SCSI Frequently Asked Questions (FAQ) List (LONG)

SCSI Frequently Asked Questions (FAQ) List

What is SCSI?
SCSI stands for Small Computer System Interface. It’s a standard for connecting peripherals
to your computer via a standard hardware interface, which uses standard SCSI commands. The
SCSI standard can be divided into SCSI (SCSI1) and SCSI2 (SCSI wide and SCSI wide and fast)
and now SCSI-3 which is made up of at least 14 separate standards documents.

SCSI2 is the most popular version of the SCSI command specification and allows for scanners,
hard disk drives, CD-ROM players, tapes [and many other devices]. SCSI-3 resolves many long
time "gray areas" and adds much new functionality and performance improvements. It also adds
new types of SCSI busses like fibre channel which uses a 4 pin copper connection or a
pair of glass fibre optic cables instead of the familiar ribbon cable connection.

What do all these SCSI buzzwords mean?
Host adapter, also called a Host Bus Adapter or HBA is the card that connects your computer to the
SCSI-bus. May be called a SCSI-controller by marketing . An example would be a PCI SCSI
host adapter like the Adaptec 2940UW. Terminators (passive) are a group of resistors on the physical
ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from
the ends of the bus. Each terminated signal is connected by:
220 Ohm to +5 volt (TERMPWR) 330 Ohm to ground. For NARROW SCSI the 18 signals that are
terminated are: I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) … DB(0).

For WIDE SCSI there are 9 more signals; DB(p1), DB(8) … DB(15)Terminators (active)
rather than passive terminators that use TERMPWR which may not be exactly +5v, active
terminators use a voltage regulator. Basically it is a set of 110 Ohm resistors from each
signal to a 2.8 Volt regulated Voltage source. Single ended "Normal" electrical signals.
Uses open collector drivers to drive the SCSI bus. [usually] survives wrong cable insertion.
DIFFSENSE signal is used to detect connection of differential devices and prevent damage.
The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm allowed to connect a
device to the main cable. Most devices are single ended. Differential (Now called High
Voltage Differential to distinguish it from LVD) Uses two wires to drive one signal. Max.
cable length of 25 meters. Electrically incompatible with single ended devices! Much more
expensive than single ended. Used from SCSI-1 upwards.

Asynchronous 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 sent asynchronously over the 8 bit part of the SCSI-bus. Synchronous SCSI
Rather than waiting for an ACK, devices that both support synchronous SCSI can send multiple
bytes over the bus in the following way: send data1 : send data2 : … : send data3
(max outstanding bytes) : wait : wait : response1 : response2: … This improves throughput,
especially if you use long cables. (The time that a signal travels from one end of the cable
to the other end of the cable IS relevant.) Fast SCSI Fast SCSI allows faster timing on the
bus. ( 10MHz instead of 5MHz ) On a 8 bit SCSI-bus this increases the *theoretical* maximum
speed from 5MB/s to 10MB/s. Ultra SCSI Synchronous data transfer option which allows up to
20MHz data clocking on the bus. Also called FAST20.

What is SCSI-2 ?
This is a term describing the latest published ANSI standard (X3.131-1994). This document
describes several connectors (both shielded and unshielded) that include 1 byte wide data
bus, defines FAST transfer speeds, defines SCSI protocol for wider data transfers, defines
the parallel SCSI messages, and command structure. This provides the base on which future
SCSI features are compared against.

What is SCSI-3 ?
This term describes a set of related standards that are currently being developed. The SCSI-2
document is very large (400+ pages) and covers the full range of topics. SCSI-3
split this large document into a series of smaller documents that each covers a "layer" of
the interface definition. The basic layers are: physical (connectors, pin assignments,
electrical specifications), protocol (description of how physical layer activity is organized
into bus phases, packets, etc.), architecture (a description of how command requests are
organized, queued, responded to by any protocol), primary commands (description of commands
that must be supported by all SCSI devices), and device specific commands (commands that are
specific to a particular class of devices; CD-ROMS or WORM drives, for example).

The set of standards needed to do a SCSI-3 parallel interface disc drive implementation is
SPI (SCSI Parallel Interface) for the physical layer, SIP (SCSI Interlocked Protocol) for
the protocol layer, SAM (SCSI Architecture Model) for the architecture, SPC (SCSI Primary
Commands) for the primary command set, and SBC (SCSI Block Commands) for the disc drive
specific command set.

The SCSI-3 standards are layered in this manner to allow substitution of parts of the
structure as new technology emerges. For example, a comparable set of standards for a
SCSI Fiber Channel interface disc drive replaces the physical and protocol layers with new
documents but uses the same documents for the other 3 "layers". The main point to
remember here is that the terms SCSI-2 or SCSI-3 don’t imply any particular performance per
se — it refers to the generation of documents that a product conforms to. Since the
newest features are only in SCSI-3 and they tend to be higher performing, however, SCSI-3
devices should be better performing.

What is SCSI FAST ?
This refers to timings defined in SCSI-2 for 10 MegaTransfer/sec transfer rate. A
"MegaTransfer" refers to the rate of signals on the interface regardless of the width
of the bus. For example, 10 MT/sec rate on 1 byte wide bus results in 10 MB/sec transfer
rate but on a 2 byte wide bus results in a 20 MB/sec transfer rate.

