Subject: comp.periphs.scsi FAQ (TOC & Volume I)

From: [email-masked] (Gary Field)
Date: Thu, 2 Dec 1993 16:00:48 GMT

SCSI FAQ:
	   Frequently Asked Questions for comp.periphs.scsi

FAQ history:	Created by Johnathan Vail ([email-masked]) from articles
		submitted to him by comp.periph.scsi readers.
		Maintained by Jonathan Vail until November 1993.

Current Editor: Gary Field ([email-masked])

Table of contents:

Volume I
    Is it possible for two computers to access the same SCSI disks?
    What is the problem with the Adaptec 1542C and external cables?
    Where can I get SCSICNTL.EXE and other Adaptec files?
    What is the difference between the Adaptec 1542A and 1542B?
    What kinds of Optical Drives are available?
    Where can I get FTP/download SCSI documents?
    Where can I get SCSI documents?
    Where can I get information on various disk drives and controllers?
    What is the telephone number of Adaptec?
    What is the telephone number of Archive Corporation?
    What is the address and telephone number for Quantum?
    What is the telephone number for Seagate?
    What is the telephone number and address of Conner Peripherals?
    What is the number for NCR?
    What is the address and telephone number of UltraStor?
    What is the address and telephone number of Wangtek?
    What is the address and telephone number of Western Digital?
    What is FAST SCSI?
    SCSI terminators should measure 136 ohms?
    Can someone explain the difference between 'normal' and differential scsi?
    What are the pinouts for differential SCSI?
    What are the pinouts for SCSI connectors?

Volume II
    What is the difference between SCSI-1 and SCSI-2?
    Is SYNCHRONOUS faster than ASYNCHRONOUS?
    Is the 53C90 Faster than spec?
    What are the jumpers on my Conner drive?
    What are the jumpers for my Wangtek 5150 drive?
    What is CAM?
    What is FPT (Termination)?
    What is Active Termination?
    Why Is Active Termination Better?
    Why is SCSI more expensive than IDE?

====
QUESTION: Is it possible for two computers to access the same SCSI disks?
ANSWER From: [email-masked] (Michael Burke)
====

Yes, two (or more) systems can be on the same scsi bus as scsi disk and
tape drives. As long as the scsi requirements are met - cable lengths,
termination and type - the devices can share the scsi bus.

The question should be - Are there any O/S' that will allow the sharing of
file systems? It would not make sense for two hosts to go about treating
shared disks as if they owned the device. Data would be destroyed pretty
quickly.

On the issue of tape devices, however, O/S' tend to give exclusive usage
to an application. In this way, tape drives can be shared much more easily.

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

====
QUESTION: What is the problem with the Adaptec 1542C and external cables?
ANSWER From: Scot Stelter, Adaptec  (Product Manager for the AHA-1540)
====

Several articles lately have cited the importance of SCSI-2-compliant
cables when cabling SCSI bus subsystems.  Perhaps the most accurate
and technically detailed one was published in Computer Technology
Review in March (Volume XIII, No. 3. PP. 6).  In short, it explains
the double-clocking mechanism that can occur due to cables whose
impedance falls below the 90-Ohm SCSI-2 spec.  Steep edge speeds on
the REQ and ACK lines of the SCSI bus exacerbate the problem, but
non-compliant cables are the root cause. Both LAN TIMES in the US
(5/24/93, page 115) and CT Magazine in Germany (7/93, page 18) cite
this cable problem.

In an extensive survey of cables available in the US and Europe, we
found that more than half of the cables available have single-ended
impedances in the 65 to 80 Ohm range -- below the 90 to 132 Ohms
specified in the SCSI-2 spec.  It seems that some (not all) cable
vendors do not understand the specification, describing their cables
as SCSI-2 compliant when they are not.  A common misconception is that
SCSI-2 means a high-density connector.  In fact, there are several
connector options.  I have published a technical bulletin that
summarizes the critical requirements (TB 001, April 1993). An artifact
of its faster design left the AHA-1540C with faster edge-speeds than
its predecessor, the AHA-1540B.  As I have said, this can exacerbate
the effect of bad cables.  This explains why some users could get
their AHA-1540B to work when an early AHA-1540C might not.
Essentially, the 1540B was more forgiving than the early 1540Cs.  Good
cables fixed the problem, but unfortunately for the user, good cables
are hard to find.

