SCSI spec. says?
- Why are my 80MB/s Ultra2/LVD drives only listed as 40MB/s [SE] by my OS?
- My system says my new 9.1 GB disk is only 8.5 GB. Did I get cheated ?
- How much overhead does the SCSI protocol impose ?
- How can I calculate the performance I'll get with mixed SCSI devices?
SCSI Manufacturer Contact Information:
How can I contact:
- Adaptec
- Archive Corporation (now Seagate)
- BusLogic/Bustek/Mylex
- Conner Peripherals (now Seagate)
- Corel
- DPT (Distributed Processing Technology)
- Future Domain
- Fujitsu
- HP
- IBM
- Initio
- Legacy Storage System
- Micropolis
- Maxtor
- NCR
- Philips
- Quantum
- Seagate
- Symbios Logic
- UltraStor
- Tecmar Technologies (formerly Wangtek, WangDAT, Sytron, and Rexon)
- Western Digital
Manufacturer Specific Questions:
- Where can I get obsolete Adaptec files and utilities?
- Who manufactures SCSI extenders and Single-Ended to Differential converters ?
- What are the general steps I need to do to install a SCSI disk to be used with Windows?
- My SCSI CDROM only works when Windows 95 is installed. How can I get Windows 95 installed? Is this a catch 22?
- Are there any storage related reasons to upgrade to Windows 95 OSR2?
- Under Windows 95 OSR2 I can only see the first 8 GB of my 9 GB disk. What's going on?
- I'm having problems with my Adaptec 2940xx under Windows 95 but it works OK under other O/Ses (like Linux or Windows NT).
- My drive letters map to different disk partitions than I think they should. What's going on?
Host Adapter Model Specific Questions:
- What is the problem with the Adaptec 1542C and external cables?
- What is the difference between the Adaptec 1542A and 1542B?
- What are the differences between the Adaptec 1542B and the 1542C?
- What are the differences between the 1542C and the 1542CF?
- Where can I get drivers (ASPI and other) for the WD7000 FASST2 host adapter?
- How to replace Macintosh internal HD and terminate the SCSI chain properly?
- I changed the host adapter in my system and now my disk doesn't work. Why?
- What is Low Level Formatting(LLF)?
- My host adapter says it "auto-terminates". How does it do that?
- Can I flash update a new BIOS into my OEM Adaptec host adapter?
- Although the Adaptec BIOS identifies my drives, I cannot access my drives under DOS. What am I doing wrong?
The Future of SCSI and Storage in General:
Table of ContentsAnswers to the Questions:
QUESTION:What is SCSI?
ANSWER From: LSD, L.J.Sak@Kub. Edited by Gary
Field (scsifaq@engineer.com)
SCSI stands for Small Computer System Interface. It's a standard for connecting peripherals to your computer via a standard hardware interface, which uses standard SCSI commands. The SCSI standard can be divided into SCSI (SCSI1) and SCSI2 (SCSI wide and SCSI wide and fast) and now SCSI-3 which is made up of at least 14 separate standards documents.
SCSI2 is the most popular version of the SCSI command specification and allows for scanners, hard disk drives, CD-ROM players, tapes [and many other devices]. SCSI-3 resolves many long time "gray areas" and adds much new functionality and performance improvements. It also adds new types of SCSI busses like fibre channel which uses a 4 pin copper connection or a pair of glass fibre optic cables instead of the familiar ribbon cable connection.
QUESTION: SCSI sounds interesting. What do I need to get started?
ANSWER From: Gary Field (scsifaq@engineer.com)
In order to put together a PC with SCSI I/O you'll need:
- A SCSI host adapter (also called a SCSI controller by sales types)
- A SCSI cable (either WIDE, 68 pin, or narrow, 50 pin, external or internal)
- A SCSI device of some sort (disk, tape, CD-ROM, CD-RW, DVD-ROM, scanner)
QUESTION: What information should I provide when asking a question in the comp.periphs.scsi newsgroup?
ANSWER From: Gary Field (scsifaq@engineer.com)
In order for most SCSI problems to be resolved, one needs to provide at least the following:
- Type of system (PC, SPARC or Alpha Workstation, etc.)
- If PC, what type of motherboard?
- Operating System (DOS, Windows 3.x, Win 95/98, Win NT 4/5, Linux, other UNIX)
- Specific SCSI host adapter (Symbios xxxx, Adaptec xxxx, etc)
- List of attached devices (and for disks, whether they're WIDE or NARROW)
- Which connectors on the host adapter the cables are connected to
- Length of SCSI bus
- Where the terminators are located (90% of all SCSI problems result from mis-placed terminators).
- Whether the configuration is new, or was working before.
If you don't know what some of these things mean, read the rest of this document until you do. You'll get much more help if you appear to have made an effort to find the answer on your own before asking for help.
Asking a question like "My scanner doesn't work, how come?" may not even get you a response.
PLEASE DO NOT ASK "Which is better IDE or SCSI"?
Please spare us all the aggravation of the week long tirade that will
result from asking this seemingly innocent question!
QUESTION: What do all these SCSI buzzwords mean?
ANSWER From: hennes@stack.urc.tue.nl (Hennes
Passmann)[Editor(GF)]
- Host adapter
- Also called a Host Bus Adapter or HBA. The card that connects your computer to the SCSI-bus. Usually called SCSI-controller by marketing droids. An example would be a PCI SCSI host adapter like the Adaptec 2940UW.
- Terminators (passive)
- A group of resistors on the physical ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from the ends of the bus. Each terminated signal is connected by:
- 220 Ohm to +5 volt (TERMPWR)
- 330 Ohm to ground.
- For NARROW SCSI the 18 signals that are terminated are:
- Terminators (active)
- Rather than passive terminators that use TERMPWR which may not be exactly +5v, active terminators use a voltage regulator. Basically it is a set of 110 Ohm resistors from each signal to a 2.8 Volt regulated Voltage source.
- Single ended
- "Normal" electrical signals. Uses open collector drivers to drive the SCSI bus.
- [usually] survives wrong cable insertion.
- DIFFSENSE signal is used to detect connection of differential devices and prevent damage.
- The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm allowed to connect a device to the main cable. Most devices are single ended.
- Differential (Now called High Voltage Differential to distinguish it from LVD)
- Uses two wires to drive one signal.
- Max. cable length of 25 meters.
- Electrically incompatible with single ended devices!
- Much more expensive than single ended.
- Used from SCSI-1 upwards.
- Apple kludge
- The single ended 50 pins cable has been reduced to 25 pins by tying most grounds together. DB25 connector (like a parallel port). Often used as the external SCSI connector. Unfortunately, this abomination is being perpetuated by being used on devices like the Zip drive!
- A way of sending data over the SCSI-bus.
- The initiator sends a command or data over the bus and then waits until it receives a reply (e.g. an ACKnowledge). All commands are sent asynchronously over the 8 bit part of the SCSI-bus.
- Synchronous SCSI
- Rather than waiting for an ACK, devices that both support synchronous SCSI can send multiple bytes over the bus in the following way:
- send data1 : send data2 : ... : send data3 (max outstanding bytes)
- : wait : wait : response1 : reponse2: ...
- This improves throughput, especially if you use long cables. (The time that a signal travels from one end of the cable to the other end of the cable IS relevant.)
- Fast SCSI
- Fast SCSI allows faster timing on the bus. ( 10MHz instead of 5MHz )
- On a 8 bit SCSI-bus this increases the *theoretical* maximum speed from 5MB/s to 10MB/s.
- Ultra SCSI
- Synchronous data transfer option which allows up to 20MHz data clocking on the bus. Also called FAST20.
I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ... DB(0).
For WIDE SCSI there are 9 more signals; DB(p1), DB(8) ... DB(15)
Asynchronous SCSI:
- Synchronous data transfer option which allows up to 40MHz data clocking on the bus. Also called FAST40.
- Use of this option also requires the use of LVD bus drivers.
Ultra3 SCSI
- Synchronous data transfer option which allows up to 80MHz data clocking
on the bus. Also called Ultra160. Use of this option also requires the
use of LVD bus drivers.
- Wide SCSI
- Uses an extra cable (or more commonly a 68 pin P cable) to send the data 16 or 32 bits wide. This allows for double or quadruple speed over the SCSI-bus.
- RAID [Added by Editor(GF) Corrected by Fredrik Bjork (ace@varberg.se)]
- A Redundant Array of Independent Disks is a set of disk drives connected in such a way as to allow certain types of access optimization, or data security. This can be accomplished in hardware using a special dual ported SCSI adapter, or completely in software in a special device driver.
- A RAID 0 array stripes the data across multiple drives to decrease data latency. A RAID 1 array mirrors the data on multiple drives for increased data integrity. A RAID 5 array uses extra drives in a distributed manner to store parity information that can be used to apply data correction and recover any data in the event of any individual disk failure. This provides high reliability.
- The following was submitted by RAIDER@ultrafast.net:
- The minimum number of drives required for each RAID level is:
- RAID 0 - TWO (2) drives
- RAID 1 - TWO (2) drives
- RAID 0+1 - FOUR (4) drives
- RAID 3 - THREE (3) drives
- RAID 4 - THREE (3) drives
- RAID 5 - THREE (3) drives
- Disconnect/reconnect (also called reselect)
- This feature of the SCSI protocol allows a device to temporarily give up control of the SCSI bus. This is typically done when the device is performing an operation which will take some time. For example, it is very important for tape drives which would otherwise lock out other devices during long operations such as rewind.
- Bus Segment
- A portion of a SCSI bus isolated by a signal conditioner chip. A bus segment is logically part of a single SCSI bus (e.g. SCSI IDs must be unique) but electrically separated such that reflections on the segment do not affect other segments. Using bus segments allows longer busses because the signals get cleaned up (edges re-clocked etc) by going through the signal conditioner chips. Each segment must have its own terminations; One at the signal conditioner chip, and one at the far end of the segment.
- Logical Unit Number (LUN)
- A LUN is a sub-unit of a target. Most of the time, the LUN is just 0 since most types of target devices don't have sub-units. One example of where you might use LUNs is with multi-disc CDROM changers. Many of these units refer to each disc in the changer as a LUN. e.g. with the CDROM drive set as target ID 4, the first CD disc would be ID 4, LUN 0, the next would be ID 4, LUN 1 and so forth.
- Another example is a optical disk jukebox where the optical drive might be LUN 0 and the changer might be LUN 1.
- Some host adapters ignore LUNs unless the "Enable LUNs" option is set in the host adapter BIOS or operating system driver config. They default to not using LUNs because it speeds up the bus scan process and most targets don't support LUNs anyway.
- LUN numbers are generally defined by the manufacturer and can't be changed by the user.
- The Adaptec 2940 series BIOS has changed the place in the BIOS that LUN support is controlled several times.
- A sketchy history:
- AHA-2940U BIOS ver 1.11 = No LUN support
- AHA-2940UW BIOS ver 1.25 = Under Advanced Config, "Multiple LUN support (enable/disable)"
- ASUS P2B-LS* Adaptec AIC-7890 BIOS = Under SCSI device config, "Support Multiple LUNs (on/off)" on a per device basis.
- * Note: The built-in SCSI adapter on this motherboard is quite similar to the Adaptec 2940U2W.
QUESTION: What is the history of SCSI (What is SASI)?
ANSWER From: hennes@stack.urc.tue.nl (Hennes
Passmann)
#include <stddisclaimer.h>
- 1979
- The disk drive manufacturer Shugart begin working on a new drive interface with logical rather than physical adressing. It used 6 byte commands.
- Shugart Associates Systems Interface (20 pages long) made public.
- A few SASI drives are developed
- 1980
- Attempt to make SASI an ANSI standard failed.
- 1981
- Shugart and NCR request an ANSI committee be formed for SASI.
- 1982
- ANSI committee X3T9.2 is formed.
- SCSI adds the ATN signal to the bus and creates the message protocol.
- 1983
- Development of SCSI drives and ST-506 to SCSI bridges begins.
- 1985
- CCS (Common Command Set) used in most disk drives.
- Only disk and tape commands were adequately specified.
- 1986
- Work begins on SCSI-2.
- SCSI-1 becomes official as ANSI X3.131-1986 (yes, after the work had begun on SCSI-2)
- 6 and 10 byte commands.
- SCSI-2 specifies CDROM commands.
- 1988
- Production of SCSI-2 devices begins.
- 1993
- Work begins on SCSI-3.
- 1994
- SCSI-2 becomes official as X3.131-1994.
- SCSI-2 is backward compatible with SCSI-1 and adds the following:
- Fast SCSI-2. Optional bus speed of 10MHz instead of 5MHz.
- Wide Optional 16 or 32 bit cable instead of 8 bits.
- more commands defined, many optional (I'm not going to type the entire list here)
- broader support for non-disk devices (tape.CDROM,Scanners....)
- SCSI-2 devices can talk to the host adapter on their own inititive. (e.g. to set in which mode they should operate, FAST or not, wide, extra wide or normal ...) This can confuse some older SCSI-1 HA.
- 1995
- Production of drives that have some SCSI-3 enhancements.
- Ultra SCSI: Bus speed of 20MHz?
- 1996
- SCSI-3 proposals include:
- Support for graphical commands.
- Fibre channel protocol (fibre channel) (FCP)
- Serial packet protocol (IEEE 1394) (SBP)
- SCSI-3 general packet protocol (almost all serial interfaces) and of course the old SCSI-2 commands and more.
- Low Voltage Differential Parallel interface (SPI-2)
- CD-R command set and algorithms (MMC)
1998
- Ultra2: Bus Speed of 40 MHz. LVD only.
- 1999
- Ultra3: Bus Speed of 80 MHz. LVD only.
- Future (after 1998):
- SCSI-3 becomes official
- SCSI becomes a more network-like environment where devices can be physically distributed and shared more easily.
QUESTION: Can I access SASI drive with SCSI controller?
ANSWER From: Gary Field (scsifaq@engineer.com)
Well, the answer is a definite maybe, but very unlikely. Old low performance SCSI adapters and drivers that use only a minimal subset of the SCSI commands may work with SASI devices that happen to support the INQUIRY command. Newer adapters and drivers expect to be able to use messages and will get very upset with a SASI device that doesn't understand them.
In reality, there is no practical reason to do this. Any SASI device is so obsolete that is has no real value in a system being used in 1990 or later.
QUESTION:How should I lay out my SCSI bus? What should I avoid?
QUESTION: Where do I put the terminators?
QUESTION: Where should the adapter card be placed?
Answers From: Nick Kralevich <nickkral@cory.eecs.berkeley.edu>
edited by Gary Field (scsifaq@engineer.com)
One confusing thing about SCSI is what the SCSI bus is supposed to look like, and how devices should be placed on the bus.
The SCSI bus MUST run continuously from one device to another, like this:
DEVICE A --------- DEVICE B --------- DEVICE C -------- DEVICE D
Where device A, B, C, and D can either be internal or external devices.
The devices on the SCSI bus should have at least 4 to 6 inches of cable between devices. This is to satisfy the SCSI-2 requirement that "stubs" be placed at least .1 meters apart. Some devices that have a lot of internal wiring between the connector and the SCSI chip can look like a "stub" or bus discontinuity. The reason for all these requirements is that a SCSI bus is really 18 "transmission lines" in the wave theory sense. A pulse propagating along it will "reflect" from any part of the transmission line that is different from the rest of it. These reflections add and subtract in odd combinations and cause the original pulse to be distorted and corrupted. The terminators "absorb" the energy from the pulses and prevent reflections from the ends of the bus. They do this because they (hopefully) have the same impedance as the rest of the transmission line.
The SCSI bus must not have any "Y" shape cabling. For example, setting up a cable that looks like this is NOT allowed:
DEVICE B
\
\
\
>------------- DEVICE C ----------- DEVICE D
/
/
/
DEVICE A
Termination must be present at two and ONLY two positions on the SCSI bus, at the beginning of the SCSI bus, and at the end of the SCSI bus. There MUST be no more than two, and no less than two, terminators on the bus.
Termination must occur within 4 inches (.1 meter) of the ends of the SCSI bus.
The following ARE acceptable: +------------+----------+-----------+-----------+---------+ | | | | | | DEVICE A Unconnected Unconnected DEVICE B DEVICE C Adapter Terminated Terminated +------------+----------+-----------+-----------+---------+ | | | | | | DEVICE A Unconnected DEVICE B Unconnected Adapter DEVICE C Terminated Terminated +------------+----------+-----------+-----------+---------+ | | | | | | Adapter DEVICE A DEVICE B Unconnected Unconnected DEVICE C Terminated Terminated
+------------+----------+-----------+-----------+---------+ | | | | | | Adapter DEVICE A DEVICE B Unconnected Unconnected Termination Terminated
The following ARE NOT allowed: +------------+----------+-----------+-------------------+ | | | | | DEVICE A DEVICE B Adapter Unconnected Unconnected Dangling cable end Terminated Terminated
+------------+----------+-----------+-----------+ | | | | | Termination DEVICE A DEVICE B DEVICE C Adapter Termination in middle of bus Terminated
- [Editor GF]
- Helpful hint:
- I have found that it is much better in the long run to always disable the internal terminators in all of your devices and place a terminator block at the end of the cable itself. I'll grant you that this costs a little more because you need to buy a separate terminator. But, you never need to be concerned in the future when you re-arrange devices in your system, which device had its terminator enabled (none of them do). With the arrival of LVD and SCA, devices are starting to be shipped which don't even have internal terminators anyway, so getting used to the idea of terminating the cable end and not the device is a good practice. This is just my two cents worth... (backed by 15 years of tinkering with SCSI...).
- Old wive's tale:
- I still hear people say "If you put a terminator part way down your SCSI bus, the devices beyond it won't be seen". This is a total misconception of what terminators do. Putting a termination part way down the bus is incorrect and does cause problems, but it is quite unpredictable what the effect will be. It's not simply a matter of making the devices beyond the terminator invisible to the host adapter. Many people believe this myth and it will probably never go away, but I hope to convince at least a few people that this is not a valid way to envision how termination works.
The placement of the SCSI adapter card can be on the end, at the beginning, or somewhere in the middle of the SCSI bus.
Quite frankly, placement of the controller card isn't special.
The adapter card is just another device on the SCSI bus.
As long as the rules above and in other sections of this FAQ are followed, there should be no problem placing the adapter card anywhere on the SCSI bus.
However, if you place the adapter card somewhere in the middle of the SCSI bus, you must be sure to disable termination on the adapter card. As noted previously, a SCSI device is only allowed to have termination if it's at the end of the bus. Only two terminators are allowed to terminate the SCSI bus, one at each end.
One last note: It doesn't make any difference where each SCSI ID is placed along the bus. It only matters that no two devices have the same ID. Don't forget that the adapter has an ID too. (Usually ID 7).
QUESTION: What is a SCSI terminator? Why do I need them?
ANSWER From: Gary Field (scsifaq@engineer.com)
Updated: May, 1999
A SCSI bus is a transmission line. To prevent reflections from the ends of the bus, you need a device which makes the transmission line appear to be of infinite length. This is done by attaching resistors, which have the same resistance as the characteristic impedance of the transmission line, to the ends of the bus. Also, since SCSI line drivers are open-collector (which can only pull a signal low), a pull-up resistor is needed to pull the signal high when it's not asserted.
If the ends of the bus are not terminated, the signal pulses will reflect off these open ends and travel back along the bus in the other direction. The resultant adding and cancelling of signal amplitudes distorts and corrupts the SCSI signals.
There are two basic types of terminators, active and passive:
- Passive terminators consist of pairs of resistors. A 220 Ohm pulling each signal up to TERMPWR and a 330 Ohm pulling each signal down to GROUND. Passive terminators were considered adequate in SCSI-1 when the bus only ran at 5 MHz. In SCSI-2, passive terminators were given the name "Alternative 1".
- Active terminators consist of 110 Ohm resistors connected from each signal line to a common 2.85 Volt regulated power supply. Active terminators both terminate the bus better (less reflection), and supply cleaner pull-up current (due to their Voltage regulation). They were first defined in SCSI-2 and were given the name "Alternative 2" to distinguish them from passive terminators.
Recommendations and requirements:
In SCSI-2 when the fastest defined speed was 10 MHz, passive terminators
were allowed, but active terminators were recommended.
In SCSI-3, the "alternative X" terminology has been discarded, and
the SPI-2 standard only allows active termination for single-ended buses
regardless of speed.
My personal recommendation is not to buy any new passive terminators.
If you want to use up the old ones you have lying around, on older systems,
with short buses and no more than 4 devices, that don't have any devices
faster than 10 MHz, I can't argue with that, but ONLY BUY ACTIVE (or preferrably
LVD) terminators for any new systems. If you run into problems, switching
to an active terminator might well solve them.
Other people will tell you that only active terminators are ever acceptable
at any speed. I leave the choice up to the individual at Fast10 and below,
above that, active is absolutely the only acceptable choice.
I often hear the whine "It seems to be working, why should I bother
with the terminators?"
The following appropriate analogy was given to me by Kevin Kilzer:
"It only seems to work fine because you have not seen an error.
It's like having mice in your house. If you never see one,
you don't realize they are there."
Suddenly a problem will arise and you won't even realize it's associated
with the fact that you added a device to your SCSI bus two weeks ago. Termination
problems can manifest themselves in many ways. The best solution is to
avoid them by following the rules to the letter.
A final nit to pick:
As I was reminded in looking back at the standards, technically SCSI-2
did not sanction Fast10 on single ended buses. It was only spec'd for differential.
However, as was the case with WIDE SCSI using the 68 pin P cable, the industry
latched onto it and it later became standardized in SCSI-3 SPI.
QUESTION: What is terminator power (TERMPWR)? Why do I need it? Where does it come from?
ANSWER From: Roger J. Hamlett (Roger@ttelmah.demon.co.uk)
Updated: November, 1999
TERMPWR is the power source for the SCSI terminators. Terminators (both active and passive) require power because in addition to providing the correct impedance to prevent reflections on the SCSI bus, they source pull-up current to the SCSI signals.
The SCSI spec. allows for multiple devices to supply power, but also limits the maximum current that should be available. The "rule" is that "initiators shall supply TERMPWR". Hence a SCSI controller (host adapter) should supply TERMPWR, and on longer buses it is worth having a device near the end to also supply it. However, no more than about four devices should supply it, because in the event of a failure (shorted cable etc), there could be dangerous currents available. Not all devices are designed to be able to supply TERMPWR, but many can. Usually this is done by setting one or two jumpers to select where TERMPWR will go. For example:
- TERMPWR to on drive terminator only
- TERMPWR to SCSI bus
- On drive terminator gets its TERMPWR from SCSI bus
QUESTION: Is the spacing of connectors on a SCSI cable important?
ANSWER From: Gary Field (scsifaq@engineer.com)
The ANSI SCSI spec's say that "stubs" on a SCSI bus must not be any more than .1 meters (4 in.) long. In SCSI-2 there are also guidelines that say you shouldn't place "stubs" any closer than .3 meters (12 in.) apart. Since each device attached acts as a "stub", you really shouldn't place connectors any closer than this. This gets to be more important as your bus performance goes up. i.e. with Fast20 it is very important, but with SCSI-1 it doesn't really matter much. Since Fast20 also limits your overall bus length to 1.5 meters (for single ended) this also means you shouldn't really connect more than 5 devices for best reliability.
Another minor enhancement involves altering the spacing of adjacent connectors to prevent reflection resonance.
e.g. place connectors at one end, then .3m, then .56m then .86m then 1.12m
Overall, the cable impedance and configuration (straight vs. twisted pair) are probably more significant factors than connector spacing. However, if there is room for the extra cable, I recommend spacing the connectors as described above for best reliability.
Excess cable length is also a bad thing, so basically all these factors must traded off against each other to build the best SCSI cable for a given situation.
QUESTION: How long can my SCSI bus be?
ANSWER From: Gary Field (scsifaq@engineer.com)
The SCSI bus length limits are based on the speed of the fastest device attached to the bus.
Here's a table which shows the limits:
| Speed of FASTEST device | Max. Single-Ended bus length | Max. HV Diff. bus len. | Max. LVD bus length |
| 5 MHz (SCSI1 synch.) | 6 meters | 25 meters | 12 meters |
| 10 MHz (SCSI2 FAST) | 3 meters (not recommended in SCSI-2) | 25 meters | 12 meters |
| 20 MHz (Ultra or Fast20) | I recommend no more than 1.5 meters. The SCSI-3 SPI spec. gives a much more complicated reommendation. | 25 meters | 12 meters |
| 40 MHz (Ultra2 or Fast40) | Not recommended | 12 meters | 12 meters |
| 80 MHz (Ultra160) | Not recommended | Not recommended | 12 meters |
These limits assume the use of good quality cable, and the use of active terminators or LVD/SE terminators at each end of the bus.
