Date   : Fri, 18 Nov 2005 11:26:53 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: SASI/SCSI

Mark Usher wrote:
> The big question though is what 'extra gubbins' is needed to turn the SASI
> controllers for the Beeb into SCSI compatible controllers?

On simple controllers, not much - IIRC at the hardware level the interface 
connector has one extra control pin for SCSI (it's grounded for SASI), and 
there's an extra handshaking step performed between one of the common control 
lines and this SCSI-only one when transferring data.

Can't remember exact signal names off the top of my head - but the extra gate 
count ends up just being an inverter and a flip-flop from what I remember (for 
a target device the handshake would be more complex and require processing in 
between the asserted control signal and subsequent tweaking of the 'reply' 
signal, but in the case of a simple host device - controller - it's possible 
to just do the acknowledge automatically in hardware via the flip-flop)

The software I recall differs though in the way that things are driven - the 
protocol's very close between SASI and SCSI, but not *quite* the same. There's 
an extra protocol phase IIRC with SCSI which SASI doesn't have.

>> I have a few different SASI controllers. One from Australia, one home grown
> and another non-acorn one. Really want one though that works with SCSI, and
> as yet, have not been able to find one.

See above - if you can figure out schematics then it's probably possible to 
modify relatively easily. But you'll end up having to tweak any driving 
software too.

I'm not sure if ADFS supports the Master SCSI controller - anyone know? I've 
only ever heard of it being used to control the VP-415 laserdisk player in a 
Domesday setup. If it is supported by ADFS, then your best bet might be to 
build a copy of one as then you wouldn;t be left writing your own drivers. I 
can't remember if the schematic's out there or not - if it isn't I could 
probably be persuaded to look at my Master's SCSI board and trace it out...

cheers

Jules