After surveying the cable market and many of our customers, we decided
that bad cables were going to be here for a while, and we had to make
the 1540C as forgiving as the 1540B was.  At the end of April we made
a change to the AHA-1540C that involved using a passive filter to
reduce the slew rate of the ACK line, the signal that the host adapter
drives during normal data transfers.  Extensive testing with many
intentionally illegal configurations confirms that we succeeded. Prior
to release, we tested the AHA-1540C with over 200 peripherals, systems
and demanding software programs with no failures.  Then, a second team
retested the AHA-1540C across a wild combination of temperatures,
humidities and other stresses.  This testing gives me confidence that
the AHA-1540 line continues to serve as the gold standard for SCSI
compatibility.

====
QUESTION: What is the difference between the Adaptec 1542A and 1542B?
ANSWER From: [email-masked] (Harvey Fishman)
====

The AHA-1542A is obsolete and no longer supported by Adaptec.  They
stopped providing firmware upgrades at some level prior to the equivalence
to the 3.10 level of the AHA-1542B firmware.  I am not sure just where
though.  The present latest AHA-1542B firmware is version 3.20, and
supports drives up to 8GB under MS-DOS.

====
QUESTION: Where can I get SCSICNTL.EXE and other Adaptec files?
ANSWER From: [email-masked] (Randy Bush)
        and  Timothy Hu [email-masked]
====

New files from Roy as follows:

ftp.psg.com:~/pub/adaptec/...

-rw-rw-r--  1 randy    staff      110689 Feb 25 00:29 SCSICNTL.EXE.Z
-rw-rw-r--  1 randy    staff      368640 Feb 25 00:27 adse.dd
-rw-rw-r--  1 randy    staff        1959 Feb 25 00:25 adse.dd.readme
-rw-rw-r--  1 randy    staff       17896 Feb 25 00:37 list
-rw-rw-r--  1 randy    staff       99545 Feb 25 00:20 os2drv.zip
-rw-rw-r--  1 randy    staff       70801 Feb 25 00:20 scsi_drv.Z
-rw-rw-r--  1 randy    staff       66508 Feb 25 00:24 scsi_drv.readm
-rw-rw-r--  1 randy    staff      118697 Feb 25 00:17 update.pkg.Z

You can get the ASPI specs from Adaptec's Bulletin Board (408)945-7727.

====
QUESTION: What kinds of Optical Drives are available?
ANSWER From: [email-masked] (John Kim)
====

As I promised I am posting the summary of what I learned about 128mb optical
drives through many kind replies and some effort on my part. The purpose of
this informal survey was to aid people (starting from myself) in deciding on
which 128mb optical drive to buy.

When I posted my questions, it was done only on comp.mac.sys.hardware and
forgot to do the same also on comp.arch.storage and comp.periphs.scsi where
are less traffic than c.m.s.h. However, as a Macintosh owner myself, this
survey was biased toward the Mac world and the mail order houses mentioned
specializes in Mac-related products, although the below mentioned optical
drives might be usable also with non-Mac platforms (Sun, NeXT,
PC-compatibles).

My questions were:

o what kind of drive you bought from whom at what price 
o what drive mechanism (MOST, Epson, Fujitsu, Sony, ...) it uses
o how fast it is in terms of average seek time & data transfer rate
o how noisy the drive is
o how large and heavy the drive is
o what drive formatting program (eg, FWB or Silver Lining) you use what its
  goods/bads 
o the quality of the service of the seller (mail order company, retail store,
  etc.) 

Summary

In general, these days, some magneto-optical (MO) drives seem to be
almost as fast as (if not faster than) ordinary hard drives (HD). The
access time of fastest 128MB MO drives (around 30ms) are slower than
average HD's access time (15ms) but the transfer rate seems to be
about the same (764KBytes/sec) or not much slower. The advantages of
the MO drives over the HDs are that your storage space is almost
limitless, expandable at a relatively cheap price ($40/120MB = 34
cents/MB) compared to $1/MB rate of HDs or that of SyQuest drives, and
the life of the media is very long (they say it's 30 years or
rewritable 100,000 times.)

