Path: utzoo!utgpu!watserv1!watdragon!tiger!mwandel
From: mwandel@tiger.waterloo.edu (Markus Wandel)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Multiple Amigas on a SCSI Bus.
Message-ID: <22999@watdragon.waterloo.edu>
Date: 8 Apr 90 02:54:28 GMT
References: <1356@lpami.wimsey.bc.ca>
Sender: daemon@watdragon.waterloo.edu
Organization: University of Waterloo
Lines: 33

In article <1356@lpami.wimsey.bc.ca> lphillips@lpami.wimsey.bc.ca (Larry Phillips) writes:
> 
> I agree that the best bet would be, as you say, to write some form of
> arbitration making one host adapter look like a disk, but I am wondering if
> the 2091 supports SCSI bus arbitration (lower level than the arbitration you
> are speaking of), so that, for example, an IO device could act as a master
> on the bus.
>
> Disconnect/reconnect... I noticed a posting from Dave Haynie that mentioned
> the 2091 disconnecting between the command and data phases in case something
> else needs to use the bus.

Just thought I'd mention that one implies the other.  You can't have a SCSI
system which supports disconnection/reselection without having arbitration
implemented, since a SCSI device processing a command while disconnected must
be able to reselect the host at any time, and thus arbitrate for use of the
bus.  This is the same arbitration as is required when you want to have
multiple initiators on the bus.

In other words, the only issue would be to change the 2091's SCSI device ID,
and how easy that is depends on its design.  It's probably just a 1-byte
patch to the device driver, if you can get it to run out of RAM.

Further clarification:  The 2091 does not "disconnect between command and
data phases".  The target device (disk) controls that, and it can disconnect
and reconnect as many times as it wants, even between command bytes and
after the data phase.  The initiator and the target device negotiate the
disconnect capability at the beginning of each command, after that everything
is up to the target to decide.

Markus Wandel
mwandel@tiger.waterloo.edu
(519) 884-9547