Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!jarthur!nntp-server.caltech.edu!toddpw
From: toddpw@nntp-server.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: Sharing a SCSI HD
Message-ID: <1991May4.064334.3019@nntp-server.caltech.edu>
Date: 4 May 91 06:43:34 GMT
References: <74719@eerie.acsu.Buffalo.EDU> <15350@darkstar.ucsc.edu> <52434@apple.Apple.COM>
Organization: California Institute of Technology, Pasadena
Lines: 28

mattd@Apple.COM (Matt Deatherage) writes:

No offense, Matt, but I've been wrong on this more often then you :) ...
Here's the Scoop.

>The SCSI specs say that SCSI implementations may or may not have more than
>one master on the SCSI bus.  Apple chooses not to.

Actually, Apple chose not to do it in the Mac SCSI manager ROMs. It is possible
for Mac SCSI drivers to implement arbitration, although most don't. I had that
vehemently pointed out to me by someone who writes those drivers.

The DMA SCSI card ROMs, however, are totally ANSI compliant, including support
for full arbitration. This I learned from Matt Gulick himself face-to-face.

>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 correct. The only way to get around it is to have ALL the bus masters
support arbitration.

So if you want to share a SCSI drive, make sure you have a DMA SCSI and a
Mac with good SCSI drivers installed. Unfortunately, the Mac HD vendors who
write drivers that support arbitration aren't being very vocal about it. The
Mac magazines don't seem to include it in their reviews, either.

Todd Whitesel
toddpw @ tybalt.caltech.edu