Fijitsu 128 REM Portable: At this moment, to my knowledge, 128 MByte
optical drives based on Fujitsu mechanism seems to be the fastest,
roughly having average seek time of 30ms and average transfer rate of
768KBytes/sec. Another good thing about this Fujitsu drive is that it
is more compact in size than previous 128mb optical drives, ie,
"portable". I don't know how Fujitsu mechanism (FM) is different from
Epson mechanism (EM) and how FM provides a similar performance at a
cheaper price in a smaller frame. Maybe using split-head
implementation to make the read-write head lighter? Could anybody post
info on this? One person tells me that the eject mechanism is too
strong, sometimes shooting the cartridge out making them land on the
floor. He says Fujitsu told him that the FM's coming out in April will
have gentler eject.

Epson: The next fastest (or maybe just about the same speed) are Epson
mechanism (EM) drives, having average access time of 34ms and transfer rate of
768KB/sec.  These achieve faster speed compared to other old mechanisms by
having a higher rpm (3600rpm vs. past 2400 rpm).

Slower ones: Other mechanisms (Sony, Panasonic, etc.) seem to have
been dominating the optical drive market before FM and EM's advent.
These have a typical access rate of ~45ms.  I don't know if now there
are new implementations that make them perform better then FM and EM.
Maybe someone can tell us.

Noise Level: One thing to consider might be noise of the drive. Different
mechanisms may have typical noise level, but one thing sure is that different
resellers/companys' drive's noise level differs even for the same drive
mechanism, eg, Sony. It looks like different casing produces different noise
levels? (Could someone confirm/disconfirm this aspect?)  Base on the report
in Nov '92 issue of MacWorld, the noise level of MacDirect, MacProduct and
DGR 128mb MO drives seem to be OK or quite quiet.

This issue of MacWorld deals with removable media drives (optical drives of
various capacity, SyQuest, Bernoulli and Flopticals) and you can get some
idea on what the differences among different drive mechanisms are.

Formatting Software: Another thing to consider is what kind of media
formatting software you will use. All companys (or mail order places) seemed
to provide for free formatting program with their drives. I don't have the
details on this. But an inefficient formatting can result in slow drive
performance. The most popular one used to be FWB's Hard Disk Toolkit but
Anubis (advertised to improve performance up to 35% [compared to what?]) is
beginning to be used also. I don't know if all formatting program and the
drive hardware allows to have read and write verify off but by having these
turned off you can obtain significant speed boost at the risk of less secure
data transfer. MacWorld's report warned that drives from some companys don't
let you turn on/off the verify. In the worst case, some come with verify off
and no option to toggle it back to ON.

256mb MO drives: In general these have better transfer rate (1.23MB/sec) and
a little slower access time (35ms). I feel that this capacity will soon be
the next standard. These drives are able to also read/write 128mb cartridges
and 256mb will soon be new ANSI and ISO standard. I once heard from a
saleperson at a mail order place that these are not reliable yet and he saw
many they sold came back with complaints. This may be a non-general instance
on a typical drive mechanism (seems to be MOST mechanism). Personally, I feel
128mb is accomodating enough for personal usage at home unless you are
dealing with very large data files (eg, large graphic images).

====
QUESTION: Where can I get FTP/download SCSI documents?
ANSWER From: [email-masked] (News Administator)
====

Last Changed:	Thu Sep 24 23:31:09 CDT 1992 (New BBS Phone number)

This is a periodic posting of information about some of the archives at
ftp.cs.tulane.edu and the available files from the SCSI-BBS, including
SCSI, ESDI, IPI, and Fiber Channel documents from the standards committees.

These files are available for FTP from ftp.cs.tulane.edu in the directory
pub/scsi. Files are stored in file areas as they are found in the BBS with
each area having a file named 'files.bbs' that tells what each file is. The
file pub/scsi/index.Z list each file area, it's descriptions and it's files.

Thanks to John Lohmeyer of NCR, a majority of the SCSI related files from the
SCSI BBS are now available for anonymous ftp. These files were sent to me by
Mr. Lohmeyer at his expense so that more people would have access to them.
The SCSI BBS (719-574-0424) contains a large amount of data relating to SCSI,
and ESDI as well as SCSI-2, IPI, and Fiber Channel, as well as the last
revision of the SCSI-1 standard before it went to publication by ANSI.

Most of the files in the SCSI archive are either archived with the ZIP utility
or compressed with the 'compress' program. Most of the text files are stored
as Wordstar word processing files. PKzip for PC/MS-DOS is included in the 
archive to allow users to break up the .ZIP files, and the PC/MS-DOS binaries
and .C source are also in the archive to convert the Wordstar documents to
ASCII text.

====
QUESTION: Where can I get SCSI documents?
ANSWER From: [email-masked] (Kevin Jones)
         and [email-masked] (John Matrow
====

The only literature that I'm aware of is:                  

  The SCSI specification: Available from:

      Global Engineering Documents
      15 Inverness Way East
      Englewood Co  80112-5704
      (800) 854-7179
	SCSI-1: X3.131-1986
	SCSI-2: X3.131-199x
	SCSI-3 X3T9.2/91-010R4 Working Draft

(Global Engineering Documentation in Irvine, CA (714)261-1455??)

SCSI-1: Doc # X3.131-1986 from ANSI, 1430 Broadway, NY, NY 10018

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

====
QUESTION: Where can I get information on various disk drives and controllers?
ANSWER: [email-masked] (Eric Krieger) (Updated Nov. 29, 1993)
====

        Drive and Controller Guide,  Version 4.3

   THEREF(tm) is a comprehensive Directory of Hard Drives, Floppy Drives,
Optical Drives, and Drive Controllers & Host Adapters.  It is designed to
help the novice and pro alike with integration problems and system setups.

   Information is provided in two handy formats; Portrait mode, for those
who prefer a normal book-binding type print format, and(or) do not have a
printer with Landscape capability.  And Landscape mode, for those who pre-
fer a computer-printout type format.

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

By F. Robert Falbo

The archive is available at the following sites, all members of AmiNet
(list valid in November '93):

USA (MO)     ftp.wustl.edu           128.252.135.4   pub/aminet/    
USA (TX)     ftp.etsu.edu            192.43.199.20   pub/aminet/    
USA (CA)     ftp.cdrom.com           192.153.46.2    pub/aminet/    
Scandinavia  ftp.luth.se             130.240.18.2    pub/aminet/    
Germany      ftp.uni-kl.de           131.246.9.95    pub/aminet/    
Germany      ftp.uni-erlangen.de     131.188.1.43    pub/aminet/    
Germany      ftp.cs.tu-berlin.de     130.149.17.7    pub/aminet/    
Germany      ftp.th-darmstadt.de     130.83.55.75    pub/aminet/    
Germany      ftp.uni-paderborn.de    131.234.2.32    pub/aminet/    
Germany      ftp.uni-oldenburg.de    134.106.40.9    pub/aminet/
Switzerland  ftp.eunet.ch            changing        pub/aminet/    
Switzerland  litamiga.epfl.ch        128.178.151.32  pub/aminet/ (*)
UK           src.doc.ic.ac.uk        146.169.2.1     pub/aminet/    
Australia    splat.aarnet.edu.au     192.107.107.6   pub/aminet/ (*)
   (*) closed 6:30am to 4pm weekdays

The archive is present at:

pub/aminet/text/docs/TheRef43.lha    [197 K]
pub/aminet/text/docs/TheRef43.readme

Note:  When extracting the archive on non-Amiga machines (i.e.
       MS-DOS with LHA213, etc.) please use the option -a0 to disable
       the preservation of the file-attributes (otherwise LHA will
       skip the files inside the archive with the message "special
       attributes").

The file is also on ftp.funet.fi:

pub/doc/disks/TheRef43.lha.2         [197 K]
pub/doc/disks/TheRef43.readme.2

====
QUESTION: What is the telephone number of Adaptec?
ANSWER From: [email-masked] (Jon D Caples)
====

	800 959 7274	tech support
	800 442 7274	orders doc, new bios, etc.

	Adaptec's general inquiry number, 800-959-7272, affords access
	to a FAX-based information retrieval system.  In order to
	preserve the accuracy of this information, I won't go into details
	about how to use it (since Adaptec may change things without telling 
	me :).

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

	There are three general topics as of this writing:

		General Information 
		Sales Information 
		Technical Information

	Give it a call and request the directory!  As of this writing
	there are over 130 documents available.  You need a touchtone phone
	and the fax number.  You'll also be asked for an extension number to 
	stamp on the FAX which will be used to identify the recipient.

====
QUESTION: What is the telephone number of Archive Corporation?
ANSWER From: [email-masked] (Jim Pritchett)
====
Archive Corporation          (800) 537 2248
     Tech Support            (800) 227 6296

====
QUESTION: What is the address and telephone number for Quantum?
ANSWER From: [email-masked] (Kevin Martinez)
====

Quantum Corporation
500 McCarthy Blvd.
Milpitas, CA
95035

Technical Support Telephone Numbers:

800 826-8022   Main Technical Support Number
408 894-3282   Technical Support Fax
408 894-3214   Technical Support BBS V.32 8N1
408 434-9262   Technical Support for Plus Development Products
408 894-4000   Main Quantum Phone number

====
QUESTION: What is the telephone number for Seagate?
ANSWER From: [email-masked] (Hale Landis)
====

Here are the numbers for Seagate's Technical Support.

SeaBOARD - Bulletin Board System available 24 hours.  Use 8 data
           bits, no parity, 1 stop bit (8-N-1).

   USA/Canada  408-438-8771    9600 baud*
   England     44-62-847-8011  9600 baud*
   Germany     49-89-140-9331  2400 baud*
   Singapore   65-292-6973     9600 baud*
   Australia   61-2-756-2359   9600 baud*

   * - Maximum baud rate supported.

SeaFAX 408-438-2620

   Use a touch-tone phone to have information returned to you via
   FAX.  Available 24 hours.

Technical Support Fax 408-438-8137

   FAX your questions or comments 24 hours.  Responses are sent
   between 8:00AM and 5:00PM PST Monday through Friday.

SeaFONE 408-438-8222

   Provides recorded information 24 hours or talk to a technical
   specialist between 8:00AM to 5:00PM PST Monday through Friday.

SeaTDD 408-438-5382

   Using a Telecommunications Device for the Deaf, you can send
   questions or comments 24 hours or have a dialog with a
   technical support specialist between 8:00AM and 5:00PM PST
   Monday through Friday.

====
QUESTION: What is the telephone number and address of Conner Peripherals?
ANSWER From: [email-masked] (Eric Krieger)
====

CONNER PERIPHERALS, Incorporated        WATTS LINE:
3081 Zanker Road                          PAY LINE: (408)456-4500
San Jose   CA  95134                      FAX LINE:
                                          BBS LINE: (408)456-4415 (V.32)
CONNER                                                  (408)456-3200

====
QUESTION: What is the number for NCR?
ANSWER From: [email-masked] (Guy Kendall)
====

For data manuals for any NCR chips, please call 800-334-5454 or
719-630-3384.

====
QUESTION: What is the address and telephone number of UltraStor?
====

     UltraStor Corporation
     47061 Warm Springs Blvd.
     Fremont, CA 94539

     General   	    (714)581-4100
     Tech. Support  (510)623-8955
     FAX       	    (510)623-8953
     BBS       	    (510)623-9091

====
QUESTION: What is the address and telephone number of WANGTEK?
ANSWER From: "Terry Kennedy, Operations Mgr" <uunet!spcvxa.spc.edu!TERRY>
====

Wangtek can be reached at:

  WANGTEK Incorporated
  41 Moreland Road
  Simi Valley, CA 93065
  (805) 583-5255 [voice]
  (805) 583-8249 [FAX]
  (805) 582-3370 [BBS]

