Author: Gary Field (scsifaq@bigfoot.com)
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
http://www.scsifaq.org/
is my own personal web site that I've registered as scsifaq.org, 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!
Where to get the latest copy of this guide:
SCSIFAQ.ORG - Now at a browser near you!
Categories:
Generic SCSI Questions:Answers to the Questions:
ANSWER From: LSD, L.J.Sak@Kub. Edited by Gary
Field (scsifaq@bigfoot.com)
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.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
ANSWER From: Gary Field (scsifaq@bigfoot.com)
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!
ANSWER From: hennes@stack.urc.tue.nl (Hennes
Passmann)[Editor(GF)]
For WIDE SCSI there are 9 more signals; DB(p1), DB(8) ... DB(15)
Answers From: Nick Kralevich <nickkral@cory.eecs.berkeley.edu>
edited by Gary Field (scsifaq@bigfoot.com)
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:
Termination must occur within 4 inches (.1 meter) of the ends of the SCSI bus.
The following ARE acceptable: +------------+----------+-----------+-----------+---------+ | | | | | | DEVICE A Unconnected Unconnected DEVICE B DEVICE C Adapter Terminated Terminated +------------+----------+-----------+-----------+---------+ | | | | | | DEVICE A Unconnected DEVICE B Unconnected Adapter DEVICE C Terminated Terminated +------------+----------+-----------+-----------+---------+ | | | | | | Adapter DEVICE A DEVICE B Unconnected Unconnected DEVICE C Terminated Terminated
+------------+----------+-----------+-----------+---------+ | | | | | | Adapter DEVICE A DEVICE B Unconnected Unconnected Termination Terminated
The following ARE NOT allowed: +------------+----------+-----------+-------------------+ | | | | | DEVICE A DEVICE B Adapter Unconnected Unconnected Dangling cable end Terminated Terminated
+------------+----------+-----------+-----------+ | | | | | Termination DEVICE A DEVICE B DEVICE C Adapter Termination in middle of bus Terminated
Old wive's tale:
I still hear people say "If you put a terminator part way down your
SCSI bus, the devices beyond it won't be seen". This is a total
misconception of what terminators do. Putting a termination part way
down the bus is incorrect and does cause problems, but it is
quite unpredictable what the effect will be. It's not simply a matter
of making the devices beyond the terminator invisible to the
host adapter. Many people believe this myth and it will probably never
go away, but I hope to convince at least a few people that this
is not a valid way to envision how termination works.
The placement of the SCSI adapter card can be on the end, at the beginning, or somewhere in the middle of the SCSI bus.
Quite frankly, placement of the controller card isn't special.
The adapter card is just another device on the SCSI bus.
As long as the rules above and in other sections of this FAQ are followed, there should be no problem placing the adapter card anywhere on the SCSI bus.
However, if you place the adapter card somewhere in the middle of the SCSI bus, you must be sure to disable termination on the adapter card. As noted previously, a SCSI device is only allowed to have termination if it's at the end of the bus. Only two terminators are allowed to terminate the SCSI bus, one at each end.
One last note: It doesn't make any difference where each SCSI ID is placed along the bus. It only matters that no two devices have the same ID. Don't forget that the adapter has an ID too. (Usually ID 7).
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Updated: May, 1999
A SCSI bus is a transmission line. To prevent reflections from the ends of the bus, you need a device which makes the transmission line appear to be of infinite length. This is done by attaching resistors, which have the same resistance as the characteristic impedance of the transmission line, to the ends of the bus. Also, since SCSI line drivers are open-collector (which can only pull a signal low), a pull-up resistor is needed to pull the signal high when it's not asserted.
If the ends of the bus are not terminated, the signal pulses will reflect off these open ends and travel back along the bus in the other direction. The resultant adding and cancelling of signal amplitudes distorts and corrupts the SCSI signals.
There are two basic types of terminators, active and passive:
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.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
The ANSI SCSI spec's say that "stubs" on a SCSI bus must not be any more than .1 meters (4 in.) long. In SCSI-2 there are also guidelines that say you shouldn't place "stubs" any closer than .3 meters (12 in.) apart. Since each device attached acts as a "stub", you really shouldn't place connectors any closer than this. This gets to be more important as your bus performance goes up. i.e. with Fast20 it is very important, but with SCSI-1 it doesn't really matter much. Since Fast20 also limits your overall bus length to 1.5 meters (for single ended) this also means you shouldn't really connect more than 5 devices for best reliability.
