Author: Gary Field (

Created:    October 19,1999
Last updated: April 16, 2002

Consider this the "Reader's Digest" version of the SCSI FAQ, or FAQ Lite if you prefer. :-)

This document is intended to give you just enough information to install a SCSI subsystem
into a PC and get it to work.
I created this guide to accomodate those who are new to SCSI and were just too damned lazy to read the
entire SCSI FAQ or just don't really care how SCSI works, so long as it does. :-)

What is the SCSI FAQ:
The SCSI FAQ is a living document that attempts to serve as a reference for people who are trying to learn about SCSI
and/or troubleshooting a SCSI system. I don't look at the FAQ as a standards document. There are many topics for which I could
simply regurgitate the SCSI standards documents, but instead, I generally choose to describe a useful, "practical" answer instead of
an obtuse, detailed, answer which would only further confuse a newcomer to SCSI. I feel more people are served by this method. I will NEVER knowingly include wrong information in the interest of simplicity however. Believe me, when I put something in  that's
incorrect (it has happened on rare occasions), I get plenty of email pointing out the error to me. Sometimes my opinion even even
plays into an answer (Oh well, you get what you pay for).

Occasionally, something I have written strikes controversy, usually either due to a gray area in one of the standards, or due to a
misintrepretation of what I wrote or included. When I get notified of the disagreement, I research the topic, listen to reasonable
arguments, and make the corrections or not as I see fit.

Articles get into the FAQ in several ways:
 1) I see a question appear a number of times and write an  answer for that topic and put it in.
 2) Someone else writes up an answer for a question they feel needs answering and sends it to me. I edit it for accuracy
           if necessary, correct the grammar/spelling somewhat, and put it in.
 3) I see what I feel is a well written response to a question posted in comp.periphs.scsi and ask the author if I can include it.

About the editor:
My credentials in SCSI technology are pretty substantial. I've been working with this stuff on a daily basis, continuously since 1985
on both PCs and various UNIX platforms. I write and enhance SCSI device drivers for a living. All of my own computers are all SCSI.
I also wrote/re-wrote "The Book of SCSI: I/O for the New Millennium", and wrote the UNIX chapter in Brian Sawert's book "The
Programmer's Guide to SCSI".

There are areas of SCSI which I am not expert on, and when a question of fact comes up in one of those areas, I research the issue
using the SCSI standards documents, or, ask my colleagues who are expert in that area, about it.

In general I get nothing but compliments about the FAQ. The most common complaint is that it's not always up to date on certain
topics. I try my best to keep it updated, but SCSI marches on...

"Who pays for the "SCSI Info Central" Web site where the FAQ is distributed from"? you ask.

SCSI Info Central

is my own personal web site that I've registered as, connected to the Internet via AT&T Broadband cable modem. I spend a
considerable amount of my own time and money maintaining the site, and I hope people benefit from it.

The Eternal Question

I am continually asked "Why should I buy SCSI drives instead of EIDE?"

I hope this will summarize my thoughts on that issue:

For someone to who doesn't need a real multi-tasking workstation or server, the only reason for paying the extra money for SCSI is flexibility. EIDE/ATA is strictly for "inside the case" peripherals. SCSI allows you to attach a large collection of add-ons like scanners, CD recorders, tape drives (or even devices not conceived of yet), either inside or outside the CPU case in whatever manner suits your needs or wishes.

If you like non-technical analogies:

SCSI is like a palace, with an architecture that was well thought out from the beginning and built upon over a period of time to make it even greater than originally envisioned.

IDE/ATA is like a log cabin, with a dirt floor, built from whatever was found lying around in late fall just before the snow came. It can't be expanded because it has no foundation and would collapse under its own weight.

Both provide shelter. SCSI costs more (but not as much as a palace :-)).

Take your pick.

If automobile analogies are more to your liking:

A Ford Escort will get you to work just as fast as a Volvo station wagon. Which would you rather go on vacation in? Which would you rather be in if an accident occurs?

If your computer is nothing more than a machine that's only purpose is to perform a certain set of tasks, and you don't expect to want any more out of it, IDE is probably for you.

