Altair 8800c Build Troubleshooting Request

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

Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » February 28th, 2024, 6:55 pm

I wanted to see what my system does if I pull everything out but the CPU and panel control boards. On start up, all data and address lights, plus INTE are lit. After doing Stop + Reset the address lights are off, all data lights plus WAIT, MEMR, M1 and W0 are lit. If I flip the switch to run, all the address lights go on with A13-A15 flickering. All data lights plus MEMR, M1, STACK and W0 are lit. When stopped, I'm able to use the examine, deposit and single step switches.

It looks like what you are seeing is normal when the CPU is running and there is no RAM installed. You seem to have a problem halting the CPU. You should check pins 71 and 72 on the bus, RUN and READY respectively, to make sure they respond to the run/stop switch. If not, check the signals working back from there in the circuit until you find the problem. Also check the bus for shorts if you haven't already.
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » March 3rd, 2024, 8:33 pm

Thanks for the ideas, I will investigate those as soon as I can. Unfortunately most of my gear is packed away while we do a small renovation but I hope to get back to it soon.
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » April 4th, 2024, 10:13 am

Hello,

I'm Back. I had to pack up my workbench for a month while we were renovating. I now have the 8800c on the bench and am ready to figure out why this isn't working.

I have looked at READY, HOLD, INT AND RESET. The only line that changes is RESET, which follows the front panel RESET switch. All the other lines stay the same regardless of RESET, STOP or RUN. I also confirmed that there are no shorts on adjacent bus pins.
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Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » April 5th, 2024, 11:07 am

Next step is to check the logic signals using the schematic. Page 21 of the Front Panel manual has a schematic of the "CPU run/stop and single cycle circuit". Trace the signal starting at the run/stop input at IC F pin 2. Check the signal at each step of the path thru IC N pin 13 (RUN) to verify that the signals change as you toggle run/stop.
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » April 10th, 2024, 11:05 am

I appreciate your help and patience, I am learning the 8080 by stating from a non-working system which is not optimal.

After a Power on then STOP + RESET

The logic seems to flow ok. If I perform the above and look at N-13 which is the S-100 bus RUN it is L until I press RUN on the front panel and then it goes H and stays high even if I select STOP on the front panel. As far as I can tell the state of PRDY never changes, it is always high.
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » April 10th, 2024, 3:50 pm

Also, I re-read your post from Feb 28 and absorbed it this time:

"I wanted to see what my system does if I pull everything out but the CPU and panel control boards. On start up, all data and address lights, plus INTE are lit. After doing Stop + Reset the address lights are off, all data lights plus WAIT, MEMR, M1 and W0 are lit. If I flip the switch to run, all the address lights go on with A13-A15 flickering. All data lights plus MEMR, M1, STACK and W0 are lit. When stopped, I'm able to use the examine, deposit and single step switches."

When I have only the CPU card installed and power up all data and address lights, plus INTE are lit which is the same as you. But after a stop + reset only my address lights are on with A13-16 flickering. Nothing else is lit. If I select RUN, nothing changes, only address lines are lit with A13-15 flickering.
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Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » April 10th, 2024, 5:32 pm

mafisher wrote:Also, I re-read your post from Feb 28 and absorbed it this time:

"I wanted to see what my system does if I pull everything out but the CPU and panel control boards. On start up, all data and address lights, plus INTE are lit. After doing Stop + Reset the address lights are off, all data lights plus WAIT, MEMR, M1 and W0 are lit. If I flip the switch to run, all the address lights go on with A13-A15 flickering. All data lights plus MEMR, M1, STACK and W0 are lit. When stopped, I'm able to use the examine, deposit and single step switches."

When I have only the CPU card installed and power up all data and address lights, plus INTE are lit which is the same as you. But after a stop + reset only my address lights are on with A13-16 flickering. Nothing else is lit. If I select RUN, nothing changes, only address lines are lit with A13-15 flickering.


The lights are flickering because the CPU isn't stopped. That is the problem you need to trace. Flipping to STOP isn't stopping the CPU. The flip-flop, chip F, might not be acting correctly. Could be the chip or the signals that are feeding it.

Check that these pins toggle between 0 and 1 as you flip RUN and STOP.
chip F, pin 6 -
chip F, pin 10 -
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » April 10th, 2024, 6:19 pm

Pin 6 follows the RUN/STOP switch (H. = RUN, L = STOP).
PIn 10 is Low after a power up and still low after STOP + RESET and goes high after pressing RUN. After that it stays high regardless of STOP or RUN.

So presumably one of the signals coming into K 1, 2 or 13 is not correct.

Additional information. I have purchased a Ithaca Audio IA-1010 Z80 CPU card from ebay. Looking at the manual it is in 8080 compatibility mode. I plugged it in and while everything is not working the system appears to be behaving much closer to "correct". It seems to stop and run ok and pin 10 on IC F on the Display/Control Interface board is now following the RUN/STOP switch. This leads me to believe that my 8080 CPU board has a fault.
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Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » April 11th, 2024, 12:32 pm

It looks like the syncRUN/STOP is working since IC-F pin 6 responds to RUN and STOP

My guess is that the second flipflop isn't getting cleared IC-F pin 15 should go low when STOP is toggled.

The conditions to clear the flipflop are:

1) syncRUN/STOP =0 latches to 0 when STOP is switched - check at IC-L pin 2

2) phi2 =0 - clock should be constantly changing between 0 and 1 - check at IC-L pin 3

3) pSYNC = 1 at start of machine cycle. should be constantly changing while CPU is running. check at IC-K pin 2

4) M1 on DO5 = 1 machine cycle #1 should be constantly changing while CPU is running. check at IC-K pin 13

phi2, pSYNC and M1 are generated on the CPU board, so that could explain why it works when you change to the Z80 board. The problem would be somewhere on the 8080 CPU board.
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » April 18th, 2024, 6:07 pm

Thanks for the great suggestions knowing the various states helps a lot. I have discovered something strange, pSYNC is stuck low so I traced the signal all the way back to the 8080 where it is in fact changing, as is the input to pin 2 on J (CPU card.) However pin 3 on J is stuck low. If I lift the input to J pin 2 then the output which is pin 3 floats at around 3v and believe it or not RUN and STOP work. Don't get me wrong, the system is not working properly but now when I press RUN the address lights blink and WAIT is off and when I press STOP the address lights stop changing and the WAIT LED is lit. I have replaced the 8T97 and nothing changes. Progress I guess.
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