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comp.periphs.scsi FAQ part 1 of 2

( Part1 - Part2 )
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Archive-name: scsi-faq/part1
Posting-Frequency: monthly
Last-modified: 1998/3/31

See reader questions & answers on this topic! - Help others by sharing your knowledge
The FAQ has undergone a facelift. The source file is now a Microsoft Word
for Windows (Office '97) document so that it is be possible to
generate both a text and an HTML version from the same document.
I'm not thrilled with the text file that I'm getting out of Microsoft Word,
but I believe all the information is there.
The HTML version will make it much easier and faster to look up information
by providing hyperlinks. The text version will continue to be produced
until there is no significant interest in it. If you would be unable to
read an HTML version of the FAQ and want to make sure I continue to
produce the plain text version, email me at:
explaining why. So far I have only received one request for the text
version, so it may not live much longer.

	Gary Field

Frequently Asked Questions List for comp.periphs.scsi
Current Editor: Gary Field (
(Where you see reference to [Editor(GF)] that means me.)
Last updated: March 31, 1998
Skip to Table of Contents
FAQ history: Created by Johnathan Vail ( from 
articles submitted to him by comp.periph.scsi readers.
Maintained by Johnathan Vail until November 1993.
Where to get the latest copy of this FAQ:
The comp.periphs.scsi FAQ is posted to Usenet during the first week 
of each month. As of February 1998 it is available in both a text 
file version and an HTML version. Some sites may not yet be archiving 
the HTML version.
A recent version can be obtained via anonymous ftp from:
pub/gfield/scsi/scsifaq.txt and scsifaq.html
Note: the gfield directory will not show up using DIR, but it's 
actually there. Just CD to it.
via World Wide Web (WWW): 
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.
Table of Contents:
Generic SCSI Questions
SCSI Documentation and Books
SCSI Manufacturer Contact Information
Manufacturer Specific Questions
Platform Specific Questions
Device Model Specific Questions
Host Adapter Model Specific Questions
Generic SCSI Questions:
What is SCSI? 
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 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? 
Where Should I place the SCSI adapter on the SCSI bus? 
Is the spacing of connectors on a SCSI cable important? 
How long can my SCSI bus be? 
What are the pros and cons regarding SCSI vs IDE/ATA ? 
Should I spend the extra money on SCSI or just get IDE? 
Can I have both IDE/ATA drives and SCSI in the same system? 
Is it possible for two computers to access the same SCSI disks? 
Is it possible for two computers to access the same SCSI tape? 
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? 
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 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 

Table of Contents
SCSI Documentation and Books:
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? 
Table of Contents
SCSI Manufacturer Contact Information:
How can I contact:
Archive Corporation (see Seagate) 
BusLogic /Bustek / Mylex 
Future Domain 
Conner Peripherals 
Symbios Logic 
Tecmar Technologies (formerly Wangtek, WangDAT, Sytron, and Rexon) 
Western Digital 
DPT (Distributed Processing Technology) 
Legacy Storage System 
Table of Contents
Manufacturer Specific Questions:
Where can I get SCSICNTL.EXE and other Adaptec files? 
Where can I get technical information and jumper settings for HP 
drives ? 
Who manufactures SCSI extenders and Single-Ended to Differential 
converters ? 

Table of Contents
Platform Specific Questions:
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. 
Whatís 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). 
Table of Contents
Device Model Specific Questions:
What are the jumpers on my Conner drive? 
What are the jumpers for my Wangtek 5150 drive? 
How do I configure my HP DDS DAT tape drive? 

Table of Contents
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 
How to replace Macintosh internal HD and terminate the SCSI chain 
I changed the host adapter in my system and now my disk doesnít work. 
Table of Contents
Answers to the Questions: 
ANSWER From: LSD, L.J.Sak@Kub. Edited by Gary 
SCSI stands for Small Computer Systems 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 
SCSI2 is the most recent version of the SCSI command specification 
and allows for scanners, hard disk drives, CD-ROM players, tapes [and 
many other devices] to connect.

Table of Contents
Question: What information should I provide when asking a question in 
the comp.periphs.scsi newsgroup?
Answer From: Gary Field(
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 
Length of SCSI bus
Where the terminators are located
Whether the configuration is new, or was working before.

 It may seem like a lot of information to provide, but unless you 
have some SCSI experience, you may not realize how many factors can 
affect whether the system works properly or not.
 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.

Table of Contents
Question: What do all these SCSI buzzwords mean?
Answer From: (Hennes Passmann)[Editor(GF)]
-Host adapter:
The card that connects your computer to the SCSI-bus.
Usually called SCSI-controller by marketing droids.
-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.
The 18 signals that are terminated are:
I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ... DB(0).
-Terminators (active).
Rather than passive terminators that use TERMPWR which may not be 
exactly +5v, active terminators use a voltage regulator. 
-Single ended:
"Normal" electrical signals. Uses open collector to the SCSI bus, 
[usually] survives wrong cable insertion. DIFFSENSE signal is used to 
detect connection of wrong type devices.
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.
Uses two wires to drive one signal.
Max. cable length of 25 meters.
Electrically incompatible with single ended devices! 
SCSI-1 and 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.
-Asynchronyous SCSI:
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 
send asynchronously over the 8 bit part of the SCSI-bus.
-Synchronous SCSI.
Rather then waiting for an ACK, devices that both support synchronous 
SCSI can send multiple bytes over the bus in the folowing 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. I know of no single drive that reaches these 
- RAID:[Added by Editor(GF) Corrected by Fredrik Bjork 
A Redundant Array of Independant Disks is a set of drives connected 
to a special dual ported SCSI adapter that allows certain types of 
access optimization. A RAID 0 array stripes the data accross 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.
-Ultra SCSI:
Allows up to 20MHz signals on the bus.
-Wide SCSI:
Uses an extra cable (or 68 pin P cable) to send the data 16 or 32 
bits wide. This allows for double or quadruple speed over the SCSI-
bus. Note that no *single* drive reaches these speeds, but groups of 
several drives can. 
Table of Contents
Question: What is the history of SCSI (What is SASI)?
Answer From: (Hennes Passmann)
#include <stddisclaimer.h>
1979 The disk drive manufacturer Shugart begin working on a new drive 
interface with logical rather then 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.
1986 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 backwards 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 adaptor on their own inititive.
(e.g. to set in which mode they shoud 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)
-Serial packet protocol (IEEE P1394)
-SCSI-3 general packet protocol (almost all serial interfaces) and of 
course the old SCSI-2 commands and more.
-Low Voltage Differential Parallel interface
-CD-R command set and algorithms
Future(after 1996): SCSI-3 becomes official
SCSI becomes a more network-like environment where devices can be 
physically distributed and shared more easily.
Table of Contents
Question: Can I access SASI drive with SCSI controller?
Answer From: Gary Field(
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.
Table of Contents
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 <>
	 edited by Gary Field (
One confusing thing about SCSI is what the SCSI bus is supposed to 
like, and how devices should be placed on the bus.
The SCSI bus MUST run continuously from one device to another, like 
DEVICE A --------- DEVICE B --------- DEVICE C -------- DEVICE D
Where device A, B, C, and D can either be internal or external
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 relections 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 
relections from the ends of the bus. They do this because they 
(hopefully) have the same impedance as the rest of the transmission 
The SCSI bus must not have any "Y" shape cabling. For example, 
setting up a cable that looks like this is NOT allowed:
           >------------- DEVICE C ----------- DEVICE D
Where do I put the terminators?

