Re: Apple Rev.C SCSI Jumpers

[dempson@actrix.gen.nz (David Empson)]

Stingray <stingray@webzone.net> wrote:

> David Empson wrote:
> 
> >
> > And the "priority" is exactly the same thing as the SCSI ID, so we're
> > both right.
> 
> I've never heard SCSI ID referred to as priority. Just goes to show, you
> learn something new every day, even on your day off, when you're trying
> real hard not to! :-)

It is only a "priority" in the specific case of the SCSI Arbitration
phase.  If two SCSI devices want access to the bus at the same time, the
arbitration phase is used to determine who wins.  Both devices activate
the data bus line corresponding to their SCSI ID (e.g. bit 7 for SCSI ID
7).  Each device then checks to see if any higher numbered data bus bit
is active.  If the device's own ID is the highest active bit, then it
wins arbitration and gets control of the bus for the next transaction.

Thus the highest SCSI ID has the highest priority.

The RamFast's lack of a SCSI ID means that it can't use arbitration, and
also can't support the disconnect/reconnect mechanism, in which a SCSI
device can be left to execute a slow command, then it reconnects to the
host when it is finished.  The bus can be used for something else in the
meantime (with another device).

This isn't a major issue with the Apple II, since the operating system
doesn't have much need for simultaneous SCSI operations, and multiple
SCSI devices aren't all that common anyway.

Because of this lack of arbitration phase support, it is not valid to
talk about the priority of SCSI IDs in conjunction with a RamFast.

The SCSI ID is also used to identify which devices are communicating -
the SCSI ID of the host and target appear on the bus at the appropriate
times.  This is a two-way operation with the Apple cards (allowing
disconnect/reconnect), but only the drive's SCSI ID appears on the bus
with a RamFast.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand