UD182050@NDSUVM1.BITNET (Mike Aos) (08/05/90)
I'd prefer to have the standard pin-outs of the DB25 version, but I can
figure it out from the 50-pin version. Either way, I'd like to know what
the purpose of each wire is.
Thanks,
-------
Mike
UD182050@NDSUVM1 (.Bitnet?) "Share and Enjoy"
UD182050@VM1.NoDak.Edu -Sirius Cybernetics Corporation
Apple IIgs 'till I can afford a NeXT!
I got LOTSA opinions. You want one? You can have it!thad@cup.portal.com (Thad P Floryan) (08/06/90)
UD182050@NDSUVM1.BITNET (Mike Aos) in <4460UD182050@NDSUVM1> writes:
I'd prefer to have the standard pin-outs of the DB25 version, but I
can figure it out from the 50-pin version. Either way, I'd like to
know what the purpose of each wire is.
Purpose, NO; for that you get the SCSI specs either from the SCSI BBS in
Wichita, the mgse archive server, or the ANSI and/or IEEE.
If you are seeking the PINNINGS, that's a different matter, and I've enclosed
something I posted several years ago (hmmm, to the predecessor of this
newsgroup :-) that HAS been useful to many. John Lohmeyer (the Chair of the
ANSI SCSI Committee) commented favorably on the original posting and had some
comments about my comments regarding pin 26; his reply is on floppy (somewhere)
otherwise I'd enclose it, too.
The final arbiter IS the SCSI standards document; suggest you get a copy.
Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]
========================= begin included material =========================
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From: thad@cup.portal.com
Newsgroups: comp.periphs,comp.sys.att,unix-pc.general
Subject: Re: Questions on SCSI device interfacing...
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Date: 29 Aug 88 11:12:06 GMT
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Many systems supporting SCSI (Amiga, Mac, etc.) use a DB25 connector at
the host-adapter end to save real-estate; the SCSI devices themselves
use the "correct" 50-pin connectors (either Centronics 50-pin or the
dual-25 row kind).
The so-called "Mac System Cable" has a DB-25 on one end and a Centronics
50-pin connector on the other. SCSI interconnects are 50-wire.
At the end of this posting are the pinouts of the DB25 and the 50-wire.
One thing I've noticed (after adding over 1GB HD and a Fujitsu 190MB tape
drive to my Amiga's SCSI bus) is that some device manufacturers take liberties
with SCSI wires 20,22,24,28,30, and 34; some tie these to ground, some tie
them to +5VDC, and some leave them (properly) unconnected.
Another caveat with multiple devices on a given bus is to be SURE that only
one is supplying +5VDC on SCSI pin 26 for terminator power. Some mfrs provide
jumpers to enable/disable, some feed thru a diode, some feed the power out
"raw", and some do nothing.
What I do on my system is supply terminator power from the host adapter, and
use an external terminator at the END of the bus.
From my experience, what I suggest is that when you're making the cabling,
pull pins 20,22,24,26,28,30,34 from the connector that attaches to the embedded
SCSI drive and/or SCSI translator. I've noticed that Apple's SCSI interconnect
cables do NOT pass pins 20,22,24,28,30,34 (for the reason I noted above).
Another observation I'd like to make is that MANY of the 3rd party consumer-
grade mfrs of SCSI HDs wire their boxes INCORRECTLY. SCSI specs mandate no
more than a 10cm "stub" off the bus, yet many Mac-type drives I've opened up
(just gotta see what's in there! :-) have a 2" 50-wire connection joining the
two Centronics connectors continuing with a 15" stub to the 50-wire connector
on either an embedded SCSI drive or an Adapter 4000A or 4070 translator. This
mis-wiring would account for the problems people have running a SCSI bus out
to 20 feet (I have no problems since I wired my systems correctly, and the
last device is 19' from the host adapter). Remember: SCSI wiring is
supposed to be DAISY-CHAINED, per:
WRONG: CORRECT:
____________________ __________________
/ \ / \
[] | [] |
\ \_DISK \_DISK
/ /
[] [] |
\__________________/
The "[]" is a 50-pin Centronics connector
SCSI Connector (DB-25)
Pin Name
-------------
1 REQ
2 MSG
3 I/O
4 RST
5 ACK
6 BSY
7 GND
8 DB0
9 GND
10 DB3
11 DB5
12 DB6
13 DB7
14 GND
15 C/D
16 GND
17 ATN
18 GND
19 SEL
20 DBP
21 DB1
22 DB2
23 DB4
24 GND
25 N.C. (or) Terminator Power
SCSI Connector (50 pin header)
Pin Name Pin Name
-----------------------------------
1 GND 2 DB0
3 GND 4 DB1
5 GND 6 DB2
7 GND 8 DB3
9 GND 10 DB4
11 GND 12 DB5
13 GND 14 DB6
15 GND 16 DB7
17 GND 18 DBP
19 GND 20 GND <- caution: not always
21 GND 22 GND <- caution: not always
23 GND 24 GND <- caution: not always
25 N.C. 26 Terminator Power
27 GND 28 GND <- caution: not always
29 GND 30 GND <- caution: not always
31 GND 32 ATN
33 GND 34 GND <- caution: not always
35 GND 36 BSY
37 GND 38 ACK
39 GND 40 RST
41 GND 42 MSG
43 GND 44 SEL
45 GND 46 C/D
47 GND 48 REQ
49 GND 50 I/O
Thad Floryan [thad@cup.portal.com (or) ..!sun!portal!cup.portal.com!thad]urlichs@smurf.sub.org (Matthias Urlichs) (08/10/90)
In comp.periphs.scsi, article <32474@cup.portal.com>,
thad@cup.portal.com (Thad P Floryan) writes:
<
< Another caveat with multiple devices on a given bus is to be SURE that only
< one is supplying +5VDC on SCSI pin 26 for terminator power. Some mfrs provide
< jumpers to enable/disable, some feed thru a diode, some feed the power out
< "raw", and some do nothing.