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
The following ARE NOT allowed:
   |            |          |           |                   |
DEVICE A    DEVICE B    Adapter    Unconnected        Unconnected 
Terminated             Terminated
   |            |          |           |           |
Termination DEVICE A   DEVICE B  DEVICE C       Adapter 
Where Should I place the SCSI adapter on the SCSI bus?
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 
terminations 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).
Table of Contents
Question: What is a SCSI terminator? Why do I need them?
ANSWER From: Gary Field (
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 
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 resulting adding and cancelling of signal 
amplitudes distorts and destroys the SCSI signals.
There are two basic types of terminators, active and passive.
Table of Contents
Question: Is the spacing of connectors on a SCSI cable important?
ANSWER From: Gary Field (
The ANSI SCSI spec's say that "stubs" on a SCSI bus must not be any 
more than .1 meters (4 in.) long. In the most recent spec's 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.
Table of Contents
QUESTION: How long can my SCSI bus be?
ANSWER From: Gary Field (
The SCSI 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.
5 MHz (SCSI1 synch.)		6 meters		            25 
10 MHz (SCSI2 FAST)		3 meters(not rec.)	      25 meters	 
20 MHz (Ultra or Fast20)	1.5 meters(not rec.)	       ?
40 MHz (Ultra2 or Fast40)	Only differential connections allowed
These limits assume the use of good quality cable which maintains its 
characteristic impedance between 90 and 130 Ohms and the use of 
active 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.
When Low Voltage Differential(LVD) devices are available, this will 
allow lengths between the single-ended numbers and the HV Diff. 
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 
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.
Table of Contents
QUESTION: What are the pros and cons regarding SCSI vs IDE/ATA ?
ANSWER From: Gary Field (
Pros of IDE/ATA:
	Inexpensive due to high volume of production
	Supported directly by system BIOS in most cases
	Less overhead per command
Cons of IDE/ATA:
Very limited device attachment (two drives including CDROMs) 
Only supports disk, CDROM (and limited support for tape) 
Single threaded (commands do not overlap even with a second drive) 
CPU is tied up transferring all data 
IDE/ATA and ATAPI evolved as one kludge on top of another
(so compatibility is not always good)
Cannot handle scatter/gather operations well
Pros of SCSI:
Flexible device attachment (up to 7 or 15 devices per SCSI bus) 
Support for almost any peripheral type (disk, tape, CDROM, scanner 
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) (Win 95 ?)
Cons of SCSI:
Generally more expensive than IDE/ATA 
Slightly more complicated to install than IDE/ATA
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 <>
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 
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?
Table of Contents
Should I spend the extra money on SCSI or just get IDE?
ANSWER From: Gary Field (
For home users this is a difficult question to answer in general.
It totally depends on how you use your system, what operating system 
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/EIDE is single threaded by nature. The current command must 
complete before additional commands can start. With most IDE adapters 
the processor must be involved in reading/writing the data from/to 
memory. Another drawback is that only two drives can be attached. In 
a single drive single-tasking system IDE will probably be slightly 
faster and is definitely less expensive.
When you start talking about multi-tasking operating systems (like 
Win95, WinNT, Unix, OS/2 and Netware) SCSI is now a big advantage. As 
disk drives get bigger, backup devices are becoming even more 
important. In my opinion floppy tapes just aren't satisfactory. 
They're too slow, too unreliable, non-portable(media exchange wise 
not physically), and have low storage capacities. SCSI tape drives 
are more expensive, but have none of these problems. SCSI devices 
share the bus bandwidth efficiently by allowing one device to 
transfer data while another is seeking or rewinding its media. Early 
SCSI implimentations had some compatibility problems but these days 
SCSI is simpler to install than EIDE. Each user needs to make this 
choice individually, but if you don't consider all the issues, you 
can find yourself needing to re-vamp all your I/O to add a device 
later on. Before you decide to go with IDE, ask yourself if you will 
ever want to add a CDROM, CD-R, scanner, or tape drive or need more 
than two hard disk drives.
Here's a discussion that shows some of the advantages of SCSI in more 
from: Greg Smith (
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 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 aslo 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 
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 Ė 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 
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 is 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 
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.
Table of Contents
QUESTION: Can I have both IDE/ATA drives and SCSI in the same system?
ANSWER From: Gary Field (
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.
Table of Contents
QUESTION: Is it possible for two computers to access the same SCSI 
ANSWER From: burke@seachg.uucp (Michael Burke)
Yes, two (or more) systems can be on the same SCSI bus as SCSI disk 
and tape drives. As long as the SCSI requirements are met - cable 
lengths, termination and type - the devices can share the SCSI bus.
[Editor(GF): Each host adapter needs to have a unique ID just as the 
devices do. Some adapters don't let you set this. ]
The question should be - Are there any O/S' that will allow the 
sharing of file systems? 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.
[Editor(GF): CDROM drives can be shared pretty easily because they 
are by definition READ-ONLY]
Disks can be best shared by having two (or more) partitions on a 
disk. Each host "owning" its own file system.
[Editor(GF): You also need to watch out for host adapters that reset 
the bus when booting. Some adapters let you control this. ]
[ Additional editorial comment Editor(GF):
The above discussion refers primarily to PCs. There are high end 
systems that do allow 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, Digital UNIX, and Digital VMS are examples of systems 
that allow this.
- Thanks to Cees de Groot for suggesting this addition.]
Table of Contents
QUESTION: Is it possible for two computers to access the same SCSI 
ANSWER From: Gary Field (
Yes, this is not usually as problematic as sharing disks as long as 
the operator is sensible about what is attempted.
Some things you need to watch out for:
