Ideas for a simple, but somewhat extendable computer bus

Eric Smith spacewar at
Mon Nov 20 18:47:45 CST 2017

On Nov 20, 2017 7:41 AM, "Tapley, Mark via cctalk" <cctalk at>

        Catching up late, sorry if this is an old question, but what did
the Digital Group computers use? My recollection is that they offered cards
with 6800, 6502, 8080, and Z-80 CPUs on the same bus, and that part of the
system seemed to work reasonably well.

The Digital Group had two separate buses, a memory bys and an I/O bus, as
well as two other slot types incompatible with either bus, for a CPU card
and a TVC (video and cassette) card. They didn't support interrupts or DMA
on any bus. If you wanted to use an interrupt, you had to wire it over the
top. Doc Suding said that he didn't put interrupts on the bus because
(paraphrasing) they are complicated and you don't need them.

As you say, they did support various CPUs, but not more than one in a
system. I wouldn't recommend that anyone consider The Digital Group as an
example of good bus design.

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