Notice that I used the term MHz to specify speed since MB/sec. changes with the bus width.
Note: Bus width doesn't change the maximum allowable length. The bus width is independent of bus length or speed.
The above table assumes that you know the max. speed of your devices (usually by looking in the manuals). Some software (like Adaptec EZ-SCSI) provides a driver status monitor which will tell you what mode the devices are actually in. This is important, since any synchronous speed must be negotiated by either the device, or the adapter. The speed actually used will be the least common denominator between the two.
For example, if a Fast20 disk is attached to a "SCSI2" host adapter that only goes up to Fast10, the device will only run at 10 MHz.
In systems with high performance disks and external peripherals which
require long cables (i.e. external scanners, tapes or CDROM changers),
you may want to put the external devices on their own bus to avoid having
to slow down the fast disks. There are dual channel host adapters to make
this simpler (avoids using multiple IRQs etc).
The SCSI Trade Association also has a handy table.
QUESTION:How should I set the IDs of my devices?
ANSWER From: Gary Field (scsifaq@engineer.com)
(updated February 2001)
The main rule of SCSI IDs is that they all need to be unique on a per bus basis. Each device on a particular bus must be set to a different ID so that they can address each other without confusion. Some devices have a sticker on the drive which shows the ID pins, but if your does not, you'll need the data sheet for the device. The pins to jumper are obvious if you know how to count in binary, if not there is usually a table of jumper combinations on the data sheet for each ID setting.
There is a secondary consideration in setting IDs though; Higher ID
numbers have a higher priority on the bus.
The overall ID priority order on a WIDE bus is as follows (highest
to lowest): 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.
There are at least two philosophies on how to use the device priorities
to best advantage:
Method 1:
Set the host adapter's ID to 7. The next lower IDs (6, 5, 4 ...) would
then be used for any time critical devices you may have such as streaming
tape drives or CD-RW drives. Your hard disks would be set to lower priority
IDs because they are generally the fastest devices on the bus and if given
too high a priority will hog all the bus bandwidth and "starve out" the
slower but time critical devices.
Method 2:
This philosophy maintains that devices that create the load should
be given low priorities and devices that relieve the load should be given
higher priorities. In this view, the host adapter creates the load (I/O
to be done), therefore, set the host adapter's ID to 0(or even 15 if no
narrow devices will be attached). The time critical devices (streaming
tape and CD-RW) would then be assigned highest priorities. Everything else
(including disks) would be assigned IDs in between. The placement of the
load creator at low priority pretty much prevents the "starvation" scenario.
To my knowledge no benchmarks have been published that show one method to be superior to the other. I would appreciate it if anyone would run some good tests to prove what the best method is or point to existing published results supporting one of these methods (or even another method).
Method 1 is apparently supported by Adaptec since they set all their
host adapters to ID 7 by default.
I personally doubt that it makes very much difference which method
you choose except on very heavily loaded systems where the drivers take
full advantage of tagged command queueing etc.
Special consideration for older host adapters:
Many older host adapters make the assumption that the boot disk will
be at ID 0. Most newer ones however, allow the user to specify which ID
to boot from.
See also 1
QUESTION:How do I connect up the device ID switch in an external case?
ANSWER From: Gary Field (scsifaq@engineer.com)
February 2001
The ID switches in external SCSI cases are designed to be as flexible as possible because there is no standard for how ID pins on SCSI devices are to be layed out. This flexibility however means that the user has to be creative!
Usually there are 4 wires coming from the switch (five if it's WIDE). They are:
- ID bit 0 (value of 1)
- ID bit 1 (value of 2)
- ID bit 2 (value of 4)
- ID bit 3 (if WIDE) (value of 8)
- Common ground
QUESTION: What are the pros and cons regarding SCSI vs IDE/ATA ?
ANSWER From: Gary Field (scsifaq@engineer.com)
Updated: April, 2001
Pros of IDE/ATA:
- Inexpensive due to high volume of production and simplified firmwar development and testing requirements.
- Supported directly by system BIOS in most cases. (unless you want DMA support)
- Less overhead per command
- Very limited device attachment (two drives (including CDROMs) per channel, and two channels per system max.) (Recent versions of Linux (and I hear Win NT) support four or more ATA adapters)
- Single threaded (commands do not overlap even with a second drive)
- CPU is tied up transferring all data (actually newer EIDE controllers can do DMA as well if special drivers are loaded)
- IDE/ATA and ATAPI evolved from the ancient ST-506 interface as one kludge on top of another
- Cannot handle scatter/gather operations well (important in good Virtual Memory operating systems)
- Maximum disk drive capacity of 128 GB
- Flexible device attachment (up to 7 or 15 devices per SCSI bus) (inside or outside of case)
- Longer cable lengths allowed (up to 12 meters using LVD)
- Support for almost any peripheral type (disks, tape, CDROM, optical, scanner etc)
- All commands can overlap with commands on other devices. Usually uses DMA to transfer data (which frees CPU for other tasks)
- Interface and protocol is carefully specified by ANSI.
- Largest, highest performance devices are available in SCSI before IDE
- Most adapters can do scatter/gather DMA which is a necessity in virtual memory systems (Like Unix, NT, 2000) (Win 95/98 ?)
- Max drive capacity of 2048 GB
- Generally more expensive than IDE/ATA, due to more complex firmware and extra testing required. (not to mention greater performance commanding a higher price).
- Slightly more complicated to install than IDE/ATA, due to termination requirements.
- Seems scary to novice users because of amount of terminology and connector/protocol options.
Some people point to the need to set IDs in SCSI as making it more
complicated, but it's really no more complicated than choosing master/slave
jumpers in IDE.
---------------
Now that I've said that, here's an article to show that there's more than one opinion on this subject:
From: Ed Schernau <mithrandir@ids.net>
Subject: FYI: EIDE and DMA/Scatter-Gather
The Western Digital Caviar EIDE drive that came in what is now the file server in our office came with a Win3.x 32 BDA driver which allowed the user to select DMA type (B or F) and to implement scatter-gather.
Also, the Intel Triton chipset implements 2 EIDE controllers, and I know that at least the 1 on the PCI bus supports bus-mastering, as well as DMA. However, PIO transfers can be faster, the infamous Mode 4 can in theory, do 16.6 MB/sec and I've heard of a Mode 5 which can do 22 MB/sec. Which [PIO] is only a benefit in single-tasking systems like DOS or Win3.x. Sounds like Intel is trying to make EIDE into SCSI, eh?
QUESTION:Should I spend the extra money on SCSI or just get IDE?
ANSWER From: Andrew Korn (korn@eik.bme.hu)
For home users this is a difficult question to answer in general. It totally depends on how you use your system, what operating systems are installed, and whether you will add more I/O devices in the future.
For server systems in a corporate environment the only sensible answer is to go with SCSI peripherals.
IDE has been improved a lot in the past few years, so in most cases it will be an acceptable choice for home users. You should consider the following
(we are mostly talking PC hardware from now on):
1. Your motherboard probably has an integrated EIDE controller capable of supporting up to four devices. (Older motherboards may not have a dual-channel IDE controller, in which case only two drives can be connected; even older motherboards may not be equipped with an IDE controller at all.) If not, an IDE controller for your system should cost less than $30, which is about half of what a decent SCSI host adapter (Symbios 53C810 based) would cost you. On the other hand, some high-end motherboards come with integrated SCSI host adapters.
2. EIDE is a single threaded architecture. This means that of the two drives connected to an IDE channel, one will always be idle while the other is executing a command. If you only want a hard disk and a CD-ROM drive, you can install the CD-ROM on the secondary IDE channel (the hard disk will probably be the primary 'master' drive); in this case, the aforementioned limitation does not affect you. Also, if you only plan on using single-tasking operating systems such as DOS, you needn't be concerned about this single-threadedness.
SCSI devices share the bus bandwidth efficiently by allowing one device to transfer data while another is seeking or rewinding its media. This will, however, only gain you performance if you use a proper multi-tasking operating system (such as Linux/UNIX [Editor(GF): or Windows NT/2000]).
3. By default, IDE devices use PIO (Programmed I/O) to communicate with the rest of the system. This has the drawback of consuming a lot of CPU time. However, most newer EIDE controllers support bus-mastering and most drives support DMA or even UDMA transfer modes. Using bus-master DMA decreases CPU consumption to almost zero. (It may not be easy to activate the DMA transfer mode under DOS, however.)
Early SCSI host adapters had much the same problem, but all newer ones support DMA transfers.
4. If you plan to use only two drives (one per IDE channel), IDE will probably be slightly faster and definitely less expensive than SCSI.
If you think you need more than two drives, plan to use a multi-tasking environment (such as Unix, OS/2, Netware or Windows 95/98/NT/2000), and think that performance is more important than cost, SCSI is the way to go.
Anything bigger than a small low-cost Linux-based server should probably use SCSI.
5. IDE tapes are not as cool as SCSI tapes. They tend to be slower, less reliable and less compatible with each other than SCSI tape drives. SCSI tapes are more expensive, however.
6. IDE is probably slightly easier to install. Termination is not an issue, and it will probably require no effort on your part to make the system aware of any new devices you add. In some increasingly rare cases this may not be true for SCSI. (You know what SCSI stands for? "System Can't See It." :))
Especially with older systems it may not be trivial (or, in rare cases, even possible) to make the computer boot from a SCSI drive.
7. It is problematic to add more than four IDE drives to a system. If you think you will need more than that, SCSI is probably the choice for you.
If your motherboard came with an integrated EIDE controller, however, there is no need to ignore that feature; you can have a mixed system with both IDE and SCSI devices. (Remember to buy SCSI where performance and parallelism is an issue; but there is no need to buy an expensive SCSI CD-ROM drive if an IDE drive would suit your needs.)
8. If you need high reliability, you want to buy a RAID capable SCSI host adapter. Be aware, however, that it is possible to emulate RAID from software; Linux can do RAID 0, 1, 4 and 5 with any mixture of SCSI and IDE disks. This software-based solution is probably less reliable and slower than a true RAID controller, but certainly also much less expensive. [Editor(GF): You can't boot from a software RAID set either].
[Editor(GF): ATA and IDE are basically the same thing, and the terms are used interchangably in this document.]
Here's a discussion that shows some of the advantages of SCSI in more detail:
from: Greg Smith (GREGS@lss-chq.mhs.compuserve.com)
Under DOS (and DOS/win3.1), there is very little useful work the host can do while waiting for a disk operation to complete. So handing off some work from a 66 MHz 486 to, say, an 8 MHz Z80 (on the controller) does result in a performance loss. Under EVERY other OS worth discussing (Unix, Netware, NT, OS/2, Win95/98 etc) the processor can go off and do something else while the access is in progress, so the work done by the other CPU can result in a performance increase. In such systems, due to virtual memory, a 64K byte 'contiguous' read requested by a process may be spread to 16 separate physical pages. A good SCSI controller, given a single request, can perform this 'scatter/gather' operation autonomously. ATA requires significant interrupt service overhead from the host to handle this.
Another big issue: ATA does not allow more than one I/O request to be outstanding on a single cable, even to different drives. SCSI allows multiple I/O requests to be outstanding, and they may be completed out of order. For instance, process 'A' needs to read a block. The request is sent to the drive, the disk head starts to move, and process 'A' blocks waiting for it. Then, process 'B' is allowed to run; it also reads a block from the disk. Process B's block may be sitting in a RAM cache on the SCSI controller, or on the drive itself. Or the block may be closer to the head than process A's block, or on a different drive on the same cable. SCSI allows process B's request to be completed ahead of process A's, which means that process B can be running sooner, so that the most expensive chip - the system CPU - tends to spend less time twiddling its thumbs. Under ATA, the process B request cannot even be sent to the drive until the process A request is complete. These SCSI capabilities are very valuable in a true multi-tasking environment, especialy important in a busy file server, and useless under DOS, which cannot take advantage of them.
I tend to hear from people, 'Well, I never use multitasking' because they never actively run two programs at once - all but one are 'just sitting there'. Consider what happens though, when you minimize a window which uncovers parts of four other application windows. Each of those applications is sent a message telling it to update part of its window; under win95, they will all run concurrently to perform the update. If they need to access disk (usually because of virtual memory) the smoothness of the update can depend a lot on the disk system's ability to respond to multiple independent read requests and finish them all as quickly as possible; SCSI is better at this.
So, yes, ATA can be faster under DOS; but SCSI provides advantages which are inaccessible to DOS. They will benefit Win95 however. The cost of intelligent, fast SCSI controllers and drives should decrease as people discover these advantages and start buying them.
I should add that many of SCSI's advantages are NOT available with some of the simpler SCSI controllers which were targeted only to the DOS market or part of cheap CDROM add-on kits.
Furthermore, SCSI allows far greater flexibility of interconnect. I concede that for the mass market, which likes to buy pre-configured machines, this is but a small advantage.
QUESTION:Why do SCSI disks cost so much more than IDE/ATA disks?
ANSWER From: Gary Field (scsifaq@engineer.com)
The increased cost of SCSI disks is primarily due to four factors:
- SCSI drives are developed first for the high performance market (servers and high end workstations). After the drive technology matures, they slap an IDE controller card onto it and start shipping high volume. That's why IDE/ATA drives are mostly 5400 and 7200 RPM when SCSI drives are mostly 10k and 15k RPM.
- SCSI drive firmware is far more complicated than IDE firmware. This is because SCSI supports multiple initiators, disconnect/reconnect and tagged command queueing. SCSI protocol also has many more commands. So, it costs more to develop, debug and test the firmware.
- The drive manufacturers know that for high end systems SCSI is required and IDE isn't really even a choice, so they tack on a premium to pay for the above two factors (charge what the traffic will bear).
- Because SCSI drives are produced in lower volume they cost more to produce. Conversely, since SCSI drives cost more, people buy IDE instead and SCSI drives never get the advantage of high volume production.
Interestingly, with other types of SCSI devices, the price difference isn't as great. SCSI CD-RW drives for example are usually only a little more expensive than IDE/ATAPI ones of similar ratings.
QUESTION: Can I have both IDE/ATA drives and SCSI in the same system?
ANSWER From: Gary Field (scsifaq@engineer.com)
The short answer is YES. There are a few issues to consider however.
The main issue is which device will be used for booting the system. Under MSDOS, The system BIOS determined this completely. A couple third party BIOSes (like MRBIOS) allowed the user to choose the boot source, but most conventional BIOSes just booted from the IDE if it was present. If no IDE was present then the standard option card BIOS scan would find the SCSI card's BIOS and use it to boot.
Under Windows 95 and Windows NT, there are more options. Since the motherboard BIOS is used to load the boot sector that will still happen according to the same rules as under MSDOS described above. After the boot sector is loaded, the O/S's device drivers take over and those can be unloaded or drive letters re-ordered via the O/S configuration tools.
Update: As of 1999, this issue has largeley been solved. Most BIOSes allow the user sufficient control to boot from whatever device they want to (either SCSI or IDE).
QUESTION: Is it possible for two computers to access/share the same SCSI devices?
ANSWER From: Gary Field (scsifaq@engineer.com)
with input from Michael Burke and Cees de Groot and Sheldon E Smith.
Updated: Jan, 2001
Yes, two (or more) hosts can be on the same SCSI bus as other SCSI devices. As long as all the SCSI hardware requirements are met, multiple hosts can share the SCSI bus. With that said, let's look at what you're getting yourself into.
This discussion refers primarily to PCs. There are high end systems
that do allow full of sharing SCSI devices. Usually, this is to allow
fault tolerance. Two systems are connected to the same set of SCSI storage
devices and when one of them fails, the other takes control. AIX with HACMP,
Compaq Tru64 UNIX with TruClusters, and Compaq VMS clusters are examples
of systems that allow this. The combined group of systems with shared
storage is often referred to as a cluster. Open VMS and Tru64 V5.x and
newer also have a feature called the Ditributed Lock Manager which allows
multiple hosts to actually share the filesystems on shared disks locking
individual records as needed.
I am told Windows 2000 server now supports host fail-over for fault
tolerant support of shared disks/RAIDs.
Some basic things you need to watch out for:
- Each host's device drivers must use SCSI RESERVE/RELEASE commands to lock access. This locks the drive for access by only one host adapter. The conflicting host gets a RESERVATION CONFLICT status until the controlling host sends a RELEASE cmd or a bus reset is issued. The bus reset is necessary when the controlling host has failed and left the shared device reserved.
- The host adapters on each host as well as each device on the shared bus must have unique IDs. Some host adapters may not let you set their ID.
- Good and common electrical grounding of both systems and the devices.
- SCSI length limits are not violated. For practical bus lengths you may need differential (either HVD or LVD but not mixed).
- Make sure both hosts select the same data transfer mode (synch or asynch).
- Only shareable devices can be on the shared bus.
- Neither host adapter resets the SCSI bus except when needing to break stuck reservations.
For Disks:
It would not make sense for two hosts to go about treating shared disks
as if they each owned the device. Data would be destroyed pretty quickly.
Even if the user tries to only access the shared disk from one host at
a time, each host retains a cache of filesystem meta-data(directories etc)
and neither host can know what the other host is changing on the disk.
Therefore, unless one of the hosts restricts itself to read-only access,
the filesystem will get corrupted.
Another way of letting multiple hosts share a disk is to break up the
disk into partitions that are reserved for each host. Each host "owning"
its own file system. Some provision needs to be made to prevent either
host from using the wrong partition however. I'm not aware of a good way
to do this.
For CD-ROMs and scanners:
CD-ROM drives and scanners can be shared (for data access) pretty easily
because they are by definition READ-ONLY, but you can't be reading data
from one host and playing music on the same drive from another. With scanners,
discipline will be required to avoid attempting to access the scanner from
both hosts simultaneously.
For Tapes:
Sharing tape devices is straightforward unless you need to handle failing-over
from one host to another. If you want to do this the tape backup application
needs to be written to know how to determine the position on the tape where
writing was last successful and take up from there. If the RESERVE/RELEASE
mechanism is used to reserve the tape drive, a bus reset will need to be
used to break the reservation upon host fail-over. The reset will start
the tape drive rewinding, so the application will need to find where it
left off before it starts writing again, or start the backup over again.
For host to host communication:
Some people get the bright idea that it would be cool to transfer data
directly from one host to another via the SCSI bus. While this might be
cool indeed, you'll have your work cut out for you to get it to work! In
order to do this one of the host adapters needs to flip into "target mode".
In this mode the "target mode" host is made to appear like a disk or tape
to the other host and data can be written/read to/from it in the usual
manner. The snag is that software that does this is very rare and only
works on specific host adapters. See here for
more info on target mode and vendors that supply software.
Conclusions:
Before considering implementing a shared SCSI bus configuration, you
should examine your motives for wanting to do this. If you simply want
to transfer data between the systems, a network (10/100 base T) is a MUCH
simpler solution. A pair of ethernet cards costs about $50 and all the
software you need is built into both Win95/98/NT/ME/2000 or Linux.
If you need fault tolerance, maybe you really do need a shared bus,
but be prepared for lots of expense or years of painful software
development.
Table of Contents
QUESTION:What
is the problem with the Adaptec 1542C and external cables?
ANSWER From: Scot Stelter, Adaptec (Product
Manager for the AHA-1540)
Several articles lately have cited the importance of SCSI-2-compliant cables when cabling SCSI bus subsystems. Perhaps the most accurate and technically detailed one was published in Computer Technology Review in March '93 (Volume XIII, No. 3. PP. 6). In short, it explains the double-clocking mechanism that can occur due to cables whose impedance falls below the 90-Ohm SCSI-2 spec. Steep edge speeds on the REQ and ACK lines of the SCSI bus exacerbate the problem, but non-compliant cables are the root cause. Both LAN TIMES in the US (5/24/93, page 115) and CT Magazine in Germany (7/93, page 18) cite this cable problem.
In an extensive survey of cables available in the US and Europe, we found that more than half of the cables available have single-ended impedances in the 65 to 80 Ohm range -- below the 90 to 132 Ohms specified in the SCSI-2 spec. It seems that some (not all) cable vendors do not understand the specification, describing their cables as SCSI-2 compliant when they are not. A common misconception is that SCSI-2 means a high-density connector. In fact, there are several connector options. I have published a technical bulletin that summarizes the critical requirements (TB 001, April 1993). An artifact of its faster design left the AHA-1540C with faster edge-speeds than its predecessor, the AHA-1540B. As I have said, this can exacerbate the effect of bad cables. This explains why some users could get their AHA-1540B to work when an early AHA-1540C might not.
Essentially, the 1540B was more forgiving than the early 1540Cs. Good cables fixed the problem, but unfortunately for the user, good cables are hard to find.
After surveying the cable market and many of our customers, we decided that bad cables were going to be here for a while, and we had to make the 1540C as forgiving as the 1540B was. At the end of April '93 we made a change to the AHA-1540C that involved using a passive filter to reduce the slew rate of the ACK line, the signal that the host adapter drives during normal data transfers. Extensive testing with many intentionally illegal configurations confirms that we succeeded. Prior to release, we tested the AHA-1540C with over 200 peripherals, systems and demanding software programs with no failures. Then, a second team retested the AHA-1540C across a wild combination of temperatures, humidities and other stresses. This testing gives me confidence that the AHA-1540 line continues to serve as the gold standard for SCSI compatibility.
QUESTION: What is the difference between the Adaptec 1542A and 1542B?
ANSWER From: fishman@panix.com (Harvey Fishman)
The AHA-1542A is obsolete and no longer supported by Adaptec. They stopped providing firmware upgrades at some level prior to the equivalence to the 3.10 level of the AHA-1542B firmware. I am not sure just where though. The present latest AHA-1542B firmware is version 3.20, and supports drives up to 8GB under MS-DOS.
QUESTION:What are the differences between the Adaptec 1542B and the 1542C?
ANSWER from: Terry Kennedy (terry@spcvxa.spc.edu)
The 1542C is an an updated model which replaces the 1542B. The 1542C features jumperless setup, having only 8 DIP switches. All other configuration options are set using the 1542C's built-in BIOS configuration utility. Configurable features not found on the 1542B are:
- Ability to enable/disable sync negotiation on a per-ID basis (the 1542B could only do it for all ID's on the SCSI bus)
- Ability to send "start unit" commands on a per-ID basis
- BIOS works with alternate I/O port settings on the adapter.
- Ability to boot from ID's other than 0
- Software-selectable termination
- Software-selectable geometry translation
- Additional DMA speeds of 3.3 and 10 MB/sec
QUESTION: What are the differences between the 1542C and the 1542CF?
ANSWER from: Terry Kennedy (terry@spcvxa.spc.edu)
The 1542CF includes all of the 1542C features, and adds "Fast" SCSI operation, providing SCSI data rates of up to 10MB/sec (compared with an upper limit of 5MB/sec on the 1542C). This is unrelated to the host DMA rate. It also has a software configurable address for the floppy controller and a "self-healing" fuse for termination power.
QUESTION: Where can I get obsolete Adaptec files and utilities?
ANSWER From: Gary Field (scsifaq@engineer.com)
Try here or here.
You can get ASPI spec's from Adaptec's
web site in the developer section.]
QUESTION:What kinds of optical drives are available?
ANSWER From: Psycho Bob <honge@creighton.edu>[Editor(GF)]
UPDATE: November 1999
Optical storage has good points going for it; Immunity to stray
magnetic fields; Potential for higher storage capacity per unit area; and
relatively low media cost.
Current optical storage solution offers two different types of storage
--rewritable and non-rewritable. The non-rewritable represents storage
method in which the data becomes permanent after being written onto the
disc. Rewritables, on the other hand, allows you to alter the data after
it has been written -- just like the magnetic storage devices. And for
rewritables, two different technologies are available -- magneto-optical
("MO") and phase-change ("PD").
Magneto-Optical
As the name implies, MO uses both magnetic and optical technology to store data on the disc. The disc itself is rare earth metal substrate. When data is to be written, the particular spot is first heated by the laser to the Curie point, and the magnetic field is generated while the spot cools. By varying the magnetic field angle, the substrate is polarized in a certain way that it will reflect back the laser beam differently depending on the magnetic field angle present when the particular spot was cooling down.
