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From: jjg@walden.UUCP
Newsgroups: comp.sys.mac
Subject: Re: Connecting two MacPlusses to one SCSI drive!
Message-ID: <83@walden.UUCP>
Date: Sat, 11-Apr-87 09:35:09 EST
Article-I.D.: walden.83
Posted: Sat Apr 11 09:35:09 1987
Date-Received: Sun, 12-Apr-87 19:37:42 EST
References: <243@sering.mcvax.cwi.nl>
Reply-To: jjg@walden.UUCP (John Grana)
Distribution: world
Organization: Awcs Inc.   Henrietta, New York
Lines: 46

In article <243@sering.mcvax.cwi.nl> frankk@cwi.nl.UUCP (Frank Kuiper) writes:
>
>Here is a nice question.
>
>How reliable is it, to connect two Mac's to the same SCSI hard disk?
 ...
>
>The big question is: what will happen if both of the Mac's want to
>write to the drive?

	If both Mac's arbitrate for the SCSI bus at the exact same time,
	I can see a real problem. From what I remember, during the SCSI
	arbitration time ( the request for the bus by the Initiator (Mac))
	the first step is to check Busy (a SCSI bus signal). No problem
	so far... (neither Mac is on the bus). Then, the Mac(s) should
	put their SCSI I.D. on the data bus and 'glance' around to see if
	it wins arbitration ( it has the highest I.D. on the bus). Since
	BOTH Mac's are addressed the same, each thinks it has the bus...
	what follows would be disaster. Since the arbitration/selection
	phase is done before any command to a Target ( the hard disk) it
	doesn't matter what the command is (i.e. both attempt write or read
	or mixed).
>
>I know that SCSI devices need different numbers to perform ok when
>connected, so now there are two devices (i.e. the Mac's) with the
>same device number (=7) connected!
>
		see above

>Anybody any idees?
>
	One idea.. I think the SCSI I.D. of the Mac is 'stored' in a
	register in the NCR 5380 (SCSI Proto. Cont. I.C.) You may be
	able to hack the SCSI init to change one of the Mac's to a
	different I.D. Maybe I.D. 6. Since there are 8 allowed I.D.'s
	just pick one that doesn't collide with the hard disk. I know
	this would work with the NCR 5385/6 (the ID REGISTER can be used
	in "strapped" or "programmed" id mode). You would have to check
	the data sheet on the NCR 5380 (sorry I don't have that one 
	handy!)

						good luck

						John Grana
						jjg@walden
				..!seismo!rochester!cci632!walden!jjg