On the other hand, if you enjoy computing and are always looking for more things your computer can do for you, SCSI will help enhance the experience for you. You won't regret the investment.

Just as with a palace however, you need to learn your way around. That's where this document comes in!

Thanks to David Sanderson and Roland Bauer for helping me improve the quality and compatibility of the HTML in this document. I would also like to thank the denizens of comp.periphs.scsi for their stimulating banter which triggered the writing of many of the articles in the FAQ.

Where to get the latest copy of this guide:

SCSIFAQ.ORG - Now at a browser near you!

If you just can't get enough SCSI, you might also want to look at:
SCSI Info Central where you will also find The SCSI Game Rules

Table of Contents:


Generic SCSI Questions: Table of Contents

SCSI Documentation and Books:

Table of Contents

Platform Specific Questions:

Table of Contents


Answers to the Questions:


ANSWER From: LSD, L.J.Sak@Kub. Edited by Gary Field (

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.

Table of Contents

QUESTION: SCSI sounds interesting. What do I need to get started?

ANSWER From: Gary Field (

In order to put together a PC with SCSI I/O you'll need: The SCSI device will most likely have a built in terminator that can be enabled or disabled. If it doesn't you'll also need a terminator (active terminator perferably). You'll find that there is quite a variety of SCSI cables out there. This is due to the fact that SCSI is so flexible. You are not limited to one SCSI device of course, that's just a minimum.

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: 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.

PLEASE DO NOT ASK "Which is better IDE or SCSI"?
Please spare us all the aggravation of the week long tirade that will result from asking this seemingly innocent question!

Table of Contents

QUESTION: What do all these SCSI buzzwords mean?

ANSWER From: (Hennes Passmann)[Editor(GF)]

Host adapter
Also called a Host Bus Adapter or HBA. The card that connects your computer to the SCSI-bus. Usually called SCSI-controller by marketing droids. An example would be a PCI SCSI host adapter like the Adaptec 2940UW.
Terminators (passive)
A group of resistors on the physical ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from the ends of the bus. Each terminated signal is connected by:
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.
Apple kludge
The single ended 50 pins cable has been reduced to 25 pins by tying most grounds together. DB25 connector (like a parallel port). Often used as the external SCSI connector. Unfortunately, this abomination is being perpetuated by being used on devices like the Zip drive!

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 : 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 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.
Ultra2 SCSI
Synchronous data transfer option which allows up to 40MHz data clocking on the bus. Also called FAST40.
Use of this option also requires the use of LVD bus drivers.
Uses an extra cable (or more commonly a 68 pin P cable) to send the data 16 or 32 bits wide. This allows for double or quadruple speed over the SCSI-bus.

RAID [Added by Editor(GF) Corrected by Fredrik Bjork (]
A Redundant Array of Independent Disks is a set of disk drives connected  in such a way as to allow certain types of access optimization, or data security. This can be accomplished in hardware using a special dual ported SCSI adapter, or completely in software in a special device driver.
A RAID 0 array stripes the data across multiple drives to decrease data latency. A RAID 1 array mirrors the data on multiple drives for increased data integrity. A RAID 5 array uses extra drives in a distributed manner to store parity information that can be used to apply data correction and recover any data in the event of any individual disk failure. This provides high reliability.
The following was submitted by
The minimum number of drives required for each RAID level is:
  RAID 0 - TWO (2) drives
  RAID 1 - TWO (2) drives
  RAID 0+1 - FOUR (4) drives
  RAID 3 - THREE (3) drives
  RAID 4 - THREE (3) drives
  RAID 5 - THREE (3) drives
Addition by:
Disconnect/reconnect (also called reselect)
This feature of the SCSI protocol allows a device to temporarily give up control of the SCSI bus. This is typically done when the device is performing an operation which will take some time. For example, it is very important for tape drives which would otherwise lock out other devices during long operations such as rewind.
Addition by: Editor GF (
Bus Segment
A portion of a SCSI bus isolated by a signal conditioner chip. A bus segment is logically part of a single SCSI bus (e.g. SCSI IDs must be unique) but electrically separated such that reflections on the segment do not affect other segments. Using bus segments allows longer busses because the signals get cleaned up (edges re-clocked etc) by going through the signal conditioner chips. Each segment must have its own terminations; One at the signal conditioner chip, and one at the far end of the segment.
Logical Unit Number (LUN)
A LUN is a sub-unit of a target. Most of the time, the LUN is just 0 since most types of target devices don't have sub-units. One example of where you might use LUNs is with multi-disc CDROM changers. Many of these units refer to each disc in the changer as a LUN. e.g. with the CDROM drive set as target ID 4, the first CD disc would be ID 4, LUN 0, the next would be ID 4, LUN 1 and so forth.
Another example is a optical disk jukebox where the optical drive might be LUN 0 and the changer might be LUN 1.
Some host adapters ignore LUNs unless the "Enable LUNs" option is set in the host adapter BIOS or operating system driver config. They default to not using LUNs because it speeds up the bus scan process and most targets don't support LUNs anyway.
LUN numbers are generally defined by the manufacturer and can't be changed by the user.
The Adaptec 2940 series BIOS has changed the place in the BIOS that LUN support is controlled several times.
A sketchy history:
* Note: The built-in SCSI adapter on this motherboard is quite similar to the Adaptec 2940U2W.
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 look like, and how devices should be placed on the bus.