MO comes in many sizes and capacities. Consumers were first exposed to MO in Steve Jobs' NeXT computer in the mid-1980s. Although 5.25" had a slow start due to initial high cost, it has been evolving quite nicely.
The more popular ISO capacities for 5.25" MO are 4.8GB/5.2GB, 2.4GB/2.6GB, and 1.2GB/1.3GB. In 3.5" form, MO is available in 540MB/640MB, 230MB,and the 128MB. There are also some 12" MO, 14" MO, and other odd sizes in odd capacities -- particularly the hybrid 3.5" 1.3GB MO/PD drive. But they are limited to niche markets due to high cost and rarity.
Sony MiniDisc-Data is derived from the Mini-Disc (MD) audio format cartridge introduced earlier. MD-Data is to MD as CD-ROM is to digital audio compact disc (CD-DA). MD-Data (and digital audio MD) is based on the same magneto-optical technology, which partially explains the initial high-cost of the consumer MD audio recordable units. Fow now, MD-Data is the smallest of the MO family. With 2.5" form factor, it can store either 140MB or 650MB of uncompressed data.
Sony pushed the original MD-Data in the mid-'90s, but it did not catch on due to high cost (for the capacity offered) and Sony's decision to separate MD audio function from MD-Data. And for few years, MD format has lagged behind the capacity and speed of fellow MO breathens. In November 1999, Sony announced the MD-Data-2 format and has gotten the format up-to-date. It now has 650MB storage capacity with equal increase in transfer speed. The MD-Data-2 debuts with Sony's MPEG-2 camcorder in the Japanese market in December of 1999. The most important technical advancement MD-Data brought for MO in general is the one-pass recording. Prior to 5.25" 2.4GB/2.6GB MO and 3.5" 540MB/640MB MO, practically all MO used two passes to write data onto the disc -- one pass to erase the whole track, and a follow-up pass to write the updated data. MD's one pass recording, called light intensity modulation, direct over-write (LIM-DOW, ISO 14517) has been incorporated into several later-generation MO formats to speed up the writing speed.
Anyway, what's the limit of erase/write cycle can MO endure? Well, it doesn't look like anyone is really sure about it. Few years past, it is guessed to be around 1 million times with 30 years of archival stability. Today, Maxoptix says MO can sustain "greater than 1 trillion" cycles with greater than 50 years of archival storage life.
Today, the popular MO formats are 3.5" and 5.25" that follow the ISO standard. (Yes, there are others. But they're far and few in between...) The drives and medias are available from Fujitsu, IBM, Maxoptix, Pinnacle Micro, Pioneer Sony, Toshiba, and others.
Panasonic phase-change double-function (PD)
In around mid-'95, Panasonic released a proprietary optical storage format called phase-change double-function (PD) drive. The PD uses substrate that will reflect the light differently when heated to different temperatures (and then cooled). Write-once-read-multiple (WORM) medias were actually the first phase-change formats, but PD is the first *reversible* (that is, "re-writable") phase-change format. Panasonic PD stores 650MB per PD cartridge. Panasonic's own PD drive has also gone away with Sony's MD-Data, but the technology lives on in forms of CD-RW and DVD-RAM. The PD media is said to take approximately 1,000 erase/write cycles. After about 1,000 cycles, the substrate will be fatigued to the point where the two different states of the crystalline structure will become difficult to differentiate reliably.
WORM
Write-once-read-multiple (WORM) format is a *write once* format -- once you have written the data to the disc, the written data cannot be changed. Put it another way, the data recorded on the disc media is *permanent*.
WORM was the first popular format for optical storage, before being eclipsed by MO. WORM is still used by big companies and the government for archival purposes since it has the characteristic of not being able to be altered without damaging the media (good audit trail).
The new WORM formats being introduced are tending to be proprietary. There is rarely any interchangability between different vendor's drives and media. During the WORM to MO transition period, a curious format called continuous composite write-once (CCW) appeared. CCW cartridges function as WORM cartridges, writable using the installed base of WORM drives. But put it into MO drive, CCW cartridges becomes rewritable. Simply put, CCW is MO in WORM's clothing. Many of today's 5.25" MO drives still have the capability to read CCW cartridges. And practically all WORM cartridges sold today are CCW variety.
Sony-Philips Compact Disc (CD-R/CD-RW)...
WORM (in "MO" form) was once limited to niche market, but made one heck of a come-back with form of CD-R. CD-R is based on the Sony-Philips' proprietary CD-DA, commonly referred as "CD" (you know, those shiny disc things that America-On-Line sends you). CD-R offers standard capacity of 650MB of data per disc, and can be used to store data or record music (and be played in common CD players). But here, only the data-storage facet of the CD-R/CD-RW is discussed.
As far as data storage is concerned, the specifications are written in the "Orange Book." The Orange Book established three physical format for recordable CDs -- CD-MO, CD-R (previously known as "CD-WO"), and CD-RW (previously known as "CD-E").
CD-MO is a MO medium in CD format. As far as I know, this format only exists only on paper. The popular formats to come out of it are CD-R and CD-RW.
CD-R/CD-RW Incompatibility
Sony and Philips finally agreed on a standard for compact disc re-writable
(CD-RW), together with HP, Matsushita, etc. Long story short, the CD-RW
uses phase-change media -- same as Panasonic proprietary PD format. Not
only that, it also stores 650MB like PD. And also like the PD, the CD-RW
media cannot be read in existing CD-ROM drives! CD-ROM drives manufactured
in 1997 and after will read CD-RW discs though.
CD-R and CD-RW are known for their incompatibilities. There are combinations
of CD-R media, CD-R recorder, and CD-ROM player that simply wouldn't work.
CD-RW is worse -- virtually no audio CD players will play CD-RW disc. The
problem stems from the fact that reflectivity of the CD-R is less than
a factory-pressed CD-ROM. And CD-RW is worse in that respect than the CD-R.
As such, only the very recent CD-ROM player labled "Multi-Read" can read
the CD-RW discs.
DVD
Possibly the most soap-operatic of all data-storage formats. With convergence of computers and audio/video equipment, DVD was the most talked about format for years as several companies fighting for what "DVD" format should be.
Writable DVD formats:
For now, DVD-RAM is not made to be playable in DVD-ROM players that
are so popular for its good pictures. If you've looked at it, you'll notice
DVD-RAM is encased in a carriage case (a la MO-style) but "video" DVDs
aren't. Although I think it may be possible to take the DVD-RAM media out
of the case and stick it into computer DVD-ROM to read the recorded data.
What's the difference between DVD-RAM, DVD-RW, and DVD+RW? (March 2000)
The names differ depending on whose specification the DVD storage is
based on. If it's Matsushita (Panasonic), then it's DVD-RAM; if it's Sony/Philips,
then it's DVD+RW; or if it's Pioneer, then the name becomes DVD-RW. The
majority of current DVD storage devices follow Matsushita's DVD-RAM standard.
Pioneer currently has its DVD-RW in the form of a DVD video recorder in
Japan. Each has slightly different storage capacities. It's still unknown
whether they'll be truly compatible with each other. But all three specs
have been submitted and all are regarded as DVD re-writable "standards."
The Future?
Future optical storage will likely get bigger and bigger capacities, and faster and faster transfer rates.
Anyway, MO is here to stay, so are CD and the DVD family of formats.
As for DVD... The competitions should prove to be entertaining (not). DVD
is not much about technology but more about politics. But since so many
electronic and entertianment giants are backing the DVD, you probably won't
go wrong if you buy one. (Just hope the one you buy will not be orphaned
at the turn of the hat by DVD consortium.) Some form(s) of DVD recordable
will eventually standardized, but don't expect it to have more storage/speed
than what MO/PD/WORM formats offer.
Summary of optical disk formats
Format * |
Physical Size |
Capacity per disk |
Bytes per sector |
# of sides |
Capacity per side |
Standard |
MO 1p |
2.5" |
140MB |
2048/ 2336 |
Single |
140MB |
Sony MD-Data |
| MO 1p | 2.5" | 650MB | 2048/2336 | single | 650MB/720MB | Sony MD-Data-2 |
MO 2p |
3.5" |
128MB |
512 |
Single |
128MB |
ISO/IEC 10090, ECMA 154 |
MO 2p |
3.5" |
230MB |
512 |
Single |
230MB |
ISO/IEC 13963, ECMA 201 |
MO 1p |
3.5" |
540MB 640MB |
512 2048 |
Single Single |
540MB 640MB |
DIS(ISO/IEC) 15041 |
| MO/PD 1p | 3.5" | 1.3GB | 2048 | single | 1.3GB | ISO/IEC 14760 |
MO 2p |
5.25" |
600MB 650MB |
512 1024 |
Dual Dual |
296MB 322MB |
ISO/IEC 10089 ANSI X3.2121-1992 |
|
MO 1p |
5.25" | 654MB | ?? | single | 654MB | Pioneer |
MO 2p |
5.25" |
1GB 1GB |
512 1024 |
Dual Dual |
463MB 510MB |
ISO 13481 |
MO 2p |
5.25" |
1.2GB 1.3GB |
512 1024 |
Dual Dual |
595MB 650MB |
ISO/IEC 13549 and ECMA 184 |
MO 1p |
5.25" |
2.4GB 2.6GB |
512 1024 |
Dual Dual |
1.2GB 1.3GB |
DIS(ISO/IEC) 14517 |
MO 2p |
5.25" |
1.5GB |
4096 |
Dual |
750MB |
Panasonic |
|
MO 1p |
5.25" | 1.7GB | ?? | Dual | 654MB | Pioneer |
MO 1p |
5.25" |
4.6GB |
1024 |
Dual |
2.3GB |
Pinnacle Micro "Apex" |
| MO 2p | 5.25" | 4.1GB
4.8GB 5.2GB |
512
1024 2048 |
Dual | 2.1GB | ISO/IEC 15286 |
MO |
12" |
8GB |
?? |
Nikon |
||
MO |
12" |
3.2GB |
?? |
Sony |
||
MO |
14" |
6.8GB 10.2GB 14.8GB |
1024 1024 1024 |
Dual Dual Dual |
3.4GB 5.1GB 7.4GB |
Kodak System 2000 |
| WORM | 5.25" | 1.3GB | ?? | Dual | 650MB | ISO/IEC 11560 |
WORM |
5.25" |
2.6GB |
DIS(ISO/IEC) 15486 |
|||
WORM |
5.25" |
650MB |
Single |
650MB |
ISO/IEC 9171 Format A |
|
|
WORM |
5.25" | 654MB | ?? | single | 654MB | Pioneer |
WORM |
5.25" |
470MB 940MB 1.3GB |
Single Dual |
470MB 650MB |
Panasonic ISO/IEC 10091 |
|
| WORM | 5.25" | 1.4GB | ?? | Dual | 700MB | Panasonic |
|
WORM |
5.25" | 1.7GB | ?? | Dual | 654MB | Pioneer |
| WORM | 5.25" | 2.6GB | ?? | Dual | 1.3GB | ISO/IEC 15486 |
|
WORM |
5.25" | 5.2GB | ?? | Dual | 2.6GB | DIS (ISO/IEC) 18093 |
WORM |
12" |
15GB |
Sony |
|||
| WORM | 12" | 19GB | ?? | Dual | ?? | Pioneer LD-R |
|
WORM |
14" | 14.8GB
25GB |
DIS (ISO/IEC) 15898 | |||
PD 1p |
5.25" |
650MB |
4096 |
Single |
650MB |
Panasonic |
| CD-R | 3.5" | 130MB | 2048 | Single | 130MB | Orange Book |
CD-R |
5.25" |
650MB |
2048 |
Single |
650MB |
Orange Book |
CD-RW 1p |
5.25" |
650MB |
2048 | Single | 650MB |
Orange Book |
DVD-ROM |
5.25" |
4.7GB 9.4GB 17GB |
2048 2048 2048 |
Single 2layer dual |
4.7GB 9.4GB 9.4GB |
UDF ISO-13346 |
| DVD-R | 3.5" | 2.4GB | 2048 | dual | 1.2GB | 1.2GB, ECMA-279 |
|
DVD-R |
5.25" | 3.95GB
4.7GB |
2048 | Single | 3.95GB
4.7GB |
Pioneer |
|
DVD-R |
5.25" | 8GB | 2048 | dual | 4GB | ECMA-279 |
|
DVD-RAM 1p |
5.25" | 2.6GB
5.2GB |
2048 | Single
dual |
2.6GB | ECMA-272, ECMA-273 |
| DVD-RAM 1p | 5.25" | 4.7GB
9.4GB |
2048 | Single
dual |
4.7GB
9.4GB |
UDF ISO-13346 |
| DVD-RW 1p | 5.25" | 3GB
6GB |
ECMA-274 |
*technology: 1p -- one-pass write 2p -- two-pass writeStandards for storage are set by many organizations. International Standards Organization (ISO), European Computer Manufacturers Association (ECMA), Deutsche Institut fur Normung (DIN), Japanese Industrial Standards Committee (JISC), and American National Standards Institute (ANSI) set the main optical disc storage standards. The ISO standards take precedence over all other standards.
In the above table, the heading defines one standard -- e.g. 5.25" MO 1.2GB/1.3GB has both ISO 13549 and ECMA 184 listed for it. IT IS NOT THAT 1.2GB FOLLOWS ISO 13549 AND 1.3GB FOLLOWS ECMA 184.
Of CD standards...
Funny as it seems, CD is actually considered as a proprietary format made by Sony and Phillips. The physical format for derivatives like CD-ROM and CD-R are "written in mutual agreement" in form of Red Book, Yellow Book, Orange Book, etc.
Of bytes/sector and usability...
As many of you might notice (especially on 5.25" MOs), there are different sized sectors. Many O/Ses assume one sector to contain 512 bytes. If you buy any of the media that use different than 512 byte/sector, you will need a software driver of some sort to use the media.
In optical media, the sectors are "hard sectored" at factory -- in other words, you cannot change the number of sectors by reformatting (low-level formatting) them. Take the 5.25" 1.2GB/1.3GB MO for example again. The 1.3GB media is sectored at 1024 bytes per sector. So the 1.3GB media has total of 637,041 sectors (per side) on it. If you do not use a software driver and your operating system does not properly recognize it, the 1.3GB media will become a 650MB cartridge (~325MB per side)!!
The safest bet is to use the 512 bytes/sector media. That should make the drive and media usable on most operating systems.
QUESTION: Where can I get various SCSI documentation?
QUESTION: How can I find out about the emerging SCSI standards?
Updated: August, 2000
Thanks to John Lohmeyer of LSI Logic (formerly Symbios Logic, AT&T GIS, NCR Microelectronics), a number of SCSI related files are freely available.
This is the place to find more information about I/O Interfaces, especially
SCSI, SCSI-2, and SCSI-3 including SPI, Fast-20 (Ultra
SCSI), Fast-40 (Ultra2 SCSI), Low Voltage Differential (LVD), SPI-3
(Ultra3 SCSI or Ultra160), SPI-4 (Ultra320), CAM, and much more. There
are also pointers here to other web sites on Fibre Channel, ATA (IDE),
and ATAPI.
The information is accessible from:
WWW: http://www.t10.org
SASI Spec. - (.PDF format): ftp://ftp.t10.org/t10/drafts/sasi/sasir0C.pdf
SCSI-1 draft standard - (Plain text, no figures, Dec. 1985): ftp://ftp.t10.org/t10/drafts/s1/s1-r17b.txt OR here
SCSI-2 draft standard (converted to HTML) -
From: Gary Watson (trimm@netcom.com)
Small Form Factor (SFF) Committee documents (like the SCA spec's) are available by FaxAccess at:
(408) 741-1600 You will be asked to order documents by number.
For example: to get information on the Single Connector Attach spec.
The SCA-1 spec. is document #8015
The SCA-2 spec. is document #8046 (8451?)
document #8000 is an index to the other documents.
SCA-2 pinout data can also be found in the SCSI3 SPI-4 document referred
to as "Alternative 4".
This FaxAccess service is available to all, but please keep in mind that unless you have engineering-level understanding of peripheral interfaces, you _will_not_ be able to understand any of it and you are wasting your own time and the bandwidth of these resources. If you are trying to learn more about SCSI, you are better off reading the magazine articles and books listed elsewhere in this FAQ.
The SCSI, SFF, SSA, and Fibre Channel reflectors:
A list of these is available on the T10 site.
"The SCSI, SFF, SSA, and Fibre Channel reflectors are for review and commentary on the respective specifications, not for asking questions about the interfaces (unless related to a specific ambiguity in a specification) nor for recruiting nor for technical support nor any purpose other than what is stated. The reflectors _are_ available for public review and commentary as required by ANSI and ISO."
Any spec on the reflectors or on the BBS or on the ftp sites are **proposed**
or **preliminary** and are often subject to major substantive changes during
the committee process. Actual, released, final specs are *only* available
from Global Engineering Documents.
For Fibre Channel Info:
For Firewire (IEEE-1394) Info:
QUESTION:Where can I get official ANSI SCSI documents?
ANSWER #1 From: kev@hpcpbla.bri.hp.com (Kevin Jones)
and jmatrow@donald.WichitaKS.NCR.COM (John
Matrow)
The SCSI specification: Available from:
ANSI
11 West 42nd St. - 13th floor
New York, NY 10036
Sales Dept. (212) 642-4900
OR
Global Engineering Documents
15 Inverness Way East
Englewood Co 80112-5704
(800) 854-7179 or (303) 792-2181
Int'l Sales Fax: (303) 397-2740
SCSI-1: X3.131-1986
SCSI-2: X3.131-199x
SCSI-3 X3T9.2/91-010R4 Working Draft
[Editor(GF):] The official ANSI standards are NOT available free of charge from any source. Only draft versions are freely distributable.
QUESTION: What SCSI books and tutorials are available?
ANSWER From: Gary Field (scsifaq@engineer.com)
Updated: June, 2000
The Book of SCSI : I/O for the New Millennium, by Gary Field, Peter Ridge et al
Published by No Starch Press, San Francisco, CA
ISBN # 1-886411-10-7 , List Price $49.95.
A very complete reference and tutorial on almost all aspects of SCSI,
including all the latest advances like Ultra2WIDE/LVD, and all the previous
standard SCSI features. It addresses everything you need to know to install
and debug SCSI I/O on a PC running Windows 95/98/NT and information
on Linux as well. Also includes a CD-ROM with useful SCSI utilities and
information.
The technical editor was none other than John Lohmeyer (chairman of
the ANSI SCSI committee since the beginning of SCSI), so you know the facts
are straight!
IN-DEPTH EXPLORATION OF SCSI can be obtained from Solution Technology, Attn: SCSI Publications, POB 104, Boulder Creek, CA 95006, (408)338-4285, FAX (408)338-4374
THE SCSI ENCYLOPEDIA and the SCSI BENCH REFERENCE can be obtained from ENDL Publishing, 14426 Black Walnut Ct., Saratoga, CA 95090,
(408)867-6642, FAX (408)867-2115
SCSI: UNDERSTANDING THE SMALL COMPUTER SYSTEM INTERFACE was published by Prentice-Hall, ISBN 0-13-796855-8 (Seems to be out of print)
A neat little book called "Basics of SCSI" second edition, was sent to me free of charge by Ancot Corporation, Menlo Park, CA (415) 322-5322. It gives a simplified description of how most aspects of the SCSI bus work and includes some discussion of SCSI-2 issues.
"Programmer's Guide to SCSI" with CDROM - by Brian Sawert.
Published by Addison Wesley, Reading, MA. SRP $39.95
ISBN # 0-201-18538-5
Includes a chapter on UNIX SCSI subsystems written by Gary Field.
Addition by: (kyrrin2@wizards.net)
'The SCSI Bus and IDE Interface' 2nd edition by Friedhelm Scmidt,
Addison-Wesley Publishing, $34.95 (I think). It includes a diskette with examples of source code to handle SCSI and IDE devices from a low-level programmer's perspective, and it has very detailed technical descriptions of both subsystems.
Not a book for beginners, but I heartily recommend it for anyone who's
serious about learning the technical ropes.
ANSWER #2 From: Runar Jorgensen (runar.jorgensen@fys.uio.no)
There was a two part article in Byte Magazine. The first part was in Feb 1990 issue, p. 267-274 and the second was in Mar 1990 issue, p. 291-298.
Another two part article appeared in Byte in May 1986 and June 1986.
QUESTION: Where can I find SCSI info on the Web?
ANSWER FROM: Gary Field (scsifaq@engineer.com)
Try some of these: Table of Contents
QUESTION: Where can I get information on various disk drives and controllers?
ANSWER: ekrieger@quasar.xs4all.nl (Eric Krieger)
(Updated Sep. 30, 1994)
Drive and Controller Guide, Version 4.3
THEREF(tm) is a comprehensive Directory of Hard Drives, Floppy Drives, Optical Drives, and Drive Controllers & Host Adapters. It is designed to help the novice and pro alike with integration problems and system setups.
Information is provided in two handy formats; Portrait mode, for those who prefer a normal book-binding type print format, and(or) do not have a printer with Landscape capability, and Landscape mode, for those who pre-fer a computer-printout type format.
For printing, a Laserjet is preferred, but not necessary, and setup info is provided. For viewing, LIST(tm) by Vernon Buerg, will provide an excellent result, and allow text searches for finding specific models.
By F. Robert Falbo
Due many reports about the unavailablity of this file/archive I made sure that the file does exist at the following site:
You should find the archive at:
/pub/doc/hardware/harddisks/theref43.tar.gz
/pub/doc/hardware/harddisks/theref43.readme
(In that directory-path there is also a sub-directory Seagate, where you also can find info/files about Seagate-drives).
Before you actually get this file, be sure to get/read the file /README.FILETYPES since it explains the used file-extension and which (de-)archiver should be used (and where to find/get them!).
Note: In the archive there are files containing Extended ASCII or ANSI characters (mostly used with IBM- and compatible PC's), so it may be a bit unreadable when reading it on non-PC systems, or without using a proper Characterset/Font!
QUESTION: Where can I get technical information and jumper settings for HP drives?
ANSWER From: Rodney Brown (RBrown@cocam.com.au)
Update From: Martin C Mueller (mcm@mathematik.uni-kl.de
)
QUESTION: How can I contact Adaptec?
Also: Future Domain, Corel CD Creator, Trantor, Incat systems.
ANSWER From: jcaples@netcom.com (Jon D Caples)
408 945-8600 Main number
800 959 7274 tech support
800 442 7274 orders, doc, new bios, etc.
408 945-7727 BBS
Adaptec's general inquiry number, 800-959-7274, affords access to a FAX-based information retrieval system. In order to preserve the accuracy of this information, I won't go into details about how to use it (since Adaptec may change things without telling me :) ).
For those outside the CAN-US area, or local to Adaptec the direct FAX info number is (408) 957-7150.
There are three general topics as of this writing:
- General Information
- Sales Information
- Technical Information
[Editor(GF): As of July 1993 Adaptec bought Trantor.
Try (800) 872-6867 (TRA-NTOR)]
World Wide Web (WWW) URL: http://www.adaptec.com/
[(from: Andrew Lockhart (andrew@interact.manawatu.planet.co.nz) ]
You can address Adaptec support by email. The address is support@adaptec.com. An auto-responder will bounce a message back acknowledging receipt of your email. This message will also detail other current forms of Adaptec Technical support. They promise a, no more than, 5 day turn-around. We have found the response brief, but satisfactory to our needs. We should add, we mention we are dealers in our email (which may improve Adaptec's response).
QUESTION:How can I contact Archive Corporation?
[Editor(GF)]
Archive was bought by Conner Peripherals in 1993
QUESTION: How can I contact BusLogic/Bustek/Mylex?
ANSWER From: Gary Field (scsifaq@engineer.com)
QUESTION: How can I contact Corel?
ANSWER From: Gary Field (scsifaq@engineer.com)
Main Number: (800) 772-6735
Tech. Support: (613) 728-1010
ANSWER From: Gerrit Visser (gerrit@isgtec.com)
WWW: http://www.corel.ca/
ftp: ftp.corel.ca:/pub (SCSI is under Multimedia)
For Corel CD Creator Software contact Adaptec
QUESTION: How can I contact Fujitsu?
ANSWER From: Ken Porter (72420.2436@compuserve.com)
Fujitsu FactsLine FAX Back service (408) 428-0456
A six page catalog of available documents can be ordered.