Both host's device drivers must use RESERVE/RELEASE commands to lock 
access. This locks the drive for access by only one system, the 
conflicting host gets BUSY status until the currently accessing host 
sends a RELEASE cmd. 
The adapter on both hosts have unique IDs. 
Good and common grounding of both systems and the devices. 
SCSI length limits are not violated. 
Make sure both hosts select the same data transfer mode (synch or 
Both hosts can be told which disks and other devices to access and 
not to attempt to access the ones owned by the other host. 
Neither host adapter resets the SCSI bus.
Table of Contents
QUESTION: What is the problem with the Adaptec 1542C and external 
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 Ď93 (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.
Table of Contents
QUESTION: What is the difference between the Adaptec 1542A and 1542B?
ANSWER From: (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.
Table of Contents
QUESTION: What are the differences between the Adaptec 1542B and the 
ANSWER from: Terry Kennedy (
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 
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
Additionally, the 1542C uses a Z80 CPU and 8Kb buffer instead of an 
8085 and 2Kb buffer as on the 1542B.
Table of Contents
QUESTION: What are the differences between the 1542C and the 1542CF?
ANSWER from: Terry Kennedy (
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.
Table of Contents
QUESTION: Where can I get SCSICNTL.EXE and other Adaptec files?
ANSWER From: (Randy Bush)
and Timothy Hu
"list" is a file that describes all the files in this directory.
You can get the ASPI specs from Adaptec's Bulletin Board (408)945-
[Editor(GF): You can also get ASPI spec's from Adaptec's WWW server.]
Table of Contents
QUESTION: What kinds of Optical Drives are available?
The previousanswer From: (John Kim) has been
replaced with an updated version.
ANSWER From: Psycho Bob <>[Editor(GF)]
DATE: Sep 18, 1996
As magnetic recording approaches the current engineering limit, more 
and more attention is paid to optical storage solutions. Optical 
storage has good points going for it -- immunity to stray magnetic 
field, potential for higher storage capacity per unit area, and 
relatively low media cost.
Although CD-ROM and CD-R are also optical storage units, they are not
rewritable -- that puts them out as either secondary storage or 
backup storage for most of us. There is an upcoming sub-format called 
CD-E ("E" for erasable) that is suppose to become available in late 
1996, but I haven't seen much news or even definite rumors. With the 
advent of DVD, the CD-E may only be a temporary stepping stone to 
recordable DVDs.
Currently, the most popular magnetic storage format is magneto-
(MO) format. It was the only popular rewritable optical storage disc
technology before Panasonic's phase-change double-function (PD) 
came out in 1995.
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 certain way that it will reflect the 
laser beam differently depending on the magnetic field angle when the 
particular spot was cooling down.
MO comes in many sizes and capacities. Consumers were first exposed 
MO in Steve Jobs' NeXT computer in the mid-1980s. Although 5.25" had 
slow start due to initial high cost, it has been evolving quite 
The more popular ISO capacities for 5.25" MO are 2.4GB/2.6GB,
1.2GB/1.3GB, and the 600MB/650MB. 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. But they are limited to 
niche markets.
Sony MiniDisc-Data
Derived from the Mini-Disc (MD) audio format Sony introduced, 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 explains the high-cost of the consumer MD audio 
MD-Data is the smallest of the MO family. With 2.5" form factor, it 
can store 140MB of uncompressed data. Current MD-Data drives are 
rather slow at 150KB/sec sustained transfer rate, but Sharp is hoping 
to change that.
Sharp will (hopefully) ship a 300KB/sec by the end of 1996, with a 
second generation of MD-Data available by sometime in 1997. The 
current schedule from Sharp indicates the second generation MD-Data 
will be able to store up to 700MB with 600KB/sec transfer rate.
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, almost all MO used two passes to write data onto 
the disc 
-- one pass to erase the whole track, and a second pass to write the
updated data. MD's one pass recording, called light intensity 
modulation, direct over-write (LIM-DOW, ISO 14517) will be in almost 
all the future MO formats until another better technology comes 
Just like CD, MD-Data comes in various flavors -- rewritable, write-
once, and read-only cartridges. There is also a hybrid disc for MD 
and MD-Data that is part read-only, and part rewritable.
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. Write-once-read-multiple (WORM) drives were 
actually the first phase-change formats, but PD is the first 
*reversible* (that is, re-writable) phase-change format. Current PD 
stores 650MB per PD cartridge.
Currently, PD's only advantage over its MO brethren is the PD drive's 
ability to read regular CD-DA and CD-ROMs. The PD rewritable 
cartridge is not usable in regular CD-ROM drives.
Both write-once-read-multiple (WORM) and compact disc recordable (CD-
are both write-once formats -- once you have written the data to the
disc, the data cannot be changed. Put another way, the disc media can
only be used once. For long term archival of data that need not be
changed, it makes sense -- as CD-R media price is unbeatable [As of 
mid 1996, 650 MB CD-R media sells for $6 to $8 each or about 1 cent 
per MB!] . Current CD-R offers maximum of 650MB per disc.
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 wihout damaging the media (good audit 
The new WORM formats being introduced are tending to be more
proprietary. There is rarely any interchangability between different 
vendor's drives and media.
During the WORM to MO transition, 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 
MO in WORM's clothing. Many of today's 5.25" MO drives still have the
capability to read CCW cartridges.
The future
Almost all the formats mentioned above have future plans -- usually 
"improved version" with faster and more storage capacity. The 5.25" 
MO camp is shooting for the 4.8GB/5.2GB range, with faster sustained 
transfer rate in writing data. 3.5" may double their 650MB soon by 
using both sides of the disc. PD may also double the storage space by 
using both sides of the disc. But currently it's doubtful as DVD has 
pretty much been finalized. It'll be interesting to see how Panasonic 
will interpret the PD in the DVD marketplace (DVD-PD?). DVD-RAM is 
rumored to use phase-change technology.
The same goes for CD-E, the latecomer of the bunch. If the CD-E is 
truly playable in ordinary CD-ROM (and audio CD player), it'll 
probably become the optical storage standard in all but the high-
capacity, high-end/server market.
Format Phys.  Capacity      Bytes      # of sides   Capacity        
       size   per disk     per sector               per side 

