Cromemco ZPU with Altair 8800c

Peter Schmidl

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Sep 17, 2026, 2:36:53 PMSep 17

to Altair 8800

First of all, I have a running Altair 8800c with these hardware booting into CP/M:

(set to FF00 for Combined Disk Boot Loader (CDBL))

MITS 8080 CPU board
Front Panel Interface
FDC+
88-2SIOJP

Which changes are needed to just boot into CP/M with the ZPU card instead of the genuine MITS board? (the mod provided by Mike is just mandatory for the front panel operations, isn't it?)

Thank you,

Peter

PS:

JUMP ENABLE is not connected

Walt Perko

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Sep 17, 2026, 2:51:32 PMSep 17

to Altair 8800

Hi,

Okay, that's where the problem arises ... The Cromemco ZPU works fine in a real MITS Altair 8800 without needed any modifications other than the Jump Address Jumper being cut-OPEN.

OTOH, using the Cromemco ZPU in an Altair 8800c computer (Mike's Front Panel board set) you need to make the modifications that were discussed in a previous thread here.

Now I guess I should post this on my website as well.

https://deramp.com/downloads/altair/hardware/altair_8800c/Cromemco%20ZPU%20Issues%20and%20Fix.pdf

The old thread: https://groups.google.com/g/altair-8800/c/25HQbe0NsFg/m/gMZxcpN2AQAJ

.

Peter Schmidl

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Sep 17, 2026, 2:56:27 PMSep 17

to Altair 8800

I'm aware of the mod provided by Mike, but as I mentioned in my post I thought this is just needed for the front panel operations not booting into CP/M?

Walt Perko

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Sep 17, 2026, 3:01:24 PMSep 17

to Altair 8800

Hi,

I don't know for sure, but I think the Comemco ZPU modification is need to get the front panel to let you BOOT to CP/M. My phraseology may be slightly incorrect, but basically, IF the Altair 8800c front panel from Mike isn't working with the Cromemco ZPU, then I would expect CP/M to fail too.

Now we need either a confirmation or a better answer from a more knowledgeable member.

.

Patrick Linstruth

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Sep 17, 2026, 3:53:49 PMSep 17

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Have you tried it and it’s not working or are you just inquiring?

Patrick

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Peter Schmidl

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Sep 17, 2026, 5:09:18 PMSep 17

to Altair 8800

I did the mod now and have a similar behaviour as described in this thread: https://forum.vcfed.org/index.php?threads/cromemco-zpu-mystery.1249815/

(heavy blinking of A15,14 and 1) see YT video: https://www.youtube.com/watch?v=RL9WdwdGibo

I tried to replace IC34 (LS00), IC36(367) and the Z80 CPU as well => no change at all.

Any suggestions are very welcome!

BR,

Peter

Peter Schmidl

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Sep 17, 2026, 5:10:44 PMSep 17

to Altair 8800

Correction: * (heavy blinking of A15,14 and 13) *

Peter Schmidl

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Sep 18, 2026, 9:36:57 AMSep 18

to Altair 8800

Update:

Stop/Reset is now working but it is not booting into CP/M. I tested the MITS 8080 board as well after the Mod and it's working fine.

Turned on or after STOP-RESET: (looks OK)

Examine:

Examine Next:

RUN (after STOP-RESET):

Any ideas what to check next or what could be wrong?

Thank you,

BR Peter

Patrick Linstruth

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Sep 18, 2026, 10:07:29 AMSep 18

to Altair 8800

Since you are using a front panel, I would disable any power on jumps enabled on all boards and Examine FF00, verify the opcode looks right, and then Run.

Sent from my iPhone