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

DEVICE A --------- DEVICE B --------- DEVICE C -------- DEVICE D

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

The devices on the SCSI bus should have at least 4 to 6 inches of cable between devices. This is to satisfy the SCSI-2 requirement that "stubs" be placed at least .1 meters apart. Some devices that have a lot of internal wiring between the connector and the SCSI chip can look like a "stub" or bus discontinuity. The reason for all these requirements is that a SCSI bus is really 18 "transmission lines" in the wave theory sense. A pulse propagating along it will "reflect" from any part of the transmission line that is different from the rest of it. These 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 line.

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


            >------------- DEVICE 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
   |            |          |           |           |         |
Adapter    DEVICE A   DEVICE B Unconnected Unconnected   Termination

The following ARE NOT allowed:
   |            |          |           |                   |
DEVICE A    DEVICE B    Adapter    Unconnected        Unconnected    Dangling cable end
Terminated             Terminated
   |            |          |           |           |
Termination DEVICE A   DEVICE B  DEVICE C       Adapter             Termination in middle of bus
[Editor GF]

Helpful hint:
I have found that it is much better in the long run to always disable the internal terminators in all of your devices and place a
terminator block at the end of the cable itself. I'll grant you that this costs a little more because you need to buy a separate
terminator. But, you never need to be concerned in the future when you re-arrange devices in your system, which device had its
terminator enabled (none of them do). With the arrival of LVD and SCA, devices are starting to be shipped which don't even have
internal terminators anyway, so getting used to the idea of terminating the cable end and not the device is a good practice. This is
just my two cents worth... (backed by 15 years of tinkering with SCSI...).

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

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 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).

Table of Contents

QUESTION: What is a SCSI terminator? Why do I need them?

ANSWER From: Gary Field (
Updated:  May, 1999

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

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

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

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

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.

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

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

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

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

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

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.
Max. LVD bus length
5 MHz (SCSI1 synch.)
6 meters
25 meters
12 meters
3 meters (not
 recommended in SCSI-2)
25 meters
12 meters
20 MHz (Ultra or Fast20)
1.5 meters (not
 recommended until SCSI3 SPI)
25 meters 
12 meters
40 MHz (Ultra2 or Fast40)
Not recommended
12 meters
12 meters

These limits assume the use of good quality cable,  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.

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 slowing down the fast disks. There are dual channel host adapters to make this simpler (avoids using multiple IRQs etc).
Table of Contents

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

ANSWER From: Gary Field (

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 devices.

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

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

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

See also 1

Table of Contents

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

ANSWER From: Gary Field (

Pros of IDE/ATA:

Cons of IDE/ATA: Pros of SCSI: Cons of SCSI: ---------------

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 scatter-gather.

Also, the Intel Triton chipset implements 2 EIDE controllers, and I know that at least the 1 on the PCI bus supports bus-mastering, as well as DMA. However, PIO transfers can be faster, the infamous Mode 4 can in theory, do 16.6 MB/sec and I've heard of a Mode 5 which can do 22 MB/sec. Which [PIO] is only a benefit in single-tasking systems like DOS or Win3.x. Sounds like Intel is trying to make EIDE into SCSI, eh?