Another minor enhancement involves altering the spacing of adjacent connectors to prevent reflection resonance.
e.g. place connectors at one end, then .3m, then .56m then .86m then 1.12m
Overall, the cable impedance and configuration (straight vs. twisted pair) are probably more significant factors than connector spacing. However, if there is room for the extra cable, I recommend spacing the connectors as described above for best reliability.
Excess cable length is also a bad thing, so basically all these factors must traded off against each other to build the best SCSI cable for a given situation.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
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 |
10 MHz (SCSI2 FAST) |
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
ANSWER From: Gary Field (scsifaq@bigfoot.com)
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
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Pros of IDE/ATA:
Now that I've said that, here's an article to show that there's more than one opinion on this subject:
From: Ed Schernau <mithrandir@ids.net>
Subject: FYI: EIDE and DMA/Scatter-Gather
The Western Digital Caviar EIDE drive that came in what is now the file server in our office came with a Win3.x 32 BDA driver which allowed the user to select DMA type (B or F) and to implement scatter-gather.
Also, the Intel Triton chipset implements 2 EIDE controllers, and I know that at least the 1 on the PCI bus supports bus-mastering, as well as DMA. However, PIO transfers can be faster, the infamous Mode 4 can in theory, do 16.6 MB/sec and I've heard of a Mode 5 which can do 22 MB/sec. Which [PIO] is only a benefit in single-tasking systems like DOS or Win3.x. Sounds like Intel is trying to make EIDE into SCSI, eh?
ANSWER From: Andrew Korn (korn@eik.bme.hu)
For home users this is a difficult question to answer in general. It totally depends on how you use your system, what operating systems are installed, and whether you will add more I/O devices in the future.
For server systems in a corporate environment the only sensible answer is to go with SCSI peripherals.
IDE has been improved a lot in the past few years, so in most cases it will be an acceptable choice for home users. You should consider the following
(we are mostly talking PC hardware from now on):
1. Your motherboard probably has an integrated EIDE controller capable of supporting up to four devices. (Older motherboards may not have a dual-channel IDE controller, in which case only two drives can be connected; even older motherboards may not be equipped with an IDE controller at all.) If not, an IDE controller for your system should cost less than $30, which is about half of what a decent SCSI host adapter (Symbios 53C810 based) would cost you. On the other hand, some high-end motherboards come with integrated SCSI host adapters.
2. EIDE is a single threaded architecture. This means that of the two drives connected to an IDE channel, one will always be idle while the other is executing a command. If you only want a hard disk and a CD-ROM drive, you can install the CD-ROM on the secondary IDE channel (the hard disk will probably be the primary 'master' drive); in this case, the aforementioned limitation does not affect you. Also, if you only plan on using single-tasking operating systems such as DOS, you needn't be concerned about this single-threadedness.
SCSI devices share the bus bandwidth efficiently by allowing one device to transfer data while another is seeking or rewinding its media. This will, however, only gain you performance if you use a proper multi-tasking operating system (such as Linux [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 (GREGS@lss-chq.mhs.compuserve.com)
Under DOS (and DOS/win3.1), there is very little useful work the host can do while waiting for a disk operation to complete. So handing off some work from a 66 MHz 486 to, say, an 8 MHz Z80 (on the controller) does result in a performance loss. Under EVERY other OS worth discussing (Unix, Netware, NT, OS/2, Win95 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.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
The short answer is YES. There are a few issues to consider however.
The main issue is which device will be used for booting the system. Under MSDOS, The system BIOS determined this completely. A couple third party BIOSes (like MRBIOS) allowed the user to choose the boot source, but most conventional BIOSes just booted from the IDE if it was present. If no IDE was present then the standard option card BIOS scan would find the SCSI card's BIOS and use it to boot.
Under Windows 95 and Windows NT, there are more options. Since the motherboard BIOS is used to load the boot sector that will still happen according to the same rules as under MSDOS described above. After the boot sector is loaded, the O/S's device drivers take over and those can be unloaded or drive letters re-ordered via the O/S configuration tools.
