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From: cramer@clem.UUCP
Newsgroups: comp.periphs,comp.sys.mac
Subject: Re: Making a Mac talk SCSI to a Sun
Message-ID: <13413@sun.uucp>
Date: Mon, 16-Feb-87 14:21:51 EST
Article-I.D.: sun.13413
Posted: Mon Feb 16 14:21:51 1987
Date-Received: Tue, 17-Feb-87 07:03:32 EST
References: <2594@phri.UUCP> <1784@hoptoad.uucp> <540211194.20192@minster.UUCP>
Sender: news@sun.uucp
Reply-To: cramer@sun.UUCP (Sam Cramer)
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Organization: Sun Microsystems, Mountain View
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Xref: watmath comp.periphs:208 comp.sys.mac:1324

In article <540211194.20192@minster.UUCP> martin@minster.UUCP (martin) writes:
>In article <1784@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
>>I believe there would be problems with having two SCSI masters.  A better
>
>No, SCSI (as opposed to SASI) was explicitly designed to allow any, or
>all, of the devices to be masters. If having two masters dosn't work, then
>one of the machines dosn't implement SCSI correctly (or completely!).
>
>I don't know about the implementations on the Sun, or the Mac, but all
>high performance (which I assume the Sun interface, at least is) interfaces
>should implement disconnect-reconnect to allow overlapped seeks, and
>this automatically gives one the multi-master capability (again so long
>as the software is not broken :-).

This is not exactly correct.  Disconnect/reconnect support is not related
to having multiple initiators (referred to above as "masters").
Disconnect/reconnect is a SCSI facility that allows a target controller
to give up the SCSI bus during the execution of a command if it is not needed.

Here is sketch of how disconnect/reconnect works:

	Initiator (usually the host CPU) selects the target controller
	(in this case assume a disk controller).  As part of the selection 
	protocol, the initiator sends a message to the disk controller 
	indicating it may disconnect if it would like.

	The intiator sends a command to the target.  Let's assume it is 
	a read command.

	The target realizes that performing the read will require a seek.
	It sends a disconnect message to the initiator to indicate that
	it wants to disconnect from the SCSI bus.

	Time passes and the head finally reaches the appropriate track.
	The target signals that it wants to reconnect to the SCSI bus.

	The initiator and target perform the reconnection protocol, and
	the SCSI bus goes into DATA IN phase.  Data is transfered
	from the target to the initiator.

	The target indicates command completion by sending the status
	and command completion messages.

Please note that disconnect/reconnect support is optional according the SCSI
spec.  Sun introduced disconnect/reconnect support in our 3.3 SunOS release.

Supporting multiple initiators is optional under the SCSI spec.  Many SCSI 
implementations, including Sun's, do not support multiple initiators. 

Due to the fact that Sun does not support multiple initiators, interfacing
a Mac to a Sun SCSI port would be non-trivial.  I suspect it would be more
productive to try to run the AppleTalk protocol through a serial port -
the Sun 3/50 uses the same Zilog serial chip as the Mac.

Sam Cramer	uucp:	{cbosgd,decwrl,hplabs,seismo,ucbvax}!sun!cramer
		arpanet: cramer@sun.com