Table of Contents

QUESTION:Should I spend the extra money on SCSI or just get IDE?

ANSWER From: Andrew Korn (

For home users this is a difficult question to answer in general. It totally depends on how you use your system, what operating systems are installed, and whether you will add more I/O devices in the future.

For server systems in a corporate environment the only sensible answer is to go with SCSI peripherals.

IDE has been improved a lot in the past few years, so in most cases it will be an acceptable choice for home users. You should consider the following

(we are mostly talking PC hardware from now on):

1. Your motherboard probably has an integrated EIDE controller capable of supporting up to four devices. (Older motherboards may not have a dual-channel IDE controller, in which case only two drives can be connected; even older motherboards may not be equipped with an IDE controller at all.) If not, an IDE controller for your system should cost less than $30, which is about half of what a decent SCSI host adapter (Symbios 53C810 based) would cost you. On the other hand, some high-end motherboards come with integrated SCSI host adapters.

2. EIDE is a single threaded architecture. This means that of the two drives connected to an IDE channel, one will always be idle while the other is executing a command. If you only want a hard disk and a CD-ROM drive, you can install the CD-ROM on the secondary IDE channel (the hard disk will probably be the primary 'master' drive); in this case, the aforementioned limitation does not affect you. Also, if you only plan on using single-tasking operating systems such as DOS, you needn't be concerned about this single-threadedness.

SCSI devices share the bus bandwidth efficiently by allowing one device to transfer data while another is seeking or rewinding its media. This will, however, only gain you performance if you use a proper multi-tasking operating system (such as Linux [Editor(GF): or Windows NT]).

3. By default, IDE devices use PIO (Programmed I/O) to communicate with the rest of the system. This has the drawback of consuming a lot of CPU time. However, most newer EIDE controllers support bus-mastering and most drives support DMA or even UDMA transfer modes. Using bus-master DMA decreases CPU consumption to almost zero. (It may not be easy to activate the DMA transfer mode under DOS, however.)

Early SCSI host adapters had much the same problem, but all newer ones support DMA transfers.

4. If you plan to use only two drives (one per IDE channel), IDE will probably be slightly faster and definitely less expensive than SCSI.

If you think you need more than two drives, plan to use a multi-tasking environment (such as Unix, OS/2, Netware or Windows 95/NT), and think that performance is more important than cost, SCSI is the way to go.

Anything bigger than a small low-cost Linux-based server should probably use SCSI.

5. IDE tapes are not as cool as SCSI tapes. They tend to be slower, less reliable and less compatible with each other than SCSI tape drives. SCSI tapes are more expensive, however.

6. IDE is probably slightly easier to install. Termination is not an issue, and it will probably require no effort on your part to make the system aware of any new devices you add. In some increasingly rare cases this may not be true for SCSI. (You know what SCSI stands for? "System Can't See It." :))

Especially with older systems it may not be trivial (or, in rare cases, even possible) to make the computer boot from a SCSI drive.

7. It is problematic to add more than four IDE drives to a system. If you think you will need more than that, SCSI is probably the choice for you.

If your motherboard came with an integrated EIDE controller, however, there is no need to ignore that feature; you can have a mixed system with both IDE and SCSI devices. (Remember to buy SCSI where performance and parallelism is an issue; but there is no need to buy an expensive SCSI CD-ROM drive if an IDE drive would suit your needs.)

8. If you need high reliability, you want to buy a RAID capable SCSI host adapter. Be aware, however, that it is possible to emulate RAID from software; Linux can do RAID 0, 1, 4 and 5 with any mixture of SCSI and IDE disks. This software-based solution is probably less reliable and slower than a true RAID controller, but certainly also much less expensive.

[Editor(GF): ATA and IDE are basically the same thing, and the terms are used interchangably in this document.]

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

from: Greg Smith (

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, especialy important in a busy file server, and useless under DOS, which cannot take advantage of them.

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

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

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

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

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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.

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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.

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QUESTION: What SCSI books and tutorials are available?