ANSWER From: Mike Henry (anonymous)
A while back, Fujitsu created a product called Fujitsu Knowledge System (FKS) (long available on Compuserve (GO FUJITSU)). It is a Windows Help File (.HLP) listing of many Fujitsu disk, tape, and optical products. It includes drive switch/jumper settings and meanings. It is available via anonymous ftp from ftp.intellistor.com in the /pub/fks directory, filename: fks.exe
It is self-extracting and mostly self-documenting.
QUESTION: How can I contact Quantum?
ANSWER From: kmartine@qntm.com (Kevin Martinez)
Quantum Corporation
500 McCarthy Blvd.
Milpitas, CA
95035
Technical Support Telephone Numbers:
800 826-8022 Main Technical Support Number
408 894-3282 Technical Support Fax
408 894-3214 Technical Support BBS V.32 8N1
408 434-9262 Technical Support for Plus Development Products
408 894-4000 Main Quantum Phone number
800 4DISKFAX FAX on demand (From Thanh Ma tma@encore.com)
WWW: http://www.quantum.com/
Note: In October, 2000 Maxtor merged with Quantum.
QUESTION: How can I contact Seagate?
ANSWER From: John McDonald (John_McDonald@notes.seagate.com)
Technical Support Services
Online Services
Using a modem, you can obtain troubleshooting tips, free utility programs, drive specifications, and jumper settings for Seagate's entire product line. You can also download software for installing and analyzing your drive.
- SeaNET
- SeaBOARD
- Australia 61-2-9756-2359
- France 33 1-48 25 35 95
- Germany 49-89-1409331
- Taiwan 886-2-2719-6075
- Thailand 662-531-8111
- UK 44-1628-478011
- USA Disc: 405-936-1600
- Tape: 405-936-1630
You can obtain technical information about Seagate products over the Internet from Seagate's World Wide Web home page (http://www.seagate.com), Seagate's FTP server (ftp://ftp.seagate.com) or e-mail. Send your e-mail questions to DiscSupport@seagate.com or TapeSupport@seagate.com.
SeaBOARD is a computer bulletin board system that contains information about Seagate disc and tape drive products and is available 24 hours daily. Set your communications software to eight data bits, no parity and one stop bit (8-N-1).
Location Phone number
- SeaFAX
- Australia 61-2-9756-5170
- Germany 49-89-14305102
- UK 44-1628-894084
- USA 1-800-SEAGATE or
- Disc: 405-936-1620
- Tape: 405-936-1640
- Seagate Technical Support FAX
- Australia 61-2-9725-4052
- France 33 1-46 04 42 50
- Germany 49-89-14305100
- Hong Kong 852-2368 7173
- Japan 81-3-5462-2979
- Korea 82-2-556-7395
- 82-2-556-4251
- Singapore 65-488-7528
- Taiwan 886-2-2715-2923
- UK 44-1628-890660
- USA Disc: 405-936-1685
- Tape: 405-936-1683
You can use a touch-tone telephone to access Seagate's automated FAX system to receive technical support information by return FAX. This service is available 24 hours daily.
Location Phone number
You can FAX questions or comments to technical support specialists 24 hours daily. Responses are sent during business hours.
Location Phone number
- SeaFONE
- Seagate Telephone Technical Support
- Australia 61
1-800-SEAGATE
Seagate's 800 number (1-800-732-4283) allows toll-free access to automated
self-help services that provide answers to commonly asked questions, troubleshooting
tips, and specifications for disc drives and tape drives. This service
is available 24 hours daily and requires a touch-tone phone. International
callers can reach this automated self-help service by calling 405-936-1234.
For one-on-one help, you can talk to a technical support specialist during local business hours. Before calling, note your system configuration and drive model number (STnnnn).
Location Phone number
QUESTION: How can I contact Conner Peripherals?
Conner Peripherals was bought by Seagate
QUESTION: How can I contact Maxtor?
ANSWER From: David G North (D_North@tditx.com)
In October 2000, Maxtor merged with Quantum.
Their web site is still available however.
QUESTION: How can I contact NCR?
NCR Microelectronics division was bought by AT&T and then by Symbios Logic.
See "How can I contact Symbios Logic"
QUESTION:How can I contact Philips?
ANSWER From: S. C. Mentzer (smentzer@anes.hmc.psu.edu)
Philips Consumer Electronics Co.
One Philips Drive
Knoxville, TN 37914-1810
(615) 521-4316
(615) 521-4891 (FAX)
[Editor(GF)]
QUESTION: How can I contact Symbios Logic?
ANSWER From: Symbios Logic
Update From: Wade Adams (link@vantek.net)
[Editor(GF)]: In Feb., 1998, Adaptec attempted to purchase Symbios Logic. The Federal Trade Commission told them they couldn't and subsequently, Symbios was sold to LSI Logic. In 2014 LSI was acquired by Broadcom Limited.
For literature on any Symbios Logic product please contact:
Phone: (800) 636-8022
(800) 856-3093
(719) 536-3300
Fax: (719) 536-3301
email: literature@symbios.com
Technical Support:
Phone: (719) 533-7230
WWW: http://www.symbios.com/
also: https://www.broadcom.com
QUESTION: How can I contact UltraStor? (Out of business)
ANSWER From: Ultrastor
UltraStor Corporation
13766 Alton Parkway suite 144
Irvine, CA 92718
General (714) 581-4100
Tech. Support (714) 581-4016
FAX (714) 581-4102
BBS (714) 581-4125
email: ultrastor@primenet.com
finger: ustor@primenet.com
ftp: ftp.primenet.com:users/u/ustor
QUESTION: How can I contact Tecmar Technologies (formerly Wangtek, WangDAT,Sytron, and Rexon)?
ANSWER FROM: from: Jay Long - (jayl@mfltd.co.uk) and
Peter Dyballa (pete@riese.thi.informatik.uni-frankfurt.de)
Tecmar Technologies, Inc.
1900 Pike Rd., Bldg. E
Longmont, CO USA
phone: (303) 682-3700
(303) 776-7706
FAX: (303)776-1085
faxback: (800) 4BACKUP
European Office
Unit 15 Suttons Business Park
Suttons Park Avenue
Earley, Reading, UK RG6 1AZ
(44) 1189-660063
(44) 1189-660065 FAX
Singapore Office
Blk. 35 Marsiling Industrial Estate Road 3 #05-01/ 06
Singapore 739257
(65) 269-2228
(65) 360-0888 fax
QUESTION:How can I contact Western Digital?
ANSWER From: FILIPG@PARANOIA.COM
Address:
Western Digital Corporation
8105 Irvine Center Drive
Irvine, CA USA 92718
Online Services:
Tech Support BBS 714-753-1234 (up to 28.8 KBS)
WWW: http://www.wdc.com/
FTP ftp.wdc.com
AOL (keyword) WDC or Western Digital
MSN (go word) WDC
QUESTION: How can I contact DPT (Distributed Processing Technology)?
ANSWER: From: Gary Field (scsifaq@engineer.com)
voice: (407) 830-5522
FAX: (407) 260-6690
[Editor(GF)]
WWW: http://www.dpt.com/
QUESTION: How can I contactMicropolis?
ANSWER: From: Richard Ravich (Richard_Ravich@microp.com)
[Editor(GF): As of late 1997 Micropolis is now out of business -- RIP.
QUESTION: How can I contact Legacy Storage Systems?
ANSWER: From: Gregory Smith (GREGS@lss-hq.mhs.compuserve.com)
General: (905) 475-1077
Sales/Tech support/Service: (905) 475-0550
U.S. Tech Support: (800) 361-5685
Fax: (905) 475-1088
Mail:
Legacy Storage Systems
43 Riviera Drive
Markham, ON Canada L3R 5J6
QUESTION: How can I find out about IBM hard disks?
ANSWER: From: Gary Field (scsifaq@engineer.com)
QUESTION: How can I contact Initio?
ANSWER: From: Gary Field (scsifaq@engineer.com)
QUESTION: What is FAST SCSI?
ANSWER From: kev@hpcpbla.bri.hp.com (Kevin
Jones)
There are 2 handshaking modes on the SCSI bus, used for transferring data:
ASYNCHRONOUS is a classic Req/Ack handshake.
SYNCHRONOUS is "sort of" Req/Ack, only it allows you to issue multiple Req's before receiving Ack's. What this means in practice is that SYNCHRONOUS transfers are approx 3 times faster than ASYNCHRONOUS.
SCSI1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec and synchronous transfers at up to 5.0 Mbytes/Sec.
SCSI2 had some of the timing margins "shaved" in order that faster handshaking could occur. The result is that asynchronous transfers can run at up to 3.0 Mbytes/Sec and synchronous transfers at up to 10.0 Mbytes/Sec.
The term "FAST" is generally applied to a SCSI device which can do syncrhonous transfers at speeds in excess of 5.0 Mbytes/Sec. This term can only be applied to SCSI2 devices since SCSI1 didn't have the timing margins that allow for FAST transfers.
QUESTION: How can I check a passive SCSI terminator?
ANSWER From: stevel@coos.dartmouth.edu (Steve
Ligett)
With an Ohmmeter:
The terminator contains 18 220-ohm resistors from signals to TERMPWR, and 18 330-ohm resistors from those signals to GROUND. I've drawn that below:
TERMPWR --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | | | | | | | | | | | | | | | | | R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 | | | | | | | | | | | | | | | | | | sig o o o o o o o o o o o o o o o o o o | | | | | | | | | | | | | | | | | | R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 | | | | | | | | | | | | | | | | | | --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ GROUND R1 = 220 Ohms, R2 = 330 OhmsWhen you measure from any one signal to termpower, you aren't measuring that resistor in isolation, you are measuring that resistor IN PARALLEL with the combination of the corresponding 330 ohm resistor plus 17 220+330 ohm resistor pairs in series.
I've redrawn the schematic to make this easier to see: TERMPWR /+---+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | | | | | | | | | | | | | | | | | | R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 | | | | | | | | | | | | | | | | | | | o o o o o o o o o o o o o o o o o | | | | | | | | | | | | | | | | | | | R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 | | | | | | | | | | | | | | | | | | | --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | / GROUND R1 | | | | R2 | / o <--------- 17 other pairs in parallel ----------> sigWe're trying to measure that one resistor from a signal to TERMPWR, but there's a ton of other stuff in parallel. The resistance of that "stuff" is 330 + 550/17 ohms (the 330 ohm resistor, in series with a parallel combination of 17 550 ohm resistors). The general formula for the equivalent of two resistances in parallel is R1*R2/(R1+R2).
Whipping out my trusty spreadsheet, I find that the "stuff" has a resistance of about 362 ohms, and that, in parallel with 220 ohms is about 137 ohms.
QUESTION:Can someone explain to me the difference between 'normal' SCSI and differential SCSI?
ANSWER From: ralf@wpi.WPI.EDU (Ralph Valentino)
"Normal" SCSI is also called "Single-ended" SCSI. For each signal that needs to be sent across the bus, there exists a wire to carry it. With differential SCSI, for each signal that needs to be sent across the bus, there exists a pair of wires to carry it. The first in this pair carries the same type of signal the single-ended SCSI carries. The second in this pair, however, carries its logical inversion. The receiver takes the difference of the pair (thus the name differential), which makes it less susceptible to noise and allows for greater cable length.
QUESTION: What are the pinouts for differential SCSI?
ANSWER From: ralf@wpi.WPI.EDU (Ralph Valentino)
Differential SCSI Connector Pinouts ---------------------------------------- ---------------------------------------- | SCSI | | MINI | | | SCSI | | MINI | | | SIGNAL | DD-50P | MICRO | DD-50SA | | SIGNAL | DD-50P | MICRO | DD-50SA | ---------------------------------------- ---------------------------------------- | -GND | 2 | 26 | 34 | | (open) | 1 | 1 | 1 | | -DB(0) | 4 | 27 | 2 | | +DB(0) | 3 | 2 | 18 | | -DB(1) | 6 | 28 | 19 | | +DB(1) | 5 | 3 | 35 | | -DB(2) | 8 | 29 | 36 | | +DB(2) | 7 | 4 | 3 | | -DB(3) | 10 | 30 | 4 | | +DB(3) | 9 | 5 | 20 | | -DB(4) | 12 | 31 | 21 | | +DB(4) | 11 | 6 | 37 | | -DB(5) | 14 | 32 | 38 | | +DB(5) | 13 | 7 | 5 | | -DB(6) | 16 | 33 | 6 | | +DB(6) | 15 | 8 | 22 | | -DB(7) | 18 | 34 | 23 | | +DB(7) | 17 | 9 | 39 | | -DB(P) | 20 | 35 | 40 | | +DB(P) | 19 | 10 | 7 | | GND | 22 | 36 | 8 | |DIFSENS | 21 | 11 | 24 | | GND | 24 | 37 | 25 | | GND | 23 | 12 | 41 | |TERMPWR | 26 | 38 | 42 | |TERMPWR | 25 | 13 | 9 | | GND | 28 | 39 | 10 | | GND | 27 | 14 | 26 | | -ATN | 30 | 40 | 27 | | +ATN | 29 | 15 | 43 | | GND | 32 | 41 | 44 | | GND | 31 | 16 | 11 | | -BSY | 34 | 42 | 12 | | +BSY | 33 | 17 | 28 | | -ACK | 36 | 43 | 29 | | +ACK | 35 | 18 | 45 | | -RST | 38 | 44 | 46 | | +RST | 37 | 19 | 13 | | -MSG | 40 | 45 | 14 | | +MSG | 39 | 20 | 30 | | -SEL | 42 | 46 | 31 | | +SEL | 41 | 21 | 47 | | -C/D | 44 | 47 | 48 | | +C/D | 43 | 22 | 15 | | -REQ | 46 | 48 | 16 | | +REQ | 45 | 23 | 32 | | -I/O | 48 | 49 | 33 | | +I/O | 47 | 24 | 49 | | GND | 50 | 50 | 50 | | GND | 49 | 25 | 17 | ---------------------------------------- ----------------------------------------
Please note that I can only verify the DD-50P connector. The Mini
Micro and DD-50SA pinout above is a pin for pin mapping from the SCSI
pinout in this FAQ.
====
How can I tell if I have a single ended or a differential drive?
ANSWER From: Gary Field (scsifaq@engineer.com)
Most times the model number of the drive will end with "D".
Use an Ohm meter to check the resistance between pins 21 & 22.
On a single ended system, they should both be tied together and tied to GND.
On the differential drive, they should be open or have a significant resistance between them. Differential drives are less common than single-ended ones, because they are mainly used only where longer cable runs are necessary, and they are not generally used in PCs, but state of the art drives are available with differential interfaces. Generally only the higher performance drives have a differential option because of the added cost.
QUESTION: Who manufactures SCSI extenders and Single-Ended to Differential converters ?
ANSWER From: Gary Field (scsifaq@engineer.com)
----------------------
Apcon Inc.
17938 SW Boones Ferry Road
Portland, OR 97224
Phone: (503) 639-6700 Fax: (503) 639-6740
Email: info@apcon.com
----------------------
Paralan Corporation
4655 Ruffner Street
San Diego CA 92111
Phone: 619-560-7266 Fax: 619-560-8929
email: scsi@paralan.com
---------------------
Rancho Technology Inc.
10783 Bell Court-Rancho
Cucamonga-CA-91730
Phone: (909)987-3966; Fax: (909)989-2365;
E-Mail: scsi@rancho.com; BBS: (909)980-7699
URL: http://www.rancho.com
QUESTION: I've got a SCSI disk with an 80 pin connector. Someone called it an SCA drive. Can I connect this to my SCSI bus?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: Updated July 1999
SCA stands for "Single Connector Attachment". It is a standard being worked on by the ANSI Small Form Factor (SFF) committee. It combines WIDE SCSI signals, Power connections and ID switch connections onto one connector.
The main reason for creating this standard was to make it easier to connect drives in a hot-swappable RAID configuration.
SCSI vendors sell adapters that bring out the three sets of signals to conventional connectors.
Some places that sell such adapters are:
(There aren't any host adapters with 80 pin connectors, so don't ask )
See: SCA Specifications for more information about SCA.
SCA adapters for LVD drives:
If your SCA drive is an Ultra2 WIDE or LVD type drive, you need to
make sure to get an SCA adapter designed to work with LVD drives. Some
SCA adapters don't connect the DIFFSENS signal through which prevents proper
sensing of whether single ended devices are on the bus. So if your drive
is LVD, be sure to ask the vendor for an adapter that's compatible with
LVD.
QUESTION:What are the pinouts for SCSI connectors?
ANSWER From: snively@scsi.Eng.Sun.COM (Bob Snively)
[ Edited and expanded by Gary Field (scsifaq@engineer.com)
]
Originally dated May 23, 1990
The connector families described by the drawings have standard pin numberings which are described the same way by all vendors that I have encountered. The SCSI-2 specification identifies the standard numbering, using that convention. It happened to be documented by AMP, but all the vendors use the same convention.
The following diagrams have the outline drawings of connector sockets at the bottom. This is really for reference only, because the connector sockets and plugs are both specified as to their numbering and usually are labeled.
-------------------- microSCSI to SCSI Diagram --------------------------- SCSI Connector Pinouts (single-ended) ------------------------------------ ------------------------------------- | SCSI | | MINI | | | SCSI | | MINI | | | SIGNAL| DD-50P | MICRO | DD-50SA | | SIGNAL | DD-50P | MICRO | DD-50SA | ------------------------------------ ------------------------------------- | -DB(0)| 2 | 26 | 34 | | GND | 1 | 1 | 1 | | -DB(1)| 4 | 27 | 2 | | GND | 3 | 2 | 18 | | -DB(2)| 6 | 28 | 19 | | GND | 5 | 3 | 35 | | -DB(3)| 8 | 29 | 36 | | GND | 7 | 4 | 3 | | -DB(4)| 10 | 30 | 4 | | GND | 9 | 5 | 20 | | -DB(5)| 12 | 31 | 21 | | GND | 11 | 6 | 37 | | -DB(6)| 14 | 32 | 38 | | GND | 13 | 7 | 5 | | -DB(7)| 16 | 33 | 6 | | GND | 15 | 8 | 22 | | -DB(P)| 18 | 34 | 23 | | GND | 17 | 9 | 39 | | GND | 20 | 35 | 40 | | GND | 19 | 10 | 7 | | GND | 22 | 36 | 8 | | GND | 21 | 11 | 24 | | RSR | 24 | 37 | 25 | | RSR | 23 | 12 | 41 | |TERMPWR| 26 | 38 | 42 | | OPEN | 25 | 13 | 9 | | RSR | 28 | 39 | 10 | | RSR | 27 | 14 | 26 | | GND | 30 | 40 | 27 | | GND | 29 | 15 | 43 | | -ATN | 32 | 41 | 44 | | GND | 31 | 16 | 11 | | GND | 34 | 42 | 12 | | GND | 33 | 17 | 28 | | -BSY | 36 | 43 | 29 | | GND | 35 | 18 | 45 | | -ACK | 38 | 44 | 46 | | GND | 37 | 19 | 13 | | -RST | 40 | 45 | 14 | | GND | 39 | 20 | 30 | | -MSG | 42 | 46 | 31 | | GND | 41 | 21 | 47 | | -SEL | 44 | 47 | 48 | | GND | 43 | 22 | 15 | | -C/D | 46 | 48 | 16 | | GND | 45 | 23 | 32 | | -REQ | 48 | 49 | 33 | | GND | 47 | 24 | 49 | | -I/O | 50 | 50 | 50 | | GND | 49 | 25 | 17 | ------------------------------------ ------------------------------------- * NC = NOT CONNECTED CONNECTOR TYPES: DD-50SA ________________________ MINI-MICRO DD-50P | ------------------- | ______________________ ______ ______ |17 \. . . . . . . . . /1 | | _________________ | 49| . . . . . .| 1 |33 \. . . . . . . . /18 | | 1\ - - - - - - - /25 | 50| . . . . . .|2 |50 \. . . . . . . / 34 | | 26\- - - - - - -/50 | ------------- | ------------- | | ------------- | ------------------------- ---------------------- ribbon cable Old style Sun SCSI "SCSI-2" male male __________________ ( 1 25 ) \ ++++++++++++++ / \ 26 50/ -------------- "Centronics" 50 male (use pin numbers for MINI-MICRO) (VIEWED FROM FACE OF CONNECTOR - USE VENDOR NUMBERING SYSTEM AS SPECIFIED) 16 bit Wide SCSI-3 "P" (Primary) Connector pinout (single-ended) -------------------- -------------------- | SCSI | HIGH DEN | | SCSI | HIGH DEN | | SIGNAL | 68 PIN | | SIGNAL | 68 PIN | -------------------- -------------------- | GND | 1 | | -DB(12)| 35 | | GND | 2 | | -DB(13)| 36 | | GND | 3 | | -DB(14)| 37 | | GND | 4 | | -DB(15)| 38 | | GND | 5 | | -DB(P1)| 39 | | GND | 6 | | -DB(0) | 40 | | GND | 7 | | -DB(1) | 41 | | GND | 8 | | -DB(2) | 42 | | GND | 9 | | -DB(3) | 43 | | GND | 10 | | -DB(4) | 44 | | GND | 11 | | -DB(5) | 45 | | GND | 12 | | -DB(6) | 46 | | GND | 13 | | -DB(7) | 47 | | GND | 14 | | -DB(P) | 48 | | GND | 15 | | GND | 49 | | GND | 16 | | GND | 50 | |TERMPWR | 17 | |TERMPWR | 51 | |TERMPWR | 18 | |TERMPWR | 52 | | RSRVD | 19 | | RSRVD | 53 | | GND | 20 | | GND | 54 | | GND | 21 | | -ATN | 55 | | GND | 22 | | GND | 56 | | GND | 23 | | -BSY | 57 | | GND | 24 | | -ACK | 58 | | GND | 25 | | -RST | 59 | | GND | 26 | | -MSG | 60 | | GND | 27 | | -SEL | 61 | | GND | 28 | | -C/D | 62 | | GND | 29 | | -REQ | 63 | | GND | 30 | | -I/O | 64 | | GND | 31 | | -DB(8) | 65 | | GND | 32 | | -DB(9) | 66 | | GND | 33 | | -DB(10)| 67 | | GND | 34 | | -DB(11)| 68 | --------------------- --------------------- ____________________________ | _______________________ | | 1\ - - - - - - - - - - /34 | | 35\- - - - - - - - - -/68 | | ------------------- | ---------------------------- "WIDE SCSI-3 P" male --------------------------------------------------------------------------- IBM's "Not really SCSI" connectors: [Editor(GF)] Note that this connector is NON-COMPLIANT WITH ANY SCSI STANDARD! 60 pin Burndy connector as used on IBM RS/6000 systems: Pin Signal Pin Signal --- ------ --- ----- 1 Gnd 31 Gnd 2 -DB(0) 32 -ATN 3 Gnd 33 Gnd 4 -DB(1) 34 Gnd 5 Gnd 35 Gnd 6 -DB(2) 36 -BSY 7 Gnd 37 Gnd 8 -DB(3) 38 -ACK 9 Gnd 39 Gnd 10 -DB(4) 40 -RST 11 Gnd 41 Gnd 12 -DB(5) 42 -MSG 13 Gnd 43 Gnd 14 -DB(6) 44 -SEL 15 Gnd 45 Gnd 16 -DB(7) 46 -C/D 17 Gnd 47 Gnd 18 -DB(P) 48 -REQ 19 Gnd 49 Gnd 20 Gnd 50 -I/O 21 Gnd 51 Gnd 22 Gnd 52 Reserved 23 Gnd 53 Reserved 24 Gnd 54 Reserved 25 N/C 55 Reserved 26 TERMPWR 56 Reserved 27 Gnd 57 Reserved 28 Gnd 58 Reserved 29 Gnd 59 Reserved 30 Gnd 60 ReservedTable of Contents
===
ANSWER From: Gary Field (scsifaq@engineer.com)
Macintosh Plus SCSI Connector Pinouts
Note that this connector is NON COMPLIANT WITH ANY SCSI STANDARD!
The grounding is insufficient and does not allow for proper twisted-pair
transmission line implementation. It is recommended that a short adapter cable
be used to convert to the more common Centronics style 50 pin connection,
rather than extend the 25 pin connection any further than necessary.
The Macintosh Plus used a NCR 5380 SCSI chip controlled by the MC68000
processor.
