Date   : Thu, 17 Nov 2005 13:58:22 +0000
From   : Richard Gellman <splodge@...>
Subject: Re: Best Emulator??

Jules Richardson wrote:

>>I wrote these as part of my work on the Leeds university domesday
>>emulator. It has a few things hacked into it (like video commands) but
>>they are easily removed. It also needs either the data register read
>>or write operation (can't remember which) modified to invert the bits,
>>as the SCSI "adapter" in the domesday box used a different chip for
>>one of the data register buffers to that on the hard drive SCSI
>>adapter.
>>    
>>
>
>This purely on the Master? I seem to recall the BBC B SCSI board being
>significantly more complex than the Master one, or at least in terms of
>chip count.
>
>I suppose it's possible they used 4 bit registers and buffers everywhere
>in the BBC B one which is why the chip count's so high! I've always
>meant to look at the schematic as I've never figured out why it's so
>complex.
>
>I believe that the SCSI controller in the Viglen BBCs is different yet
>again, plus of course Torch made both a SASI and SCSI controller for the
>BBC... so there's at least 5 different flavours out there!
>  
>
I don't know about "purely on the Master", but my work was based on the 
information in the Acorn Winchester Service Manual. Said unit uses a 
SCSI<->Winchester adapter board with a Winchster HD on the business end. 
The SCSI end is then attached to Acorn's Host Adapter board, which 
basically uses demuxes to latch data onto the SCSI bus itself. IIRC, 
this read data/write data, control, and status - all 8-bit. The first 
two being mapped to the data pins, the control register being mapped to 
the control lines that go TO the SCSI adapter, and the last being mapped 
to the status lines coming FROM the SCSI adapter. A logic circuit taps 
into the read/write data mechanism to flip the REQ/ACK lines when the 
bus is in a data phase.

All of this then maps onto the 1Mhz bus, and the Acorn ADFS in the 
default Master 128 picks it up perfectly.

The disk format btw is identical to that of an ADFS floppy - ie. no 
partitions or extraneous data. I have in fact had BeebEm believing it 
had a 640K hard disc attached at one point....

-- Richard