Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!mips!apple!mattd
From: mattd@Apple.COM (Matt Deatherage)
Newsgroups: comp.sys.apple2
Subject: Re: Sharing a SCSI HD
Message-ID: <52434@apple.Apple.COM>
Date: 4 May 91 01:59:00 GMT
References: <74719@eerie.acsu.Buffalo.EDU> <15350@darkstar.ucsc.edu>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 37

In article <15350@darkstar.ucsc.edu> unknown@cats.ucsc.edu (Matt Ackeret) writes:
>>	Anyway, can a mac and an Apple IIgs share the same HD?
>
>	Apparently it can be done, but Apple won't tell us how... heh...
>
>	actually, I've heard it can be done, but it's never really been
>documented.. (I was told this by someone who used to work at Apple)

This always turns into a bloody holy war because everyone (except me) thinks
they know a whole lot about SCSI.  I don't pretend to, but I know my way
around our systems, so let me give it a shot.

The SCSI specs say that SCSI implementations may or may not have more than
one master on the SCSI bus.  Apple chooses not to.  This makes the cost of
some of the SCSI stuff lower since we don't have to include pounds of
SCSI bus arbitration software/hardware/voodoo magic.  This is perfectly legal
within the SCSI spec but really annoys people who want to hook multiple CPUs
to one drive, so they often cite the "you can do this" clause and say Apple
is not following the SCSI spec because we don't do that.  Read it more
carefully, prospective flamers.

So, if you try this, you'll run into bus arbitration problems whenever more
than one of the masters tries to access the drive at the same time.  This is
similar to the old CMS SCSI card (which did kind of support this) problem, which
they solved by telling users to only let one CPU write to any given partition,
and they did hard-partitioning things.  I think.  Remember, I'm not a SCSI
expert and don't play one on TV.

At any rate, you might be able to pull it off for a short period of time, but
unless you always exact extreme care something will eventually go boom.

-- 
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Matt Deatherage, Developer Technical  | The opinions expressed herein are
Support, Apple Computer, Inc.         | not those of Apple Computer, and
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