MO 1p  2.5"    140MB       2048/2336    single       140MB     Sony 
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         512        single       540MB    
DIS(ISO/IEC) 15041
               640MB         2048       single       640MB
MO 2p 5.25"    600MB         512        dual         296MB    ISO/IEC 
               650MB         1024       dual         322MB    ANSI 
MO 2p 5.25"    1GB           512        dual         463MB    ISO 
               1GB          1024        dual         510MB
MO 2p 5.25"   1.2GB          512        dual         595MB    ISO/IEC 
              1.3GB         1024        dual         650MB    ECMA 
MO 1p 5.25"   2.4GB          512        dual         2.298GB  
DIS(ISO/IEC) 14517
              2.6GB         1024        dual         1.3GB
MO 2p 5.25"   1.5GB         4096        dual         750MB    
MO 1p 5.25"   4.6GB         1024        dual         2.3GB    
Pinnacle Micro "Apex"
MO 12"        8GB                                             Nikon
MO 12"       3.2GB                                            Sony
MO 14"       6.8GB          1024        dual         3.4GB    Kodak 
System 2000
             10.2GB         1024        dual         5.1GB
             14.8GB         1024        dual         7.4GB
WORM 5.25"   2.6GB                                            
DIS(ISO/IEC) 15486
WORM 5.25"   650MB                     single        650MB    ISO/IEC 
9171 Format A
WORM 5.25"   470MB                                            
WORM 12"     15GB                                             Sony 
PD 1p 5.25"  650MB         4096        single        650MB    
CD-R 5.25"   550MB         2048        single        553MB
             650MB         2048        single        650MB
CD-E 5.25"   650MB pending...