ANSWER #1 From: kev@hpcpbla.bri.hp.com (Kevin Jones)
and jmatrow@donald.WichitaKS.NCR.COM (John
Matrow)
The SCSI specification: Available from:
ANSI
11 West 42nd St. - 13th floor
New York, NY 10036
Sales Dept. (212) 642-4900
OR
Global Engineering Documents
15 Inverness Way East
Englewood Co 80112-5704
(800) 854-7179 or (303) 792-2181
Int'l Sales Fax: (303) 397-2740
SCSI-1: X3.131-1986
SCSI-2: X3.131-199x
SCSI-3 X3T9.2/91-010R4 Working Draft
[Editor(GF):] The official ANSI standards are NOT available free of charge from any source. Only draft versions are freely distributable.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Updated: June, 2000
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
(408)867-6642, FAX (408)867-2115
"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.
'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.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: Updated July 1999
SCA stands for "Single Connector Attachment". It is a standard being worked on by the ANSI Small Form Factor (SFF) committee. It combines WIDE SCSI signals, Power connections and ID switch connections onto one connector.
The main reason for creating this standard was to make it easier to connect drives in a hot-swappable RAID configuration.
SCSI vendors sell adapters that bring out the three sets of signals to conventional connectors.
Some places that sell such adapters are:
(There aren't any host adapters with 80 pin connectors, so don't ask )
See: http://playground.sun.com/pub/SCA/SCAR3-2.txt for more information about SCA.
ANSWER From: Gary Field (scsifaq@bigfoot.com
)
See also: 1
ANSWER From: eric@telebit.com (Eric Smith)
and brent@auspex.com (Brent R. Largent)
An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than using resistor voltage dividers.
This is a passive terminator:
TERMPWR ------/\/\/\/------+------/\/\/\/----- GND | | SCSI signalNotice that the termination voltage varies with the voltage on the TERMPWR line. One voltage divider (two resistors) is used for each SCSI signal.
An active terminator looks more like this (supply filter caps omitted):
2.85 Volt Regulator +-----------+ +2.85V 110 Ohms TERMPWR -----| in out |------+------/\/\/\/-------SCSI signal | gnd | | +-----------+ | | +------/\/\/\/-------SCSI signal | | GND ---------------+ | +------/\/\/\/-------SCSI signal | etc.Assuming that the TERMPWR voltage doesn't drop below the desired termination voltage (plus the regulator's minimum drop), the SCSI signals will always be terminated to the correct voltage level.
Several vendors have started making SCSI active terminator chips, which contain the regulator and the resistors including Dallas Semiconductor, Unitrode Integrated Circuits and Motorola.
[Editor(GF): Another nice feature of activer termination is that it can be disabled by a single jumper instead of needing to unplug resistor arrays.]
ANSWER brent@auspex.com (Brent R. Largent)
Typical passive terminators (resistors) allow signals to fluctuate directly in relation to the TERM Power Voltage. Usually terminating resistors will suffice over short distances, like 2-3 feet, but for longer distances active termination is a real advantage.
Active termination provide the following advantages:
- Helps reduce noise.
- A logic bit can be used to effectively disconnect the termination.
- Regulated termination voltage.
- SCSI-2 spec. recommends active termination on both ends of the scsi bus.
- Improved resistance tolerances (from 1% to about 3%)
[Editor(GF):
- Reduces current drawn from TERMPWR line.
In FPT form:
- Provides signal overshoot/undershoot clamping on all signal lines. ]
ANSWER From: Rodney Brown (RBrown@cocam.com.au)
Info taken from Usenet postings by:
John Zatler (JPZ@Popmail.mcs.com)
Steve Schreppler (schrep@oasys.dt.navy.mil)
Dave Nadler (nadler@ug.eds.com)
DataMate / Methode
Methode Electronics, Inc.
dataMate Division
7444 West Wilson Avenue
Chicago, IL 60656
(708) 867-9600
(800) 323-6858
(708) 867-3149 FAX
WWW: http://www.methode.com/datamate/dmhome.htm
Brief description of terminators available.
Passive, Active, SLICK (Elaboration of FPT)) in:
Centronics 50 pin (SCSI-1) DM8[05]0-09-[0RS]
Male 3 row D-Sub (Old Sun) DM950-??-?
Male 50 position .050" Centres (SCSI-2 HD) DM20[05]0-02-[0RS]
Male 68 position .050" Centres (SCSI-3 P cable) DM2050-02-68[RS]
Male & Female for ribbon cables DM1050-02-[0RS] (M),
DM650-06-[0RS] (F)
Male/Female for pass through between device and ribbon cable DM550-06-[0RS]
Newark Electronics stocks the DataMate product line.
