SASI device <-> PC ?

Jules Richardson julesrichardsonuk at
Thu Jun 30 16:53:24 CDT 2005

On Thu, 2005-06-30 at 20:39 +0100, Tony Duell wrote:
> > 
> > 
> > Has anyone here successfully interfaced a SASI device to a PC at the
> > hardware level?
> > 
> > I've got a few classic systems which use SASI (or not-quite-SCSI)
> > controllers to talk to SASI-ST506 bridge boards and from there to ST506
> > type drives.
> I was under the impression that SASI was sufficiently close to SCSI that 
> a SCSI interface could talk to a SASI device given the right software. It 
> would probably make things simpler if the SASI device and the controller 
> were the only things on the bus.

Far as I recall, it misses out some of the things found in SCSI, such as
multiple targets and extent reservation. I can't remember if it supports
parity or not either, which might mess with some SCSI controllers that
expect it.

At to the software level - again from memory - it's close, but there are
a few holes (ISTR there's no notion of an INQUIRY command, so any host
that tries to gain knowledge of what's connected to it that way is going
to get upset - this rules out the use of all Adaptec controllers that I
know of which have an on-board BIOS, for instance)

> > Presumably inventing a simple SASI board to hang off a PC parallel port
> > (say) is a lot easier than mucking around with the equivalent for floppy
> > drives - or is the data rate still likely too low to cause timeout
> > problems within the bridge board's firmware?
> I am pretty sure you can drive it as slowly as you like. All the bridge 
> boards I've seen have intenral buffer memory, certainly enough for one 
> block. 

Yep - I think there may be a few cases where drive/controller assert a
line and the 'other' thing is supposed to respond within a certain time
period though; it's cases like that with the basic handshaking where I'm
not sure if a PC parallel port would cope or not. For actual shifting
data around I believe you're absolutely correct and it doesn't matter if
it takes a second or a week between bytes...

> > I've got several Acorn, Torch and RML machines which use bridge boards
> FWIW the Torch XXX uses a SCSI interface chip on the mainboard, and I 
> thought the drive bus was SCSI on that machine.

Yep, the XXX is SCSI (NCR interface chip of some sort IIRC) - their
machines which use the BBC as the I/O processor tend to be SASI though
(such as the C520 'giant brown turd' machine and the 725). As well as
the machines, I've got a few ex-workshop drives and so it'd be nice to
save any useful data that may be on them!

Actually, if I'm remembering right (without venturing into the loft and
pulling lids!), the XXX typically shipped with a bridge board in it and
used ST506 type drives, it just used a SCSI bridge rather than SASI (I
think it was an OMTI)

I have a feeling it might have been a customer option, but at the time
it was possible to buy a XXX through official channels with a much
larger capacity disk via the ST506 route than it was one with SCSI disk.
I'd have to go back through old notes and emails to be sure!



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