What is SCSI FAST -20?
This refers to timings defined in SCSI-3 physical document for 20 MT/sec transfer rate. This
achieves data rates twice as fast as SCSI FAST rates.

What is SCSI FAST -40?
This refers to timings being defined for a future revision of the SCSI-3 physical documents
that achieves 40 MT/sec.

What is ULTRA SCSI FAST -40?
This is an old term for the FAST-20 data rate. This term was dropped by the committee because
the company UltraStore owned a trademark on the term Ultra SCSI, so it could not be used
by the standards committee to describe its work. If you see this term anywhere, read carefully
to see if it refers to the UltraStore trademark (which refers to a SCSI FAST class of
product) or if the writer is incorrectly using this term for FAST-20.

This term usually refers to the two byte wide (68 pin) connector that is defined in the
SCSI-3 Parallel Interface (SPI) document. This technically makes it a SCSI-3 feature.
The term can be generically applied to any implementation wider than 1 byte, but there are
no implementations wider than 2 bytes today. I don’t expect wider implementations
because faster transfer rates are giving plenty of life to 2 byte transfers until serial
interfaces (like Fiber Channel or FireWire) become more popular.

This refers to a combination of FAST transfer rate with 2 byte wide connector, which results
in 20 MB/sec data transfer rate. Remember that we will have wide FAST-20 (40 MB/sec)
products this year and wide FAST-40 (80 MB/sec) products might be available in late 1996.

This refers to products that use a fiber channel physical and protocol characteristics with
SCSI command set. The interface is completely different than parallel SCSI. It is a serial
interface, meaning command and data information is transmitted on one signal. That signal

uses a 1 GHz rate, however, so it achieves 100 MB/sec over coaxial cable. Even faster currently
rates are possible if optical fiber is used, but the optical transmitter/receiver is too
expensive for disk drive use. Information over fiber channel is organized into packets. This
interface has more in common with local area networks than with parallel SCSI.

What is the history of SCSI (What is SASI)?
1979 The disk drive manufacturer Shugart begin working on a new drive
interface with logical rather than physical addressing. It used 6 byte
commands. Shugart Associates Systems Interface (20 pages long) made
public. A few SASI drives are developed 1980. Attempt to make SASI an ANSI
standard failed. 1981 Shugart and NCR request an ANSI committee be formed
for SASI. 1982 ANSI committee X3T9.2 is formed. SCSI adds the ATN signal
to the bus and creates the message protocol. 1983 Development of SCSI
drives and ST-506 to SCSI bridges begins. 1985 CCS (Common Command Set)
used in most disk drives. Only disk and tape commands were adequately
specified. 1986 Work begins on SCSI-2. SCSI-1 becomes official as ANSI
X3.131-1986 (yes, after the work had begun on SCSI-2) 6 and 10 byte
commands. SCSI-2 specifies CDROM commands. 1988 Production of SCSI-2
devices begins. 1993 Work begins on SCSI-3. 1994 SCSI-2 becomes official
as X3.131-1994. SCSI-2 is backward compatible with SCSI-1 and adds the
following: Fast SCSI-2. Optional bus speed of 10MHz instead of 5MHz.
Wide Optional 16 or 32 bit cable instead of 8 bits. more commands defined,
many optional (I’m not going to type the entire list here) broader support
for non-disk devices (tape.CDROM,Scanners….) SCSI-2 devices can talk to
the host adapter on their own initiative. (e.g. to set in which mode they
should operate, FAST or not, wide, extra wide or normal …) This can
confuse some older SCSI-1 HA. 1995 Production of drives that have some
SCSI-3 enhancements. Ultra SCSI: Bus speed of 20MHz? 1996 SCSI-3 proposals include:

Support for graphical commands. Fibre channel protocol (fibre channel)
(FCP) Serial packet protocol (IEEE 1394) (SBP)

SCSI-3 general packet protocol (almost all serial interfaces) and of course
the old SCSI-2 commands and more.

Low Voltage Differential Parallel interface (SPI-2) CD-R command set and algorithms (MMC)

1998 Ultra2: Bus Speed of 40 MHz. LVD only. 1999 Ultra3: Bus Speed of 80 MHz. LVD only.
Future (after 1998): SCSI-3 becomes official SCSI becomes a more network-like environment
where devices can be physically distributed and shared more easily.

How should I lay out my SCSI bus? What should I
avoid? Where do I put the terminators? Where should the
adapter card be placed?
One confusing thing about SCSI is what the SCSI bus is supposed to look
like, and how devices should be placed on the bus.

The SCSI bus MUST run continuously from one device to another, like this:


Where device A, B, C, and D can either be internal or external devices.

The devices on the SCSI bus should have at least 4 to 6 inches of cable
between devices. This is to satisfy the SCSI-2 requirement that "stubs" be
placed at least .1 meters apart. Some devices that have a lot of internal
wiring between the connector and the SCSI chip can look like a "stub" or
bus discontinuity. The reason for all these requirements is that a SCSI
bus is really 18 "transmission lines" in the wave theory sense. A pulse
propagating along it will "reflect" from any part of the transmission line
that is different from the rest of it. These reflections add and subtract
in odd combinations and cause the original pulse to be distorted and
corrupted. The terminators "absorb" the energy from the pulses and prevent
reflections from the ends of the bus. They do this because they
(hopefully) have the same impedance as the rest of the transmission line.