___________________ | SCSI DB-25S | | SIGNAL pin(s) | +------------------+ DB-25S (female) | -DB(0) | 8 | _____________________________ | -DB(1) | 21 | 13\ o o o o o o o o o o o o o /1 | -DB(2) | 22 | 25\ o o o o o o o o o o o o /14 | -DB(3) | 10 | ------------------------ | -DB(4) | 23 | View from rear of computer. | -DB(5) | 11 | | -DB(6) | 12 | | -DB(7) | 13 | | -DB(P) | 20 | | GND | 7,9,14 | | GND |16,18,24 | | -ATN | 17 | | BSY | 6 | | -ACK | 5 | | -RST | 4 | | -MSG | 2 | | -SEL | 19 | | -C/D | 15 | | -REQ | 1 | | -I/O | 3 | +------------------+Pin 25 is NOT CONNECTED in the Mac Plus implementation. Newer Macs connect TERMPWR to pin 25, but are otherwise the same.
Future Domain 25 pin connector pinout Used on TMC-830/845 and TMC-850/860/885.Note:
Use the Macintosh pinout above for TMC-850M, TMC-1610M, TMC-1650/1670 or MCS-600
___________________ | SCSI | DB-25S | | SIGNAL| pin(s) | +-----------------+ DB-25S (female) | -DB(0)| 14 | _____________________________ | -DB(1)| 2 | 13\ o o o o o o o o o o o o o /1 | -DB(2)| 15 | 25\ o o o o o o o o o o o o /14 | -DB(3)| 3 | ------------------------ | -DB(4)| 16 | View from rear of computer. | -DB(5)| 4 | | -DB(6)| 17 | | -DB(7)| 5 | | -DB(P)| 18 | | GND |1,6,8,13 | | GND |13,19,25 | | -ATN | 20 | | BSY | 23 | | -ACK | 22 | | -RST | 10 | | -MSG | 21 | | -SEL | 7 | | -C/D | 11 | | -REQ | 24 | | -I/O | 12 | +-----------------+ Pin 9 is NOT CONNECTEDTable of Contents
QUESTION: Where can I find pictures of the various SCSI connectors?
ANSWER From: Charlie (MotahCity@netzero.net)
It isn't easy for a newcomer to identify the multitude of SCSI connectors
by name alone.
This URL
will show diagrams and photos to help with identifying them:
QUESTION:What is the difference between SCSI-1 and SCSI-2?
ANSWER From Dal Allen:
SCSI-1_versus_SCSI-2
In 1985, when the first SCSI standard was being finalized as an American National Standard, the X3T9.2 Task Group was approached by a group of manufacturers. The group wanted to increase the mandatory requirements of SCSI and to define further features for direct-access devices. Rather than delay the SCSI standard, X3T9.2 formed an ad hoc group to develop a working paper that was eventually called the Common Command Set (CCS). Many products were designed to this working paper.
In parallel with the development of the CCS working paper, X3T9.2 sought permission to begin working on an enhanced SCSI standard, to be called SCSI-2. SCSI-2 would include the results of the CCS working paper, caching commands, performance enhancement features, and whatever else X3T9.2 deemed worthwhile.
While SCSI-2 was to go beyond the original SCSI standard (now referred to as SCSI-1), it was to retain a high degree of compatibility with SCSI-1 devices.
How is SCSI-2 different from SCSI-1?
1. Several options were removed from SCSI-1:
a. Single initiator option was removed.
b. Non-arbitrating Systems option was removed.
c. Non-extended sense data option was removed.
d. Reservation queuing option was removed.
e. The read-only device command set was replaced by the CD-ROM command
set.
f. The alternative 1 shielded connector was dropped.
2. There are several new low-level requirements in SCSI-2:
a. Parity must be implemented.
b. Initiators must provide TERMPWR -- Targets may provide TERMPWR.
c. The arbitration delay was extended to 2.4 us from 2.2 us.
d. Message support is now required.
3. Many options significantly enhancing SCSI were added:
a. Wide SCSI (up to 32 bits wide using a second cable)
b. Fast SCSI (synchronous data transfers of up to 10 Mega-transfers per second -- up to 40 MegaBytes per second when combined with wide SCSI)
c. Command queuing (up to 256 commands per initiator on each logical unit)
d. High-density connector alternatives were added for both shielded and non- shielded connectors.
e. Improved termination for single-ended buses (Alternative 2)
f. Asynchronous event notification
g. Extended contingent allegiance
h. Terminate I/O Process messaging for time-critical process termination.
4. New command sets were added to SCSI-2 including:
a. CD-ROM (replaces read-only devices)
b. Scanner devices
c. Optical memory devices (provides for write-once, read-only, and
erasable media)
d. Medium changer devices
e. Communications devices
5. All command sets were enhanced:
a. Device Models were added
b. Extended sense was expanded to add:
+ Additional sense codes
+ Additional sense code qualifiers
+ Field replaceable unit code
+ Sense key specific bytes
c. INQUIRY DATA was expanded to add:
+ An implemented options byte
+ Vendor identification field
+ Product identification field
+ Product revision level field
+ Vital product data (more extensive product reporting)
d. The MODE SELECT and MODE SENSE commands were paged for all device types
e. The following commands were added for all device types:
+ CHANGE DEFINITION
+ LOG SELECT
+ LOG SENSE
+ READ BUFFER
+ WRITE BUFFER
f. The COPY command definition was expanded to include information on how to handle inexact block sizes and to include an image copy option.
g. The direct-access device command set was enhanced as follows:
+ The FORMAT UNIT command provides more control over defect management
+ Cache management was added:
- LOCK/UNLOCK CACHE command
- PREFETCH command
- SYNCHRONIZE CACHE command
- Force unit access bit
- Disable page out bit
+ Several new commands were added:
- READ DEFECT DATA
- READ LONG
- WRITE LONG
- WRITE SAME
+ The sequential-access device command set was enhanced as follows:
- Partitioned media concept was added:
* LOCATE command
* READ POSITION command
- Several mode pages were added
- Buffered mode 2 was added
- An immediate bit was added to the WRITE FILEMARKS command
+ The printer device command set was enhanced as follows:
- Several mode pages defined:
* Disconnect/reconnect
* Parallel printer
* Serial printer
* Printer options
+ The write-once (optical) device command set was enhanced by:
- Several new commands were added:
* MEDIUM SCAN
* READ UPDATED BLOCK
* UPDATE BLOCK
- Twelve-byte command descriptor blocks were defined for several
commands to accommodate larger transfer lengths.
=======================================================
The following article was written by Dal Allan of ENDL in April 1990. It was published nine months later in the January 1991 issue of "Computer Technology Review". While it appeared in the Tape Storage Technology Section of CTR, the article is general in nature and tape-specific. In spite of the less than timely publication, most of the information is still valid.
It is reprinted here with the permission of the author. If you copy this article, please include this notice giving "Computer Technology Review" credit for first publication.
What's New in SCSI-2
Scuzzy is the pronunciation and SCSI (Small Computer System Interface) is the acronym, for the best known and most widely used ANSI (American National Standards Institute) interface.
Despite use of the term "Small" in its name, everyone has to agree that
Scuzzy is large - in use, in market impact, in influence, and unfortunately, in documentation. The standards effort that began with a 20-page specification in 1980 has grown to a 600 page extravaganza of technical information.
Even before ANSI (American National Standards Institute) published the first run of SCSI as a standards document in 1986, ASC (Accredited Standards Committee) X3T9.2 was hard at work on SCSI-2.
No technical rationale can be offered as to why SCSI-1 ended and SCSI-2
began, or as to why SCSI-2 ended and SCSI-3 began. The justification is much more simple - you have to stop sometime and get a standard printed. Popular interfaces never stop evolving, adapting, and expanding to meet more uses than originally envisaged.
Interfaces even live far beyond their technological lifespan. SMD (Storage Module Drive) has been called technically obsolete for 5 years but every year there are more megabytes shipped on the SMD interface than the year before. This will probably continue for another year or so before the high point is reached, and it will be at least a decade before SMD is considered to be insignificant.
If SCSI enhancements are cut off at an arbitrary point, what initiates the decision? Impatience is as good an answer as any. The committee and the market get sick of promises that the revision process will "end soon," and assert pressure to "do it now."
The SCSI-3 effort is actively under way right now, and the workload of the committee seems to be no less than it was a year ago. What is pleasant, is that the political pressures have eased.
There is a major difference between the standards for SCSI in 1986 and SCSI-2 in 1990. The stated goal of compatibility between manufacturers had not been achieved in SCSI in 1986 due to a proliferation of undocumented "features."
Each implementation was different enough that new software drivers had to be written for each device. OEMs defined variations in hardware that required custom development programs and unique microcode. Out of this diversity arose a cry for commonality that turned into CCS (Common Command Set), and became so popular that it took on an identity of its own.
CCS defined the data structures of Mode Select and Mode Sense commands,
defect management on the Format command, and error recovery procedures. CCS succeeded because the goals were limited, the objectives clear and the time was right.
CCS was the beginning of SCSI-2, but it was only for disks. Tape and optical disks suffered from diversity, and so it was that the first working group efforts on SCSI-2 were focused on tapes and optical disks. However, opening up a new standards effort is like lifting the lid on Pandora's Box - it's hard to stay focused on a single task. SCSI-2 went far beyond extending and consolidating CCS for multiple device types.
SCSI-2 represents three years of creative thought by some of the best minds in the business. Many of the new features will be useful only in advanced systems; a few will find their way into the average user's system. Some may never appear in any useful form and will atrophy, as did some original SCSI features like Extended Identify.
Before beginning coverage of "what's new in SCSI-2," it might be well to list some of the things that aren't new. The silicon chips designed for SCSI are still usable. No new features were introduced which obsolete chips. The cause of silicon obsolescence has been rapid market shifts in integrating functions to provide higher performance.
Similarly, initiators which were designed properly, according to SCSI in 1986, will successfully support SCSI-2 peripherals. However, it should be pointed out that not all the initiators sold over the last few years behaved according to the standard, and they can be "blown away "by SCSI-2 targets.
The 1986 standard allows either initiators or targets to begin negotiation for synchronous transfers, and requires that both initiators and targets properly handle the sequence. A surprisingly large percentage of SCSI initiators will fail if the target begins negotiation. This has not been as much of a problem to date as it will become in the future, and you know as well as I do, that these non-compliant initiators are going to blame the SCSI-2 targets for being "incompatible."
Quirks in the 1986 standard, like 4 bytes being transferred on Request
Sense, even if the requested length was zero have been corrected in SCSI-2. Initiators which relied on this quirk instead of requesting 4 bytes will get into trouble with a SCSI-2 target.
A sincere effort has been made to ensure that a 1986-compliant initiator does not fail or have problems with a SCSI-2 target. If problems occur, look for a non-compliant initiator before you blame the SCSI-2 standard.
After that little lecture, let us turn to the features you will find in
SCSI-2 which include:
o Wide SCSI: SCSI may now transfer data at bus widths of 16 and 32 bits.
Commands, status, messages and arbitration are still 8 bits, and the B-Cable has 68 pins for data bits. Cabling was a confusing issue in the closing days of SCSI-2, because the first project of SCSI-3 was the definition of a 16-bit wide P-Cable which supported 16-bit arbitration as well as 16-bit data transfers. Although SCSI-2 does not contain a definition of the P-Cable, it is quite possible that within the year, the P-Cable will be most popular non-SCSI-2 feature on SCSI-2 products. The market responds to what it wants, not the arbitrary cutoffs of standards committees.
o Fast SCSI: A 10 MHz transfer rate for SCSI came out of a joint effort
with the IPI (Intelligent Peripheral Interface) committee in ASC X3T9.3.
Fast SCSI achieves 10 Megabytes/second on the A-Cable and with wider data paths of 16- and 32-bits can rise to 20 Megabytes/second and even 40 Megabytes/second. However, by the time the market starts demanding 40 Megabytes/second it is likely that the effort to serialize the physical interface for SCSI-3 will attract high-performance SCSI users to the Fiber Channel.
A word of caution. At this time the fast parameters cannot be met by the Single Ended electrical class, and is only suitable for Differential. One of the goals in SCSI-3 is to identify the improvements needed to achieve 10 MHz operation with Single Ended components.
o Termination: The Single Ended electrical class depends on very tight
termination tolerances, but the passive 132 ohm termination defined in 1986 is mismatched with the cable impedance (typically below 100 ohms). Although not a problem at low speeds when only a few devices are connected, reflections can cause errors when transfer rates increase and/or more devices are added. In SCSI-2, an active terminator has been defined which lowers termination to 110 ohms and is a major boost to system integrity.
o Bus Arbitration, Parity and the Identify Message were options of SCSI, but are required in SCSI-2. All but the earliest and most primitive SCSI implementations had these features anyway, so SCSI-2 only legitimizes the de facto market choices. The Identify message has been enhanced to allow the target to execute processes, so that commands can be issued to the target and not just the LUNs.
o Connectors: The tab and receptacle microconnectors chosen for SCSI-2 are available from several sources. A smaller connector was seen as essential for the shrinking form factor of disk drives and other peripherals. This selection was one of the most argued over and contentious decisions made during SCSI-2 development.
o Rotational Position Locking: A rose by any other name, this feature
defines synchronized spindles, so than an initiator can manage disk targets which have their spindles locked in a known relative position to each other.
Synchronized disks do not all have to be at Index, they can be set to an offset in time relative to the master drive. By arraying banks of
synchronized disks, faster transfer rates can be achieved.
o Contingent Allegiance: This existed in SCSI-1, even though it was not
defined, and is required to prevent the corruption of error sense data.
Targets in the Contingent Allegiance state reject all commands from other initiators until the error status is cleared by the initiator that received the Check Condition when the error occurred.
Deferred errors were a problem in the original SCSI but were not described. A deferred error occurs in buffered systems when the target advises Good Status when it accepts written data into a buffer. Some time later, if anything goes wrong when the buffer contents are being written to the media, you have a deferred error.
o Extended Contingent Allegiance (ECA): This extends the utility of the
Contingent Allegiance state for an indefinite period during which the
initiator that received the error can perform advanced recovery algorithms.
o Asynchronous Event Notification (AEN): This function compensates for a deficiency in the original SCSI which did not permit a target to advise the initiator of asynchronous events such as a cartridge being loaded into a tape drive.
o Mandatory Messages: The list of mandated messages has grown:
+----------------------+--------------------------+-------------------+ | Both | Target | Initiator | +----------------------+--------------------------+-------------------| | Identify | Abort | Disconnect | | | | | | Message Reject | No Operation | Restore Pointer | | | | | | Message Parity Error | Bus Device Reset | Save Data Pointer | | | | | | | Initiator Detected Error | | +----------------------+--------------------------+-------------------+o Optional messages have been added to negotiate wide transfers and Tags to support command queueing. A last-minute inclusion in SCSI-2 was the ability to Terminate I/O and receive the residue information in Check Condition status (so that only the incomplete part of the command need be re-started by the initiator).
o Command Queueing: In SCSI-1, initiators were limited to one command per LUN e.g. a disk drive. Now up to 256 commands can be outstanding to one LUN.
The target is allowed to re-sequence the order of command execution to optimize seek motions. Queued commands require Tag messages which follow the Identify.
o Disk Cacheing: Two control bits are used in the CDB (Command descriptor Block) to control whether the cache is accessed on a Read or Write command, and some commands have been added to control pre-fetching and locking of data into the cache. Users do not have to change their software to take advantage of cacheing, however, as the Mode Select/Mode Sense Cache page allows parameters to be set which optimize the algorithms used in the target to maximize cache performance. Here is another area in which improvements have already been proposed in SCSI-3, and will turn up in SCSI-2 products shipping later this year.
o Sense Keys and Sense Codes have been formalized and extended. A subscript byte to the Sense Code has been added to provide specifics on the type of error being reported. Although of little value to error recovery, the additional information about error causes is useful to the engineer who has to analyze failures in the field, and can be used by host systems as input to prognostic analysis to anticipate fault conditions.
o Commands: Many old commands have been reworked and several new commands have been added.
o Pages: Some method had to be found to pass parameters between host and target, and the technique used is known as pages. The concept was introduced in CCS and has been expanded mightily in SCSI-2.
A number of new Common Commands have been added, and the opcode space for 10-byte CDBs has been doubled.
o Change Definition allows a SCSI-2 initiator to instruct a SCSI-2 target to stop executing according to the 1986 standard, and provide advanced SCSI-2 features. Most SCSI-2 targets will power on and operate according to the 1986 standard (so that there is no risk of "disturbing" the installed initiators), and will only begin operating in SCSI-2 mode, offering access to the advanced SCSI-2 capabilities, after being instructed to do so by the initiator using the Change Definition command.
o The Mode Select and Mode Sense pages which describe parameters for
operation have been greatly expanded, from practically nothing in 1986 to hundreds of items in SCSI-2. Whenever you hear of something being described as powerful and flexible tool, think complicated. Integrators are advised to be judicious in their selection of the pages they decide to support.
o The Inquiry command now provides all sorts of interesting data about the target and its LUNs. Some of this is fixed by the standard, but the main benefit may be in the Vendor Unique data segregated into the special designation of Vital Product Data, which can be used by integrators as a tool to manage the system environment.
o Select Log and Sense Log have been added so that the initiator can gather both historical (e.g. all Check Conditions) and statistical (e.g. number of soft errors requiring ECC) data from the target.
o Diagnostic capabilities have been extended on the Read/Write Buffer and Read/Write Long commands. The ways in which the target can manage bad blocks in the user data space have been defined further and regulated to reduce inconsistencies in the 1986 standard. A companion capability to Read Defect Data permits the initiator to use a standard method to be advised of drive defect lists.
o A new group of 12-byte command blocks has been defined for all optical devices to support the large volume sizes and potentially large transfer lengths. The Erase command has been added for rewritable optical disks so that areas on the media can be pre-erased for subsequent recording. Write Once disks need Media Scan, so that the user can find blank areas on the media.
o New command sets have been added for Scanners, Medium Changers, and CDROMs.
All of this technical detail can get boring, so how about some "goodies" in SCSI-2 which benefit the common man and help the struggling engineer? First, and probably the best feature in SCSI-2 is that the document has been alphabetized. No longer do you have to embark on a hunt for the Read command because you cannot remember the opcode.
In the 1986 standard, everything was in numeric sequence, and the only
engineers who could find things easily were the microprogrammers who had memorized all the message and opcode tables. Now, ordinary people can find the Read command because it is in alphabetic sequence. This reorganization may sound like a small matter but it wasn't, it required a considerable amount of effort on the part of the SCSI-2 editors. It was well worth it.
Another boon is the introduction for each device class of models which describe the device class characteristics. The tape model was the most needed, because various tape devices use the same acronym but with different meanings or different acronyms for the same meaning.
The SCSI-2 tape model defines the terms used by SCSI-2, and how they correspond to the acronyms of the different tapes. For example, on a 9-track reel, End of Tape is a warning, and there is sufficient media beyond the reflective spot to record more data and a trailer. Not so on a 1/4" tape cartridge. End of Tape means out of media and no more data can be written. This sort of difference in terms causes nightmares for standardization efforts.
So there it is; a summary of what is in SCSI-2. It's not scary, although it is daunting to imagine plowing through a 600-page document. Time for a commercial here. The "SCSI Bench Reference" available from ENDL Publications (408-867-6642), is a compaction of the standard. It takes the 10% of SCSI-2 which is constantly referenced by any implementor, and puts it in an easy-to-use reference format in a small handbook. The author is Jeff Stai, one of the earliest engineers to become involved with SCSI implementation, and a significant contributor to the development of both the 1986 standard and SCSI-2.
SCSI-2 is not yet published as a standard, but it will be available later this year. Until then, the latest revision can be purchased from Global Engineering (800-854-7179).
Biography
Consultant and analyst I. Dal Allan is the founder of ENDL and publisher of the ENDL Letter and the "SCSI Bench Reference." A pioneer and activist in the development and use of standard interfaces, he is Vice Chairman of ASC X3T9.2 (SCSI) and Chairman of the SCSI-2 Common Access Method Committee.
QUESTION: What is the difference between SCSI-2 and SCSI-3?
ANSWER From: excerpts of postings by Jeff Stai and others:
(Mohit K Goyall - goyall@utdallas.edu)
Are SCSI-3 hard drives and/or controllers available yet?
Allegedly, Previous postings have said "I heard that SCSI-3 has been standardized," but I haven't seen anything firm about it. I've seen controllers advertised by JDR Microdevices and some cheap clones; the Quantum "Empire" drives are also advertised as SCSI-3 by some mail order vendors. Seagate and IBM call their fastest drives (probably comparable in speed to the Quantums, if not faster) "Wide SCSI-2."
That's a misnomer. See below.
What is the difference between SCSI-3 and Fast & Wide SCSI-2?
Wide SCSI-2 required two cables to do 16 bit wide transfers. SCSI-3 defined a single cable, single REQ/ACK 16 bit, WIDE transfer. The reason you are hearing 16-bit single cable being called SCSI-3 is that they CAN. The fact that single cable 16-bit has been around for a while just shows you how much the standardization process lags behind the real world.
SCSI-3 is really a family of standards. SCSI was broken up from a single document into different layers and command sets. This was done to allow for different physical transport layers (like fibre channel and SSA) to be defined, and to allow for smaller "bite-sized" projects that maybe get done a little faster ;-)
The family includes the following members with TLAs:
- SCSI-3 Parallel Interface (SPI): Defines the mechanical, timing, phases, and electrical parameters of the parallel cable we all know and love. Some of the electrical and cable parameters are tightened/improved over SCSI-2.
- SCSI-3 Interlock Protocol (SIP): Defines the messages and how the phases are invoked. No real change from SCSI-2, except for some new messages.
- SCSI-3 Architectural Model (SAM): In a nutshell, defines a common set of functions and services and definitions for how a physical transport properly gets commands, data, and status exchanged between two devices, complete with error handling and queueing.
- SCSI-3 Primary Commands (SPC): All of the commands executed by any and all SCSI devices, like REQUEST SENSE and INQUIRY, etc.
- SCSI-3 Block Commands (SBC): Disk commands.
- SCSI-3 Stream Commands (SBC): Tape commands.
- SCSI-3 Controller Commands (SCC): RAID box commands.
- SCSI-3 Multimedia Commands (MMC): For CDROMS etc.
- SCSI-3 Fibre Channel Protocol (FCP): SCSI commands over gigabit Fibre Channel.
- SCSI-3 Serial Bus Protocol (SBP): SCSI commands over IEEE 1394 High Speed Serial Bus (Apple's "Firewire").
- SCSI-3 Serial Storage Protocol (SSP): SCSI commands over SSA.
whew.
=== QUESTION: After perusing the latest issue of Computer Shopper, I came away with the impression that companies are calling F&W SCSI-2 HD's SCSI-3. Is this an incorrect assumption, or is F&W SCSI-2 known as SCSI-3?
Is this really mostly marketing hype?
Actually, there is something to that. TECHNICALLY, what is out there is often a hybrid: SCSI-3 "SPI" silicon with some other hodgepodge of SCSI-3 proposals, all mixed in with SCSI-2 stuff.
An earlier posting said that the Quantum Empire ("SCSI-3") drives contain some commands from the SCSI-3 command set, and Adaptec suggested a specific setting on its 2940W controller to work properly with the drive.
I understand there are some drives with proposed SCSI-3 command features. These are mostly in the MODE SELECT and in error codes, as I recall. Perhaps someone who knows more about this could elaborate?
Note also that the major players (like DC Drives) don't have any "SCSI-3" stuff advertised; only JDR and some cheap clones are promoting it.
Besides, Wide SCSI-2 has yet to really catch on (mostly because only a few drives are fast enough to take advantage of it).
There is no "wide SCSI-2" because that would mean two cables. Single cable wide SCSI has always been SCSI-3, it just took too d*** long to get into a standard! :-)
QUESTION: Is SYNCHRONOUS faster than ASYNCHRONOUS?
ANSWER From: Gary Field (scsifaq@engineer.com
)
Yes, the asynchronous transfer option waits for each byte to be transferred before it is acknowledged. With synchronous protocol, the device sending the data is allowed to get ahead of the device receiving the data by a number of bytes (called the offset). The offset is negotiated between the initiator and the target some time prior to the transfer beginning. The synchronous protocol is considerably more efficient and therefore faster than asynchronous.
QUESTION: Is the NCR 53C90 Faster than spec?