  WANGTEK-Europe
  Unit 1A, Apollo House
  Calleva Industrial Park
  Aldermaston, Reading
  RG7 4QW England
  (44) 734-811463 [voice]
  (44) 734-816076 [FAX]
  851-848135 [telex]

====
QUESTION: What is the address and telephone number of Western Digital?
ANSWER From: uunet!whale.st.usm.edu!rniess (Rick Niess)
====

    1-714-753-1068  Western Digital
    1-714-756-8176  Western Digital
    1-714-753-1234  Western Digital
    1200/2400 baud:    714-753-1234
    9600 baud (Hayes): 714-753-1068

====
QUESTION: what is FAST SCSI?
ANSWER From: [email-masked] (Kevin Jones)
====

There are 2 handshaking modes on the SCSI bus, used for transferring data:
ASYNCHRONOUS and SYNCHRONOUS. ASYNCHRONOUS is a classic Req/Ack handshake.
SYNCHRONOUS is "sort of" Req/Ack, only it allows you to issue multiple 
Req's before receiving Ack's. What this means in practice is that 
SYNCHRONOUS transfers are approx 3 times faster than ASYNCHRONOUS.

SCSI1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec and 
synchronous transfers at up to 5.0 Mbytes/Sec.

SCSI2 had some of the timing margins "shaved" in order that faster handshaking
could occur. The result is that asynchronous transfers can run at up to
3.0 Mbytes/Sec and synchronous transfers at up to 10.0 Mbytes/Sec. 
The term "FAST" is generally applied to a SCSI device which can do 
syncrhonous transfers at speeds in excess of 5.0 Mbytes/Sec. This term can
only be applied to SCSI2 devices since SCSI1 didn't have the timing margins
that allow for FAST transfers.

====
QUESTION: SCSI terminators should measure 136 ohms?
ANSWER From: [email-masked] (Steve Ligett)
====

Yes, that is what you should measure.  Let's see how that is so.  The
terminator contains 18 220-ohm resistors from signals to termpower, and
18 330-ohm resistors from those signals to ground.  I've drawn that
below:

termpower--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
220 ohms-> R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
signals -> o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
330 ohms-> R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ground   --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

When you measure from any one signal to termpower, you aren't measuring
that resistor in isolation, you are measuring that resistor IN PARALLEL
with the combination of the corresponding 330 ohm resistor plus 17
220+330 ohm resistor pairs in series.  I've redrawn the schematic to
make this easier to see:

termpower--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
           |  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R <- 220 ohms
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
           |  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
           |  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  R <- 330 ohms
  220 ohms R  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
           |  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- ground
           |  |
           |  R <-- 330 ohms
           |  /
signal ->  o-/

<our resistor><--------- other stuff that's in parallel ---------->

We're trying to measure that one resistor from a signal to termpower,
but there's a ton of other stuff in parallel.  The resistance of that
"stuff" is 330 + 550/17 ohms (the 330 ohm resistor, in series with a
parallel combination of 17 550 ohm resistors).  The general formula for
the equivalent of two resistances in parallel is r1*r2/(r1+r2).
Whipping out my trusty spreadsheet, I find that the "stuff" has a
resistance of about 362 ohms, and that in parallel with 220 ohms is
about 137 ohms.

====
QUESTION: Can someone explain to me the difference between 'normal' scsi
           and differential scsi?
ANSWER From: [email-masked] (Ralph Valentino)
====

"Normal" SCSI is also called "Single-ended" SCSI.  For each signal
that needs to be sent across the bus, there exists a wire to carry it.
With differential SCSI, for each signal that needs to be sent across
the bus, there exists a pair of wires to carry it.  The first in this
pair carries the same type of signal the single-ended SCSI carries.
The second in this pair, however, carries its logical inversion.  The
receiver takes the difference of the pair (thus the name
differential), which makes it less susceptible to noise and allows for
greater cable length.

