SCSI Adapter Hardware Design

[John B. Milton]

In article <5255 at bnrmtv.UUCP> miket at bnrmtv.UUCP (Michael Thompson) writes:
[ Three SCSI designs ]

I would go with the single chip processor design, with some mods:

Add a latch to hold the processor in the reset state. Make the program memory
for the processor dual port RAM. Download the code with a program in /etc/rc.
This would eliminate the need to burn ROMs to change the firmware. With PALs,
this change would not be too hard to implement. DMA could also be added to this
design. Because one of the prime devices we want access to over SCSI would be
a tape drive, I think that 16k would be WAY too small. Large memory means
dynamic RAM, more complexity and cost.

Another completely different design would fall along the lines of a project
I have started working on: An IBM PC bus adapter. Some thoughts:
May have to be connected "closer" into the UNIXpc than the expansion bus for
speed and noise reduction. By closer, I mean it would probably be a piggy
back card in the 68000 socket. It would not be as messy as my hard disk board.
There would be some kind of ribbon cable over to a PC box (the same one you
will have your second hard drive in :) There you would have a driver card to
receive the signals from the UNIXpc and drive the PC bus. There should not be
too many more problems supporting an AT 16 bit bus.

There are bound to be some boards that just won't work right. Software would
be an ongoing bitch. Most vendors and so afraid of being put out of business,
they will not cooperate with someone trying to write new code for the hardware,
even if it is to their advantage. Those of you who have tuned into what the
MINIX folks are doing know what I mean. The obvious reason to go this route is
the plentiful and cheap hardware available for the PClones. A SCSI board
for $25 is a typical example.

I have thought of a high speed connection to a regular PC, and it doesn't seem
like the proper (or cheap) way to go. From the programming point of view, I
would much rather talk directly to a VGA card, rather than telling a PC over
there to do it. For those of you who are wondering, I don't think it's going
to be possible to use PC RAM cards, as they expect full memory cycles to do
refresh. The UNIXpc uses more efficient short (RAS only) cylces to do refresh.
But, there might be a way.

Well folks what do you think? What am I mising? You might end up spending more
money going the PC bus interface route, then again maybe not.

John
-- 
John Bly Milton IV, jbm at uncle.UUCP, n8emr!uncle!jbm at osu-cis.cis.ohio-state.edu
(614) h:294-4823, w:764-2933; AMPR: 44.70.0.52;  Don't FLAME, inform!