ANSWER From: kstewart@ncr-mpd.FtCollins.NCR.COM
(Ken Stewart)
I've seen a few comments about our 54C90 being faster than spec. While I doubt the author was really complaining (I got twice as much as I paid for - sure makes me mad ;) I'd like to explain the situation. Along the way, I'll also show that asynchronous is faster on short cables, while synchronous is faster on long cables. The cross-over point occurs somewhere around six feet--assuming that you have our 53C90 family devices at both ends of the cable. The reason has to do with the propagation delay of the cable; the turn around time of the silicon; and the interlocked nature of the asynchronous handshake.
1) We have measured propagation delays from various cables and found an average of 1.7 nanoseconds per foot, which is roughly 5.25 ns per meter. 2) The turn-around time is the amount of time the SCSI chip takes to change an output in response to an input. If REQ is an input then ACK is an output. Or if ACK is an input then REQ is an output. Typical turn-around time for the 53C90 is 40 nanoseconds.
3)The asynchronous transfer uses an interlocked handshake where a device cannot do the next thing until it receives positive acknowledgment that the other device received the last thing.
First REQ goes true /* driven by Target */
then ACK is permitted to go true /* driven by Initiator */
then REQ is permitted to go false
then ACK is permitted to go false
Thus we have four "edges" propagating down the cable plus 4 turn-around delays. Asynchronous transfer requires 55 ns setup and no hold time (paragraph in 5.1.5.1 in SCSI-1 or SCSI-2) which gives an upper speed limit around 18 MB/s. A detailed analysis (assuming 53C90 family) shows that the setup time subtracts out. This is mostly because we are running at one-third the max rate, but also because setup for the next byte can begin anytime after ACK is received true or REQ is received false, depending on who is receiving. You can either take my word for it or draw the waveforms yourself. Thus, the asynchronous transfer reduces to:
(4 * 1.7 * 1) + (4 * 40ns) = 167 ns /* 1 foot cable */
= 6 MB/s
(4 * 5.25 * 6) + (4 * 40ns) = 286 ns /* 6 meter cable */
= 3.5 MB/s
(4 * 5.25 * 25) + (4 * 40ns) = 685 ns /* 25 meter cable */
= 1.5 MB/s
note: cables longer than 6 meters require external differential transceivers which add delay and degrade the performance even more than indicated here.
Our simulations say that under very best conditions (fast silicon, low temperature, high voltage, zero length cable) we can expect more than 8 MB/s asynchronously. In the lab, I routinely measure 5 MB/s on 8 foot cables. So, if you were writing the data manual for this, how would YOU spec it?
The framers of the SCSI spec threw in synchronous mode to boost the performance on long cables. In synchronous mode, the sending device is permitted to send the next byte without receiving acknowledgment that the receiver actually received the last byte. Kind of a ship and pray method.
The acknowledgment is required to come back sometime, but we just don't have to wait for it (handwave the offset stuff and the ending boundary conditions). In this mode any external transceivers add a time shift, but not a delay. So if you negotiate for 5 MB/s, you get 5MB/s regardless how long the cable is and regardless whether you are single-ended or differential. But you can't go faster than 5.5 MB/s, except in SCSI-2.
Synchronous mode does have a hold time (unlike asynch) but again, setup and hold times subtract out. In SCSI-1 synchronous mode, the speed limit comes from the combined ASSERTION PERIOD + NEGATION PERIOD which is 90ns + 90ns = 180ns = 5.5 MB/s. Our 53C90 family doesn't quite hit the max, but we do guarentee 5.0 MB/s. In SCSI-2, anything above 5.0 MB/s is considered to be FAST. Here the maximum transfer rate is explicitly limited to 100 ns or 10MB/s; you don't have to read between the lines to deduce it.
Interesting tid-bit: given a SCSI-2 FAST period of 100 ns and a cable delay of 131 ns on a 25 meter cable, you can actually stack 1.31 bytes in the 8-bit cable. In FAST and WIDE SCSI you can stack 5.24 bytes in this copper FIFO.
Hummm...
QUESTION: What is ASPI?
ANSWER From: Gary Field (scsifaq@engineer.com)
Updated: May, 2000
ASPI stands for Advanced SCSI Programming Interface. It was developed by Adaptec (and the "A" originally stood for Adaptec). It is a calling convention and set of commands that can be used to send SCSI commands via any SCSI host adapter that supports it. It is strictly for use with machines running MSDOS, Windows( 3.1x, 95/98/NT/2000), Netware, or OS/2. There is no UNIX version of ASPI. The error reporting and recovery mechanisms are much more limited than in CAM, but ASPI gained much wider acceptance because it was available earlier.
The ASPI subsystem is divided into layers:
- ASPI Manager Driver (the layer closest to the host adapter)
- ASPI peripheral Driver (the code which knows the details of the SCSI peripheral, e.g. DISK, CD-ROM, scanner etc)
- ASPI applications (utilities like SCSITool, or CD Recording packages)
ASPI manager drivers are available from the manufacturers of most SCSI host adapters.
QUESTION: What is CAM?
ANSWER From: ctjones@bnr.ca (Clifton Jones)
CAM stands for Common Access Method.
It is an ANSI standard to make it easier to program SCSI applications by encapsulating the SCSI functions into a standardized calling convention.
[Editor(GF): It is similar to ASPI but much more elaborate and complete].
ANSWER From: landis@sugs.tware.com (Hale Landis)
====
You can get the CAM spec(s) from the ANSI T10 Web site
QUESTION: What is FPT (Termination)?
ANSWER From: jvincent@bnr.ca (John Vincent)
FPT stands for Forced Perfect Termination. FPT is actually really simple, I wish I had thought of it. What it does is use diode clamps to eliminate over and undershoot. The "trick" is that instead of clamping to +5 and GND they clamp to the output of two regulated voltages. This allows the clamping diodes to turn on earlier and is therefore better at eliminating overshoot and undershoot. The block diagram for a FPTed signal is below. The resistor value is probably in the 110 Ohm range. The actual output voltages of the regulators may not be exaclty as I have shown them but ideally they are matched to the diode characteristics so that conduction occurs when the signal voltage is greater than 3.0 V or less than 0.2 V.
+-----------*--- TERMPWR | | ____|___ | | | | | Vreg 1 |---------------------------------* 2.8 V |________| | | | | | ----- | | --- | \ - | / term resistor | \ (110 Ohms) | / ____|___ | | | | | Vreg 2 |-*--------* 2.4 V | |________| | | | | --+-- | | / \ | +------+ /___\ | | | | | | | terminated | *----------*------------- signal | | | | | --+-- | / \ | /___\ Both diodes are fast silicon | | switching diodes (.6 V drop) ___|____ | | | | | Vreg 3 |----------* 0.8 Volts |________|The diagram shows the circuit for terminating one signal. In a complete FPT there would be 36 diodes and 18 110 Ohm resistors plus the regulator chips.
Using the values shown, transients would be clamped at 0.2V and 3.0V.
[Editor(GF)]:
Some errors in the above diagram were corrected as suggested by
Wietze van Winden (wietze@ittpub.nl)
QUESTION: What is Active Termination?
ANSWER From: eric@telebit.com (Eric Smith)
and brent@auspex.com (Brent R. Largent)
An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than using resistor voltage dividers.
This is a passive terminator:
TERMPWR ------/\/\/\/------+------/\/\/\/----- GND | | SCSI signalNotice that the termination voltage varies with the voltage on the TERMPWR line. One voltage divider (two resistors) is used for each SCSI signal.
An active terminator looks more like this (supply filter caps omitted):
2.85 Volt Regulator +-----------+ +2.85V 110 Ohms TERMPWR -----| in out |------+------/\/\/\/-------SCSI signal | gnd | | +-----------+ | | +------/\/\/\/-------SCSI signal | | GND ---------------+ | +------/\/\/\/-------SCSI signal | etc.Assuming that the TERMPWR voltage doesn't drop below the desired termination voltage (plus the regulator's minimum drop), the SCSI signals will always be terminated to the correct voltage level.
Several vendors have started making SCSI active terminator chips, which contain the regulator and the resistors including Dallas Semiconductor, Unitrode Integrated Circuits and Motorola.
[Editor(GF): Another nice feature of active termination is that it can be disabled by a single jumper instead of needing to unplug resistor arrays.]
QUESTION: Why Is Active Termination Better?
ANSWER brent@auspex.com (Brent R. Largent)
Typical passive terminators (resistors) allow signals to fluctuate directly in relation to the TERM Power Voltage. Usually terminating resistors will suffice over short distances, like 2-3 feet, but for longer distances active termination is a real advantage.
Active termination provide the following advantages:
- Helps reduce noise.
- A logic bit can be used to effectively disconnect the termination.
- Regulated termination voltage.
- SCSI-2 spec. recommends active termination on both ends of the scsi bus.
- Improved resistance tolerances (from 1% to about 3%)
[Editor(GF):
- Reduces current drawn from TERMPWR line.
In FPT form:
- Provides signal overshoot/undershoot clamping on all signal lines. ]
QUESTION:How can I tell whether an unmarked terminator is active or passive?
ANSWER From: Gary Field (scsifaq@engineer.com)
If you have an Ohm-meter of one kind or another, measure the resistance from the TERMPWR pin to an adjacent GROUND pin. Reverse the probes and take another reading.
If the reading is about 30.5 Ohms, with the probes both ways, you have a passive single-ended terminator.
If the reading is about 45 Ohms, with the probes both ways, you have a passive differential terminator.
Active terminators should read much higher and give very different readings
with the probes interchanged.
Another method: (Suggested by Rico Tudor)
If you measure the resistance from one signal pin to another, a reading
of 264 Ohms indicates a passive terminator.
A reading of 220 Ohms indicates an active terminator. This will be
true for either 50 or 68 pin terminators.
If you measure the resistance from a signal pin to a GROUND pin on a
50 pin terminator, a reading of 143 Ohms indicates a passive terminator.
The value would be closer to 139 Ohms on a 68 pin passive terminator. A
reading of greater than 400 Ohms indicates an active terminator.
For those geeks who must know how these values were arrived at:
For passive terminators
Signal to Signal formula: 1 / ((220 +
220) * (330 + 330) ) = 264 Ohms
Signal to Ground formula: 1 / ( (1/330) + ( 1/(220+((1/(N-1)) * 550) ) ) )= 153 Ohms (for N=18) or 145 (for N=27)
For active terminators
Signal to Signal formula: 110 + 110 = 220 Ohms
Signal to Ground formula: It is not practical to guess what the
output circuit of a 2.85 Volt regulator might look like and attempt to
derive an exact formula for it. Actual measurements indicate that it will
generally be high resistance.
QUESTION: Where can I buy terminators ?
ANSWER From: Rodney Brown (RBrown@cocam.com.au)
Info taken from Usenet postings by:
John Zatler (JPZ@Popmail.mcs.com)
Steve Schreppler (schrep@oasys.dt.navy.mil)
Dave Nadler (nadler@ug.eds.com)
DataMate / Methode
Methode Electronics, Inc.
dataMate Division
7444 West Wilson Avenue
Chicago, IL 60656
(708) 867-9600
(800) 323-6858
(708) 867-3149 FAX
WWW: http://www.methode.com/datamate/dmhome.htm
Brief description of terminators available.
Passive, Active, SLICK (Elaboration of FPT)) in:
Centronics 50 pin (SCSI-1) DM8[05]0-09-[0RS]
Male 3 row D-Sub (Old Sun) DM950-??-?
Male 50 position .050" Centres (SCSI-2 HD) DM20[05]0-02-[0RS]
Male 68 position .050" Centres (SCSI-3 P cable) DM2050-02-68[RS]
Male & Female for ribbon cables DM1050-02-[0RS] (M),
DM650-06-[0RS] (F)
Male/Female for pass through between device and ribbon cable DM550-06-[0RS]
Newark Electronics stocks the DataMate product line.
Newark Electronics (International orders)
4801 N. Ravenswood Ave. 500 N. Pulaski St.
Chicago IL 60640-4496 Chicago IL 60624-1019
(312)-784-5100, (FAX (312)-638-7652, TLX 6718690 NEWARK U).
Selectronix Ltd
Minerva House, Calleva Park,
Aldermaston, Reading, RG7 8NE, UK
Tel: +44 (0)118 9817387
Fax: +44 (0)118 9817608
WWW: http://www.selectronix.co.uk/
Technical Cable Concepts Inc.
QUESTION: What is Plug and Play SCSI?
ANSWER From: leefi@microsoft.com (Lee Fisher)
Date: August 1998
Plug and Play is the technology that supports automatic configuration of PC hardware and attached devices.
For more info see:
QUESTION:Where can I get drivers (ASPI and other) for the WD7000 FASST2 host adapter?
ANSWER From: Gary Field (scsifaq@engineer.com)
(Update - March 2000)
The following information is outdated but I'm leaving it here for now just for historical purposes. My current opinion is that these cards are totally obsolete and should be replaced. The only support for these cards I know of is under Linux and FreeBSD.
Western Digital stopped producing WD7000 FASST2 cards some time in 1990. Future Domain bought the rights to produce them. Future Domain was later bought out by Adaptec and the boards are no longer produced. Columbia Data Products Inc. of Altamonte Springs, Florida still provides driver support for the card. Their SST IV driver package provides support for many types of SCSI devices including disks, tapes, and CDROM. Also included in this package is an ASPI manager driver (equivalent to the Adaptec ASPI4DOS.SYS). I have personally tested this ASPI manager and it works with GNU tar w/ASPI and the Corel CDROM driver, so most other ASPI stuff should work too. Versions of SSTASPI.SYS prior to Oct 1993 do NOT work with the above mentioned programs so be sure to check the file date. There are other useful programs in the package as well. For instance I find the TAPEUTIL program very handy for duplicating tapes. The price of this package is $99 or $85 as an upgrade of a previous version.
A pre-requisite to run this software is that the adapter card must have a BIOS ROM version of 3.36 or newer. I don't think cards manufactured before 1989 or so are compatible.
Columbia Data Products Inc.
1070 B Rainer Dr
Altamonte Springs, FL 32714 (407) 869-6700 (main number)
(407) 862-4725 (fax)
http://www.cdp.com (Inactive)
cdp@cdp.com (Columbia e-mail)
[Update to above information 1/20/97][Editor(GF)]
From: "Alan L. Welsh" <snapback@ix.netcom.com>
Subject: Western Digital 7000-Fasst SCSI Cards and CDP's SST software
Alan L. Welsh, President
Columbia Data Products, Inc.
We don't usually recommend that users purchase the upgrade for the 7000 software today. Development has ceased, Windows 95 is not supported except in DOS mode, and today I would rather recommend a popular currently manufactured Local-bus SCSI board and not an ISA 7000 board. However, there are still some companies that we do support that have standardized on 7000s and need to keep them in service for years to come. So please buy the software, sell the board, use it as-is, or buy a new board.
http://www.cdp.com (Inactive) cdp@cdp.com
---------------------
HISTORY OF THE WD-7000 SCSI HOST ADAPTER AND COLUMBIA DATA PRODUCTS, INC.
Starting in early 1987, Western Digital (WD) manufactured virtually all of the 100,000+ 7000 SCSI boards, except for a few hundred that were made by Future Domain. The first few thousand, known as 7000-ASC boards went out with no software and only a ROMBIOS that was actually written by John Sponger of WD. In the summer of 1987, Columbia Data Products (CDP) completed and shipped its first ROMBIOS for the card that enabled it to boot and operate in DOS. At that same time, CDP also completed a DOS ram-resident driver, so that DOS would recognize and operate the card without the slowness of the ROMBIOS, a DASD driver so that DOS could access additional drive letters, and to break the (then) 32 meg barrier, and partitioning software to perform the FDISK function for SCSI.
It was CDP's goal at that time to develop and provide SCSI software that would enable: any SCSI host adapter, to run any SCSI peripheral, on any operating system, in any PC-based bus. Since at that time WD had 80% of the hard drive controller market, CDP chose WD as the most logical choice to strategically market with, and so CDP supported their cards almost exclusively. During that following year, CDP continued to develop the software for the 7000 host adapters, enabling it to run faster than any other board of its time, including Adaptec's new 1540, whose hardware was actually faster.
In the fall of 1988, CDP exclusively licensed its SCSI software suite, called SST to WD. The WD 7000-asc SCSI host adapter was renamed 7000-FASST. WD was the first OEM to ship software with all SCSI boards distributed as part of the package. CDP's SST software was well received, even though SCSI was still a relatively small market. CDP was paid a royalty for each card shipped and CDP provided complete software support and limited hardware support throughout the world.
By 1991 CDP had developed support for all SCSI peripherals known, all PC operating systems such as Unix, Xenix, Windows, Dos, Netware, and even AIX, although never officially released, and a SCSI toolkit utility package.
All of the 7000-FASST's shipped had multiboot capability that allowed all of these operating systems to simultaneously coexist on a single hard drive so that one OS can be selectively booted each session.
CDP's exclusive was ending with WD, and CDP was porting the software to 25 of the most popular SCSI host adapters. Unfortunately, most of software had to be re-architected and rewritten to embrace not only all the new adapters but also the new SCSI software standards such as CAM, LADDR, ASPI, INT-4b, as well as CDP's own standard since 1987, SDLP. During the next few years WD was losing a considerable amount of money and sold many of their product lines, which included selling the SCSI board business to Future Domain. Future Domain did very little sales of the 7000 as they had competing product lines and didn't understand the value of a bus mastering SCSI board. (Bus mastering gives the card the ability to move data to and from the card and system memory directly without the CPU's involvement, making it as fast as the peripherals driving it, even on an old slow 80286!) The bus mastering 1542 product line from Adaptec is still being produced today, very popular, and is based on the same basic design as the 7000. From a pricing standpoint, the prices for this class of product has declined less than 50% in ten years. This is only amazing if you compare the price of 1MB of memory at $300 in 1987 to that of today.
CDP has continued to develop and support for the 7000-FASST continuously, even though the board hasn't been manufactured for quite a number of years.
Our last major revision of our SST-IV software was done in late 1993, although there have been some minor revisions since then. To enable CDP to continue to develop software and support the board, CDP has been selling upgrades to the large installed user base for years. Without this revenue, development and support would have ceased long ago. There are no plans to continue development at this time, as SCSI is moving from the ISA bus to Local Bus. Although Window-95 development and support was considered, the potential upgrade business wouldn't have covered the cost of development.
In 1994 CDP entered the server backup software market, shipping the first version of Snapback in March of that year. Many of our customers for years had been begging us to write our own backup software and were complaining that "restoring" their servers sometimes took days with the current backup products. For SCSI software development purposes only, CDP had been backing up and restoring hard drives containing multiple operating systems for years. CDP adapted and then rewrote this software in this first release to provide the ability to backup and restore any hard drive that contained any operating system, from DOS. CDP later wrote a device driver in Netware, that could make the backup tape look, act and perform like a hard drive from a Netware workstation. This enabled direct file retrieval and use through Netware from the backup tape, making it appear to a workstation to be just another drive letter. Since all the directories and FATs are cached, the tape is almost as fast as a hard drive. Another feature, resize, allows a Netware server's hard drive to be replaced with a larger one in an hour instead of a day's labor.
At fall COMDEX 1996, CDP released its latest version, Snapback Live! That backs up a live image of a Netware file server's hard drive, capturing all open files in the process, without impacting system performance. Watch your Computer magazine for Snapback reviews in 1997, as well as a version for NT. Innovating backup software has now become CDP's new life--from an innovative SCSI software company.
For more information, contact us at:
http://www.cdp.com (Inactive) OR cdp@cdp.com
QUESTION: What if I have a SCSI drive larger than a gigabyte (1024MB)?
ANSWER From: Gary Field (scsifaq@engineer.com)
The IBM PC/AT BIOS Int 13h disk interface was specified in about 1986 when a large disk drive was about 60 MB. IBM decided that disks wouldn't have more than 1024 cylinders and only allocated 10 bits for the CYL parameter to the INT 13h interface. By 1989, this was already a problem. When vendors began to support SCSI drives under INT 13h, they needed to come up with a translation algorithm between the CYL, HEAD, SECT parameters of INT 13h and the linear block numbers used by SCSI devices. Various vendors chose to map the two such that each INT 13h "cylinder" contained 1 MB.
In other words they emulated a drive with 32 heads and 63 sectors per track.
At the time, large drives were at about 300 MB, so this worked OK. Once drives larger than 1024 MB arrived, a problem developed. They couldn't provide cylinder values greater than 1023! Changing algorithms became necessary.
This is painful since any disk formatted with the old algorithm can't be read using the new algorithm.
By the way, different vendors chose different mappings, so drives formatted with one adapter can't necessarily be moved to a different one.
Adaptec's newer adapters (e.g. the 154xC and the 154xCF) provide a BIOS control to select the old algorithm or the new one, and they also provide BIOS PROMs for the 154xB that will use the new algorithm.
There is an absolute limit of 16 M sectors which means 8 GB assuming 512 byte sectors. Also DOS (actually the FAT 16 filesystem) only allows 2 GB per partition.
The day when this presents another problem is not too far away (1995?)
Hopefully, we'll all be running more sophisticated O/Ses that bypass this limitation by then.
QUESTION: My SCSI bus works, but is not reliable. What should I look at?
ANSWER From: Gary Field (scsifaq@engineer.com)
If you still have problems after you're sure that you have all the ID and termination and cable issues resolved, it's time to dig a little deeper (Voltmeter and Oscilloscope required).
If you get your SCSI bus to the point where it basically works, but it isn't reliable I have found that the gremlin can be the TERMPWR Voltage.
With your system fully powered up, and both terminators attached, measure the TERMPWR Voltage at the far end of your bus. It needs to be between 4.25 and 5.25 Volts. Many vendors start with the system's +5 VDC and add a regular silicon rectifier diode and fuse in series. Silicon rectifiers have an inherent voltage drop of .6 to 1.0 Volts depending on the current through them.
Schottky barrier rectifiers are much better for this application. I always use a 1N5817 myself. If the diode on the host adapter is a 1N400x type, change it to a 1N5817. If you add up the drop across the diode and the fuse and 15 feet of ribbon cable and the connector contact resistances, many times you'll find yourself below 4.0 Volts. When using passive terminators, this can shift the signal threshold and decrease the signal to noise ratio on the bus.
If you aren't able to get relief with these methods, sometimes you can solve the problem by having several devices supply TERMPWR to the bus.
Sometimes the Voltage is high enough, but there is too much noise on the TERMPWR line. This can cause really strange problems! If you can see more than about 200 mV of noise on TERMPWR, add a .1 uF and 10 uF capacitor from TERMPWR to one of the adjacent GROUND lines. You need to have the bus as active as you can get it when measuring the noise. I have actually seen over 1 Volt of noise in some severe cases.
Another way you can help to solve TERMPWR problems is to use active terminators. These don't draw as much current from the TERMPWR source and they also have a built in regulator, which can operate on lower voltage than the standard passive terminators. The regulator also tends to reduce the noise.
QUESTION: Where can I find information about programming using the ASPI interface from DOS and Windows?
ANSWER From: Gary Field (scsifaq@engineer.com)
The following are the best sources of information I have found:
- Adaptec's Developer web site has some useful ASPI information. See the FAQ Question How can I contact Adaptec? for phone numbers.
- Dr Dobb's Journal March 1994 issue pg 154, has an article called "The Advanced SCSI Programming Interface" by Brian Sawert. Example code in C and x86 assembly language is included. The code can be obtained via anonymous ftp from: Dr Dobb's Journal archive web site.
- "The Programmer's Guide to SCSI" has a lot of ASPI programming info and example source.
- The new "Book of SCSI:I/O for the New Millennium" has an example ASPI application program with source code on it's CD-ROM.
- Freeware source of ASPI example code is: ASPI tar, get it from: ftp://ftp.simtel.net/pub/simtelnet/msdos/diskutil/aspisrc.zip
QUESTION: What is target mode and what host adapters and drivers support it?
ANSWER From: Gary Field (scsifaq@engineer.com)
Target mode refers to the ability of some SCSI host adapters to respond on the SCSI bus as if they were a SCSI target (device) instead of an initiator (host) which is the usual behavior. Using target mode two hosts can communicate with each other over the SCSI bus at high speeds.