Newark Electronics (International orders)
4801 N. Ravenswood Ave. 500 N. Pulaski St.
Chicago IL 60640-4496 Chicago IL 60624-1019
(312)-784-5100, (FAX (312)-638-7652, TLX 6718690 NEWARK U).
Selectronix Ltd
Minerva House, Calleva Park,
Aldermaston, Reading, RG7 8NE, UK
Tel: +44 (0)118 9817387
Fax: +44 (0)118 9817608
WWW: http://www.selectronix.co.uk/
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Attaching a SCSI-1 device to a system with a SCSI-2 host adapter and several SCSI-2 devices already attached will not hurt over-all performance significantly unless it doesn't handle disconnect/reconnect well. This assumes that the host adapter keeps track of protocol options separately for each target device. Some people have the idea that attaching a SCSI-1 device to a SCSI-2 bus will cause the entire bus to run at SCSI-1 speeds. This is not true.
Can I connect a SCSI-2 CDROM to a SCSI-3 host adapter?
Can I connect a Narrow SCSI2 disk to a WIDE SCSI3 host adapter?
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Questions of this nature really cannot be answered in a useful way. There are so many aspects and options to each of the SCSI standards, you need to be much more specific about what devices and adapters you're interested in connecting. Most of the time the best thing to do is just try it! Most combinations will work, but if you're considering a purchase and looking for a guarantee from "The Net", forget it.
The issue is further complicated by the fact that vendors like to latch onto the latest acronyms before they even know what's involved. For example SCSI3 is not approved yet, but vendors are already saying their devices are SCSI3 compatible. Since there is no standards compliance testing organization, they can pretty much say what they want.
If you buy a high end host adapter (probably called SCSI3 :-) ) from a reputable vendor, and it has enough control over the various options (like synch xfer rate 5,10,20,40 mega-xfers/sec and the ability to disable WIDE or FAST/Ultra negotiation), and you carefully think out what devices you connect to it (all WIDE devices nearest the host adapter end of the bus etc.), and you are careful to properly terminate not only both ends, but both halves (upper byte and lower byte) of the bus, and none of the older devices you might already have (like a Panasonic CDROM) do anything stupid (like not handle the WIDE negotiation message without hanging) then it will all work fine. :-)
Even though a host adapter may be called SCSI3 doesn't mean it can enable or disable each optional feature, yet this is vital for supporting older devices.
To make matters worse, you won't know which older devices do some of the stupid things unless you know someone who's been bitten already. Your best bet is to look for good deals on name brand devices and adapters and before you buy, ask in comp.periphs.scsi whether anyone has tried the combination you're considering. It's also important to buy from a well known vendor with reasonable return policies.
If you're looking at buying a Vendorxyz spiffydisk which claims to be SCSI-3 compatible and you have a Seagate ST-01 host adapter and you want to know if anyone else has tried this combination, then that's exactly what you should ask.
In general, most SCSI devices and adapters made less than 4 years apart will probably work together, but without specific information about exactly which devices there's no assurance of it. There's also the potential for poor performance even if it does work.
ANSWER FROM: Gary Field (scsifaq@bigfoot.com)
Updated: June, 2000
Yes, you just need an appropriate adapter. Most WIDE devices use the
68 pin "P" connector so you need a 68 pin to 50 pin adapter.
You do need to make sure that both the upper byte and lower byte of
the bus will be properly terminated though. Some adapters
provide Hi-9 terminators, others do not. If the wiring adapter
is placed right at the SCSI host adapter, you can usually configure the
host adapter's on-board terminators to only terminate the high byte.
You need to be clear on what type of connectors are present
where you want to do the conversion. You also need to plan your bus
so that there won't be any narrow cable between any of the
WIDE devices.
Certain host adapters with auto-termination make the assumption that
when the low byte is terminated the high byte is also. When
using WIDE/narrow adapters this assumption is not valid. Another purpose
served by the hi-9 terminator is supplying pull-up
current to the upper data lines which would otherwise be left floating.
Special note for LVD drives:
It is recommended that if you connect a WIDE LVD drive to a narrow
bus that you use a 68 to 50 pin adapter which has high byte
termination. It may seem that the termination wouldn't be needed
in this case because the bus is narrow. However, the drive needs to
have those signals "pulled up" (logically negated), to avoid the
floating signals from confusing it.
