Date   : Tue, 17 Apr 2007 09:05:33 -0500
From   : julesrichardsonuk@... (Jules Richardson)
Subject: Acorn SCSI board arbitration

Richard Gellman wrote:
> Hi,
> 
> The circuitry is there, as it is simply a matter of asserting one's SCSI
> ID bit on the data bus, and setting one or more of the phase lines to
> indicate arbitration mode. Whoever has the lowest SCSI ID wins.

In the Master AIV version? In the Winchester flavour there seems to be no way 
for a host to just assert just one of the 8 data lines (corresponding to the 
ID of that host) and leave the other lines in a high-impedance state where 
another host on the bus is free to assert them.

Unless I'm misunderstanding*, the way it's wired currently if there were two 
hosts on the bus with different IDs, then they'd end up shorting out the data 
lines corresponding to those IDs during the arbitration phase: one host would 
be outputting a logic high on the same data line as the other outputting a 
logic low, which sounds like a good way of cooking something!

* Possible. I can't for the life of me find a SCSI-1 spec anywhere, so I'm 
going from bits of info gleaned from the SASI spec and various SCSI target 
devices (which don't participate in arbitration anyway).