ANSWER From: Gary Field (
Updated: June, 2000

The Book of SCSI : I/O for the New Millennium, by Gary Field, Peter Ridge et al

Published by No Starch Press, San Francisco, CA

ISBN # 1-886411-10-7 , List Price $49.95.

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

A complete description (as well as a link to purchase the book online at a significant discount) is available at:
My Book Site

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

THE SCSI ENCYLOPEDIA and the SCSI BENCH REFERENCE can be obtained from ENDL Publishing, 14426 Black Walnut Ct., Saratoga, CA 95090,

(408)867-6642, FAX (408)867-2115

SCSI: UNDERSTANDING THE SMALL COMPUTER SYSTEM INTERFACE was published by Prentice-Hall, ISBN 0-13-796855-8 (Seems to be out of print)

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

"Programmer's Guide to SCSI" with CDROM - by Brian Sawert.

Published by Addison Wesley, Reading, MA. SRP $39.95

ISBN # 0-201-18538-5

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

Brian's own web site

Addison Wesley Web site

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. 291-298.

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

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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 (

Date: Updated July 1999

SCA stands for "Single Connector Attachment". It is a standard being worked on by the ANSI Small Form Factor (SFF) committee. It combines WIDE SCSI signals, Power connections and ID switch connections onto one connector.

The main reason for creating this standard was to make it easier to connect drives in a hot-swappable RAID configuration.

SCSI vendors sell adapters that bring out the three sets of signals to conventional connectors.

Some places that sell such adapters are:

(There aren't any host adapters with 80 pin connectors, so don't ask )

See: for more information about SCA.

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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

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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 dividers.

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 omitted):

            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.]

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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 bus.

- 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. ]

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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 DM550-06-[0RS]

Newark Electronics stocks the DataMate product line.

Newark Electronics (International orders)

4801 N. Ravenswood Ave. 500 N. Pulaski St.

Chicago IL 60640-4496 Chicago IL 60624-1019

(312)-784-5100, (FAX (312)-638-7652, TLX 6718690 NEWARK U).


Selectronix Ltd

Minerva House, Calleva Park,

Aldermaston, Reading, RG7 8NE, UK

Tel: +44 (0)118 9817387

Fax: +44 (0)118 9817608


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QUESTION: Will attaching a SCSI-1 device to my SCSI-2 bus hurt its performance?

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 adapter?

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 want.

If you buy a high end host adapter (probably called SCSI3 :-) ) from a reputable vendor, and it has enough control over the various options (like synch xfer rate 5,10,20,40 mega-xfers/sec and the ability to disable WIDE or FAST/Ultra negotiation), and you carefully think out what devices you connect to it (all WIDE devices nearest the host adapter end of the bus etc.), and you are careful to properly terminate not only both ends, but both halves (upper byte and lower byte) of the bus, and none of the older devices you might already have (like a Panasonic CDROM) do anything stupid (like not handle the WIDE negotiation message without hanging) then it will all work fine. :-)

Even though a host adapter may be called SCSI3 doesn't mean it can enable or disable each optional feature, yet this is vital for supporting older devices.

To make matters worse, you won't know which older devices do some of the stupid things unless you know someone who's been bitten already. Your best bet is to look for good deals on name brand devices and adapters and before you buy, ask in comp.periphs.scsi whether anyone has tried the combination you're considering. It's also important to buy from a well known vendor with reasonable return policies.

If you're looking at buying a Vendorxyz spiffydisk which claims to be SCSI-3 compatible and you have a Seagate ST-01 host adapter and you want to know if anyone else has tried this combination, then that's exactly what you should ask.

In general, most SCSI devices and adapters made less than 4 years apart will probably work together, but without specific information about exactly which devices there's no assurance of it. There's also the potential for poor performance even if it does work.