The SCSI bus must not have any "Y" shape cabling. For example, setting up
a cable that looks like this is NOT allowed:


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 DEVICE B DEVICE C Adapter Terminated

+————+———-+———–+———–+———+ |
DEVICE A Unconnected DEVICE B Unconnected Adapter DEVICE C Terminated

Adapter DEVICE A DEVICE B Unconnected Unconnected DEVICE C Terminated

Adapter DEVICE A DEVICE B Unconnected Unconnected Termination Terminated

The following ARE NOT allowed:
Dangling cable end
+————+———-+———–+——————-+ DEVICEA DEVICE B Adapter Unconnected Unconnected Terminated Terminated

Termination in middle of bus
Termination DEVICE A DEVICE B DEVICE C Adapter

Helpful hint: I have found that it is much better in the long run to
always disable the internal terminators in all of your devices and place a
terminator block at the end of the cable itself. I’ll grant you that this
costs a little more because you need to buy a separate terminator. But,
you never need to be concerned in the future when you re-arrange devices in
your system, which device had its terminator enabled (none of them do).
With the arrival of LVD and SCA, devices are starting to be shipped which
don’t even have internal terminators anyway, so getting used to the idea of
terminating the cable end and not the device is a good practice. This is
just my opinion.

Old wive’s tale: I still hear people say "If you put a terminator part way
down your SCSI bus, the devices beyond it won’t be seen". This is a total
misconception of what terminators do. Putting a termination part way down
the bus is incorrect and does cause problems, but it is quite unpredictable
what the effect will be. It’s not simply a matter of making the devices
beyond the terminator invisible to the host adapter. Many people believe
this myth and it will probably never go away, but I hope to convince at
least a few people that this is not a valid way to envision how termination

Where Should I place the SCSI host 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
previous ly, a SCSI device is only allowed to have termination if it’s at
the end of the bus. Only two terminators are allowed to terminate the SCSI
bus, one at each end.

One last note: It doesn’t make any difference where each SCSI ID is placed

along the bus. It only matters that no two devices have the same ID.
Don’t forget that the adapter has an ID too. (Usually ID 7).

What is a SCSI terminator? Why do I need them?
A SCSI bus is a transmission line. To prevent reflections from the ends of
the bus, you need a device which makes the transmission line appear to be
of infinite length. This is done by attaching resistors, which have the
same resistance as the characteristic impedance of the transmission line,
to the ends of the bus. Also, since SCSI line drivers are open-collector
(which can only pull a signal low), a pull-up resistor is needed to pull
the signal high when it’s not asserted.

If the ends of the bus are not terminated, the signal pulses will reflect
off these open ends and travel back along the bus in the other direction.
The resultant adding and canceling of signal amplitudes distorts and
corrupts the SCSI signals.

There are two basic types of terminators, active and passive:

Passive terminators consist of pairs of resistors. A 220 Ohm pulling each
signal up to TERMPWR and a 330 Ohm pulling each signal down to GROUND.
Passive terminators were considered adequate in SCSI-1 when the bus only
ran at 5 MHz. In SCSI-2, passive terminators were given the name
"Alternative 1". Active terminators consist of 110 Ohm resistors connected
from each signal line to a common 2.85 Volt regulated power supply. Active
terminators both terminate the bus better (less reflection), and supply
cleaner pull-up current (due to their Voltage regulation). They were first
defined in SCSI-2 and were given the name "Alternative 2" to distinguish
them from passive terminators.

Recommendations and requirements: In SCSI-2 when the fastest defined speed
was 10 MHz, passive terminators were allowed, but active terminators were
recommended. In SCSI-3, the "alternative X" terminology has been
discarded, and the SPI-2 standard only allows active termination for
single-ended buses regardless of speed. My personal recommendation is not
to buy any new passive terminators. If you want to use up the old ones you
have lying around, on older systems, with short buses and no more than 4
devices, that don’t have any devices faster than 10 MHz, I can’t argue with
that, but ONLY BUY ACTIVE (or preferably LVD) terminators for any new
systems. If you run into problems, switching to an active terminator might
well solve them. Other people will tell you that only active terminators
are ever acceptable at any speed. I leave the choice up to the individual
at Fast10 and below, above that, active is absolutely the only acceptable choice.

A final nit to pick: As I was reminded in looking back at the standards, technically SCSI-2
did not sanction Fast10 on single ended buses. It was only spec’d for differential. However,
as was the case with WIDE SCSI using the 68 pin P cable, the industry latched onto it and
it later became standardized in SCSI-3 SPI.