If for some reason you attach a WIDE device to a WIDE host adapter using
a narrow cable, you must be sure to disable WIDE
negotiation in the host adapter BIOS or the device will hang when it
is accessed.
One further caveat is that if narrow devices are attached to a WIDE
adapter, the adapter's ID must be between 0 and 7 because
narrow devices would not be able to see it if the ID was any higher
than 7.
WIDE to NARROW adapters are available from:
819 Striker Ave, Suite 6
Sacramento, CA 95834
Technical Cable Concepts
1790 E. McFadden Ave., Unit 103/104
Santa Ana, CA 92705
TEL: (714) 835-1081
FAX: (714) 835-1595
MegaHaus
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
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Narrow SCSI devices can only use IDs 0 through 7. WIDE SCSI devices on a SCSI-3 system with 68 pin P cables, can use IDs 0 through 15. It is generally wise to reserve 0-7 for narrow devices though. SCSI-2 only specified the use of IDs 0-7 even for WIDE devices, but SCSI-3 allows 0-15 for WIDE devices. All devices on one bus must have unique IDs of course.
The arbitration priorities are as follows:
highest
ID 7
...
ID 0
ID 15
...
ID 8
ID 23
...
ID 16
ID 31
...
ID 24
lowest
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.
ANSWER From: Roberto Waltman (rwaltman@bellatlantic.net)
Let me start from scratch and describe one by one all necessary steps:
ANSWER From: Gary Field (scsifaq@bigfoot.com)
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:
Once you get the system booted, select drive J (the CDROM), and run SETUP.EXE
ANSWER From: Gary Field (scsifaq@bigfoot.com)
The retail version of Windows 95 is limited to 2 GB per partition by the use of the FAT16 filesystem. Since you're getting more than 2 GB, you must be using a FAT32 filesystem.
Using FAT32 with drives larger than 8 GB requires a host adapter that supports the "INT 13 extensions". If your host adapter was built before about 1996, you may not have this feature. For example Adaptec 2940W Host adapters did not support this. Even the early 2940UW didn't have it. As of BIOS ver. 1.2x the support is present. Check with your host adapter manufacturer for an updated BIOS.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Microsoft uses the following strange algorithm to map drive letters to disk partitions:
Look at the partition tables on BOTH PHYSICAL DISKS.
First PRIMARY partition becomes Drive C:
If there is a second PRIMARY partition it becomes Drive D:
If there are no more PRIMARY partitions, look for EXTENDED partitions.
The first logical drive in the first EXTENDED partition becomes drive E:, the next logical drive in that EXTENDED partition becomes F: etc.
If there is another EXTENDED partition, the first logical drive in it becomes drive G:, the next logical drive in it becomes drive H: etc.
Then, as device drivers are loaded, any disk drives they support will be assigned consecutive drive letters. CDROMs and other "Network drives" can be assigned specific drive letters if desired, leaving holes in the lettering scheme.
Under MSDOS, Win 3.x and Windows 95 this behavior can't be changed. Under Windows NT all drive letters can be re-mapped to whatever you want.
If you don't want D: on your second disk, don't create a PRIMARY partition on it!
An example of this mapping which often confuses newbies:
INT 13h Drive 0x80:
PRIMARY -> C:
EXTENDED
Logical -> E:
Logical -> F:
Int 13h Drive 0x81:
PRIMARY -> D:
EXTENDED
Logical -> G:
Logical -> H:
CDROM( SCSI ID=5 ):
Assigned to drive M:
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: September 1998
Each platter in a disk drive is organized as tracks and sectors. Each sector contains header and trailer information as well as error detection (CRC) data in addition to the actual user data field.
When a disk is manufactured the platters are blank (no sector layout). Before shipping, a special command (usually not documented) is issued to the drive to cause it to lay down the sector headers, blank data fields and good CRC. Also many data patterns may be written to each sector to check for media errors. Any sectors with errors are put into the "manufacturer's defect list" and the drive remembers not to use those sectors in the future. Later, after the drive is shipped, a user may decide to "Low Level Format" the drive if he is having problems, or wants to start with a "clean slate". This is done using the SCSI FORMAT command via a special utility usually supplied by the host adapter manufacturer (usually in the on-board BIOS).
Some side effects of doing a LLF:
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.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: January 1999
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: January 1999
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: May, 1999
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: January 2000
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: January 2000
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.
ANSWER From: Gary Field (scsifaq@bigfoot.com)
Date: July, 2000