Front Panel Madness

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

Front Panel Madness

Postby Ttpilot » January 30th, 2022, 8:26 pm

Thanks to AltairClone and TronDD I got my 8080 CPU card sorted out. I had hopes that would take care of the problems with my 8800c. No luck. It turns on fine. When I STOP/RESET it resets to address 2h; the A1 light is on. If I toggle EXAMINE and EXAMINE NEXT, the counter starts at that point. A RESET sends me back to 2h. Also, the EXAMINE switch moves the address count up each time it's used

If I attempt to examine areas higher in memory (e.g. 0xF000000h or 0x0000F000h) I seem to be stuck back at 02h.

I can use the DEPOSIT/DEPOSIT NEXT switches to put data in RAM. However, the first flip of the DEPOSIT switch turns the D4 light on, regardless of how the data toggles were set. Any attempt to load a value in memory with also set the D4 bit.

This all has me stumped. I've taken out each board, twice, and examined it to be sure all the components are in the right place and the solder joins are okay

Anybody have similar issues?

60D7E72D-8836-4D21-A10D-BEBF1A76468E.jpeg
Status after STOP/RESET
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Re: Front Panel Madness

Postby TronDD » January 31st, 2022, 6:35 pm

I think you have a similar problem to what I had, only my data lines were all stuck high. You seem to be stuck at 020Q.

EXAMINE tries to force a JMP (303Q) onto the data lines and go to the address on the switches but the CPU is seeing the 020Q which appears to be an unused opcode that acts as a NOP so the address increments as if you had hit EXAMINE NEXT.

Does EXAMINE increment the displayed address by 1 or by 3?

Unsure how RESET works but if it tries to execute a 3 byte command (by doing the equivalent of an examine after resetting the CPU, for example) and they all are stuck to 020Q, that will land you on address 2 starting from 0.

For me, it was bad solder joint(s) on the front panel s100 board. If you can narrow down to the data bus paths, you can look at those connections and ICs. Find the D4 line and where it's getting set high.
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Re: Front Panel Madness

Postby AltairClone » February 1st, 2022, 12:00 am

Let’s start with A1 being on after reset. So after a STOP-RESET (raise and hold STOP, raise and release RESET, release STOP), then all address lights are off except for A1, correct? During the RESET, did all lights come on while RESET was raised?

After reset with just A1 on, look at 8080 pin 26 (A1). Is it high or low at the 8080? If high, then the 8080 is probably bad. If low, then look at IC-B pin 5 on the CPU board. Is it high or low? If high, look at IC-B pin 4, is it high or low?

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Re: Front Panel Madness

Postby Ttpilot » February 1st, 2022, 12:35 am

AltairClone wrote:Let’s start with A1 being on after reset...


Okay. STOP/RESET pressed in proper sequence. As RESET is flipped, all the lights come on. The A1 light remains on after the reset. Pin 26 on the 8080 is high (5v).

I stopped at that point. It sounds like maybe the 8080 is faulty

I'll get a new one ordered. Of course, I have a couple of spare Z80s, but no 8080s lol

EDIT: PS Thanks for your diagnostic skills

Also EDIT: Corrected pin 20 to 26
Last edited by Ttpilot on February 1st, 2022, 2:42 pm, edited 1 time in total.
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Re: Front Panel Madness

Postby Ttpilot » February 1st, 2022, 1:38 am

TronDD wrote:Does EXAMINE increment the displayed address by 1 or by 3?


EXAMINE increments by 1, starting from address 02q, as does EXAMINE NEXT
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Re: Front Panel Madness

Postby TronDD » February 1st, 2022, 9:11 am

Hmmm. Interesting. I don't think the problem is exactly as I described then.

I'll leave it to Mike. He knows way more about these machines than anyone. I don't want to add confusion to the debugging processes.

But you did actually check pin 26 of the CPU and not pin 20, right?
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Re: Front Panel Madness

Postby Ttpilot » February 1st, 2022, 11:10 am

TronDD wrote:But you did actually check pin 26 of the CPU and not pin 20, right?


Yup, 26. It was kind of a pain; it’s hard to see the bottom row of pins. I ended up propping one side of the computer up in the air

EDIT: Thanks for pointing out my error when I typed pin 20. Fixed that now
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Re: Front Panel Madness

Postby Ttpilot » February 5th, 2022, 5:23 pm

AltairClone wrote:...look at 8080 pin 26 (A1). Is it high or low at the 8080? If high, then the 8080 is probably bad. If low, then look at IC-B pin 5 on the CPU board. Is it high or low? If high, look at IC-B pin 4, is it high or low?


8080A pin 26 was high. I replaced the 8080A and got the same result. IC-B pins 4 and 5 both show high

I swapped IC-B and IC-F to see if the 8T97 was the problem. Same result

FWIW If I remove the cpu and front panel interface but leave the RAM board in, the A1 pin on the motherboard is low. With the front panel interface board reinstalled, A1 is still low. When the cpu board is added, A1 is high. All these after STOP/RESET. That would seem to isolate the problem to the cpu board, wouldn't it?
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Re: Front Panel Madness

Postby AltairClone » February 5th, 2022, 7:24 pm

If not the 8080, then I would have expected failure of IC-B (pin 4 internal short). However, if you’ve swapped IC-B then that’s not the problem either.

I’ve been under the impression that A1 is stuck on. If not, that changes everything. Let me verify I understand your tests before we try anything else. Stop-Reset leaves only A1 on, correct? If you then do examine next, it goes to address 3 (A1, A0 on), correct? If you then hit examine next one more time, does it go to address 4 (i.e., A1 turns off) or is A1 stuck on no matter what you do?

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Re: Front Panel Madness

Postby Ttpilot » February 6th, 2022, 12:04 am

It's doing as you said: A1 goes off as the address counter moves. RESET moves it back to 02. Here's a video of it http://hawken.sdsmt.edu/8800panel.mov.

Note that D3 lights up as soon as I hit DEPOSIT, regardless of how the switch is set. This is in addition to the A1 weirdness
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