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From: chris@com50.c2s.mn.org (Chris Johnson)
Newsgroups: comp.periphs.scsi,comp.sys.mac.hardware
Subject: Re: Pinouts for "standard" SCSI <--> Mac SCSI ??
Message-ID: <1990May25.194541.11462@com50.c2s.mn.org>
Date: 25 May 90 19:45:41 GMT
References: <3471@rodan.acs.syr.edu>
Organization: Com Squared Systems, Inc.
Lines: 49


Here's the standard single-ended SCSI cable pin assignments for
a non-shielded connector:

  SIGNAL        PIN NUMBER
  ------        ----------
  -DB(0)            2
  -DB(1)            4
  -DB(2)            6
  -DB(3)            8
  -DB(4)            10
  -DB(5)            12
  -DB(6)            14
  -DB(7)            16
  -DB(P)            18
  GROUND            20
  GROUND            22
  GROUND            24
  TERMPWR           26
  GROUND            28
  GROUND            30
  -ATN              32
  GROUND            34
  -BSY              36
  -ACK              38
  -RST              40
  -MSG              42
  -SEL              44
  -C/D              46
  -REQ              48
  -I/O              50

All odd pins except pin 25 are connected to ground.
Pin 25 is left open.

In fact, Pin 25 MUST be left unconnected, because if
the connector were plugged in upside down and Pin 25
were grounded, you'd short terminator power to ground.
And then the smoke escapes.

I'd suggest not scrimping on connecting the odd pin
grounds because single-ended SCSI is plenty sensitive
to noise even with all the grounding.  But I'm not an
expert in the area of analog/interference electronics
or signal skew or any other fun real-world transmission
media problems.  You're mileage will vary.
-- 
    ...Chris Johnson           chris@c2s.mn.org   ..bungia!com50!chris
  Com Squared Systems, Inc.    St. Paul, MN USA   +1 612 452 9522