*technology: 1p -- one-pass write
2p -- two-pass write
Standards 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 
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 
Of CD standards...
Funny as it seems, CD is actually considered as proprietary a 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 
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.  
Addendum: (11/15/96)
Sony and Phillips have just announed finalization of 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 regular CD 
and CD-ROM drives (surprise!)!!
So, the good news is that CD-RW is here. The bad news is that it's as
proprietary as Panasonic's PD in compatibility with current installed 
base of CD and CD-ROM players.
Table of Contents
QUESTION: Where can I get various SCSI documentation?
Thanks to John Lohmeyer of Symbios Logic, a number of SCSI related 
files are available for anonymous ftp.
The archive contains a large amount of data relating to SCSI, and 
ESDI as well as SCSI-2, IPI, and Fiber Channel, as well as the last 
revision of the SCSI-1 and SCSI-2 standards before they went into 
publication by ANSI.
This information server is maintained by Symbios Logic (formerly NCR 
Corp., formerly AT&T Global Information Solutions) in the hope of 
returning some value to the Internet community. It contains 
information about commercial products, and also about computing-
related topics in which Symbios Logic as a company, or individuals 
therein, have interest and expertise.
The information is accessible from several sources:
SCSI BBS: (719) 574-0424
anonymous ftp to
Table of Contents
QUESTION: How can I find out about the emerging SCSI standards?
ANSWER From: Milton Scritsmier (
The X3T10 committee has opened up a WWW site. It has an overview of 
SCSI-3, as well as pointers to the WWW sites for the three serial 
interfaces (FC, SSA, and P1394), and a pointer to an online copy of a 
proposed SCSI-2 spec.
Here is the original announcement:
Subject: New X3T10 Home Page
Date: Thu, 31 Aug 95 14:07:00 MDT
With a LOT of help from Carey Harrington (Thank you!), X3T10 now has 
a World Wide Web home page. If you have a web browser, you may want 
to check out:
John Lohmeyer, Chair X3T10 Technical Committee
ANSWER #2 From: Gary Bartlett (
A draft version of the SCSI-2 spec is in HTML form on the WWW at:
ANSWER #3 From: Gary Watson (
Small Form Factor (SFF) Committee documents 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
document #8000 is and index to the other documents.
[Editor(GF): you might try:
2.txt ]
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 Symbios WWW 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. 
ANSWER #4 From: Gary Field(
For Fibre Channel Association:
Table of Contents
QUESTION: Where can I get official ANSI SCSI documents?
ANSWER #1 From: (Kevin Jones)
and jmatrow@donald.WichitaKS.NCR.COM (John Matrow)
The SCSI specification: Available from:
11 West 42nd St. - 13th floor
New York, NY 10036
Sales Dept. (212) 642-4900
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.
Table of Contents
QUESTION: What SCSI books and tutorials are available?
ANSWER From: Gary Field (
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
from ENDL Publishing, 14426 Black Walnut Ct., Saratoga, CA 95090,
(408)867-6642, FAX (408)867-2115
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.
"The book of SCSI - A guide for Adventurers" by Peter M. Ridge.
Published by No Starch Press, Daly City, CA,
ISBN # 1-886411-02-6, List Price $34.95.
Contains general coverage of most aspects of SCSI.
"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: (
'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 (
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. 
Another two part article appeared in Byte in May 1986 and June 1986.
Table of Contents
QUESTION: Where can I find SCSI info on the Web?
ANSWER FROM: Gary Field (
Try some of these:
Table of Contents
QUESTION: Where can I get information on various disk drives and 
ANSWER: (Eric Krieger) (Updated Sep. 30, 
Drive and Controller Guide, Version 4.3
THEREF(tm) is a comprehensive Directory of Hard Drives, Floppy 
Optical Drives, and Drive Controllers & Host Adapters. It is designed 
to help the novice and pro alike with integration problems and system 
Information is provided in two handy formats; Portrait mode, for 
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 
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:
(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!
TheRef is also available via WWW from:
Table of Contents
QUESTION: Where can I get technical information and jumper settings 
for HP drives ?
ANSWER From: Rodney Brown (
Update From: Martin C Mueller ( )
HP SCSI Storage Device Support Pages
Table of Contents
QUESTION: How can I contact Adaptec?
	Also: Future Domain, Corel CD Creator, Trantor, Incat systems, 
Symbios Logic.
ANSWER From: (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 
	FAX info number is (408) 957-7150.
	There are three general topics as of this writing:
		General Information 
		Sales Information 
		Technical Information
	Give it a call and request the directory! As of this writing 
there are over 130 documents available. You need a touchtone phone 
and the fax number. You'll also be asked for an extension number to 
stamp on the FAX which will be used to identify the recipient.
[Editor(GF): As of July 1993 Adaptec bought Trantor.
Try (800) 872-6867 (TRA-NTOR)]
[Editor(GF): As of Feb. 19, 1998, Adaptec bought Symbios Logic]
World Wide Web (WWW) URL:
 [(from: Andrew Lockhart ( ]
You can address Adaptec support by email. The address is 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 Adaptecís response).
Table of Contents
QUESTION: How can I contact Archive Corporation?
Archive was bought by Conner Peripherals in 1993
Table of Contents
QUESTION: How can I contact BusLogic /Bustek / Mylex ?
ANSWER From: Gary Field (
Mylex Corp.
34551 Ardenwood Blvd.
Fremont, CA 94555
(510) 796-6100
Tech. Support:
Voice: (510) 608-2400
FAX: (510) 745-7715
Email: for HBA:
	 For RAID:
Table of Contents
QUESTION: How can I contact Corel?
ANSWER From: Gary Field (
Main Number: (800) 772-6735
Tech. Support: (613) 728-1010
ANSWER From: Gerrit Visser (
ftp: /pub 	SCSI is under Multimedia
For Corel CD Creator Software contact Adaptec
Table of Contents
QUESTION: How can I contact Fujitsu?
ANSWER From: Ken Porter (
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
(lot of posts requesting this info).
It is available via anonymous ftp 
in the /pub/fks directory
filename: fks.exe
It is self-extracting and mostly self-documenting.
Table of Contents
QUESTION: How can I contact Quantum?
ANSWER From: (Kevin Martinez)
Quantum Corporation
500 McCarthy Blvd.
Milpitas, CA
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
Table of Contents
QUESTION: How can I contact Seagate?
ANSWER From: (Hale Landis)
Here are the numbers for Seagate's Technical Support.
SeaBOARD - Bulletin Board System available 24 hours. Use 8 data
bits, no parity, 1 stop bit (8-N-1).
USA/Canada 408-438-8771 9600 baud*
England 44-62-847-8011 9600 baud*
Germany 49-89-140-9331 2400 baud*
Singapore 65-292-6973 9600 baud*
Australia 61-2-756-2359 9600 baud*
* - Maximum baud rate supported.
SeaFAX 408-438-2620
Use a touch-tone phone to have information returned to you via
FAX. Available 24 hours.
Technical Support Fax 408-438-8137
FAX your questions or comments 24 hours. Responses are sent
between 8:00AM and 5:00PM PST Monday through Friday.
SeaFONE 408-438-8222
Provides recorded information 24 hours or talk to a technical
specialist between 8:00AM to 5:00PM PST Monday through Friday.
SeaTDD 408-438-5382
Using a Telecommunications Device for the Deaf, you can send
questions or comments 24 hours or have a dialog with a
technical support specialist between 8:00AM and 5:00PM PST
Monday through Friday.
Table of Contents
QUESTION: How can I contact Conner Peripherals?
Conner Peripherals was bought by Seagate
Table of Contents
QUESTION: How can I contact Maxtor?
ANSWER From: David G North (
Main Number: (800) 262-9867	(Has FAXback feature for drive info etc)
ftp site: (New!)
ANSWER From: Eric Van Buren 
Table of Contents
QUESTION: How can I contact NCR?
NCR Microelectronics division was bought by AT&T and then by Symbios 
See "How can I contact Symbios Logic"
As of Feb 19, 1998, Hyundai agreed to sell Symbios to Adaptec. 
Table of Contents
QUESTION: How can I contact Philips?
ANSWER From: S. C. Mentzer (
Philips Consumer Electronics Co.
One Philips Drive
Knoxville, TN 37914-1810
(615) 521-4316
(615) 521-4891 (FAX)
Table of Contents
QUESTION: How can I contact Symbios Logic?
Answer From: Symbios Logic
Update From: Wade Adams (
Symbios is a wholly owned subsidiary or Hyundai Elecctronics.
For literature on any Symbios Logic product please contact:
Phone:	(800) 636-8022
		(800) 856-3093
(719) 536-3300
Fax:		(719) 536-3301
Technical Support:
Phone:	(719) 533-7230
Table of Contents
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
Answer From: Ben Mehling (
I am setting up a "unauthorized" UltraStor site for the orphaned 
customers and
cards still out there.
I do not think the above numbers are good anymore.
The 4100 line will get you Power I/O (an unrelated Adaptec holding)
and the 4016 line may get you a dead-end answering 
service. The company is no longer active (as far as I know). The 
primenet account is alive, but again not active. These links are to 
the "Unauthorized" UltraStor site. This site is in no way affiliated 
with UltraStor or its holding companies. It is a free "mirror" site
for distribution of drivers and information. (hint: we are trying to 
help out, not provide tech support.)
Try: (unauthorized).
	www.UltraStor (unauthorized).
	ftp.UltraStor (unauthorized).
The above three addresses are hypertext linked to these addresses:
The web site address is:
The FTP site address is:
The mail/finger address is /
The current maintainers are:
Ben Mehling (
Phil Colline (
Table of Contents
QUESTION: How can I contact Tecmar Technologies (formerly Wangtek, 
WangDAT, Sytron, and Rexon)?
ANSWER FROM: from: Jay Long - ( and
Peter Dyballa (
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
Table of Contents
QUESTION: How can I contact Western Digital?
Western Digital Corporation
8105 Irvine Center Drive
Irvine, CA USA 92718
Online Services:
Tech Support BBS 714-753-1234 (up to 28.8 KBS) 
AOL (keyword) WDC or Western Digital 
MSN (go word) WDC 
Table of Contents
QUESTION: How can I contact DPT (Distributed Processing Technology)?
ANSWER: From: Gary Field (
voice: (407) 830-5522
FAX: (407) 260-6690
Table of Contents
QUESTION: How can I contact Micropolis?
ANSWER: From: Richard Ravich (
Tech Support: (818) 709-3325
[Editor(GF): I believe that Micropolis is now (late 1997) out of 
business Ė RIP. You might try for 
drive info. ]
Table of Contents
QUESTION: How can I contact Legacy Storage Systems ?
ANSWER: From: Gregory Smith (
General: (905) 475-1077
Sales/Tech support/Service: (905) 475-0550
U.S. Tech Support: (800) 361-5685
Fax: (905) 475-1088
Legacy Storage Systems
43 Riviera Drive
Markham, ON Canada L3R 5J6
Table of Contents
ANSWER From: (Kevin Jones)
There are 2 handshaking modes on the SCSI bus, used for transferring 
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 
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.
Table of Contents
QUESTION: How can I check a passive SCSI terminator?
ANSWER From: (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:
     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
    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
     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