====
QUESTION: What are the pinouts for differential SCSI?
ANSWER From: [email-masked] (Ralph Valentino)
====

                Differential SCSI Connector Pinouts

_____________________________________  _____________________________________
| SCSI  |         | MINI  |         |  | SCSI  |        | MINI  |          |
| SIGNAL| DD-50P  | MICRO | DD-50SA |  | SIGNAL| DD-50P | MICRO | DD-50SA  |
------------------------------------  -------------------------------------
| -GND  |    2    |  26   |   34    |  | (open)|    1    |   1   |    1    |
| -DB(0)|    4    |  27   |    2    |  | +DB(0)|    3    |   2   |   18    |
| -DB(1)|    6    |  28   |   19    |  | +DB(1)|    5    |   3   |   35    |
| -DB(2)|    8    |  29   |   36    |  | +DB(2)|    7    |   4   |    3    |
| -DB(3)|   10    |  30   |    4    |  | +DB(3)|    9    |   5   |   20    |
| -DB(4)|   12    |  31   |   21    |  | +DB(4)|   11    |   6   |   37    |
| -DB(5)|   14    |  32   |   38    |  | +DB(5)|   13    |   7   |    5    |
| -DB(6)|   16    |  33   |    6    |  | +DB(6)|   15    |   8   |   22    |
| -DB(7)|   18    |  34   |   23    |  | +DB(7)|   17    |   9   |   39    |
| -DB(P)|   20    |  35   |   40    |  | +DB(P)|   19    |  10   |    7    |
|  GND  |   22    |  36   |    8    |  |DIFSENS|   21    |  11   |   24    |
|  GND  |   24    |  37   |   25    |  |  GND  |   23    |  12   |   41    |
|TERMPWR|   26    |  38   |   42    |  |TERMPWR|   25    |  13   |    9    |
|  GND  |   28    |  39   |   10    |  |  GND  |   27    |  14   |   26    |
| -ATN  |   30    |  40   |   27    |  | +ATN  |   29    |  15   |   43    |
|  GND  |   32    |  41   |   44    |  |  GND  |   31    |  16   |   11    |
| -BSY  |   34    |  42   |   12    |  | +BSY  |   33    |  17   |   28    |
| -ACK  |   36    |  43   |   29    |  | +ACK  |   35    |  18   |   45    |
| -RST  |   38    |  44   |   46    |  | +RST  |   37    |  19   |   13    |
| -MSG  |   40    |  45   |   14    |  | +MSG  |   39    |  20   |   30    |
| -SEL  |   42    |  46   |   31    |  | +SEL  |   41    |  21   |   47    |
| -C/D  |   44    |  47   |   48    |  | +C/D  |   43    |  22   |   15    |
| -REQ  |   46    |  48   |   16    |  | +REQ  |   45    |  23   |   32    |
| -I/O  |   48    |  49   |   33    |  | +I/O  |   47    |  24   |   49    |
|  GND  |   50    |  50   |   50    |  |  GND  |   49    |  25   |   17    |
----------------------------------------------------------------------------

Please note that I can only verify the DD-50P connector.  The Mini
Micro and DD-50SA pinout above is a pin for pin mapping from the SCSI
pinout in the FAQ.

How to tell if you have a single ended or differential drive:
	- Use an ohm meter to check the resistance between pins 21 & 22.
	  On a single ended system, they should both be tied together 
          and tied to GND.  On the differential drive, they should
          be open or have a significant resistance between them.  Note
          that most drives today are single ended so you usually only
          have to worry about this with old drives scavenged from
          other systems.

====
QUESTION: What are the pinouts for SCSI connectors?
ANSWER From: [email-masked] (Bob Snively)
====

Originally dated May 23, 1990

The connector families described by the drawings have standard 
pin numberings which are described the same way by all vendors
that I have encountered.  The SCSI-2 specification identifies the
standard numbering, using that convention.  It happened to be
documented by AMP, but all the vendors use the same convention.

The following diagrams have the outline drawings of connector 
sockets at the bottom.  This is really for reference only, because
the connector sockets and plugs are both specified as to their
numbering and usually are labeled.

There are some minor problems in naming the microconnector conductor
pairs, which I have corrected in the enclosed diagram.  All the conductor
pairs of the Mini-Micro (High Density) connector are in fact passed
through on the cables.  SCSI-2 defines the RSR (Reserved) lines as
maybe ground or maybe open, but they are still passed through the cable.
Most present standard SCSI devices will ground those lines.