What is terminator power (TERMPWR)? Why do I need it? Where does it come from?
TERMPWR is the power source for the SCSI terminators. Terminators (both active and passive)
require power because in addition to providing the correct impedance to prevent reflections
on the SCSI bus, they source pull-up current to the SCSI signals. The SCSI spec. allows for
multiple devices to supply power, but also limits the maximum current that should be
available. The "rule" is that "initiators shall supply TERMPWR". Hence a SCSI controller
(host adapter) should supply TERMPWR, and on longer buses it is worth having a device near
the end to also supply it . However, no more than about four devices should supply it,
because in the event of a failure (shorted cable etc), there could be dangerous currents
available. Not all devices are designed to be able to supply TERMPWR, but many can.
Usually this is done by setting one or two jumpers to select where TERMP WR will go.
For example:

TERMPWR to on drive terminator only TERMPWR to SCSI bus On drive terminator
gets its TERMPWR from SCSI bus

Even though the spec. says that host adapters should supply TERMPWR,
PCMCIA type host adapters do NOT do it. This is because PCMCIA cards are
generally plugged into laptop computers that run on batteries and can’t
afford the extra current drain. Another reason is because the contacts in
a PCMCIA connector are so tiny that the 1 Amp TERMPWR current load is
beyond their ratings. This being the case, at least one of the devices
that you wish to attach to a PCMCIA host adapter needs to be able to supply
TERMPWR, or you must provide a special terminator that has a power
connection for this purpose.

Is the spacing of connectors on a SCSI cable important?
The ANSI SCSI spec’s say that "stubs" on a SCSI bus must not be any more
than .1 meters (4 in.) long. In SCSI-2 there are also guidelines that say
you shouldn’t place "stubs" any closer than .3 meters (12 in.) apart.
Since each device attached acts as a "stub", you really shouldn’t place
connectors any closer than this. This gets to be more important as your
bus performance goes up. i.e. with Fast20 it is very important, but with
SCSI-1 it doesn’t really matter much. Since Fast20 also limits your
overall bus length to 1.5 meters (for single ended) this also means you
shouldn’t really connect more than 5 devices for best reliability.

Another minor enhancement involves altering the spacing of adjacent
connectors to prevent reflection resonance.

e.g. place connectors at one end, then .3m, then .56m then .86m then 1.12m

Overall, the cable impedance and configuration (straight vs. twisted pair)
are probably more significant factors than connector spacing. However, if
there is room for the extra cable, I recommend spacing the connectors as
described above for best reliability.

Excess cable length is also a bad thing, so basically all these factors
must traded off against each other to build the best SCSI cable for a given

How long can my SCSI bus be?
The SCSI bus length limits are based on the speed of the fastest device
attached to the bus.

Here’s a table which shows the limits:

Speed of Max Single-Ended Max HV Diff. Max LVD bus
FASTEST device bus length bus len. length
============== ================ ============ ============

5 MHz 6 meters 25 meters 12 meters
(SCSI1 synch.)

10 MHz 3 meters 25 meters 12 meters
(SCSI2 FAST) (Note 1)

20 MHz 1.5 meters 25 meters 12 meters
(Ultra or (Note 2)


40 MHz Not recommended 12 meters 12 meters
(Ultra2 or

Note 1: not recommended in SCSI-2 spec.
Note 2: 1.5 meters is my recommendation. The SCSI-3 SPI spec.
gives a much more complicated recommendation.

These limits assume the use of good quality cable, and the use of active
terminators or LVD/SE terminators at each end of the bus.

Notice that I used the term MHz to specify speed since MB/sec. changes with the bus width.

Note: Bus width doesn’t change the maximum allowable length. The bus width is independent
of bus length or speed.

The above table assumes that you know the max. speed of your devices (usually by looking in
the manuals). Some software (like Adaptec EZ-SCSI) provides a driver status monitor which
will tell you what mode the devices are actually in. This is important, since any synchronous
speed must be negotiated by either the device, or the adapter. The speed actually used
will be the least common denominator between the two.

For example, if a Fast20 disk is attached to a "SCSI2" host adapter that
only goes up to Fast10, the device will only run at 10 MHz.

In systems with high performance disks and external peripherals which
require long cables (i.e. external scanners, tapes or CDROM changers), you
may want to put the external devices on their own bus to avoid having to
slow down the fast disks. There are dual channel host adapters to make
this simpler (avoids using multiple IRQs etc).

The SCSI Trade Association also has a handy table at:

QUESTION:How should I set the IDs of my devices?

The main rule of SCSI IDs is that they all need to be unique on a per bus
basis. Each device on a particular bus must be set to a different ID so
that they can address each other without confusion. There is a secondary
consideration in setting IDs though; Higher ID numbers have a higher
priority on the bus. The overall ID priority order on a WIDE bus is as
follows (highest to lowest): 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11,
10, 9, 8. There are at least two philosophies on how to use the device
priorities to best advantage: Method 1: Set the host adapter’s ID to 7.
The next lower IDs (6, 5, 4 …) would then be used for any time critical
devices you may have such as streaming tape drives or CD-RW drives. Your
hard disks would be set to lower priority IDs because they are generally
the fastest devices on the bus and if given too high a priority will hog
all the bus bandwidth and "starve out" the slower but time critical

Method 2: This philosophy maintains that devices that create the load
should be given low priorities and devices that relieve the load should be
given higher priorities . In this view, the host adapter creates the load
(I/O to be done), therefore, set the host adapter’s ID to 0(or even 15 if
no narrow devices will be attached ). The time critical devices (streaming
tape and CD-RW) would then be assigned highest priorities. Everything else
(including disks) would be assigned IDs in between. The placement of the
load creator at low priority pretty much prevents the "starvation"

To my knowledge no benchmarks have been published that show one method to
be superior to the other. I would appreciate it if anyone would run some
good tests to prove what the best method is or point to existing published
results supporting one of these methods (or even another method).