R1 = 220 Ohms, R2 = 330 Ohms
When 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:

|     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|    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
|     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|   --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
R1 |
|  |
| R2
| /
 o   <--------- 17 other pairs in parallel ---------->
We'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.
Table of Contents
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.
Table of Contents
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 (
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.
Table of Contents
QUESTION: Who manufactures SCSI extenders and Single-Ended to 
Differential converters ?
ANSWER From: Gary Field (
The following companies manufacture SCSI extenders and converters:
Ancot Corporation
115 Constitution Drive
Menlo Park, CA 94025
Tel: (415) 322-5322
Fax: (415) 322-0455
Apcon Inc.
17938 SW Boones Ferry Road 
Portland, OR 97224 
Phone: (503) 639-6700 Fax: (503) 639-6740 
Paralan Corporation
7875 Convoy Court, San Diego, CA 92111
Tel. (619) 560-7266 || Fax 619-560-8929
Rancho Technology Inc.
10783 Bell Court-Rancho
Phone: (909)987-3966; Fax: (909)989-2365;
E-Mail:; BBS: (909)980-7699

Table of Contents
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 (
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. SCSI vendors sell adapters that bring out the three 
sets of signals to conventional connectors.
See: for more 
information about SCA.
Table of Contents
QUESTION: What are the pinouts for SCSI connectors?
ANSWER From: snively@scsi.Eng.Sun.COM (Bob Snively)
[ Edited and expanded by Gary Field ( ]
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.
There are some minor problems in naming the microconnector conductor 
pairs, which I have corrected in the enclosed diagram. All the 
conductor pairs of the Mini-Micro (High Density) connector are in 
fact passed through on the cables. SCSI-2 defines the RSR (Reserved) 
lines as may be ground or may be open, but they are still passed 
through the cable. Most present standard SCSI devices will ground 
those lines.
-------------------- 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    
------------------------------------ --------------------------------
                       ________________________        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)
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  |
|    -------------------     |
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 Reserved
Table of Contents
ANSWER From: Gary Field (
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-
transmission line implementation. It is recommended that a short 
adapter cable
be used to convert to the more common Centronics style 50 pin 
rather than extend the 25 pin connection any further than necessary.
The Macintosh Plus used a NCR 5380 SCSI chip controlled by the 
| 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.
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    |
Table of Contents
QUESTION: What is the difference between SCSI-1 and SCSI-2?
ANSWER From Dal Allen:
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 
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 
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 
c. Command queuing (up to 256 commands per initiator on each logical 
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 
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 
e. The following commands were added for all device types:
f. The COPY command definition was expanded to include information on 
how to handle inexact block sizes and to include an image copy 
g. The direct-access device command set was enhanced as follows:
+ The FORMAT UNIT command provides more control over defect 
+ Cache management was added:
- PREFETCH command
- Force unit access bit
- Disable page out bit
+ Several new commands were added:
+ The sequential-access device command set was enhanced as follows:
- Partitioned media concept was added:
* LOCATE 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:
- 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 
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 
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 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 
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 
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 
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 
SCSI-2 which include: 
o Wide SCSI: SCSI may now transfer data at bus widths of 16 and 32 
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 
with the IPI (Intelligent Peripheral Interface) committee in ASC 
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 
o Termination: The Single Ended electrical class depends on very 
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 
defined, and is required to prevent the corruption of error sense 
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 
Contingent Allegiance state for an indefinite period during which the 
initiator that received the error can perform advanced recovery 
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 
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 
In the 1986 standard, everything was in numeric sequence, and the 
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 
describe the device class characteristics. The tape model was the 
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).
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. 
Table of Contents
QUESTION: What is the difference between SCSI-2 and SCSI-3?
ANSWER From: excerpts of postings by Jeff Stai and others:
	(Mohit K Goyall -,
	(Andrew E. Lowman -
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 
- 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.
Q: 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! :-)
Table of Contents
ANSWER From: Gary Field ( )
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.
See also: 1, 2, 3
Table of Contents
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 
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 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. 
Table of Contents
QUESTION: What are the jumpers on my Conner drive?
ANSWER From: (Eric Krieger)
Embellishment from: Henrik Stahl (
Most SCSI host adapters are compatible with Conner drives. Software 
drivers and installation instructions are provided with the host 
The drives are shipped with SCSI ID set to 7. To select a different 
ID refer to the following:
Table A          Table B
ID E-1 E-2 E-3   ID E2 E3 E4
0  out out out   0 out out out
1  in out out    1 in out out
2  out in out    2 out in out
3  in in out     3 in in out
4  out out in    4 out out in
5  in out in     5 in out in
6  out in in     6 out in in
7  in in in      7 in in in
Parity is always ENABLED on the CP3200,CP30060,CP30080,CP30100,
CP 30200, CP 3500, CP 3360, CP 30540 and CP 31370.
For the CP 340, jumper E-1 to disable parity.
All other models, jumper E-4 to disable parity.
SCSI drive parameters:
Model     Hds Cyl Sec   Table  LED
CP2020     2  642  32     A    n/a
CP340      4  788  26     B     1
CP3020     2  622  33     A     1
CP3040     2 1026  40     A     1
CP3180     6  832  33     A     1
CP3100     8  776  33     A     1
CP30060    2 1524  39     A     2
CP30080    4 1053  39     A     2
CP30100    4 1522  39     A     2
CP30200    4 2119  49     A     2
CP3200     8 1366  38     A     2
CP3360     8 1806  49     A     2
CP3540    12 1806  49     A     2
CP 30080E  2 1806  46    AA   C/E
CP 30170E  4 1806  46    AA   C/E
CP 30540   6 2249  59-89 AA    B
CP 31370  14 2094  59-95 AA    B
J-4 Pin 1 = + J-1 Pin 3 = +
Pin 2 = - Pin 4 = -
On the CP 31370, jumper E5 enables termination. Default is 
termination on. It may be the same jumper for other models.
Table of Contents
QUESTION: What are the jumpers for my Wangtek 5150 drive?
ANSWER From: Terry Kennedy (
First, the disclaimer: This is not an official representation of 
Wangtek or of my employer. This is info I've discovered by reading 
publicly available reference material. When changing jumpers, always 
observe proper anti-static precautions and be sure you have the 
current configuration written down so you have a known starting 
Ok. Here's the complete scoop on Wangtek 5150ES drives:
The current part number for a "generic" 5150ES is:
33685-201 (black faceplate)
33685-202 (beige faceplate)
These are referred to as the "ACA version" of the drive.
There are _many_ other part numbers for 5150ES drives. If you have 
one that isn't one of the above, it doesn't mean you have an old or 
an out of rev drive, it just means it's a special version created for 
a distributor or OEM, or with different default jumper settings.
You can order the Wangtek 5150ES OEM Manual from Wangtek. It is part 
number 63045-001 Revision D.
There are 5 possible logic boards. Here are the jumper options for 
Logic assembly #33678
0 - SCSI unit LSB
1 - SCSI unit
2 - SCSI unit MSB
K - not documented
J32 - Diagnostic test connector, default is not installed
E1, F1 - SCSI termination power. E1 in = power from drive and to 
