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From: perry@key.COM (Perry The Cynic)
Newsgroups: comp.sys.mac.hardware
Subject: Sharing the SCSI bus?
Summary: Can a Mac do this?
Keywords: SCSI, Mac
Message-ID: <1240@key.COM>
Date: 17 Nov 89 22:36:23 GMT
Article-I.D.: key.1240
Reply-To: perry@arkon.key.COM (Perry The Cynic)
Organization: Key Computer Laboratories, Fremont
Lines: 54

Here's a rather oddball question for all you experts out here. It's not quite
idle curiosity, but it's an interesting question in any case.

The SCSI standard specifies that you can put up to eight devices on a
SCSI bus, as long as you observe limits on cable lengths etc. The standard
describes how ANY device can then initiate communications with ANY other
(not itself). In theory, your tape drive could talk to your disk to do a
backup, without your Mac needing more than to set things up initially. Of
course, nobody (to my knowledge) makes SCSI controllers intelligent enough
to actually do that (at least in the Mac area). But that's not my question.

On a conventional Mac SCSI bus, the Mac is device #7, and the only one that
initiates communications (talker). All the other devices just listen and
(we hope) obey. I am told that the Mac's SCSI manager is borderline brain-dead
in its restrictions. I haven't looked at it myself (yet); I'm hoping somebody
who has already suffered through that can enlighten me.

Let's say I have a CD-ROM reader with a SCSI interface. (Or a large hard
drive with partitions. Or a scanner. Or whatever.) Let's say I have a PC
compatible with a SCSI interface card. Can I make a SCSI bus containing
the Mac, my CD-ROM drive, AND the PC? Yeah, I know I can, physically. But
what's the chances for this to actually work? (We could try to do this with
two Macs, if Apple hadn't hardwired the Mac's SCSI controller as device #7.
Two devices with the same address is an obvious no-no-nooooh.) Let's say I
give the PC controller a free device address. 

First of all, I'm given to wonder how to terminate this thing. Assuming
both the Mac and PC interfaces are terminated, I could put them at the ends
of the SCSI bus and line up any peripherals in between (without terminators,
of course). Should that work? Does it?

As long as either the PC or the Mac is off, I guess I have a reasonable
chance for the "living" computer to control the devices on the SCSI bus.
Yes, I know file formats are different, and a Mac-formatted disk drive
won't work for the PC and vice versa, and the partition tables are probably
incompatible too. But a CD-ROM drive should work for either (they sell the
same drive with software kits for both Macs and PCs). Another candidate
could be a SyQuest 40MB removable drive.

The interesting question, of course, is whether it'll work if I turn on both
computers. Assuming I use the CD-ROM drive only from one host at a time,
what are my chances? Will the Mac's boot-time scan of the bus confuse it
if it sees a PC host card there? Can the Mac's SCSI chip and manager (esp.
the manager!) cope with a second talker on the bus? I.e., can the PC card
talk to the CD-ROM player while the Mac talks to a harddrive? Am I expecting
much too much of the poor weak-brained SCSI manager?

Has anybody tried this? If so, did it work? What did you learn? If not,
let's get some discussion going here before I start forking over my money...
  -- perry
-- 
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Perry The Cynic (Peter Kiehtreiber)		     perry@arkon.key.com
** What good signature isn't taken yet? **	   ...!pacbell!key!perry