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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 (
Updated: June, 2000

Yes, you just need an appropriate adapter. Most WIDE devices use the 68 pin "P" connector so you need a 68 pin to 50 pin adapter.
You do need to make sure that both the upper byte and lower byte of the bus will be properly terminated though. Some adapters
provide Hi-9  terminators, others do not. If the wiring adapter is placed right at the SCSI host adapter, you can usually configure the
host adapter's on-board terminators to only terminate the high byte. You need to be clear on what type of connectors are present
where you want to do the conversion. You also need to plan your bus so that there won't be any narrow cable between any of the
WIDE devices.
Certain host adapters with auto-termination make the assumption that when the low byte is terminated the high byte is also. When
using WIDE/narrow adapters this assumption is not valid. Another purpose served by the hi-9 terminator is supplying pull-up
current to the upper data lines which would otherwise be left floating.

Special note for LVD drives:

It is recommended that if you connect a WIDE LVD drive to a narrow bus that you use a 68 to 50 pin adapter which has high byte
termination. It may seem that the termination wouldn't be needed in this case because the bus is narrow. However, the drive needs to
have those signals "pulled up" (logically negated), to avoid the floating signals from confusing it.

If for some reason you attach a WIDE device to a WIDE host adapter using a narrow cable, you must be sure to disable WIDE
negotiation in the host adapter BIOS or the device will hang when it is accessed.

One further caveat is that if narrow devices are attached to a WIDE adapter, the adapter's ID must be between 0 and 7 because
narrow devices would not be able to see it if the ID was any higher than 7.

WIDE to NARROW adapters are available from:

 Computer Cable Makers Inc

819 Striker Ave, Suite 6
Sacramento, CA 95834

Technical Cable Concepts

1790 E. McFadden Ave., Unit 103/104
Santa Ana, CA 92705

TEL: (714) 835-1081

FAX: (714) 835-1595

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

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QUESTION: How does device ID numbering work with WIDE vs NARROW devices?

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 it.

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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 steps:

(If your host adapter is new, you can probably skip the next step, but if you want to avoid mysteries later, it's not a bad idea to do it.) (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.) Notes for mixing IDE and SCSI disks on the same system? 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:

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 SETUP.EXE

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QUESTION: Under Windows 95 OSR2 I can only see the first 8 GB of my 9 GB disk. What's going on?

ANSWER From: Gary Field (

The retail version of Windows 95 is limited to 2 GB per partition by the use of the FAT16 filesystem. Since you're getting more than 2 GB, you must be using a FAT32 filesystem.

Using FAT32 with drives larger than 8 GB requires a host adapter that supports the "INT 13 extensions". If your host adapter was built before about 1996, you may not have this feature. For example Adaptec 2940W Host adapters did not support this. Even the early 2940UW didn't have it. As of BIOS ver. 1.2x the support is present. Check with your host adapter manufacturer for an updated BIOS.

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QUESTION: My drive letters map to different disk partitions than I think they should. What's going on?

ANSWER From: Gary Field (

Caveat: The following description assumes the use of Microsoft's FDISK. The use of tools like Linux fdisk or Partition Magic changes a lot of
the rules.)

Microsoft uses the following strange algorithm to map drive letters to disk partitions:

Look at the partition tables on BOTH PHYSICAL DISKS.

First PRIMARY partition becomes Drive C:

If there is a second PRIMARY partition it becomes Drive D:

If there are no more PRIMARY partitions, look for EXTENDED partitions.

The first logical drive in the first EXTENDED partition becomes drive E:, the next logical drive in that EXTENDED partition becomes F: etc.

If there is another EXTENDED partition, the first logical drive in it becomes drive G:, the next logical drive in it becomes drive H: etc.

Then, as device drivers are loaded, any disk drives they support will be assigned consecutive drive letters. CDROMs and other "Network drives" can be assigned specific drive letters if desired, leaving holes in the lettering scheme.

Under MSDOS, Win 3.x and Windows 95 this behavior can't be changed. Under Windows NT all drive letters can be re-mapped to whatever you want.

If you don't want D: on your second disk, don't create a PRIMARY partition on it!

An example of this mapping which often confuses newbies:

INT 13h Drive 0x80:



Logical -> E:

Logical -> F:

Int 13h Drive 0x81:



Logical -> G:

Logical -> H:


Assigned to drive M:

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QUESTION: What is Low Level Formatting(LLF)?

ANSWER From: Gary Field (

Date: September 1998

Each platter in a disk drive is organized as tracks and sectors. Each sector contains header and trailer information as well as error detection (CRC) data in addition to the actual user data field.