	 E1 out - power from cable. F1 = terminator power fuse, 1.5A 
	 Default is IN.
E2 - Chassis ground. E2 in jumpers logic to chassis ground. E2 out 
isolates through a .33 uFD capacitor. Default is IN.
E5 - Master oscillator enable. Test only. Must be IN.
E20 - Factory test. Must be OUT.
RP1, RP2, RP3 - SIP terminators. Default is IN, remove for no 
Logic assembly #30559
HDR1 - Factory testing. Setting depends on drive. Don't touch.
HDR2 - Factory testing. Defaults are pins 15-16, 17-18, 19-20. Don't 
HDR3 pin 1 - A-B enables buffered mode. B-C disables. Can be 
overridden by SCSI Mode Select.
HDR3 pin 2, 3 - Default data format. Set to B-C for a 5150ES.
HDR3 pin 4 - parity enable. A-B enables, B-C disables.
0 - SCSI unit LSB
1 - SCSI unit
2 - SCSI unit MSB
K - not documented
E1 - SCSI termination power. E1 in = power from drive and to cable,
E1 out - power from cable.
E2 - Chassis ground. E2 in jumpers logic to chassis ground. E2 out 
isolates through a .33 uFD capacitor. Default is IN.
E3 - Master oscillator enable. Test only. Must be IN.
E4 - Write test mode. Test only. Must be OUT.
E5 - Write oscillator enable. Test only. Must be IN.
E6 - Disable HDR2. Test only. Must be IN.
E7 - Microcontroller clock select. In for a 5150ES.
E8 - Write precomp select. Set on a per-drive basis. Don't touch.
E9 - RAM size. Don't touch.
E10 - Erase frequency. Don't touch.
RP2, RP3 - DIP and SIP terminators. Default is IN, remove for no 
Logic assembly #30600
HDR1 - Factory testing. Setting depends on drive. Don't touch.
HDR2 - Write precomp select. Set on a per-drive basis. Don't touch.
HDR3 pin 1, 2, 3 - SCSI device address. 1 is LSB, 3 is MSB. A-B=1, B-
HDR3 pin 4 - Parity enable. IA-B is enabled.
HDR3 pin 5, 6 - Default data format. B-C for a 5150ES.
HDR3 pin 7 - Buffered mode select. A-B is enabled.
HDR3 pin 8 - Reserved. Must be OUT.
HDR4 - Write frequency select. Don't touch.
E1 - SCSI termination power. E1 in = power from drive and to cable,
E1 out - power from cable.
E2 - Chassis ground. E2 in jumpers logic to chassis ground. E2 out 
isolates through a .33 uFD capacitor. Default is IN.
E3 - Hard/soft reset. IN enables hard reset.
E4 - Write precomp select. Don't touch.
E5 - Clock speed. Don't touch.
E6 - Tape hole test. Don't touch.
Logic assembly #30552
HDR1 - Factory testing. Setting depends on drive. Don't touch.
HDR2 - Write precomp select. Set on a per-drive basis. Don't touch.
HDR3 pin 1, 2, 3 - SCSI device address. 1 is LSB, 3 is MSB. [Note - 
		 pins 1-3 are duplicated at another location on the 
HDR3 pin 4 - Parity enable. IN is enabled.
HDR3 pin 5, 6, 7, 8 - Default data format. 5,5 B-C, 7-8 A-B for a 
HDR4 - Write frequency select. Don't touch.
E1 - SCSI termination power. E1 in = power from drive and to cable,
E1 out - power from cable.
E2 - Chassis ground. E2 in jumpers logic to chassis ground. E2 out 
isolates through a .33 uFD capacitor. Default is IN.
E3 - Hard/soft reset. IN enables hard reset.
E4 - Write precomp select. Don't touch.
E5 - Clock speed. Don't touch.
E6 - Tape hole test. Don't touch.
Logic assembly #30427
HDR1 - Factory testing. Setting depends on drive. Don't touch.
HDR2 - Write precomp select. Set on a per-drive basis. Don't touch.
HDR3 pin 1, 2, 3 - SCSI device address. 1 is LSB, 3 is MSB. A-B=1, B-
HDR3 pin 4 - Parity enable. IA-B is enabled.
HDR3 pin 5, 6, 7, 8 - Default data format. 5,5 B-C, 7-8 A-B for a 
E1, E3 - Factory test. Must be IN.
E2 - SCSI termination power. E2 in = power from drive and to cable,
E2 out - power from cable.
E4 - Chassis ground. E4 in jumpers logic to chassis ground. E4 out 
isolates through a .33 uFD capacitor. Default is IN.
Firmware - There are many flavors of firmware. I have seen the 
following parts:
the -xxx suffix changes as the firmware is updated. According to the 
folks I spoke to at Wangtek, the standard firmware is the 21158. The 
latest version as of this writing is 21158-007. All of these will 
work with the Adaptec and GTAK.
The firmware options (as returned by a SCSI Identify) are on the end 
of the product string, which is "WANGTEK 5150ES SCSI ES41C560 AFD QFA 
STD" for the 21158-007 firmware. The 3-letter codes have the 
following meaning:
AFD - Automatic Format Detection - the drive will recognize the 
format (such as QIC-24, QIC-120, or QIC-150) that the tape was 
written in.
QFA - Quick File Access - the ability to rapidly locate a tape block, 
and 	to implement the "position to block" and "report block" SCSI 
commands. 	This is compatible with the Tandberg implementation.
STD - Standard feature set.
Table of Contents
QUESTION: How do I configure my HP DDS DAT tape drive?
ANSWER From: Alan Strassberg (
The HP DDS Configuration Guide (postscript) can be found at:
Table of Contents
ANSWER From: Gary Field (
ASPI stands for Advanced SCSI Programming Interface. It was developed 
by 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 Intel x86 machines running MSDOS, 
Windows( 3.1x, 95 or NT), Netware, or OS/2. There is no UNIX version. 
The error reporting and recovery mechanisms are much more limited 
than in CAM, but ASPI gained much wider acceptance because it was 
available earlier.
Table of Contents
ANSWER From: (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 
[Editor(GF): It is similar to ASPI but much more elaborate and 
ANSWER From: (Hale Landis)
You can get the CAM spec(s) from the SCSI BBS
Table of Contents
QUESTION: What is FPT (Termination)?
ANSWER From: (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.
Some errors in the above diagram were corrected as suggested by
Wietze van Winden (
Table of Contents
QUESTION: What is Active Termination?
ANSWER From: (Eric Smith)
and (Brent R. Largent)
An active terminator actually has one or more voltage regulators to 
produce the termination voltage, rather than using resistor voltage 
This is a passive terminator:
TERMPWR ------/\/\/\/------+------/\/\/\/----- GND
                         SCSI signal
Notice 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 
	    2.85 Volt Regulator
             +-----------+ +2.85V     110 Ohms
TERMPWR -----| in    out |------+------/\/\/\/-------SCSI signal
             |    gnd    |      |
             +-----------+      |
                   |            +------/\/\/\/-------SCSI signal
                   |            |
GND ---------------+            |
                                +------/\/\/\/-------SCSI signal
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 activer termination is that it 
can be disabled by a single jumper instead of needing to unplug 
resistor arrays.]
Table of Contents
QUESTION: Why Is Active Termination Better?
ANSWER (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 
- Improved resistance tolerances (from 1% to about 3%)
- Reduces current drawn from TERMPWR line.
In FPT form:
- Provides signal overshoot/undershoot clamping on all signal lines. 
Table of Contents
QUESTION: How can I tell whether an unmarked terminator is active or 
ANSWER From: Gary Field (
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.
Table of Contents
QUESTION: Where can I buy terminators ?
ANSWER From: Rodney Brown (
Info taken from Usenet postings by:
John Zatler (
Steve Schreppler (
Dave Nadler (
DataMate / Methode
Methode Electronics, Inc.
dataMate Division
7444 West Wilson Avenue
Chicago, IL 60656
(708) 867-9600
(800) 323-6858
(708) 867-3149 FAX
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-
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
Cables To Go
Technical Cable Concepts Inc.
Table of Contents
QUESTION: What is Plug and Play SCSI?
ANSWER From: (Lee Fisher) (Updated Dec 7 1993)
Plug and Play is the name of a technology that lets PC hardware and 
attached devices work together automatically. A user can simply 
attach a new device ("plug it in") and begin working ("begin 
playing"). This should be possible even while the computer is 
running, without restarting it.
Plug and Play technology is implemented in hardware, in operating 
systems such as Microsoft Windows, and in supporting software such as 
drivers and BIOS.
With Plug and Play technology, users can easily add new capabilities 
to their PCs, such as sound or fax, without having to concern 
themselves with technical details or encountering problems. For users 
of mobile PCs (who are frequently changing their configurations with 
docking stations, intermittent network connections, etc.) Plug and 
Play technology will easily manage their changing hardware 
configuration. For all users, Plug and Play will reduce the time 
wasted on technical problems and increase their productivity and 
satisfaction with PCs.
The Plug and Play technology is defined in a series of specifications 
covering the major component pieces. There are specifications for 
BIOS, ISA cards, PCI, SCSI, IDE CD-ROM, PCMCIA, drivers, and 
Microchannel. In a nutshell, each hardware device must be able to be 
uniquely identified, it must state the services it provides and the 
resources which it requires, it must identify the driver which 
supports it, and finally it must allow software to configure it.
The first Plug and Play compliant products are available now, as are 
development kits for drivers and hardware. Twenty different Plug and 
