Cromemco Z-2 and IMSAI 8080 at Purdue

dwight elvey dkelvey at
Sat Nov 21 10:49:59 CST 2009

> Date: Sat, 21 Nov 2009 09:31:00 -0500
> From: shumaker at
> To:
> Subject: Re: Cromemco Z-2 and IMSAI 8080 at Purdue
> Qas this a common mod? Didn't the Imsai have a header connector on the
> CPU board for the front panel?
> steve

 Yes, the address and data displayed are from the S100 but in order
to make the hole thing work, the front panel needs direct
access to the CPU's data bus as well. This is done with the header
 The IMSAI and Altair both used the same method.
To examine an address, they'd jam a jump instruction
into the CPU from the header. To increment the address,
they'd jam a NOP instruction.
 I'm not sure if this created the simplest front panel
but had the advantage that the processor already had
the correct address inside and only needed to be
started when the RUN switch is selected.
 The S100 data bus had the real data while the 8080
only saw the phony instructions from the frontpanel
unless RUN or SINGLE-STEP were selected.
 This is similar to the way most mini front panels work
so it may have made sense to do this with the 8080.
 I should also note that one of the common problems
I've seen on IMSAI and Altair front panels is getting
the bits reversed on the header. The address increment
seems to work but one can't select a correct address.
 The other problems that seems common is one of the
OC data drivers fails. I suspect the timing is not
exact and there is a little crowbar current but I've
never put a 'scope on it to see.
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