Method 1 is apparently supported by Adaptec since they set all their host
adapters to ID 7 by default. I personally doubt that it makes very much
difference which method you choose except on very heavily loaded systems
where the drivers take full advantage of tagged command queuing etc.

Special consideration for older host adapters: Many older host adapters
make the assumption that the boot disk will be at ID 0. Most newer ones
however, allow the user to specify which ID to boot from.

What are the pros and cons regarding SCSI vs IDE/ATA ?
Pros of IDE/ATA:

Inexpensive due to high volume of production and simplified testing
requirements. Supported directly by system BIOS in most cases. (unless
you want DMA support) Less overhead per command

Cons of IDE/ATA:

Very limited device attachment (two drives (including CDROMs) per channel,
and two channels per system max.) (Recent versions of Linux (and I hear
Win NT) support four or more ATA adapters) Single threaded (commands do not
overlap even with a second drive) CPU is tied up transferring all data
(actually newer EIDE controllers can do DMA as well if special drivers are
loaded) IDE/ATA and ATAPI evolved from the ancient ST-506 interface as one
kludge on top of another Cannot handle scatter/gather operations well
(important in good Virtual Memory operating systems)

Pros of SCSI:

Flexible device attachment (up to 7 or 15 devices per SCSI bus) (inside or
outside of case) Longer cable lengths allowed (up to 12 meters using LVD)
Support for almost any peripheral type (disks, tape, CDROM, optical,
scanner etc) All commands can overlap with commands on other devices.
Usually uses DMA to transfer data (which frees CPU for other tasks)
Interface and protocol is carefully specified by ANSI. Largest, highest
performance devices are available in SCSI before IDE Most adapters can do
scatter/gather DMA which is a necessity in virtual memory systems (Like
Unix, NT, 2000) (Win 95/98 ?)

Cons of SCSI:

Generally more expensive than IDE/ATA, due to more complex firmware and
extra testing required. (not to mention greater performance commanding a
higher price). Slightly more complicated to install than IDE/ATA, due to
termination requirements. Seems scary to novice users because of amount of
terminology and connector/protocol options.

Some people point to the need to set IDs in SCSI as making it more
complicated, but it’s really no more complicated than choosing master/slave
jumpers in IDE.

Here’s a discussion that shows some of the advantages of SCSI in more detail:

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 also reads a block
from the disk. Process B’s block may be sitting in a RAM cache on the SCSI
controller, or on the drive itself. Or the block may be closer to the head
than process A’s block, or on a different drive on the same cable. SCSI
allows process B’s request to be completed ahead of process A’s, which
means that process B can be running sooner, so that the most expensive chip
– the system CPU – tends to spend less time twiddling its thumbs. Under
ATA, the process B request cannot even be sent to the drive until the
process A request is complete. These SCSI capabilities are very valuable
in a true multi-tasking environment, especially important in a busy file
server, and useless under DOS, which cannot take advantage of them.

I tend to hear from people, ‘Well, I never use multitasking’ because they
never actively run two programs at once – all but one are ‘just sitting
there’. Consider what happens though, when you minimize a window which
uncovers parts of four other application windows. Each of those
applications is sent a message telling it to update part of its window;
under win95, they will all run concurrently to perform the update. If they
need to access disk (usually because of virtual memory) the smoothness of
the update can depend a lot on the disk system’s ability to respond to
multiple independent read requests and finish them all as quickly as
possible; SCSI is better at this.

So, yes, ATA can be faster under DOS; but SCSI provides advantages which
are inaccessible to DOS. They will benefit Win95 however. The cost of
intelligent, fast SCSI controllers and drives should decrease as people
discover these advantages and start buying them.

I should add that many of SCSI’s advantages are NOT available with some of
the simpler SCSI controllers which were targeted only to the DOS market or
part of cheap CDROM add-on kits.

Furthermore, SCSI allows far greater flexibility of interconnect. I
concede that for the mass market, which likes to buy pre-configured
machines, this is but a small advantage.

Is it possible for two computers to access the same SCSI disks?
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.

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.

CDROM drives can be shared pretty easily because they are by definition

Disks can be best shared by having two (or more) partitions on a disk.
Each host "owning" its own file system.

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 with ASE/TCR, and Digital VMS are examples of systems
that allow this.

Is it possible for two computers to access the same SCSI tape?
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 asynch). 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.

What is the difference between the Adaptec 1542A and 1542B?
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.

What are the differences between the Adaptec 1542B and the 1542C?
The 1542C is an an updated model which replaces the 1542B. The 1542C
features jumperless setup, having only 8 DIP switches. All other
configuration options are set using the 1542C’s built-in BIOS configuration
utility. Configurable features not found on the 1542B are:

Ability to enable/disable sync negotiation on a per-ID basis (the 1542B
could only do it for all ID’s on the SCSI bus) Ability to send "start unit"
commands on a per-ID basis BIOS works with alternate I/O port settings on
the adapter. Ability to boot from ID’s other than 0 Software-selectable
termination Software-selectable geometry translation Additional DMA speeds
of 3.3 and 10 MB/sec

Additionally, the 1542C uses a Z80 CPU and 8Kb buffer instead of an 8085
and 2Kb buffer as on the 1542B.