Play products were shown at Comdex in November 1993.
The Plug and Play specifications are now available via anonymous ftp 
at in the \drg\plug-and-play subdirectory. The 
files are compressed in .zip format, and are in Microsoft Word 
Plug and Play ISA files (.\pnpisa\*) Clarifications and corrections to pnpisa.doc MS-DOS testing tool to isloate ISA PnP hardware Plug and Play device driver interface specification Hardware spec for PnP ISA enhancement Hardware spec for PnP ISA enhancement
Plug and Play SCSI files (.\scsi_ide\*): Plug and Play SCSI specification proposal SCAM (SCSI Comnfigured Auto-Magically) specification
Plug and Play BIOS files (.\bios\*): Advanced Power management spec v.1 Plug and Play BIOS spec Spec for optional method of storing config info for PnP 
PlayList@Microsoft.COM alias:
There is an alias, PlayList@Microsoft.COM, which you can email and 
get on a Microsoft mailing list related to Plug and Play, where the 
Hardware Vendor Relations Group (HVRG) will mail out new 
specifications, announcements, information on workshops, Windows 
Hardwware Engineering Conference (WinHEC), etc...
Compuserve PlugPlay forum:
There is a forum on Compuserve, GO PLUGPLAY. This forum is the method 
for support, discussions and dialogs about Plug and Play. In 
addition, the forum's library contains all of the current 
Intel Plug and Play kits:
If you are interested in Intel's two Plug and Play kits, either "Plug 
and Play Kit for MS-DOS and Windows" or "Plug and Play BIOS 
Enhancements Kit", FAX your name and company information to Intel at 
1.503.696.1307, and Intel will send you the information.
Table of Contents
QUESTION: Where can I get drivers (ASPI and other) for the WD7000 
FASST2 host adapter?
ANSWER From: Gary Field (
	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) (Columbia home web page) (Columbia e-mail)
[Update to above information 1/20/97][Editor(GF)]
From: "Alan L. Welsh" <>
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.
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 
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 
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: OR
Table of Contents
QUESTION: What if I have a SCSI drive larger than a gigabyte 
ANSWER From: Gary Field (
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 
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 
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 
There is an absolute limit of 16 M sectors which means 8 GB assuming 
512 byte sectors. Also DOS only allows 2 GB per partition. 
The day when this presents another problem is not too far away 
Hopefully, we'll all be running more sophisticated O/Ses that bypass 
this limitation by then.
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QUESTION: My SCSI bus works, but is not reliable. What should I look 
ANSWER From: Gary Field (
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 
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 
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 
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.
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QUESTION: Where can I find information about programming using the 
interface from DOS and Windows?
ANSWER From: Gary Field (
The Adaptec BBS has some documents about ASPI. They also have a WWW 
See the FAQ Question "How can I contact Adaptec?" for phone numbers 
and URL information etc.
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: /pub/ddj/1994.03/
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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.
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QUESTION: Will attaching a SCSI-1 device to my SCSI-2 bus hurt its 
ANSWER From: Gary Field (
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.
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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 
ANSWER From: Gary Field (
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 
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 xfers/s 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.
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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 (
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 a place for 
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 all the WIDE 
devices will be at one end and all the narrow devices will be at the 
other end. 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.
If for some reason you attach a WIDE device to a narrow bus, 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:
Technical Cable Concepts
1790 E. McFadden Ave.
Unit 103/104
Santa Ana, CA 92705
TEL: (714) 835-1081
FAX: (714) 835-1595
2201 Pine Drive
Dickinson, TX 77539
Order Line 800-786-1157
Fax Line (281)534-6580
Main Line (281)534-3919
Dalco Electronics
P.O. Box 550
275 South Pioneer Blvd.
Springboro, OH 45066-1180
Warning: I am told that some 68 pin to 50 adapters have TERMPWR wired 
incorrectly such that some of the 4 TERMPWR lines on the 68 pin 
connector get connected to the pin opposite TERMPWR on the 50 pin 
side. This pin was originally a GROUND signal (in SCSI1 spec.) and 
was later changed to OPEN to prevent shorting TERMPWR if the 
connector was reversed.
Also, some of these TERMPWR lines might be connected to the RESERVED 
pins adjacent to TERMPWR. Some drives interpreted RESERVED to mean 
"OK to connect to GROUND" and therefore attaching one of these will 
also short out TERMPWR.
The proper wiring is for all 4 TERMPWR lines on the 68 pin side to 
connect to the one TERMPWR line on the 50 pin side and leave the 
RESERVED lines not connected.
Table of Contents
QUESTION: How does device ID numbering work with WIDE vs NARROW 
ANSWER From: Gary Field (
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:
ID 7
ID 0
ID 15
ID 8
ID 23
ID 16
ID 31
ID 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 
Table of Contents
QUESTION: What is spindle-sync and why would I want it?
ANSWER From: Roger J. Hamlett (
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 worse, 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 10, are the commonest configurations where 
synchronization is not advised.
Table of Contents
QUESTION: What are the general steps I need to do to install a SCSI 
disk to be used with Windows?
ANSWER From: Roberto Waltman (
This description assumes an Adaptec host adapter, but other types 
should involve about the same procedure.
Let me start from scratch and describe one by one all necessary 
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: 
Exit and reset
(If your host adapter is new, you can probably skip the next step, 
but if you want to avoid mysteries later, itís not a bad idea to do 
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 
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?)
(It is not necessary to wait for the verify function to finish, 
although it is a good idea to do it with a new disk.)
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 controller 
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.
Notes for mixing IDE and SCSI disks on the same system?
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.
Table of Contents
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 (
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 youíll need (as mentioned below) and the MSCDEX 
program to the floppy. 
Create two files on the floppy as follows:
MSCDEX /D:MSCD000 /M:12 /L:J
If you have a different host adapter, find out from your manual what 
the equivalent driver names are for your card.
Once you get the system booted, select drive J (the CDROM), and run 
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QUESTION: Under Windows 95 OSR2 I can only see the first 8 GB of my 9 
GB disk. Whatís going on?
ANSWER From: Gary Field (
The retail version of Windows 95 is limited to 2 GB per disk by the 
use of the FAT16 filesystem. Since youíre 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 
Host adapters did not support this. Even the early 2940UW didnít have 
it. As of BIOS ver. 1.2x the support is present. Check with your host 
adapter manufacturer for an updated BIOS.
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QUESTION: Are there any storage related reasons to upgrade to Windows 
95 OSR2?
ANSWER From: Gary Field (
Yes, Microsoft has made a few enhancements:
FAT-32 filesystem
which has the following features:
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 
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
For more information, see the Win95 OSR2 FAQ.
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QUESTION: I changed the host adapter in my system and now my disk 
doesnít work. Why?
ANSWER From: Gary Field (
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 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? 
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--/* Gary Field - WA1GRC, Digital Equipment Corp., 110 Spit Brook Rd
    M/S ZKO3-3/T79, Nashua, NH 03062-2698, phone: (603) 884-2543
 email:  TZ=EST5EDT
 My wife says I don't listen to her; At least I think that's what she said. */

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