When a disk is manufactured the platters are blank (no sector layout). Before shipping, a special command (usually not documented) is issued to the drive to cause it to lay down the sector headers, blank data fields and good CRC. Also many data patterns may be written to each sector to check for media errors. Any sectors with errors are put into the "manufacturer's defect list" and the drive remembers not to use those sectors in the future. Later, after the drive is shipped, a user may decide to "Low Level Format" the drive if he is having problems, or wants to start with a "clean slate". This is done using the SCSI FORMAT command via a special utility usually supplied by the host adapter manufacturer (usually in the on-board BIOS).

Some side effects of doing a LLF:

Usually it is a good idea to LLF a drive when it is installed and then verify (read each sector using a special utility) it. From then on you shouldn't need to LLF it again. A good verify utility will offer the option of re-assigning any bad blocks that are found. These will then be placed in the drive's "grown defect list". The only way to recover blocks that are mistakenly added to the grown defect list is to issue FORMAT UNIT to the drive.

LLF is NOT to be confused with running the MSDOS/Windows utilities called FDISK or FORMAT. FDISK causes a "partition table" to be created which logically divides up a disk for use by multiple filesystems and/or Operating Systems. FORMAT causes a FAT16 filesystem to be initialized in an existing partition. FORMAT is equivalent to the UNIX command mkfs (Make filesystem). FORMAT also reads the entire partition and marks any bad sectors found as unusable in the File Allocation Table. This does NOT cause the drive to add them to the drive's "grown defect list", but does prevent DOS/Windows from using them.

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QUESTION: My system says my new 9.1 GB disk is only 8.5 GB. Did I get cheated?

ANSWER From: Gary Field (

Date: January 1999

Well, not exactly. It's a matter of definitions.
Marketing people like to use the prefix Mega to mean 1,000,000 and Giga to mean 1,000,000,000.
Computer engineers define Mega to mean 1,048,576, and Giga to mean 1,073,741,824.
The reason engineers use these strange looking numbers is because they are powers of 2.
(2 to the 20th and 2 to the 30th)
The ad you read when you bought your disk was written by a marketing person. The
operating system on your computer was written by an engineer. In engineering terms,
your 9,100,000,000 byte disk is about 8.5 GB.

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QUESTION: What is LVD (Low Voltage Differential)?

ANSWER From: Gary Field (

Date: January 1999

Low Voltage Differential is a new hardware bus driver type for SCSI-3. It first becomes important
with Fast-40 (Ultra2) devices. If single ended (S.E.) bus drivers were used with Fast-40, the bus length would
be limited to about .75 meters! Since this was clearly un-acceptable, and the older High Voltage Differential
(HVD) interface adds too much cost to a system, ANSI defined a new form of differential interface
that is less expensive to implement because the bus driver logic dissipates little enough power that
it can be included in LSI chips. They also wanted to make sure they avoided the confusion caused by
HVD (HVD and S.E. devices cannot co-exist on a bus), so they specified that if an LVD device is designed properly,
it can switch to S.E. mode and operate with S.E. devices on the same bus segment.
Another difference worth noting is that LVD devices do not contain on-board terminators. You need
to attach an LVD terminator to the end of the cable instead of using a device to terminate the bus
as was commonly done with S.E. SCSI. The big advantage to LVD is that you can have a high speed
SCSI bus up to 12 meters in length!

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QUESTION:My host adapter says it "auto-terminates". How does it do that?

ANSWER From: Gary Field (
Date: May, 1999

That's a very good question! I haven't done a thorough study to determine how they all do it, but the ones I've looked at sense a ground connection on certain pins. This may work in many instances, but it's certainly not foolproof. For example many people connect scanners, or ZIP drives that have (may their little metal souls be damned) 25 pin connectors, to the end of the bus. What happens to all those ground connections now? Who knows! Another problem comes in when 68 pin buses are adapted down to 50 pins. Same problem. Another problem is if the host adapter is in the middle of the bus. It might be able to sense whether there is another terminator, but how can it know if there are too many?
In my opinion, the only reliable way to sense whether the bus is properly terminated is by looking and checking  manually, or with a Time Domain Reflectometer and I don't think they'll be putting those onto SCSI host adapters any time soon (but it's not impossible).
Certain host adapters with auto-termination make the assumption that when the low byte is terminated the high byte is also. This isn't always a good assumption, especially if 68 pin to 50 pin adapters are in use.
The bottom line is that you shouldn't depend on auto-termination to do the right thing. Set the termination manually!

