Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!nosc!ucsd!ucsdhub!hp-sdd!hplabs!hpl-opus!hpccc!hp-sde!hpfcdc!hpfclm!kinsell
From: kinsell@hpfclm.HP.COM (Dave Kinsell)
Newsgroups: comp.periphs
Subject: Re: Questions on SCSI device interfacing...
Message-ID: <1600001@hpfclm.HP.COM>
Date: 24 Jul 88 18:35:22 GMT
References: <433@icus.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 34

># A raw SCSI drive usually has a 50-pin header connection.  The standard SCSI
># connection is a 50-pin Centronics.  The Macintosh folks use a DB-25.  Don't
># ask me to explain, but it works.
>
>The 50-pin ribbon cables have ground on every other pin, which is common.  So
>there are only 25 pins worth keeping if you don't care about horizontal
>shielding between each pair of pins.

  Single-ended SCSI, which is what we're talking about here, has 9 data
  lines, 9 control lines, and a line for terminator power.  Differential
  SCSI uses some of the single-ended ground lines as opposite polarity
  signals.

  Apple sells some "SCSI" (quote/unquote) extension cables with two 50
  pin connectors, but leaving out most of the "unnecessary" ground
  lines.  Differential SCSI shifts the pin definitions by one place, and
  one of the handshake lines gets shifted into one of the unconnected
  positions.  Works really great.


>The Apple DB25 setup has a shield around all 25 pins, which is connected, at
>the Centronics-to-DB25 connection, to the bus-bar of 25 grounds found in the
>50-pin ribbon cable.
>
>It sounds flinky but it works.
 
  I suppose you could make lamp cord work, if you really wanted to, but many
  people are interested in standardization of such things.

   
>Paul Vixie

-Dave Kinsell
 kinsell@hpfclm.HP.COM
 ...!hplabs!hpfclm!kinsell