More on SCSI design

[Michael Thompson]

Hello again everyone,

I now have the "Unix PC Device Driver Development Guide" (thanks
goes to Alan Lundin) and the "Unix PC Tech. Reference Manual"
(thanks goes to John Wilkes) in my hands.  Looking over the
schematics to the Unix PC, I see that the expansion bus on the
computer is pretty complicated.  Each expansion card is expected
to have its own DMA controller, but the CPU signals are hidden
behind a lot of logic so hooking up a DMA chip such as the 
MC68840 or MC68850 doesn't look to easy or even possible.  Any DMA
chip is expected to go through the Bus Arbitration Circuit using the
correct hardware protocal.  Coming up with a simple circuit to 
implement this protocal seems like it would be pretty complicated.

On the Floppy Tape Interface Board there is a good example of how to
implement a DMA controller from an expansion slot and the circuit looks
like it would be pretty easy to adapt to a SCSI controller.  Pretty much
along the lines of yanking out the WD2797 Floppy/Tape controller chip
along with its glue logic and put the NCR 5380 or NCR 5390 SCSI controller
chip in its place.  If we did this, 75% of the design work is done for us.
The problem is that the DMA is implemented with two AT&T proprietary DMA
chips which I feel would be next to impossible to get a hold of.  Am I 
right about this, or do we just have to order them from where AT&T gets 
them.  Any help in this area would be greatly appreciated.  If we could
use these chips, problems such as conversion of the 16 bit bus to an 8 bit
bus go away.

Another idea which Tim Hogard suggested to me was to have a single chip 
microcomputer such as the 6811 control the SCSI chip and have it talk
with the 68010 through a shared memory buffer.  This would move a lot
of the complication in writing a SCSI device driver from software to 
firmware controlling the 6811.  This method would also take longer to
implement.  This would be the best route to take if we cannot get
normal DMA working since it would still offload responsibility of 
transferring information from the 68010 to other circuitry. 

I really would like to borrow as much of the circuitry for the SCSI 
controller from the floppy tape controller since it is an existing design
which works and has DMA.  Can anyone with a technical manual for the
Unix PC verify my ideas and get back to me with some thoughts.  I would
really like to hear from someone that the DMA chips (labeled 32-00379-00
and 32-00380-00 on the floppy/tape schematic) are easily available from
AT&T.  This would reduce design time of the circuit to a week or two.

Also, does anyone have the AT&T publication "UNIX PC Expansion Bus
Specification" since the technical reference manual doesn't really 
describe the expansion bus in great detail.  It instead refers people
developing hardware enhancements to that document.

Thats enough for now.  Keep in touch.

Mike Thompson
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			  |  Michael P. Thompson, Member Scientific Staff   |
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