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QUESTION: Do all my devices need to be powered up?

ANSWER From: Gary Field (

Date: January 2000

Not necessarily. It all depends on where your terminator power is being supplied from. Many users get confused between termination and terminator power. They think that if they turn off an external device that has the terminator on it, that the bus will be unterminated. This is not true. The terminator will still be there, but it might have no power to pull the SCSI signals up. This is why it's important to know where the terminator's power is coming from. If it's coming from the TERMPWR line on the SCSI bus, it is probably being supplied by the host adapter. This is just one of a number of reasons I prefer to use external stand-alone terminators rather than the ones built into devices. External terminators always get their power from the TERMPWR line. The terminators inside devices can get theirs from either the TERMPWR line or the device's own power supply. Then if you turn that device off, its terminator has no power (That's bad).There is often a jumper or switch on the device that selects the terminator power source.
The bottom line is: If you use only stand-alone terminator blocks to terminate your bus, you can turn off any device and the bus should still work. If you use one of the device's own terminators, you need to either make sure it's one you'll never turn off, or make sure its terminator power comes from the SCSI TERMPWR line.

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QUESTION: When I boot I get a message saying that the SCSI BIOS was not loaded. Why?
QUESTION: Why would I care whether my SCSI host adapter card has a SCSI BIOS or not?

ANSWER From: Gary Field (

Date: January 2000

Most SCSI host adapters have their own SCSI BIOS for two main purposes. The BIOS acts as the device driver for hard disks, and it allows certain host adapter options to be set. Some manufacturers also include handy utilities to low level format and verify hard disk drives. If a card doesn't have a BIOS, you cannot boot from a device attached to it. Some host adapters (NCR 53C810 based) depend on having the SCSI BIOS contained in the motherboard BIOS to allow them to boot.

For MSDOS (8086 real mode only) systems, the BIOS acts as the disk device driver for the operating system. For more modern (80386 protected mode) operating systems like Win95/98 and Win NT/2000, the BIOS is only used during boot and once the O/S's device drivers are loaded, the BIOS is no longer used.

So, if during boot the host adapter does not detect any hard disk drives (or other devices it could boot from like a CD-ROM or ZIP disk), it doesn't bother to load the BIOS into memory since it would serve no purpose.

If you are getting this message and you do have a SCSI hard disk attached, you probably have a termination or cabling problem that is keeping the disk from being seen on the bus. If you don't have a SCSI hard disk (maybe your only SCSI device is a scanner for example), then the message is normal and can be ignored.

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QUESTION: How Should I Choose a SCSI Host Adapter?

ANSWER From: Gary Field (
Date: July, 2000

 A SCSI Host Bus Adapter (sometimes called an HBA) is the heart of a SCSI subsystem. As the name implies, it connects the system's I/O bus (PCI or ISA) to the SCSI bus. You need to give some thought to what SCSI devices you will be connecting to your system before choosing one. There is a large variety to choose from. They range from really low end PIO ISA cards that are included with SCSI scanners, CD-RW drives, etc, to dual channel, Ultra160, 64 bit Bus Mastering PCI cards costing up to $400 or more.
If you only intend to connect low performance devices such as a scanner to the system, then a low end HBA is probably all you need.
But, if you want to connect SCSI hard disks, you'll probably want to get the best card you can afford.
Using an ISA card in a PCI capable system can cause really poor performance. PCI motherboards often implement the ISA slots in such a way that they are much slower than ISA slots on ISA only motherboards.
Another factor to consider is the type of support you require. If you really know a lot about installing and configuring PC option cards, you may want to save a few bucks and get an "OEM" card. But, if you aren't a real expert on this, you'd better stick to a "kit" packaged card that comes with manufacturer's technical support, required cables, drivers etc.
Also, you should decide what operating systems you will be running and make sure they support the specific card you are considering.

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