Date   : Tue, 17 Apr 2007 17:17:19 +0100 (BST)
From   : splodge@... (Richard Gellman)
Subject: Acorn SCSI board arbitration

> I thought SCSI used open-collector signals thoughout for that reason.
> Am I mixing it up with something else?

SCSI does, but Acorn's interpretation of SCSI is interesting. Now I can't
remember fully from memory the circuit so the following may not be
entirely accurate, but I do remember the data part of the SCSI bus is
double-tied (tied to both +5V and 0V by differing resistor values), and
the output from the BBC Micro/Master is gated by various parts on the
controller board. When it is enabled, and 0's on the data bus will drive
those bus lines to logic 0, rather than open-collector.

Now, should another initiator on the bus drive its SCSI ID to logic 1 at
the same time as the BBC Micro is driving it to logic 0 as a by-product of
its own arbitration the two will meet in a head-on collision.

Since devices typically tie all their ground wires together, this will
mean a short circuit somewhere, at the very least causing bizarre
operation.

-- Richard