Newsgroups: comp.sys.mac.hardware
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!oxtrap!oxtrap!time
From: time@oxtrap.oxtrap.UUCP (Tim Endres)
Subject: Re: Sharing the SCSI bus?
In-Reply-To: mclow@telesoft.com's message of 20 Nov 89 04:49:02 GMT
Message-ID: <TIME.89Nov27140427@oxtrap.oxtrap.UUCP>
Sender: time@oxtrap.aa.ox.com (Tim Endres)
Reply-To: time@oxtrap.UUCP
Organization: Oxtrap - Ann Arbor, MI
References: <1240@key.COM> <602@telesoft.com>
Date: 27 Nov 89 14:04:27

In article <602@telesoft.com> mclow@telesoft.com (Marshall Clow @telesoft) writes:

   From: mclow@telesoft.com (Marshall Clow @telesoft)
   In article <1240@key.COM>, perry@key.COM (Perry The Cynic) writes:
   > ... Intro
   > 
   > ... a discussion about SCSI ...
   > 
   > 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

Sorry, Marshall, but it will work. In fact it is done all the time!
Talk to anyone with an Ensoniq EPS.

Yes, _SCSISelect() can fail, but most programs deal with this.
Further, other initiators on the bus does not necessarily mean that
SCSISelect() will fail. The probability of a timeout on select
increases, but this does not translate into failure. In fact, the
opposite tends to be true, the Select almost never fails.

Multiple initiators is an *important* aspect of SCSI. I wish more
people would excersize it to be sure Apple keeps it functional.

Disclaimer: Anything can be done. It is a matter of cost.