What are the differences between the 1542C and the 1542CF?
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.

Where can I get obsolete Adaptec files and utilities?
You can get ASPI spec’s from Adaptec’s web site in the developer section.]

What kinds of optical drives are available?
Optical storage has good points going for it; Immunity to stray magnetic
fields; Potential for higher storage capacity per unit area; and relatively
low media cost.

Current optical storage solution offers two different types of storage
–rewritable and non-rewritable. The non-rewritable represents storage
method in which the data becomes permanent after being written onto the
disc. Rewritables, on the other hand, allows you to alter the data after
it has been written — just like the magnetic storage devices. And for
rewritables, two different technologies are available — magneto-optical
("MO") and phase-change ("PD").


As the name implies, MO uses both magnetic and optical technology to store
data on the disc. The disc itself is rare earth metal substrate. When
data is to be written, the particular spot is first heated by the laser to
the Curie point, and the magnetic field is generated while the spot cools.
By varying the magnetic field angle, the substrate is polarized in a
certain way that it will reflect back the laser beam differently depending
on the magnetic field angle present when the particular spot was cooling

MO comes in many sizes and capacities. Consumers were first exposed to MO

in Steve Jobs’ NeXT computer in the mid-1980s. Although 5.25" had a slow
start due to initial high cost, it has been evolving quite nicely.

The more popular ISO capacities for 5.25" MO are 4.8GB/5.2GB, 2.4GB/2.6GB,
and 1.2GB/1.3GB. In 3.5" form, MO is available in 540MB/640MB, 230MB,and
the 128MB. There are also some 12" MO, 14" MO, and other odd sizes in odd
capacities — particularly the hybrid 3.5" 1.3GB MO/PD drive. But they are
limited to niche markets due to high cost and rarity.

Sony MiniDisc-Data is derived from the Mini-Disc (MD) audio format
cartridge introduced earlier. MD-Data is to MD as CD-ROM is to digital
audio compact disc (CD-DA). MD-Data (and digital audio MD) is based on the
same magneto-optical technology, which partially explains the initial
high-cost of the consumer MD audio recordable units. For now, MD-Data is
the smallest of the MO family. With 2.5" form factor, it can store either
140MB or 650MB of uncompressed data.

Sony pushed the original MD-Data in the mid-’90s, but it did not catch on
due to high cost (for the capacity offered) and Sony’s decision to separate
MD audio function from MD-Data. And for few years, MD format has lagged
behind the capacity and speed of fellow MO breathens. In November 1999,
Sony announced the MD-Data-2 format and has gotten the format up-to-date.
It now has 650MB storage capacity with equal increase in transfer speed.
The MD-Data-2 debuts with Sony’s MPEG-2 camcorder in the Japanese market in
December of 1999. The most important technical advancement MD-Data brought
for MO in general is the one-pass recording. Prior to 5.25" 2.4GB/2.6GB MO
and 3.5" 540MB/640MB MO, practically all MO used two passes to write data
onto the disc — one pass to erase the whole track, and a follow-up pass to
write the updated data. MD’s one pass recording, called light intensity
modulation, direct over-write (LIM-DOW, ISO 14517) has been incorporated
into several later-generation MO formats to speed up the writing speed.

Anyway, what’s the limit of erase/write cycle can MO endure? Well, it
doesn’t look like anyone is really sure about it. Few years past, it is
guessed to be around 1 million times with 30 years of archival stability.
Today, Maxoptix says MO can sustain "greater than 1 trillion" cycles with
greater than 50 years of archival storage life.

Today, the popular MO formats are 3.5" and 5.25" that follow the ISO
standard. (Yes, there are others. But they’re far and few in between…)
The drives and media are available from Fujitsu, IBM, Maxoptix, Pinnacle
Micro, Pioneer Sony, Toshiba, and others.

Panasonic phase-change double-function (PD)

In around mid-’95, Panasonic released a proprietary optical storage format
called phase-change double-function (PD) drive. The PD uses substrate that
will reflect the light differently when heated to different temperatures
(and then cooled). Write-once-read-multiple (WORM) media were actually
the first phase-change formats, but PD is the first *reversible* (that is,
"re-writable") phase-change format. Panasonic PD stores 650MB per PD
cartridge. Panasonic’s own PD drive has also gone away with Sony’s
MD-Data, but the technology lives on in forms of CD-RW and DVD-RAM. The
PD media is said to take approximately 1,000 erase/write cycles. After about
1,000 cycles, the substrate will be fatigued to the point where the two
different states of the crystalline structure will become difficult to
differentiate reliably.


Write-once-read-multiple (WORM) format is a *write once* format — once you
have written the data to the disc, the written data cannot be changed. Put
it another way, the data recorded on the disc media is *permanent*.

WORM was the first popular format for optical storage, before being
eclipsed by MO. WORM is still used by big companies and the government for
archival purposes since it has the characteristic of not being able to be
altered without damaging the media (good audit trail).