Although most SCSI chips can be used in target mode, not all host adapters support it. Here are some that do:
Adaptec 2940UW (http://www.adaptec.com/)
The documents that describe how target mode works for the 2940/2944
are called:
Designer Book
AIC-7880 SCSI Chip Specification
Adaptec TARGET mode firmware programming guide
(I don't think any of these is available anymore)
NexiTech provides target mode software for the 2940 adapter. (http://www.nexitech.com/)
Qlogic QLA-1040 (http://www.qlc.com/)
QUESTION: How do I replace Macintosh internal HD and terminate the SCSI chain properly?
ANSWER From: Jie Yuan PhD (Jie.Yuan@UC.Edu)
The factory installed Macintosh internal HD should be terminated. Make sure the terminator/resitor-package is installed in the drive before using it. Most vendors will install the terminator for you if you tell them it is for use in Macintosh as the system disk. Manufacturers usually have toll free numbers for SCSI termination, ID, and such. If you don't already have the terminator, they may send you one for free. BTW, Macintosh SCSI chain starts at the system disk (ID=0), and ends at the control board (ID=7). ID numbers from 1-6 should be used for any other devices on the chain.
QUESTION: Will attaching a SCSI-1 device to my SCSI-2 bus hurt its performance?
ANSWER From: Gary Field (scsifaq@engineer.com)
Attaching a SCSI-1 device to a system with a SCSI-2 host adapter and several SCSI-2 devices already attached will not hurt over-all performance significantly unless it doesn't handle disconnect/reconnect well. This assumes that the host adapter keeps track of protocol options separately for each target device. Some people have the idea that attaching a SCSI-1 device to a SCSI-2 bus will cause the entire bus to run at SCSI-1 speeds. This is not true.
QUESTION: Can I connect a SCSI-3 disk to my SCSI-1 host adapter?
Can I connect a SCSI-2 CDROM to a SCSI-3 host adapter?
Can I connect a Narrow SCSI2 disk to a WIDE SCSI3 host adapter?
ANSWER From: Gary Field (scsifaq@engineer.com)
Questions of this nature really cannot be answered in a useful way. There are so many aspects and options to each of the SCSI standards, you need to be much more specific about what devices and adapters you're interested in connecting. Most of the time the best thing to do is just try it! Most combinations will work, but if you're considering a purchase and looking for a guarantee from "The Net", forget it.
The issue is further complicated by the fact that vendors like to latch onto the latest acronyms before they even know what's involved. For example SCSI3 is not approved yet, but vendors are already saying their devices are SCSI3 compatible. Since there is no standards compliance testing organization, they can pretty much say what they want.
If you buy a high end host adapter (probably called SCSI3 :-) ) from a reputable vendor, and it has enough control over the various options (like synch xfer rate 5,10,20,40 mega xfers/sec and the ability to disable WIDE or FAST/Ultra negotiation), and you carefully think out what devices you connect to it (all WIDE devices nearest the host adapter end of the bus etc.), and you are careful to properly terminate not only both ends, but both halves (upper byte and lower byte) of the bus, and none of the older devices you might already have (like a Panasonic CDROM) do anything stupid (like not handle the WIDE negotiation message without hanging) then it will all work fine. :-)
Even though a host adapter may be called SCSI3 doesn't mean it can enable or disable each optional feature, yet this is vital for supporting older devices.
To make matters worse, you won't know which older devices do some of the stupid things unless you know someone who's been bitten already. Your best bet is to look for good deals on name brand devices and adapters and before you buy, ask in comp.periphs.scsi whether anyone has tried the combination you're considering. It's also important to buy from a well known vendor with reasonable return policies.
If you're looking at buying a Vendorxyz spiffydisk which claims to be SCSI-3 compatible and you have a Seagate ST-01 host adapter and you want to know if anyone else has tried this combination, then that's exactly what you should ask.
In general, most SCSI devices and adapters made less than 4 years apart will probably work together, but without specific information about exactly which devices there's no assurance of it. There's also the potential for poor performance even if it does work.
QUESTION: Can I connect a WIDE device to my narrow SCSI host adapter?
QUESTION: Can I connect a narrow device to my WIDE SCSI host adapter?
ANSWER FROM: Gary Field (scsifaq@engineer.com)
Updated: June, 2000
Yes, you just need an appropriate adapter. Most WIDE devices use the
68 pin "P" connector so you need a 68 pin to 50 pin adapter. You do need
to make sure that both the upper byte and lower byte of the bus will be
properly terminated though. Some adapters provide Hi-9 terminators,
others do not. If the wiring adapter is placed right at the SCSI host adapter,
you can usually configure the host adapter's on-board terminators to only
terminate the high byte. You need to be clear on what type of connectors
are present where you want to do the conversion. You also need to plan
your bus so that there won't be any narrow cable between any of the WIDE
devices.
Certain host adapters with auto-termination make the assumption that
when the low byte is terminated the high byte is also. When using WIDE/narrow
adapters this assumption is not valid. Another purpose served by the hi-9
terminator is supplying pull-up current to the upper data lines which would
otherwise be left floating.
Special note for LVD drives:
It is recommended that if you connect a WIDE LVD drive to a narrow bus that you use a 68 to 50 pin adapter which has high byte termination. It may seem that the termination wouldn't be needed in this case because the bus is narrow. However, the drive needs to have those signals "pulled up" (logically negated), to avoid the floating signals from confusing it.
If for some reason you attach a WIDE device to a WIDE host adapter using a narrow cable, you must be sure to disable WIDE negotiation in the host adapter BIOS or the device will hang when it is accessed.
One further caveat is that if narrow devices are attached to a WIDE
adapter, the adapter's ID must be between 0 and 7 because narrow devices
would not be able to see it if the ID was any higher than 7.
WIDE to NARROW adapters are available from:
819 Striker Ave, Suite 6
Sacramento, CA 95834
Technical Cable Concepts
1790 E. McFadden Ave., Unit 103/104
Santa Ana, CA 92705
TEL: (714) 835-1081
FAX: (714) 835-1595
Dalco Electronics
P.O. Box 550
275 South Pioneer Blvd.
Springboro, OH 45066-1180
Warning: Some 68 pin to 50 adapters have TERMPWR wired incorrectly.
It seems that the manufacturers of many of these adapters (even ones
with a good reputation) have designed their adapters visually rather than
by signal description/function. I say this because I have taken a couple
apart and I can see where they went wrong. If you look at the layout of
a circuit board which makes the connections between a HD 68 pin and
a HD 50 pin you see a nice symetrical fan-like pattern, and for the most
part following this pattern gives you the correct wiring. HOWEVER; there
are 3 signals that must NOT follow the obvious pattern or TERMPWR can end
up shorted to GROUND. This is NOT a good thing. The pins in question are:
17,18 and 51 on the HD 68 connector. These are TERMPWR. If you follow
the obvious pattern:
- HD68 pin 17 connects to HD50 pin 12 (which is RESERVED in SCSI-2)
- HD68 pin 18 connects to HD50 pin 13 (which should be OPEN)
- HD68 pin 51 connects to HD50 pin 37 (which is RESERVED in SCSI-2)
To make things worse HD50 pins 12 and 37 were originally defined as
GROUND
in SCSI-1.
Also, the Pioneer DVD-U02 DVD drive neglected to leave pin 25 (which
turns into HD50 pin 13) open. Which also causes the shorted condition.
Unfortunately, most of these adapters are molded in plastic so that you can't easily open it up and cut those connections. In order to fix them you need to break off the pins in question on the HD68 connector.
QUESTION: How does device ID numbering work with WIDE vs NARROW devices?
ANSWER From: Gary Field (scsifaq@engineer.com)
Narrow SCSI devices can only use IDs 0 through 7. WIDE SCSI devices on a SCSI-3 system with 68 pin P cables, can use IDs 0 through 15. It is generally wise to reserve 0-7 for narrow devices though. SCSI-2 only specified the use of IDs 0-7 even for WIDE devices, but SCSI-3 allows 0-15 for WIDE devices. All devices on one bus must have unique IDs of course.
The arbitration priorities are as follows:
highest
ID 7
...
ID 0
ID 15
...
ID 8
ID 23
...
ID 16
ID 31
...
ID 24
lowest
(I doubt you'll ever see a system using WIDE 32 which is required for
use of IDs 16 thru 24)
A WIDE device that is set to ID 10 knows not to respond to selection for ID 2 because the parity bit P1 (for bits 8-15) will not be set by the initiator. During a selection of ID 10, the P parity bit (for bits 0-7) will not be set by the initiator, but the P1 bit will be.
To use both WIDE and narrow devices on the same bus, the host adapter must be set to ID 7 (or less) so that the narrow devices can talk to it.
QUESTION: What is spindle-sync and why would I want it?
ANSWER From: Roger J. Hamlett (Roger@ttelmah.demon.co.uk)
It fundamentally affects just one aspect of performance, the 'latency'. With a single drive, if you are waiting for a sector to 'arrive' round a track, you have (on average) to wait for approximately one half the rotational time of the drive for it to arrive. So you might arrive at the track just as the sector has gone by, and have to wait one whole rotation at the worst, or the sector might arrive just as you want it, and latency would be zero. This average time, is the minimum latency achievable. There are two methods of reducing this time. The first is to increase the rotational rate of the drive. This is why for certain types of application a 7200RPM drive, will still outperform a 5400RPM drive that has the same data rate off the drive. The other method is to have multiple copies of the required data on unsynchronized drives, and take whichever copy arrives first. This can be done with mirrored drives, and gives a small improvement in the latency time. However the 'down side' of multiple drives comes when we have to wait for all the data parts to arrive. So (for instance) on a striped array, if the drives are synchronized, the latency will remain the same as for the single drives with both data 'parts' arriving together. However, if the drives are unsynchronized, the 'total' latency goes up, to 33% 'worse' than the single drive, as we now have to wait for both parts to arrive. Similar 'extensions' take place with other RAID configurations, unless the drives are synchronized. Basically, in RAID arrays, the drives should be synchronized, _unless_ the total required data can be assembled from a small fraction of the drives.
RAID 1, and RAID 1,0, are the most common configurations where synchronization is NOT advised.
QUESTION: What are the general steps I need to do to install a SCSI disk to be used with Windows?
ANSWER From: Roberto Waltman (rwaltman@bellatlantic.net)
Let me start from scratch and describe one by one all necessary steps:
- Prepare a bootable MS-DOS floppy (SYS A: ) containing, in addition to the system files, the FDISK.EXE and FORMAT.COM programs. (Preferably the ones that came with your Win95 distribution). Make sure there isn't anything in the AUTOEXEC.BAT or CONFIG.SYS files that could make trouble later. Better still, delete these two files. Do not insert the floppy yet.
- Reset your computer and enter the BIOS setup, (not the SCSI setup) and make sure that the "disk type" is set to 'none' or 'not installed' or something similar. Verify that the boot sequence is A: first.
- Exit and reset
- Enter the SCSI setup (CTRL-A) and go to the setup menu. Press F6 to restore all the default settings.
- Exit and reset
- Enter the SCSI setup (CTRL-A) and go to the utilities menu. Make sure you see your disk in the list of devices, and the name and model look OK.
- Select the proper disk and run the "Format"
- Choose "Verify media" to build your confidence that the drive is really working right.
- If these two steps work ok, your disk and controller are fine and they are communicating correctly. If not, you have a hardware problem. (check cables?, terminators?, TERMPWR?, disk itself?)
- Exit and reset.
- Boot from the floppy this time. While the system is coming up, a message on the screen will show up saying something like "your disk model C: 80H BIOS Installed." This means that the SCSI host adapter recognized the disk, and since there is not an IDE C: disk, it installed the necessary BIOS functions to use the SCSI disk as 'C:' It does NOT mean that the C: drive is ready for DOS/Windows. If you don't get that message check that the SCSI disk is installed as device ID 0. (With newer host adapters you can use IDs other than 0)
- After getting the A:> prompt, run FDISK. Create a primary DOS partition. (2 GB max except for Win 95 OSR2 w/FAT32). Make that partition active.
- Exit and reboot from the floppy.
- At this point you already have a C: drive, but you can not use it because it has no file system. (Typing DIR C:, for example, will produce the error message 'Invalid media type', different from the 'Invalid drive specification' you got before)
- To make a file system run FORMAT C: /S /U. The /S tells the format program to copy the system files to C: at the end of the formatting. This will make C a bootable disk. (Assuming the partition was made active above )
- When the FORMAT program ends, you should be able to switch to C:, do a DIR, etc.
- Remove the floppy, reset and (hopefully) reboot from the hard disk.
- The IDE disk must be defined properly in the BIOS setup ("disk type= number" or "autodetect" instead of "not installed" as above).
- If you will only boot from the IDE disk, the SCSI disk doesn't need to be made bootable. (Some modern BIOSes let you choose to boot from SCSI even if an IDE disk is installed)
- The BIOS in the SCSI controller will install a maximum of two disks. If you have an IDE disk installed, the SCSI BIOS will still install the (first) SCSI disk. If you have 2 IDE disks
- You'll have to install SCSI drivers in the boot disk to access the SCSI disk or disks. If you have a system with 4 SCSI disks (no IDE) the controller's BIOS will install only the first two;
- Again you'll have to install drivers to access the rest, etc.
QUESTION: My SCSI CDROM only works when Windows 95 is installed. How can I get Windows 95 installed? Is this a catch 22?
ANSWER From: Gary Field (scsifaq@engineer.com)
Update: May, 2000
Since Windows 95 generally is installed from CDROM media, obviously your system needs to be able to read a CDROM before you can install Windows 95.
This is done by creating a boot diskette containing the necessary drivers to allow the SCSI adapter to talk to the CDROM drive.
For a system with an Adaptec 2940 host adapter this means:
- Make an MSDOS bootable floppy.
- Add the drivers you'll need (as mentioned below) and the MSCDEX program to the floppy.
- Create two files on the floppy as follows:
- CONFIG.SYS:
- LASTDRIVE=I
- DEVICE=ASPI8DOS.SYS /D
- DEVICE=ASPICD.SYS /D:MSCD000
- AUTOEXEC.BAT:
- MSCDEX /D:MSCD000 /M:12 /L:J
Once you get the system booted, select drive J (the CDROM), and run SETUP.EXE
You can also use a Windows 98 install floppy if you have one, since this contains drivers for most popular SCSI host adapters and CD-ROMs.
QUESTION: Under Windows 95 OSR2 I can only see the first 8 GB of my 9 GB disk. What's going on?
ANSWER From: Gary Field (scsifaq@engineer.com)
The retail version of Windows 95 is limited to 2 GB per partition by the use of the FAT16 filesystem. Since you're getting more than 2 GB, you must be using a FAT32 filesystem.
Using FAT32 with drives larger than 8 GB requires a host adapter that supports the "INT 13 extensions". If your host adapter was built before about 1996, you may not have this feature. For example Adaptec 2940W Host adapters did not support this. Even the early 2940UW didn't have it. As of BIOS ver. 1.2x the support is present. Check with your host adapter manufacturer for an updated BIOS.
QUESTION: Are there any storage related reasons to upgrade to Windows 95 OSR2?
ANSWER From: Gary Field (scsifaq@engineer.com)
Yes, Microsoft has made a few enhancements:
- FAT-32 filesystem
- Default cluster size of 4k bytes for drives up to 8 GB.
- Supports drives up to 2 Terrabytes (2048 GB).
- Will only install on drives > 512 MB.
- Can use the "backup" copy of the FAT if needed.
- Is ONLY accessible from Windows 95 OSR2. (Not supported by Windows NT)
- CDFS (ISO-9660) enhancements.
- Drive Power Management.
- 120 MB floptical support.
- The mini-port driver for the Adaptec 2940xx (\windows\system\iosubsys\aic78xx.mpd) is updated. In the retail version of Windows 95 there are problems with the Microsoft supplied driver. If the above mentioned file is older than April '96, you need a new one. The updated driver is also available from http://www.adaptec.com/.
which has the following features:
QUESTION: I changed the host adapter in my system and now my disk doesn't work. Why?
ANSWER From: Gary Field (scsifaq@engineer.com)
Each manufacturer chooses their own algorithm for converting cylinder, head and sector to a SCSI logical block number. If you run into this, you need to back up your system to tape or CD-R using the old host adapter, switch host adapters, Low Level Format (LLF)the disk (using the host adapter's BIOS), re-partition (using FDISK), and re-initialize the filesystem (using FORMAT), then restore all the data from the backup media.
Not as easy as you expected huh?
QUESTION: My drive letters map to different disk partitions than I think they should. What's going on?
ANSWER From: Gary Field (scsifaq@engineer.com)
Caveat: The following description assumes the use of Microsoft's FDISK. The use of tools like Linux fdisk or Partition Magic changes a lot of the rules.)
Microsoft uses the following strange algorithm to map drive letters to disk partitions:
Look at the partition tables on BOTH PHYSICAL DISKS.
First PRIMARY partition becomes Drive C:
If there is a second PRIMARY partition it becomes Drive D:
If there are no more PRIMARY partitions, look for EXTENDED partitions.
The first logical drive in the first EXTENDED partition becomes drive E:, the next logical drive in that EXTENDED partition becomes F: etc.
If there is another EXTENDED partition, the first logical drive in it becomes drive G:, the next logical drive in it becomes drive H: etc.
Then, as device drivers are loaded, any disk drives they support will be assigned consecutive drive letters. CDROMs and other "Network drives" can be assigned specific drive letters if desired, leaving holes in the lettering scheme.
Under MSDOS, Win 3.x and Windows 95 this behavior can't be changed. Under Windows NT all drive letters can be re-mapped to whatever you want.
If you don't want D: on your second disk, don't create a PRIMARY partition on it!
An example of this mapping which often confuses newbies:
INT 13h Drive 0x80:
PRIMARY -> C:
EXTENDED
Logical -> E:
Logical -> F:
Int 13h Drive 0x81:
PRIMARY -> D:
EXTENDED
Logical -> G:
Logical -> H:
CDROM( SCSI ID=5 ):
Assigned to drive M:
QUESTION: What benchmark tools are available for SCSI testing and where can I get them?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: August 1998
QUESTION: Will USB replace SCSI?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: Updated November 2000
If we're talking about USB 1.0 or 1.1 devices, then absolutely not!
USB 1.0 and 1.1 ports, are designed to connect miscellaneous low
speed peripherals external to a PC. It operates at 12 Mbits/sec.. It will
handle things like keyboards, mice, modems, PDAs, digital cameras, scanners,
printers etc. Some low to medium performance CD recorders also have USB
interfaces. It is NOT intended for general purpose mass storage devices
and is NOT a serious replacement for IDE or SCSI.
USB 2.0 is just starting to appear in November 2000. USB 2.0 is supposed
to be able to run at 480 Mbits/sec. If that pans out, it may be practical
to put storage devices on it. Even though that's still not as fast as parallel
SCSI's 160 MB/sec (1280 Mbits/sec.) or soon to come 320 MB/sec, it could
cut into parallel SCSI's territory, just as IDE has.
I think USB 2.0 would be more of a threat to IEEE-1394 however.
USB and IEEE-1394 both have the advantage that termination is fixed
and therefore simple. This can be done because all connections are point
to point, not daisy chained like parallel SCSI. This will be attractive
to many of the less technical users.
IEEE-1394 devices are getting to be much more available, so it's going
to be a race to see whether USB2 or IEEE-1394 garners the biggest share
of the low end I/O market.
QUESTION: Will Firewire (IEEE-1394) replace SCSI?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: September 1998 (slight changes Nov 2000)
Well, not exacty.
IEEE-1394 is a 100/200/400 Mbit/sec. serial protocol. Firewire is a trademark of Apple Computer for an early version of this protocol. There is a standard (IEEE-1394) that describes the serial interface from a hardware standpoint. There are other standards that describe the software protocol that is used to transfer data over this interface. SCSI-3 has a section (SBP) that describes one protocol.
It is becoming popular for interfacing computers to video cameras for frame grabbing and editing. Even if it starts to catch on as a storage interface, it will still be SCSI. People tend to think of SCSI as meaning only the current parallel SCSI interfaces (Single-ended, Differential, and LVD and the WIDE and NARROW variations of these.) In SCSI-3 all Hell breaks loose in terms of how many options there are (Parallel(SE, HVD, LVD), Firewire, Fibre Channel(copper or fibre), SSA)!
The serial interfaces hold a lot of promise for making the interconnection
of storage devices much simpler, and increasing the distance they can be
from the host. When the cost of these interfaces comes down, they may well
replace the parallel buses we currently use, but it will still be SCSI.
I think it's possible that IEEE-1394 storage devices will replace IDE devices
in mass market PCs in the future (probably not before 2003 or so though).
Probably the most welcome feature of IEEE-1394 is the fact that termination
is fixed and the user doesn't need to think about it.
QUESTION: What is Low Level Formatting(LLF)?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: September 1998
Each platter in a disk drive is organized as tracks and sectors. Each sector contains header and trailer information as well as error detection (CRC) data in addition to the actual user data field.
When a disk is manufactured the platters are blank (no sector layout). Before shipping, a special command (usually not documented) is issued to the drive to cause it to lay down the sector headers, blank data fields and good CRC. Also many data patterns may be written to each sector to check for media errors. Any sectors with errors are put into the "manufacturer's defect list" and the drive remembers not to use those sectors in the future. Later, after the drive is shipped, a user may decide to "Low Level Format" the drive if he is having problems, or wants to start with a "clean slate". This is done using the SCSI FORMAT command via a special utility usually supplied by the host adapter manufacturer (usually in the on-board BIOS).
Some side effects of doing a LLF:
- The logical block size and any other saved parameters that are set using MODE SELECT will remain the same as they were last set (i.e. they will NOT return to default values). This can be over-ridden with command option bits.
- The data fields of the sectors will be set to the manufacturer's default value (usually 00 but not always).
- Any bad sectors that were mapped out using RE-ASSIGN BLOCK commands will be available for use again. (i.e. the "grown defect list" is cleared). This can be over-ridden with command option bits.
- If a power failure occurs while a LLF is in progress, the drive may be left in an unusable state, requiring return to the manufacturer for repair. It is safest to do this on a system with a UPS.
LLF is NOT to be confused with running the MSDOS/Windows utilities called FDISK or FORMAT. FDISK causes a "partition table" to be created which logically divides up a disk for use by multiple filesystems and/or Operating Systems. FORMAT causes a FAT16 filesystem to be initialized in an existing partition. FORMAT is equivalent to the UNIX command mkfs (Make filesystem). FORMAT also reads the entire partition and marks any bad sectors found as unusable in the File Allocation Table. This does NOT cause the drive to add them to the drive's "grown defect list", but does prevent DOS/Windows from using them.
QUESTION:Why doesn't my Ultra WIDE drive perform at 40 MB/sec like the SCSI spec. says?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: January 1999
I am starting to hear "I see UDMA drives being advertised as 33 or 66 MB/sec, how come my brand new Ultra-WIDE SCSI drive can't even do 20 MB/sec.. SCSI is a bunch of hype". My response is "No, marketing people are a bunch of liars (or idiots, take your pick)!". Don't blindly accept what you see in ads. The UDMA drives won't run at 33 MB/sec. in any real sense either!
There is a common mis-conception that because Fast20 - WIDE is specified
at 40 MB/sec., that your drives will benchmark at that speed. People need
to understand the difference between "bus bandwidth" and "drive performance".
Bus bandwidth is the maximum speed that data can be moved across the
bus.
Drive performance is made up of many parameters; Rotational latency,
data clock rate, seek time,
cache efficiency and other factors.
The fastest disk drives (either SCSI or IDE) made in 1998 can only
transfer 20 MB/sec. even after they
have found the portion of data they need to send.
This is because the data is only moving under the heads
at about 160 Mbits/sec.
As drives start to spin their media faster and the bandwidth of read/write
heads gets higher, this number increases. Current drives can spin at 10,000
RPM. The next generation will spin at 14,400 RPM.
This will probably move the maximum transfer rate for a single drive
to about 28 MB/sec.
As you can see, there is a considerable difference between the drive
speed and the bus speed.
You ask; "Why should I have a bus that's so much faster than the drive?"
The answer is so that you can support multiple drives without slowing any
of them down.
In the example we're discussing here, a Fast 20 - WIDE bus can support
two 20 MB/sec drives before it becomes the bottleneck. This is why Fast40
- WIDE (Ultra2W or U2W) was developed. Servers and other high performance
systems need to handle more than two disks simultaneously. U2W is rated
at 80 MB/sec., so it can support four 20 MB/sec. disks before it becomes
the bottleneck. Later in 1999, Ultra3, or Fast-80, or a subset called Ultra160/m,
host adapters will become available which will allow supporting up to eight
20 MB/sec. drives or five 28 MB/sec drives. At that point the PCI bus becomes
the bottleneck.
