Cromemco ZPU Repro in Altair 8800c

[tubarespect]

Hello All!

After a long while of being busy doing life stuff, I now find myself with some free time to resume working on Altair stuff.

Now I am turning my attention to the Cromemco ZPU Reproduction board I purchased a while back. The PCB was designed by Rich Camarda of the Cromemco Google Group community. See the following link for information about this reproduction board: https://groups.google.com/g/cromemco/c/ ... UvsVPsAgAJ

I purchased the PCB from here: https://www.retrobrewcomputers.org/doku ... dinventory

I finished building the board a few days ago. The only trouble is, it does not seem to work. The lights on the front panel of my Altair 8800c flash wildly. They seem to be counting up very quickly. I have attached a link to a video of the behavior: https://www.icloud.com/iclouddrive/05ap ... A#IMG_0807. The start/stop switch and reset switch seem to cause changes in the behavior, but none of the other switches seem to do anything. Also, changing the number of wait states slows down the counting a little bit.

At first I thought I had a dead chip or made a mistake in my soldering work. I searched the board with a magnifying glass for shorts and could not find any. Then I started doing research on the problem. Eventually, I found a thread on the VCFed forum where someone who had purchased two non-reproduction ZPUs described a similar problem with using them in their Altair 8800c. Here is the link: https://forum.vcfed.org/index.php?threa ... y.1249815/

So now I am beginning to wonder if the problem is maybe not a dead chip or a soldering mistake but front panel/signal timing related. I know my Altair 8800c works fine (as far as I can tell). I have run kill-the-bit on the front panel, Altair ROM Basic, and CPM 2.2 with no issues at all using a reproduction original Altair 8080 CPU board.

Since this is Altair 8800c and Cromemco related, I also put in a request to cromemcos@gmail.com to join the Cromemco users group. I will ask these questions there as well once they let me in. I noticed from reading the posts there that no one appeared to have tested the board in a machine with a front panel. I will relay any information I get there here and vice versa.

I currently have the ZPU, front panel interface, EconoRAM II, and termination boards installed in the computer.

I have also attached a picture of the card to show y'all how it is set up. It is set for three wait states, no DRAM refresh, no address mirroring, no automatic jump on power up, and 2MHz clock frequency. I was not sure how to set the "M1 Wait". Perhaps that is the issue? Also note the Z80 clone CPU used here. It is an NEC that Unicorn Electronics sent me instead of an authentic Z80.

I am more than willing to troubleshoot signal timing or anything else and post results. My old analog oscilloscope finally died (which I used to test the clock signal timing on the 8080 card) but I will be ordering a nice new digital one very shortly.

[AltairClone]

Sounds like that forum thread is a good one and would be worth following some of the latter steps taken but the poster.

Both the original front panel and the 8800c front panel boards require status signals on the data bus during pSYNC.

Mike

[tubarespect]

Okay, so maybe scope Phi2 clock, DO5, and pSYNC? I am thinking scope these on the 8080 board and also the new Z80 board and compare.

I just checked over the resistors, chips, and capacitors installed as another troubleshooting step, everything looks good to me. No wrong resistors, chips, or capacitors installed. So no silly mistakes.

They just let me into the Cromemco Users Group so I will go ahead and post over there and report back on any insights they have.

[Wayne Parham]

Yes, please do keep us posted. I know you'll figure it out!