The new WORM formats being introduced are tending to be proprietary. There
is rarely any interchangability between different vendor’s drives and
media. During the WORM to MO transition period, a curious format called
continuous composite write-once (CCW) appeared. CCW cartridges function as
WORM cartridges, writable using the installed base of WORM drives. But put
it into MO drive, CCW cartridges becomes rewritable. Simply put, CCW is MO
in WORM’s clothing. Many of today’s 5.25" MO drives still have the
capability to read CCW cartridges. And practically all WORM cartridges
sold today are CCW variety.

Sony-Philips Compact Disc (CD-R/CD-RW)…

WORM (in "MO" form) was once limited to niche market, but made one heck of
a come-back with form of CD-R. CD-R is based on the Sony-Philips’
proprietary CD-DA, commonly referred as "CD" (you know, those shiny disc
things that America-On-Line sends you). CD-R offers standard capacity of
650MB of data per disc, and can be used to store data or record music (and
be played in common CD players). But here, only the data-storage facet of
the CD-R/CD-RW is discussed.

As far as data storage is concerned, the specifications are written in the
"Orange Book." The Orange Book established three physical format for
recordable CDs — CD-MO, CD-R (previously known as "CD-WO"), and CD-RW
(previously known as "CD-E").

CD-MO is a MO medium in CD format. As far as I know, this format only
exists only on paper. The popular formats to come out of it are CD-R and

CD-R/CD-RW Incompatibility Sony and Philips finally agreed on a standard
for compact disc re-writable (CD-RW), together with HP, Matsushita, etc.
Long story short, the CD-RW uses phase-change media — same as Panasonic
proprietary PD format. Not only that, it also stores 650MB like PD. And
also like the PD, the CD-RW media cannot be read in existing CD-ROM drives!
CD-ROM drives manufactured in 1997 and after will read CD-RW discs though.
CD-R and CD-RW are known for their incompatibilities. There are
combinations of CD-R media, CD-R recorder, and CD-ROM player that simply
wouldn’t work. CD-RW is worse — virtually no audio CD players will play
CD-RW disc. The problem stems from the fact that reflectivity of the CD-R
is less than a factory-pressed CD-ROM. And CD-RW is worse in that respect
than the CD-R. As such, only the very recent CD-ROM player d
"Multi-Read" can read the CD-RW discs.


Possibly the most soap-operatic of all data-storage formats. With
convergence of computers and audio/video equipment, DVD was the most talked
about format for years as several companies fighting for what "DVD" format
should be.

Writable DVD formats: For now, DVD-RAM is not made to be playable in
DVD-ROM players that are so popular for its good pictures. If you’ve
looked at it, you’ll notice DVD-RAM is encased in a carriage case (a la
MO-style) but "video" DVDs aren’t. Although I think it may be possible to
take the DVD-RAM media out of the case and stick it into computer DVD-ROM
to read the recorded data.

What’s the difference between DVD-RAM, DVD-RW, and DVD+RW?
The names differ depending on whose specification the DVD storage is based
on. If it’s Matsushita (Panasonic), then it’s DVD-RAM; if it’s
Sony/Philips, then it’s DVD+RW;or if it’s Pioneer, then the name becomes
DVD-RW. The majority of current DVD storage devices follow Matsushita’s
DVD-RAM standard. Pioneer currently has its DVD-RW in the form of a DVD
video recorder in Japan. Each has slightly different storage capacities.

It’s still unknown whether they’ll be truly compatible with each other.
But all three specs have been submitted and all are regarded as DVD
re-writable "standards."

The Future?
Future optical storage will likely get bigger and bigger capacities, and
faster and faster transfer rates.

Anyway, MO is here to stay, so are CD and the DVD family of formats. As
for DVD… The competitions should prove to be entertaining (not). DVD is
not much about technology but more about politics. But since so many
electronic and entertainment giants are backing the DVD, you probably won’t
go wrong if you buy on e. (Just hope the one you buy will not be orphaned
at the turn of the hat by DVD consortium.) Some form(s) of DVD recordable
will eventually standardized, but don’t expect it to have more
storage/speed than what MO/PD/WORM formats offer.

The information is accessible from:

SASI Spec. – (.PDF format):

SCSI-1 draft standard – (Plain text, no figures, Dec. 1985): OR here

SCSI-2 draft standard (converted to HTML) –

SCA Specification

The SCSI, SFF, SSA, and Fibre Channel reflectors:

A list of these is available on the T10 site.

"The SCSI, SFF, SSA, and Fibre Channel reflectors are for review and
commentary on the respective specifications, not for asking questions about
the interface s (unless related to a specific ambiguity in a specification)
nor for recruiting nor for technical support nor any purpose other than
what is stated. The reflectors _are_ available for public review and
commentary as required by ANSI and ISO."

Any spec on the reflectors or on the BBS or on the ftp sites are
**proposed** or **preliminary** and are often subject to major substantive
changes during the committee process. Actual, released, final specs are
*only* available from Global Engineering Documents.

For Fibre Channel Info:

For Firewire (IEEE-1394) Info:

Where can I get official ANSI SCSI documents?

