Date   : Tue, 23 Nov 2010 21:33:52 +1100
From   : awilliams@... (Alan Williams)
Subject: Harddisc fakery using a microcontroller

> > I'm not convinced that a AVR would be fast enough.
>
> I believe that the Adaptec and Xebec SASI/SCSI to MFM/RLL used Z80s, >
so I see no problem using an 8 bit microcontroler if you want to
> emulate SCSI.
> Cheers.
> Phill.

To keep ADFS happy you will need to emulate a ACB4000 or ACB4070, to
keep a filestore happy you will need to emulate a RO652 (hmm I seem to
have three of them) or the 60Meg drive whose numbers escape me.

None of the units on this page
http://www.reactivedata.com/Products/SCSI_Bridge_Emulators_to_CF/index.p
hp
seem to fit the bill.

The ACB4000 & ACB4070 are 8085 based.  The Xebex boards don't work with
either ADFS or Filestore.

I have been thinking about this for a while, well actually I have been
thinking of a 1MHzE bus to IDE interface which faked either of the above
controllers.  I probably could be convinced that Flash would be a better
solution. (For Filestore E01 its actually a 2MhzE bus)

There really isn't an issue with any modern single chip micro doing the
emulation job, the difficulty is that they are usually not well suited
to being bus interfaced peripherals.

In order to do this you need to provide a set of registers visible from
both the 6502 and AVR side with handshaking on transfers etc.  This is
not in the realm of the 40pin DIP PIC or AVR chips.

I did spot some AVR chips with FPGA on board, now that would be ideal,
but the licensing for the software tools was not practical for this sort
of free/open source development.  Also inherently more difficult to
solder down than a 40 pin DIL.

Of course there is the added difficulty that all the BEEB side needs to
be 5V tolerant.

Personally I would probably throw a ATMEGA162 at it that ought to have
enough grunt and enough pins, it still would need a small sea of TTL to
implement the dual port register file, or some other CPLD or low end
FPGA for that.

It may actually be easier to do it from SCSI onwards rather than 1MHzE
bus onwards as you can buy SCSI end point chips that the AVR/PIC side
could access.  You may need to choose a device that can do external
memory access, AT90S8515 or the like can do this, as can a number of the
40 pin 8051 derived devices.  Maxim have some that are quite quick.

I was thinking of eliminating the host adaptor from the picture though.

Alan