Using the 8800b CPU board in the 8800C

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

Using the 8800b CPU board in the 8800C

Postby TheoAU » September 1st, 2022, 10:35 pm

I was very interested to read that the Mike's front panel is compatible with the 8800b CPU board. I have an 8800b CPU board that came with a broken Altair B Turnkey system that I bought years ago. The CPU board is in good condition. I was able to test the 8080a and 8212 chips on my reproduction CPU board. and they both work. So I thought I'd try connecting it to the 8800c.

So far I've replaced both voltage regulators. Both were reading higher voltages than expected. From memory the 7805 was at 6.6v and the 7812 was at 14v. Prior to replacing the voltage regulators there was no response from the front panel with the 8800b CPU plugged in. But after replacing the voltage regulators, I can press run and see some activity going on.

The 8800b board uses the smaller ten pin Molex connector, so I couldn't plug it into the front panel interface board. From what I've read only eight of the pins are connected, so it should connect in the same way as the 8800 CPU. This weekend I'll make up a cable and see if it works.

Is there anything I'm missing? Should the 8800b CPU just work in this system without any other changes?
Attachments
8800b_CPU.jpg
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Re: Using the 8800b CPU board in the 8800C

Postby AltairClone » September 2nd, 2022, 3:45 pm

No changes to the front panel boards or the 8800b CPU board are required. Just make the 8 conductor harness between the FP interface board and the 8800b CPU board with the two different connectors on each end. Note also that the signal order on the
harness for the 8800b is different than for the original CPU board. I have used the 8800b CPU in my 8800c and it works fine.

Mike
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Re: Using the 8800b CPU board in the 8800C

Postby toml_12953 » September 3rd, 2022, 2:59 am

AltairClone wrote:No changes to the front panel boards or the 8800b CPU board are required. Just make the 8 conductor harness between the FP interface board and the 8800b CPU board with the two different connectors on each end. Note also that the signal order on the
harness for the 8800b is different than for the original CPU board. I have used the 8800b CPU in my 8800c and it works fine.

Mike


So is there a way to make the 8800c into an 8800b workalike? I have the Altair Clone case with an 8800 replica CPU board, your front panel interface, an FDC+ and two 88-2SIOJP boards. I'd be very interested in having an 8800b clone!
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Re: Using the 8800b CPU board in the 8800C

Postby AltairClone » September 3rd, 2022, 12:29 pm

toml_12953 wrote:So is there a way to make the 8800c into an 8800b workalike? I have the Altair Clone case with an 8800 replica CPU board, your front panel interface, an FDC+ and two 88-2SIOJP boards. I'd be very interested in having an 8800b clone!


The auto-single step repeat option of the 8800b is already an option with the 8800c. The other functions provided by the 8800b are to load/display the accumulator and to perform an IN/OUT operation from the accumulator. I think I looked at implementing these features when I first made the front panel board set. I’d have to revisit why I didn’t implement them.

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Re: Using the 8800c CPU board in the 8800c

Postby TheoAU » September 10th, 2022, 5:39 am

I've made some progress with the 8800b CPU. However, it's still not working. I suspect the board has further issues. I had some time today to make up the 8 wire harness. As mentioned by Mike, it's not a straight through harness like it is with the 8800 CPU board. I looked up the pin designations on each connector:

8800: D3 D2 D1 D0 D4 D5 D6 D7
8800b: D7 D6 D5 D4 D3 D2 D1 D0

With the 8800b CPU board installed, I get the following behaviour:

- The machine powers on with the WAIT and WO lights on, and with the value 303 on the data lights. This is exactly the same with the 8800 CPU installed.
- When pushing Examine on any selected address, the correct address lights up on A15 to A0. However, the values on D7 to D0 don't change.
- Pushing Deposit doesn't change the value into the selected location.
- Pushing Deposit Next changes the selected address but doesn't have any effect on the data lights.

I've noticed that when pushing Examine with the 8800 CPU installed, MEMR and M1 light up, but this doesn't happen with the 8800b CPU installed.

Initially with the 8800b CPU, A15 and A7 were always high. On inspecting the board, I replaced the 74367 IC at location P. It had a very corroded leg. Once I did that it fixed the issue with A15 and A7. Both locations can now be selected.

So, I suspect the issue with this board is that one or more IC's are bad, which is preventing the board accessing data values.
Attachments
20220910_173042.jpg
Lights that display with the 8800b CPU installed
20220910_173008.jpg
8800b CPU board installed with front panel harness
Last edited by TheoAU on September 10th, 2022, 8:17 am, edited 1 time in total.
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Re: Using the 8800b CPU board in the 8800C

Postby AltairClone » September 10th, 2022, 7:58 am

Remove the 2SIOJP board and make sure the front panel board does not have power on or reset auto-jump enabled. Then turn on the machine and perform the STOP-RESET operation. What shows on the front Panel at this point? Are M1 and MREAD on? If so, try an examine and similar operations, Any difference from before?

Note that examine, examine next, and deposit next will only work if M1 and MREAD are both on when you perform the operation.

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Re: Using the 8800b CPU board in the 8800C

Postby TheoAU » September 10th, 2022, 8:53 am

Hi Mike,

I removed the 882SIOJP and did a Stop/Reset, and M1 and MREAD are off. I don't get the WO on power up and all the data LEDs are high.
Hitting Examine only changes the address and does nothing to the data LEDs.

The previous behaviour was clearly the 882SIOJP triggering a reset on power up which it's configured to do.

I haven't enabled the power on jump or reset features in the front panel. I've only enabled the code injection feature.
Attachments
8800b_CPU_NO_882SIOJP.jpg
8800b CPU after stop/reset with no 882SIOJP
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Re: Using the 8800b CPU board in the 8800C

Postby AltairClone » September 10th, 2022, 9:22 am

Since you’re not getting valid status signals from the CPU board, and we now know the 2SIOJP isn’t over-riding them, we need to figure out why.

Verify IC-G pin 9 is high and pin 8 is low on the CPU board. This is the status disable signal.

If that checks out ok, then try swapping the 8080 itself and then the 8212 on the CPU board.

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Re: Using the 8800b CPU board in the 8800C

Postby TheoAU » September 10th, 2022, 10:10 am

I checked IC-G, and pin 9 was high and 8 low. So that seems ok. I previously tested the 8080 and the 8212 on my reproduction CPU and both those IC's worked fine. I do have a spare 8080 which I know works, so I put that on the board, but no change.
Last edited by TheoAU on September 10th, 2022, 10:50 am, edited 1 time in total.
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Re: Using the 8800b CPU board in the 8800C

Postby TheoAU » September 10th, 2022, 10:49 am

Here's a bit of background info on this 8800b CPU board. I got this board back in 2013. It was inside a broken Altair 8800b Turnkey system that I bought. The main reason I bought that system was to get an 882SIO board. Luckily that board worked and I now use it in my original system. Much of the Turnkey system is now gone. I sold it as parts. But I kept all the other boards it came with including the CPU board. However, I recall that the CPU board had some strange hack on it that I removed. From doing some searches it seems to have been a trimmer capacitor. I still have a picture of what the board looked like when I first got it, see below. After looking at the board recently, I saw that there was a cut trace where the trimmer capacitor was attached. So I made a point to fix that trace with a jumper wire.

But it seems this capacitor was attached to the crystal on the board. Does anyone have any thoughts why this capacitor would have been installed?
Attachments
IMG_0757 - Copy.JPG
8800b CPU with trimmer capacitor attached
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