Date   : Tue, 17 Apr 2007 13:37:26 +0100 (BST)
From   : splodge@... (Richard Gellman)
Subject: Acorn SCSI board arbitration

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.

However the SCSI code (in the VFS ROM at least, probably in the ADFS ROM
too) does not have support for arbitration, and goes straight to the
Selection phase.

If there's room in the ADFS ROM for it, arbitration phase could be fairly
easily implemented as a simple bus check and a "bus busy" state...
possibly as a wait loop - assuming any other initiators on the bus don't
take too long to complete their operation. Certainly there's no support
for the Reselection phase (target says to initiator "come back later" then
requests the initiator's attention when its ready), as this requires the
ATN line to be implemented, which is unconnected on the Acorn controller
boards.

-- Richard


> Can someone tell me if the Acorn hardware / software does the SCSI
> Arbitration
> phase (during which multiple potential hosts argue over who has control of
> the
> bus)?
>
> I suspect there's no logic there to do it (technically it is optional) and
> the
> assumption is that only one host is present on the bus and will try to
> control
> it.
>
> cheers
>
> Jules
>
>
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