seeking Motorola M68MM01A2 documentation

Chris Elmquist chrise at
Tue Jan 18 14:14:35 CST 2022

On Tuesday (01/18/2022 at 02:01PM -0600), Mike Katz wrote:
> I think it might be easier to modify the 680 prom for the I/O addresses of
> the board rather than modify the board to match the ROM.

Agreed-- except the goal, which I failed to elaborate on, is to come
up with an Altair 680 development environment so that someone can port
some code to the platform without having the real thing.  I wanted to
make that environment as close to real as possible (without having front
panel switches and LEDs)-- which means having the I/O in the same place
as the original as well as the authentic PROM code running.

> Especially if the address decoding for the I/O is done in PAL (10L8 for
> example).

No PALs on the board but there is a bipolar PROM (82S129). I'm not
adverse to making a new one of those or bodging something that drops
into that socket to modify the decoding if neccessary.  I was just hoping
to not have to butcher the board itself too much.

> Some 6800 address decoding was done with 74LS138s.  This had the potential
> to be inefficient in terms of memory usage or if the '138s were cascaded
> then propagation delay could become an issue.

Yes.  This seems to be a limited function CPU board and I suspect it takes
that approach just to get the four PROMs and I/O decoded very coarsely.

Chris Elmquist

More information about the cctech mailing list