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From: mclow@telesoft.com (Marshall Clow @telesoft)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Sharing the SCSI bus?
Summary: Mac can't share SCSI
Keywords: SCSI, Mac
Message-ID: <602@telesoft.com>
Date: 20 Nov 89 04:49:02 GMT
References: <1240@key.COM>
Organization: TeleSoft, San Diego, CA.
Lines: 34

In article <1240@key.COM>, perry@key.COM (Perry The Cynic) writes:
> 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
[stuff deleted]
> 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
[more stuff deleted]
> 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?

It won't work. As you surmised, the Mac's SCSI manager wants to be the
only "initiator" on the bus. If you want to get technical, 
_SCSISelect ( or whatever the trap is called ) will fail if the Mac
cannot get control of the bus. ( i.e, if two other devices are
communicating ).
	Sorry bout that. It would be a neat hack.
	Marshall Clow
	mclow@telesoft.com