CromemCo ZPU board in an Altair 8800 or 8800c ...

[Walt Perko]

Hi,  

I was thinking about what Peter told me the Cromemco ZPU needs to be modified to work in the Altair 8800 computers ... 

I haven't tried the Cromemco ZPU in my Altair 8800c, but there are a couple of modifications so the board is more compatible with other computers might be all the modifications needed:  

Cut the trace above IC14 that says:  JUMP ENABLE so the board doesn't goto an address normall on a Cromemco FDC board.  

Then there's raising PIN 11 on IC23 ... maybe that's needed for working with the mini-FP?  

I don't know about raising the 100Ω resistor ... it's not pulled up on my Cromemco ZPU board, and so far, the board has worked perfectly in the mini-Front Panel computers.  

From the manual:  

  The Cromemco ZPU is an S-lOO bus CPU (Central Processing Unit) using the powerful Z-80 microprocessor. This CPU is used in Cromemco computers and is also a plug-in replacement for the CPU card used in the Imsai 8080 and Altair 8800 computers.   

  4 MHz Operation. To switch from 2 MHz operation to 4 MHz opera ation simply flick the speed select switch on the ZPU from the 2 MHz to the 4 MHz position. Notice that the line previously labeled the "stack" line oQ. the 8-100 computer bus is now used as the 4 MHz indicator line. If you use the ZPU in an Altair or IM8AI computer your stack light will be on for 4 MHz opera tion and off for 2 MHz operation. (This indicator light is labeled "4MHzll on the fron.t panel of the Cromemco Z-lcomputer.) All Cromemco products are designed for reliable operation at either 2 MHz or 4 MHz clock frequencies.

Installation in Altair or Imsai Computers

 If you are using an IMSAI computer the cable from the front panel may be plugged directly into socket Jl on the ZPU card. For the Altair computer a DIP plug must be installed in place of the Molex connector on the front panel cord; in doing this be aware that the data lines are not arranged sequentially on the Altair connector but are sequential on the ZPU card as shown:

                ZPU Jl                                                               Altair Molex

  I     I    I     I     I    1    I   1                                      o    o    o   o     o    o    o   o
D7 D6 DS D4 D3 D2 Dl DO                                   D3 D2 Dl DO D4 DS D6 D7
Arrangement of data lines on Jl of ZPU          Arrangement of data lines on Molex
                                                                                connector of Altair CPU card. 

  

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[Walt Perko]

Hi, 

I hope Joe can get us good high resolution scans of both the front and back of the reproduction Cromemco ZPU board.  

One thing I'm curious about is if the two jumper holes were moved closer together to fit a standard 0.1"/2.54mm header.  That would be a very smart fix to the board.  

.

[Joseph Corda]

Here are 600dpi scan of front bare the ZPU reproduction board Rev F
Joe

[Joseph Corda]

Here are 600dpi scan of back bare the ZPU reproduction board Rev F
Due to size of image I needed to do (2) posts
Joe

[Patrick Linstruth]

None of that seems to have anything to do with the 8800c front panel. I also don’t know if the problem is with an Altair or just the 8800c. I am going to try the ZPU in a real 8800 is weekend. The ZPU documentation doesn’t say anything needs to be done to the board to make it work with an Altair. It also says you can examine the JUMP ENABLE instructions on the Altair front panel, which also doesn’t work on the 8800c.

When people say, “do X and y”, they need to say why.

What appears to be happening is the flip-flop on the front panel interface board that drives PRDY isn’t being reset, which indicates a possible timing issue with PHI2, M1 status and PSYNC. The firmware on the PIC assumes that when it lowers SS the CPU has already been put in WAIT state at the next M1 by the front panel’s TTL logic, which isn’t happening. Both SS and RUN are low while PRDY is high, which is not a valid state.

I need to get a logic analyzer on the front panel interface board and see what is happening.

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[Joseph Corda]

I was told the PDF's was blurry ..so here is a front jpeg image
Back will be posted coming shortly 
Joe

[Joseph Corda]

Here is the ZPU back jpeg image as requested
Joe

[Walt Perko]

Hi, 

I totally agree!  When people talk about stuff I want to see links to the source, the sales, the videos etc.  Not just that they saw something really cool, or they found a fix for something etc.  

.

[Peter Higgins]

On Monday, December 15, 2025 at 12:02:34 PM UTC-8 Walt Perko wrote:

I was thinking about what Peter told me the Cromemco ZPU needs to be modified to work in the Altair 8800 computers ... 
I haven't tried the Cromemco ZPU in my Altair 8800c, but there are a couple of modifications so the board is more compatible with other computers might be all the modifications needed:  

Cut the trace above IC14 that says:  JUMP ENABLE so the board doesn't goto an address normall on a Cromemco FDC board.  
Then there's raising PIN 11 on IC23 ... maybe that's needed for working with the mini-FP? 

To clarify... that is a summary of the modifications others have described were necessary to get the Cromemco ZPU working with their Altair front panel. The ZPU is a well-designed board, and as long as you are aware of the interaction between the ZPU power-on-jump feature and the functioning of the Altair front panel (as described in the ZPU manual) it should not be necessary to make any modifications - which is also my experience.

On Monday, December 15, 2025 at 12:02:34 PM UTC-8 Walt Perko wrote:

[Patrick Linstruth]

I got the 'ol girl up and running. Both Cromemco ZPUs I have, an original and repro, work fine with the front panel on an original 8800.

Now to figure out why they don't work on the 8800c.

Running Kill the Bit with ZPU Repro.

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[Joseph Corda]

Patrick

That is GREAT progress..
Also thanks for all your effort on getting my ZPU card to work !

Joe

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