By the way, you'll need a 66 MHz (or 64 bit) PCI bus to handle Ultra3/Ultra160
host adapters, since 33 MHz, 32 bit PCI can only transfer 133 MB/sec.
QUESTION:My system says my new 9.1 GB disk is only 8.5 GB. Did I get cheated?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: January 1999
Well, not exactly. It's a matter of definitions.
Marketing people like to use the prefix Mega to mean 1,000,000 and Giga to mean 1,000,000,000.
Computer engineers define Mega to mean 1,048,576, and Giga to mean 1,073,741,824.
The reason engineers use these strange looking numbers is because they are powers of 2.
(2 to the 20th and 2 to the 30th)
The ad you read when you bought your disk was written by a marketing person. The
operating system on your computer was written by an engineer. In engineering terms,
your 9,100,000,000 byte disk is about 8.5 GB.
QUESTION: How much overhead does the SCSI protocol impose?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: September 1999
At first glance the SCSI protocol may seem rather verbose with 6, 10 or 12 byte commands being sent to read a block of data. Let's look at a WORST CASE example:
- Host waits for the bus to be free (800 ns)
- Host arbitrates for the bus (2400 ns)
- Host selects target(900 ns)
- Target enters MESSAGE OUT phase
- Host sends IDENTIFY message (1 byte @asynch. = 800 ns)
- Target enters COMMAND phase
- Host sends 12 byte READ command requesting one 512 byte block (12 bytes @asynch. = 10,000 ns)
- Target enters MESSAGE IN phase
- Target sends DISCONNECT message (1 byte @asynch. = 800 ns)
- Target re-selects host(900 ns)
- Target sends RESTORE POINTERS message (1 byte @asynch. = 800 ns)
- Target enters DATA IN phase
- Target sends 512 bytes of data using U2W synchronous xfer mode(6400 ns)
- Target enters STATUS phase
- Target sends STATUS byte (1 byte @asynch. = 800 ns)
Total transfer time = 24.6 us. Of that time, 18.2 us was due to
the protocol. This comes out to 74% overhead!
Fortunately, in the real world, multiple blocks are read together,
which brings the overhead way down.
If asynch protocol was used for the data phase, this would only be 4.2% overhead! The problem is that the synchronous speeds get faster and faster, but synchronous is not used to transfer the command bytes and many of the delays are set by physical parameters of the bus (length, capacitance etc).
I see a total of 16 extra bytes that were sent in order to read those 512 bytes of data. As a percentage 16/512 is only about 3.1%.
This is for the WORST CASE with the host only asking for 1 block and
using a 12 byte command on a bus with only 1 drive active. In a more typical
case, the host would ask for many blocks to be read together and use a
10 byte command. More importantly, the bus would have several drives active
and the commands and messages would be transferred during the bus idle
times while drives were disconnected doing their seeks.
QUESTION: What is LVD (Low Voltage Differential)?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: January 1999
Low Voltage Differential is a new hardware bus driver type for SCSI-3. It first becomes important
with Fast-40 (Ultra2) devices. If single ended (S.E.) bus drivers were used with Fast-40, the bus length would
be limited to about .75 meters! Since this was clearly un-acceptable, and the older High Voltage Differential
(HVD) interface adds too much cost to a system, ANSI defined a new form of differential interface
that is less expensive to implement because the bus driver logic dissipates little enough power that
it can be included in LSI chips. They also wanted to make sure they avoided the confusion caused by
HVD (HVD and S.E. devices cannot co-exist on a bus), so they specified that if an LVD device is designed properly,
it can switch to S.E. mode and operate with S.E. devices on the same bus segment.
Another difference worth noting is that LVD devices do not contain on-board terminators. You need
to attach an LVD terminator to the end of the cable instead of using a device to terminate the bus
as was commonly done with S.E. SCSI. The big advantage to LVD is that you can have a high speed
SCSI bus up to 12 meters in length!
QUESTION: My drive stopped working. Is it dead?
ANSWER From: Gary Field (scsifaq@engineer.com)
Updated: December, 1999
Before you toss that "dead" drive, here are a few things to try:
- Substitue another power supply. Maybe it's not getting power.
- Check the cable connection. Try another connector.
- Re-check your terminators. Have you added a device lately?
- If you have a VERIFY utility (such as in the Adaptec SCSI Select BIOS), run it. Perhaps only certain blocks have gone bad. The verify utility should be able to re-assign those blocks to good areas of the disk. That still leaves holes in your files where blank sectors have been substituted though. If you have a utility that can tell you what files those block numbers were in, you can just copy over those files. Even if the disk is too damaged to read the disk media successfully, the verify utility may provide some error messages or SENSE DATA that will provide a clue to what's wrong with the drive.
- I hope it didn't have any important data on it! (You have a backup of it anyway right? Oh too bad...)
- Try doing a low level format.
- Is it under warranty? Check the serial number on the mfr web site. If it's under warranty, contact the mfr for an RMA number and send it back. Don't violate the warranty by tinkering any further.
- No warranty huh? If the drive won't spin up, sometimes it's due to "sticktion". This means that the heads are "stuck" to the media by vacuum. One method of trying to un-stick the heads is to try twisting the drive in the plane of the disk platters with a quick wrist twisting action. If that still doesn't do it, as a last resort, giving the drive a sharp rap with a screwdriver handle will sometimes free the heads. At this point it's worth a shot (so to speak). If you do succeed in getting the drive spinning again, do NOT turn it off again until you get the data on it transferred to another drive. The sticktion problem tends to be chronic so plan on replacing the drive as soon as possible.
- If the manufacturer has firmware for it on their website, try re-flashing the drive's firmware.
- If you have another identical "dead" drive lying around, try swapping the controller card from it onto the drive in question. Perhaps you can make one good one out of the two bad ones. This only works if the drives really are identical.
Additional information about what
SENSE DATA can tell you (from Folkert Rienstra):
SCSI devices provide information about their condition and any errors
that have occurred during the previous command execution.
Devices return this information in response to a REQUEST SENSE command.
Many device drivers simply throw this information away making it difficult
to analyze problems that occur during normal operation. If you can reproduce
the problem using smarter software that will issue a REQUEST SENSE command
and interpret the data for you, a device can often tell you what is wrong
with it.
What do you need:
1. A drive exerciser also known as a CSO (Customer Simulated Operations)
program that will report the error sense information.
2. A list of SCSI errors comprised of Sense Key, Additional Sense Code
(ASC) and Additional Sense Code Qualifier (ASCQ).
Or:
2. A program that translates those codes into descriptive information.
In the first category fall codeupdt from IBM and SCSI Workbench from
Western Digital. They both contain a drive exerciser.
SCSIBench, the SCSI benchmark program from Adaptec is a little exerciser
in it's own right: If the drive is not completely up to scratch it will
complain through Sense information.
* There is also SCSITool from Bart Lagerweij which is currently under
construction (Apr 2000) that has a lot of sense codes built in.
* And then there is SMARTMon from SANTOOLS (David A. Lethe) that will
show you not only translated Sense Error data on Inquiry but also accumulated
Sense Log data.
That is, when your drive is "supported" by SMARTMon. SMARTMon does
not have a drive exerciser.
(A few words of caution: Caching parameters in Device Properties MAY
crash or hang your system. Also, adding alerts to 'Alerts|Maintain rulesets
.... for statistical alerts' can result in a hung system too. David
is looking into those at the moment).
In the second category falls SCSIcode from Adaptec. This is a little DOS program (that can run in a DOS box) that accepts the Sense Key and the Additional Sense Code and translates those into the appropriate error descriptions.
Some URLs:
SMARTMon:
http://www.santools.com/smartmon.html
QUESTION: My host adapter says it "auto-terminates". How does it do that?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: May, 1999
That's a very good question! I haven't done a thorough study to determine how they all do it, but the ones I've looked at sense a ground connection on certain pins. This may work in many instances, but it's certainly not foolproof. For example many people connect scanners, or ZIP drives that have (may their little metal souls be damned) 25 pin connectors, to the end of the bus. What happens to all those ground connections now? Who knows! Another problem comes in when 68 pin buses are adapted down to 50 pins. Same problem. Another problem is if the host adapter is in the middle of the bus. It might be able to sense whether there is another terminator, but how can it know if there are too many?
In my opinion, the only reliable ways to sense whether the bus is properly terminated is by looking and checking manually, or with a Time Domain Reflectometer and I don't think they'll be putting those onto SCSI host adapters any time soon (but it's not impossible).
Certain host adapters with auto-termination make the assumption that when the low byte is terminated the high byte is also. This isn't always a good assumption, especially if 68 pin to 50 pin adapters are in use.
The bottom line is that you shouldn't depend on auto-termination to do the right thing. Set the termination manually!
QUESTION: Can I flash update a new BIOS on my OEM Adaptec host adapter?
ANSWER From: Folkert Rienstra
Date: March, 2000 (updated April, 2000)
To know that, you have to know the difference between a product made by an OEM and a product intended for delivery to an OEM (vs the retail product).
The first one is the OEM (original equipment manufacturer) himself (e.g. a card maker) supplying a product that incorporates another manufacturer's product (e.g. the chip) and sells it under his own name.
The second one is a product intended for use by an OEM where the OEM (e.g. a PC maker) will use it as a part in his product. Adaptec plays both roles. It is a chip manufacturer and it also is a card manufacturer. To complicate things it manufactures cards under its own brand name, identified as AHA-series adapters to be sold to the public, the so called retail products, and cards under the AIC-xxxx chipname umbrella, these carry different deviceIDs. (I am not sure if these were sold to OEMs or as 'Adaptec on the cheap' retail products).
To complicate things further still, they produce cards under the Adaptec brandname for use by OEMs. Some of these are recognized by the special 'S' BIOS. It is possible that you can buy these as an OEM version, although Adaptec never intended for you to buy it, unless you want yourself to be called an OEM, in which case you would need to deal with Adaptec yourself, not through your retailer (and supply you own tech. support).
And to complicate things still further they also produce cards under the Adaptec brand name that identify under the AIC-xxxx chip name umbrella, although the BIOS may say something different (some 290x and 2930-x cards). I think that those are intended to come bundled with other products.
Don't take the previous information as fact, it is just my observation.
As to warranty and service, I think that Adaptec will offer support
only for their retail products, which they may track through the serial
number or their TSID number scheme. Of course, they can not prevent you
from downloading a BIOS image for your AHA- or ASC-branded, OEM intended,
card that is physically indistinguishable from the retail product.
Is there a solution for (made by) OEM cards or inboard SCSI solutions?
Yes there is, but it is not without risk and is not for the faint of
heart:
When you have an OEM card or motherboard SCSI solution that has the
"different from AHA branded products" PnP deviceID, you may still have
a possibility to convert an Adaptec sourced retail card BIOS for use on
your card or inboard solution. There are about 5 or 6 places in a ROM image
where the PnP deviceID is stored. When a PCI BIOS extension starts, it
uses those device IDs to find and check against the card/SCSI chipset.
If that ID is not found it does not load. Change the deviceIDs in the ROM
image to the ID used for your card and you are on your way.
There is one small problem however: Adaptec BIOSes are compressed BIOSes
headed by a decompression routine. You will have to decompress the BIOS
before you can edit it, and compress it afterwards, then paste it behind
the decompression header again. Your ROM image is now ready. If you have
a problem understanding what is written here (it involves the use of LHARC
and a binary editor looking for xx-lh5- and hex IDs) don't even try
it.
An example:
You have a AIC-7880 based card identified as "Adaptec AIC-7880 PCI
SCSI Controller" the device ID is 8078(h). Look it up in the driver inf
file like Windows SCSI.INF or AIC78xx.INF that comes with the drivers.
Right behind that you find "Adaptec AHA-2940U/AHA-2940UW PCI SCSI Controller"
the device ID is 8178(h). You get the 2940UW bios (the file that ends in
.ROM) and you replace all instances of 7881(h) in the BIOS header and the
decompressed bios with 7880(h). Reassemble the ROM image and you're done.
The BIOS image can now be flashed into the card or Motherboard. For motherboard
solutions the decompression header may be absent because it is already
part of a compressed motherboard BIOS, and the name of the section may
not be obvious (e.g. the SYMBIOS BIOS in an AWARD BIOS goes under the name
of PCI32.ROM). There may be other sections that need to stay at their fixed
locations, such as the AWARD decompression BIOS @ 1B000 and the AWARD
bootblock BIOS @ 1E000.
* Some of this also applies to the SYMBIOS BIOS included in AWARD sourced
BIOSes.
Notes from the [Editor(GF)]:
Some people have told me that if you get your Adaptec card into an
unusable state when trying to re-flash the BIOS, you may be able to recover
it using an older version of the FLASH.EXE utility. Try the following links
to get older versions:
QUESTION: Although the Adaptec BIOS identifies my drives, I cannot access my drives under DOS. What am I doing wrong?
ANSWER From: Folkert Rienstra
Date: April, 2000
In order to access your drives under DOS they need to be enabled in the SCSISelect BIOS setup(CTRL-A during boot). The fact that you can see them at startup does not mean they are enabled. The BIOS does a device scan and shows any device it finds, but to be enabled for DOS access, the device needs to be "Included in BIOS Scan".
Devices that are enabled for BIOS control are identified with an additional "- Hard Disk n" at the end of the line.
QUESTION: Why are my 80MB/s Ultra2/LVD drives only listed as 40MB/s [SE] by my OS?
ANSWER From: Dana Lacoste
Date: April, 2001
Almost all LVD drives are designed to be able to be used on a Single Ended bus if desired. In this case, the drive switches to S.E. mode. The drive determines which type of bus it's on based on the voltage present on the DIFFSENSE signal. If even one S.E. device is present on an otherwise LVD bus segment, all devices will run in S.E. mode. Make sure you have only LVD/SE terminators instead of merely Active/SE terminators (LVD/SE terminators are also available which will indicate whether they are operating in LVD or SE mode).
QUESTION: What issues are involved in connecting an Ultra160 (Ultra3) drive to an Ultra2 host adapter?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: July, 2000
In general when new SCSI protocols are designed they are compatible with most of the previous protocols. This case is no exception. An Ultra3 drive attached to an Ultra2 host adapter will simply operate in Ultra2 LVD mode. If you connect that same Ultra3 drive to a SCSI-2 host adapter like an Adaptec 1540cf, it will operate in Fast-10 Single Ended mode. Essentially the host adapter and drive will negotiate for the best speed that they both have in common.
QUESTION: What is meant by PIO, DMA, and Bus Master DMA?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: July, 2000
PIO stands for Programmed (or Polled) Input/Output. This means that the CPU is used to read and write data directly to and from the host adapter chips. This is typically only used on low cost host adapter cards because it ties up the CPU during I/O which makes multi-tasking work poorly.
DMA, or Direct Memory Access means exactly that. Special circuitry is included either on the motherboard, or the host adapter card itself, that can move data between system memory and the host adapter, with the CPU only performing a simple initial setup of the transfer. There are two distinct types of DMA circuitry used in PCs, third-party DMA and Bus Mastering DMA. Third-party DMA uses chips built into all PC/AT compatible motherboards to transfer the data. Bus Mastering DMA uses circuitry on the host adapter card itself to take control of the bus and transfer the data. PCI host adapters all use Bus Mastering DMA. This is the most efficient of the three data transfer methods and consequently also the most expensive. Using DMA frees the CPU from the data movement task and interleaves data transfers into bus cycles that the CPU isn't using. This makes multi-tasking work very smoothly which is what SCSI is all about!
Another term related to DMA is "scatter/gather". This refers to the ability to perform the DMA transfer to/from a collection of separate memory blocks or pages. This is vitally important in virtual memory operating systems which keep virtual memory addresses contiguous but scatter the data throughout physical memory pages. Since DMA uses physical addresses to do the transfer, the DMA chips need to be able to accept a "linked list" of physical addresses and transfer sizes rather than simply one address and size. The term "scatter/gather" refers to what happens to the data during the DMA operation. For READs from the device, the sequence of bytes from the device are "scattered" onto discontiguous physical pages of memory. During WRITEs to the device, data is "gathered" from the various physical pages and marshalled into a sequence of bytes to transfer to the device.
In my opinion, if you're going to bother using SCSI, get the full advantage and buy a PCI Bus Mastering host adapter.
QUESTION: How Should I Choose a SCSI Host Adapter?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: July, 2000
A SCSI Host Bus Adapter (sometimes called an HBA) is the heart of a SCSI subsystem. As the name implies, it connects the system's I/O bus (PCI or ISA) to the SCSI bus. You need to give some thought to what SCSI devices you will be connecting to your system before choosing one. There is a large variety to choose from. They range from really low end PIO ISA cards that are included with SCSI scanners, CD-RW drives, etc, to dual channel, Ultra160, 64 bit Bus Mastering PCI cards costing up to $400 or more.
If you only intend to connect low performance devices such as a scanner to the system, then a low end HBA is probably all you need.
But, if you want to connect SCSI hard disks, you'll probably want to get the best card you can afford. If you will have disks and assorted other slower devices, you may even want to get a dual channel card that will allow you to isolate the slower devices onto their own bus to keep the disks performing optimally.
Using an ISA card in a PCI capable system can cause really poor performance.
PCI motherboards often implement the ISA slots in such a way that they
are much slower than ISA slots on ISA only motherboards. There is often
a temptation to use an old ISA card that came with your scanner, or CD-RW
drive. Please resist this urge. These cards rarely have proper drivers
available for other devices, plus they can degrade the performance of the
entire system which defeats the whole purpose of using SCSI in the first
place!
Another factor to consider is the type of support you require. If you
really know a lot about installing and configuring PC option cards, you
may want to save a few bucks and get an "OEM" card. But, if you aren't
a real expert on this, you'd better stick to a "kit" packaged card that
comes with manufacturer's technical support and the required cables, drivers
etc.
Also, you should decide what operating systems you will be running
and make sure they support the specific card you are considering. Also,
check the manufacturer's web site to see if they provide driver updates
on a regular basis. You will find that having updated drivers readily available
makes the difference between enjoying and hating your PC.
To be thorough, you should also read this article about determining the performance that will result from a particular combination of devices. Running through these calculations may cause you to realize that even though the SCSI protocol allows each device to run at its highest speed, having slower devices mixed with your high speed devices on the same bus, can eat up the bandwidth of even a high performance host adapter.
QUESTION: How can I calculate the performance I'll get with mixed SCSI devices?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: August, 2000
When you mix SCSI devices that have different levels of performance on the same bus it can be difficult to guess what the overall performance of the bus will be. You may even find that you run out of bus bandwidth with fewer devices than you expected.
Here are some formulas I worked out, that at least in theory should come close to telling you what speed each device will perform at, and what percentage of the available bus bandwidth you'll be using with all of the specified devices running flat out.
If all the devices on the bus run at the same speed and that speed is also the max. speed of the host adapter, you can easily estimate how it will all work. For example, if you connect three Ultra2WIDE disks that are each specified to deliver 25 MB/sec sustained, to a Ultra2WIDE host adapter card, you can figure that all three drives will be able to run at their maximum performance and most of the host adapter's bandwidth will be utilized (3 * 25 = 75 MB/sec on a 80 MB/sec host adapter which amounts to 93.75% utilization).
When you connect two Ultra drives rated at 15 MB/sec sustained, and a Fast-10, 20x CD-ROM rated at 3 MB/sec sustained, to a Ultra2 host adapter, what will be the performance of that combination?
You might be tempted to say that the two 15 MB/sec disks add up to 30
MB/sec and the CD-ROM's 3 MB/sec will bring it up to 33 MB/sec on the bus,
which is less than half of the 80 MB/sec the host adapter is specified
for.
However, this would not be even close to reality!
The real answer is that the bus will be 180% utilized, the two disks
will each benchmark at around 8.34 MB/sec and the CD-ROM at around 1.67
MB/sec!
Before we can get real answers, we need some real information:
First, you need the device specifications for all of the devices you will have connected. The two parameters you need are:
- The device's max. burst transfer rate (usually 10, 20, 40, or 80 MB/sec). e.g. a Fast-20 WIDE disk would have a burst rate of 40 MB/sec.
- The device's max. sustained transfer rate. This is the rate that data actually comes off the medium at. This can be from 150 KB/sec for a 1x CDROM up to about 35 MB/sec for a 15,000 RPM hard disk.
Next, we assume that the host adapter is capable of operating at the burst rate of the fastest device that will be attached.
Lastly, we assume that a test utility is being used that causes all attached devices to be accessed continually. This means that the results will be a "worst case" scenario but may actually reflect reality for very heavily loaded servers.
Given the above information, we can estimate the utilization of the bus bandwidth and the speed each device will operate at while running benchmarks. The following formulas ignore protocol overhead however.
My Algorithm:
The way I chose to go about figuring this is to calculate how long
it will take each device to transfer "one seconds worth" of data across
the SCSI bus. (i.e. The sustained number of Megabytes/sec times the number
of microseconds/byte at the burst rate), then, adding this figure for each
device on the bus. If the total is greater than 1 second, all the data
can't be transferred at full speed. Since there's not enough time to transfer
all the data, we need to reduce the time allotted to each device, and see
how much data it could transfer in that amount of time.
My formulae:
dev1_secs = (1/dev1_br) * dev1_sr)
dev2_secs = (1/dev2_br) * dev2_sr)
...
devN_secs = (1/devN_br) * devN_sr)
total_secs = (dev1_secs + dev2_secs + ... + devN_secs )
Percent_Utilization = total_secs * 100
Where:
- dev1_br = The first device's burst rate in MB/sec, dev1_sr = The first device's sustained rate in MB/sec
- dev2_br = The second device's burst rate in MB/sec, dev2_sr = The second device's sustained rate in MB/sec
- devN_br = The Nth device's burst rate in MB/sec, devN_sr = The Nth device's sustained rate in MB/sec
If however, Percent_Utilization is greater than 100, we need to do more:
Overutilization = Percent_Utilization - 100
Overtime = Overutilization/100
devN_actual_secs = devN_secs - ((devN_secs / total_secs) * overtime)
Real speed of DevN = devN_actual_secs * devN_br
Note: I make the assumption that each device will pay the overutilization penalty in proportion to how much data it will transfer (i.e. how much it uses the bus). In reality it would depend on other factors too, like what SCSI ID (and therefore what bus priority) each device was at. I haven't done any testing to determine whether this is the best way to divide up the "overtime". Perhaps the faster devices should pay less of a penalty. I would welcome any data on how this works out on a real bus.
An example:
We have three devices attached to an Ultra2 (80 MB/sec) host adapter:
- An Ultra WIDE hard disk with a 40 MB/sec burst rate, and a 15 MB/sec sustained transfer rate.
- An Ultra narrow hard disk with a 20 MB/sec burst rate, and a 10 MB/sec sustained transfer rate.
- An Ultra narrow 28x CD-ROM drive with a 20 MB/sec burst rate, and a 4 MB/sec sustained transfer rate.
dev2_secs = .050 * 10 = .500 secs
dev3_secs = .050 * 4 = .200 secs
total_secs = (.375 + .500 + .200 )
Percent_Utilization = total_secs * 100 = 107.5%
Since the bus is greater than 100 % utilized:
Overutilization = 107.5 - 100 = 7.5 % overutilized
Overtime = 7.5/100 = .075 seconds
dev1_actual_secs = .375 - ((.375/1.075) * .075)
Real speed of Device #1 = dev1_actual_secs * dev1_br = 13.95 MB/sec
dev2_actual_secs = .500 - ((.500/1.075) * .075)
Real speed of Device #2 = dev2_actual_secs * dev2_br = 9.3 MB/sec
dev3_actual_secs = .200 - ((.200/1.075) * .075)
Real speed of Device #3 = dev3_actual_secs * dev3_br = 3.73 MB/sec
QUESTION: What is iSCSI?
ANSWER From: Gary Field (scsifaq@engineer.com)
Date: April 12, 2001
iSCSI is a new standard for sending SCSI commands over an IP network. It is the networking industry's answer to fibre channel. Storage Area Networks are becoming very important to any company that needs large amounts of reliable storage. Fibre Channel is the defacto standard protocol for most SANs. Cisco and other networking companies are now pushing iSCSI since they ignored fibre channel for too long.