<
Any drive which feeds out power raw (or which grounds pin 26 when powered
down) is severely broken. One wonders why a simple diode is so expensive. :-(
< What I do on my system is supply terminator power from the host adapter, and
< use an external terminator at the END of the bus.
<
If you do this (and possibly even if you don't), you should insure a clean
power supply for the terminator. The new terminator from Apple (easily
recognizeable because it's black; its use on the IIfx SCSI bus ranges from
strongly recommended to absolutely necessary) has, close to the terminating
resistors, both a 2.2 uF condensator and a 10 nF ceramic condensator to
prevent spikes on the -Req line when all data wires change simultaneously.
See Technical Note 273, which was recently posted to comp.binaries.mac.
< Another observation I'd like to make is that MANY of the 3rd party consumer-
< grade mfrs of SCSI HDs wire their boxes INCORRECTLY. SCSI specs mandate no
< more than a 10cm "stub" off the bus, [...]
Correct, unfortunately. Consider that most of these drives also have
terminators built-in, and in that case they're supposed to be at the end of
the chain anyway, in which case the above is perfectly legal. (Apple strongly
recommends against shipping drives with built-in terminators, partly because
of the confusion when installing the beasts, partly because of the reason
stated above -- these condensators are uniformly missing.)
BTW, if you do it "right", these two ribbon cables are going to be darn close
together. Apple's newer SCSI devices fold these tightly and lead them through
a largish ferrite ring. Does anyone know the reason for this? (It can't be to
minimize crosstalk because you have to fold the ribbon cables tightly together
to get them through the ferrite ring, or can it?)
--
Matthias Urlichs -- urlichs@smurf.sub.org -- urlichs@smurf.ira.uka.de
Humboldtstrasse 7 - 7500 Karlsruhe 1 - FRG -- +49+721+621127(Voice)/621227(PEP)tmhoff@uokmax.uucp (Thomas Milton Hoff) (08/10/90)
Aren't the scsi pinouts in the manual that came with the computer? (Care and Feeding of your Macintosh nn)
cyliax@ecn.purdue.edu (Ingo Cyliax) (08/10/90)
In article <vq_/e2.+01@smurf.sub.org>, urlichs@smurf.sub.org (Matthias Urlichs) writes: |> BTW, if you do it "right", these two ribbon cables are going to be darn close |> together. Apple's newer SCSI devices fold these tightly and lead them through |> a largish ferrite ring. Does anyone know the reason for this? (It can't be to |> minimize crosstalk because you have to fold the ribbon cables tightly together |> to get them through the ferrite ring, or can it?) |> It's to filter out any RF that maybe generated by the drive electronics. If you don't do this you send out all kinds of RF onto the cable, which acts like an antenna. This is OK in an office, but at home it may interfere with radio & TV reception. |> -- |> Matthias Urlichs -- urlichs@smurf.sub.org -- urlichs@smurf.ira.uka.de |> Humboldtstrasse 7 - 7500 Karlsruhe 1 - FRG -- +49+721+621127(Voice)/621227(PEP) -ingo -- /* Ingo Cyliax ECN, Electrical Engineering Bldg. * * cyliax@ecn.purdue.edu Purdue University, W. Lafayette,IN 47907 * * ing@cc.purdue.edu Work: (317) 494-9523 * * cyliax@pur-ee.UUCP Home: (317) 474-0031 */