--------------------  microSCSI to SCSI Diagram   ---------------------------

                       SCSI Connector Pinouts

_____________________________________  _____________________________________
| SCSI  |         | MINI  |         |  | SCSI  |        | MINI  |          |
| SIGNAL| DD-50P  | MICRO | DD-50SA |  | SIGNAL| DD-50P | MICRO | DD-50SA  |
------------------------------------  -------------------------------------
| -DB(0)|    2    |  26   |   34    |  |  GND  |    1    |   1   |    1    |
| -DB(1)|    4    |  27   |    2    |  |  GND  |    3    |   2   |   18    |
| -DB(2)|    6    |  28   |   19    |  |  GND  |    5    |   3   |   35    |
| -DB(3)|    8    |  29   |   36    |  |  GND  |    7    |   4   |    3    |
| -DB(4)|   10    |  30   |    4    |  |  GND  |    9    |   5   |   20    |
| -DB(5)|   12    |  31   |   21    |  |  GND  |   11    |   6   |   37    |
| -DB(6)|   14    |  32   |   38    |  |  GND  |   13    |   7   |    5    |
| -DB(7)|   16    |  33   |    6    |  |  GND  |   15    |   8   |   22    |
| -DB(P)|   18    |  34   |   23    |  |  GND  |   17    |   9   |   39    |
|  GND  |   20    |  35   |   40    |  |  GND  |   19    |  10   |    7    |
|  GND  |   22    |  36   |    8    |  |  GND  |   21    |  11   |   24    |
|  RSR  |   24    |  37   |   25    |  |  RSR  |   23    |  12   |   41    |
|TERMPWR|   26    |  38   |   42    |  |  OPEN |   25    |  13   |    9    |
|  RSR  |   28    |  39   |   10    |  |  RSR  |   27    |  14   |   26    |
|  GND  |   30    |  40   |   27    |  |  GND  |   29    |  15   |   43    |
| -ATN  |   32    |  41   |   44    |  |  GND  |   31    |  16   |   11    |
|  GND  |   34    |  42   |   12    |  |  GND  |   33    |  17   |   28    |
|  BSY  |   36    |  43   |   29    |  |  GND  |   35    |  18   |   45    |
| -ACK  |   38    |  44   |   46    |  |  GND  |   37    |  19   |   13    |
| -RST  |   40    |  45   |   14    |  |  GND  |   39    |  20   |   30    |
| -MSG  |   42    |  46   |   31    |  |  GND  |   41    |  21   |   47    |
| -SEL  |   44    |  47   |   48    |  |  GND  |   43    |  22   |   15    |
| -C/D  |   46    |  48   |   16    |  |  GND  |   45    |  23   |   32    |
| -REQ  |   48    |  49   |   33    |  |  GND  |   47    |  24   |   49    |
| -I/O  |   50    |  50   |   50    |  |  GND  |   49    |  25   |   17    |
----------------------------------------------------------------------------
 
 * NC = NOT CONNECTED
 
 CONNECTOR TYPES:
                                  DD-50SA
                        ________________________            MINI-MICRO
       DD-50P          |   -------------------  |      _____________________
    ______________     |17 \o o o o o o o o o/1 |     |  _________________ |
 49| o o o o o o |1    | 33 \ o o o o o o o /18 |     |25\ o o o o o o o /1|
 50| o o o o o o |2    |  50 \o o o o o o o/ 34 |     | 50\o o o o o o o/26|
   ---------------     |      -------------     |     |   --------------   |
                       --------------------------     ----------------------
 
(VIEWED FROM FACE OF CONNECTOR -  USE VENDOR NUMBERING SYSTEM AS SPECIFIED)

--
--/*   Gary A. Field - WA1GRC, Wang Labs M/S 019-72B, 1 Industrial Ave      
   Lowell, MA 01851-5161,  (508) 967-2514, email: [email-masked], EST5EDT
                   A waist is a terrible thing to mind!			 */