Altair 8800c Build Troubleshooting Request

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

Altair 8800c Build Troubleshooting Request

Postby mafisher » February 5th, 2024, 10:14 pm

HI All,

I have just "completed" a build of an 8800c with a front panel board set, a reproduction CPU and an FDC+ and I am sorry to say that it is not working. I have the 9 slot Motherboard with 5 S-100 connectors installed (1,3,5,7 & 9). PSU voltages are good on the motherboard.

With only the front panel and the interface installed, the D0-D7 LEDs on the front are lit and all the rest are off. Is this expected behaviour? Switches do nothing, which I imaging is correct.

Adding the CPU card, all the voltages (+5, -5, 12) on the 8080 are correct. I have not yet been able to source a 74L00 and so I am using a 74LS00. Could be the issue?

When I power on; D0-D7, A0-A15 and INTE are lit. If I do a reset (STOP & RESET) INTE goes out, D0-D7 and A0-A12 are lit and A13-A15 flicker. If I use my iphone slow-mo camera it looks to me like the address lines are cycling backwards through a count. Stop doesn't do anything, in fact none of the switches do anything except reset. If I add the FDC+ I have the same behaviour.

I have messed for hours with the clock and have it as close as I can make it to the spec. (I am able to stop all activity if I take the clock out of spec.) I have checked for bent IC leads, bad solder solder joints, correct chips (other than the LS00 noted above) and chip orientation. I have even employed the "shotgun troubleshooting method" by replacing all the chips on the CPU board.

Does anyone recognize the symptoms or have any suggestions?

Thanks, Mark
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Re: Altair 8800c Build Troubleshooting Request

Postby ef1j » February 7th, 2024, 1:29 pm

Sounds like the processor is moving the PC through all of the addresses (can't think of any mechanism by which it could count down, though...)

Could it be that the PRDY line is stuck? I would check the PRDY line on the front panel interface board... and T1, IC L, E, and F, etc. if it doesn't look right.

Eric
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » February 7th, 2024, 7:08 pm

Thanks, I will try that.
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Re: Altair 8800c Build Troubleshooting Request

Postby AltairClone » February 7th, 2024, 11:45 pm

First thing to check is the CPU clock. There’s a good chance it will need some adjustment to bring it into spec. Post a scope shot that shows one cycle of the clock with phi-1 and phi-2 overlaying each other on the same voltage scale and at the same vertical position. Measure phi-1 and phi-2 on IC-Q pins 5 and 13.

Mike
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » February 11th, 2024, 6:26 pm

Thanks, I experimented with the clock again and this is as close as I can get what I think is the spec. The waveforms are pretty pathetic looking compared to what I have seen online. I have replaced every component in the circuit and it doesn't get any better. Screen grab below.

I am changing tactics for the moment and have started looking at the front panel and interface board. I started with the PIC and the state machine signals and If I look at the serial in line on the PIC I can see every switch is functioning so I think the front panel board is ok. I also assume that many of the states will depend on a properly functioning CPU board.

smRUN/STOP* responds as I would assume it should, High for RUN and low for STOP.

Under what conditions does the smRESET go high? RESET button toggled? STOP and RESET toggled? or is it dependant on some other signal from the CPU? I am not seeing any activity.

This is one of those chicken and egg situations. I want to learn the 8080 in order to have a deep enough understanding to troubleshoot and make new hardware but I need a working unit first to start learning... :)
Attachments
phi1 and 2.png
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Re: Altair 8800c Build Troubleshooting Request

Postby AltairClone » February 11th, 2024, 7:36 pm

The clock looks to be out of spec. The biggest issue the 8080 has is with the timing from the falling edge of phi2 to rising edge of phi1. It needs to be > 70ns, but yours looks less than that. It’s harder to measure without standard time divisions, but yours appears to be around 40-50ns at the midpoint.

Adjust R14 proportionally to reduce the high period of phi2. This increases the phi2 to phi1 time as we need. The high period must remain > 220ns. If more time is needed, decrease phi1 proportionally which will further increase phi2 to phi1 time. Phi1 high must remain > 60ns.

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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » February 11th, 2024, 9:12 pm

Ok I tried again basing measurements at 2.5v. As I read the spec the measurement for the time between PHI2 and PHI1 is measured from the low threshold on the trailing edge of PHI2 to high on the leading edge of PHI1. If I adjust and measure at 2.5v; PHI2 is 230nS, PHI2 to PHI1 is 107nS and PHI1 is 95nS. This capture is what I have now and should be a little easier to read. I also tried a few variations on either side if these number and, sadly, it is still not working. Thanks.
Attachments
PHI1 AND PHI2 Again.png
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Re: Altair 8800c Build Troubleshooting Request

Postby AltairClone » February 11th, 2024, 11:49 pm

Clock looks better now. It would have been a problem later anyway, so it’s good to get it right.

smRESET should go high anytime you raise the reset switch. If not, look at IC-C pin 3 on the front panel and verify it goes low when you raise reset.

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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » February 12th, 2024, 7:15 pm

smRESET does indeed go high when reset is toggled, I am not sure why I couldn't see it the other day. Which is good but now I am out of clues as to why my 8800c is not working.
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Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » February 28th, 2024, 5:08 pm

Did you get your system running? The flickering A13-A15 is an interesting clue. If they are indeed counting down, it suggest the stack pointer is being loaded to the address bus and the processor is trying to put data in the stack. The address led's are controlled only by the signals on the bus, so what's driving the address bus if not the CPU? The panel board set is able to inject code into memory, so maybe it does as well.

I would start by verifying the READY, HOLD, INT AND RESET inputs to the CPU show it is halted. There could be a problem with the logic related to the STOP switch.

Also, just for grins, make sure there aren't shorts between adjacent pins on the bus board. I found one on mine when I checked it before assembly.
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