The SCSI specification: Available from:

1 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

What SCSI books and tutorials are available?

The Book of SCSI : I/O for the New Millennium, by Gary Field, Peter Ridge
Published by No Starch Press, San Francisco, CA
ISBN # 1-886411-10-7 , List Price $49.95.

A very complete reference and tutorial on almost all aspects of SCSI, including all the
latest advances like Ultra2WIDE/LVD, and all the previous standard SCSI features. It
addresses everything you need to know to install and debug SCSI I/O on a PC running Windows
95/98/NT and information on Linux as well. Also includes a CD-ROM with useful SCSI utilities
and information. The technical editor was none other than John Lohmeyer (chairman of the
ANSI SCSI committee since the beginning of SCSI), so you know the facts are straight!

IN-DEPTH EXPLORATION OF SCSI can be obtained from Solution Technology,
Attn: SCSI Publications, POB 104, Boulder Creek, CA 95006, (408)338-4285,
FAX (408)338-4374

Publishing, 14426 Black Walnut Ct., Saratoga, CA 95090,
(408)867-6642, FAX (408)867-2115.

Prentice-Hall, ISBN 0-13-796855-8 (Seems to be out of print)

A neat little book called "Basics of SCSI" second edition, was sent to me
free of charge by Ancot Corporation, Menlo Park, CA (415) 322-5322. It
gives a simplified description of how most aspects of the SCSI bus work and
includes some discussion of SCSI-2 issues.

"Programmer’s Guide to SCSI" with CDROM – by Brian Sawert.
Published by Addison Wesley, Reading, MA. SRP $39.95.

ISBN # 0-201-18538-5

Includes a chapter on UNIX SCSI subsystems written by Gary Field.

Addison Wesley Web site

‘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.

There was a two part article in Byte Magazine. The first part was in Feb 1990 issue, p. 267-274 and the second was in Mar 1990 issue, p. 291-298.

Another two part article appeared in Byte in May 1986 and June 1986.

Where can I find SCSI info on the Web?

Try some of these:

Where can I get information on various disk drives and controllers?
Drive and Controller Guide, Version 4.3 THEREF(tm) is a comprehensive Directory of Hard
Drives, Floppy Drives, Optical Drives, and Drive Controllers & Host Adapters. It is designed
to help the novice and pro alike with integration problems and system setups. Information is
provided in two handy formats; Portrait mode, for those who prefer a normal book-binding type
print format, and(or) do not have a printer with Landscape capability, and Landscape mode,
for those who prefer a computer-printout type format.

For printing, a Laserjet is preferred, but not necessary, and setup info is
provided. For viewing, LIST(tm) by Vernon Buerg, will provide an excellent
result, and allow text searches for finding specific models.

By F. Robert Falbo

Due many reports about the unavailablity of this file/archive I made sure
that the file does exist at the following site:

You should find the archive at:


(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

The Ref is also available via WWW from: TheRef

How can I contact Adaptec?

Also: Future Domain, Corel CD Creator, Trantor, Incat systems.
408 945-8600 Main number 800 959 7274 tech support 800 442 7274 orders,
doc, new bios, etc. 408 945-7727 BBS

Adaptec’s general inquiry number, 800-959-7274, affords access to a
FAX-based information retrieval system. In order to preserve the accuracy
of this information, I won’t go into details about how to use it (since
Adaptec may change things without telling me 🙂 ).

For those outside the CAN-US area, or local to Adaptec the direct FAX info
number is (408) 957-7150.

There are three general topics as of this writing:

General Information Sales Information Technical Information

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.

As of July 1993 Adaptec bought Trantor.

Try (800) 872-6867 (TRA-NTOR)]

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 Adaptec’s response).

QUESTION: How can I contact Seagate?
Technical Support Services
Online Services

Using a modem, you can obtain troubleshooting tips, free utility programs,
drive specifications, and jumper settings for Seagate’s entire product
line. You can also download software for installing and analyzing your

SeaNET You can obtain technical information about Seagate products over the
Internet from Seagate’s World Wide Web home page (,
Seagate’s FTP server ( or e-mail. Send your e-mail
questions to or

SeaBOARD SeaBOARD is a computer bulletin board system that contains
information about Seagate disc and tape drive products and is available 24
hours daily. Set your communications software to eight data bits, no
parity and one stop bit (8-N-1).

Location Phone number Australia 61-2-9756-2359 France 33 1-48 25 35 95
Germany 49-89-1409331 Taiwan 886-2-2719-6075 Thailand 662-531-8111 UK
44-1628-478011 USA Disc: 405-936-1600 Tape: 405-936-1630

Telephone Services


Seagate’s 800 number (1-800-732-4283) allows toll-free access to automated
self-help services that provide answers to commonly asked questions,
troubleshooting tips, and specifications for disc drives and tape drives.
This service is available 24 hours daily and requires a touch-tone phone.
International callers can reach this automated self-help service by calling

Seagate Telephone Technical Support For one-on-one help, you can talk to a technical support specialist during local business hours. Before calling,
note your system configuration and drive model number (STnnnn).

Location Phone number Australia 61

QUESTION: How can I find out about IBM hard disks?

Leave a Reply

Your email address will not be published. Required fields are marked *