Status Lights Wrong

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

Status Lights Wrong

Postby toml_12953 » February 9th, 2019, 4:14 pm

When I first turn on the 8800c, all the data lights and the INTE light are on. All other lights are off.

If I do a STOP-RESET, The INTE light goes out, the WAIT light comes on, the PROT light stays off and all the status lights come on.
They don't change as a program runs, either. That's not normal.

On the Clone, MEMR, MI, and WO are on and all other status lights are off after a hard reset.

I can examine and deposit memory locations properly (as well as examine next and deposit next)

I have the FDC+ board with the monitor disabled, RAM enabled, and I'm using PINT.

Any ideas on what to check?
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Re: Status Lights Wrong SOLVED

Postby toml_12953 » February 9th, 2019, 5:35 pm

It was a simple matter of a bad CPU chip. Now that I replaced the 8080A, all the lights respond correctly. Now, on to test the floppy drives!

This is a great learning experience for me. All my other projects (Altair Duino, PiDP-8, PiDP-11 μKenbak-1 among others) worked the first time so there was no thinking or debugging necessary. As Bill Gates once said, “It’s fine to celebrate success, but it is more important to heed the lessons of failure.”
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Re: Status Lights Wrong

Postby AltairClone » February 21st, 2019, 7:38 pm

There’s a good chance the “bad” 8080 is fine and the two phase clock on the CPU board is out of spec. This is a very common problem with the Altair CPU clock circuit and its two 74123 one-shots. Fortunately, it’s very easy to measure the CPU clocks and adjust the one-shot resistor value(s) to bring the clock into spec.

Mike
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Re: Status Lights Wrong

Postby toml_12953 » February 22nd, 2019, 5:12 pm

AltairClone wrote:There’s a good chance the “bad” 8080 is fine and the two phase clock on the CPU board is out of spec. This is a very common problem with the Altair CPU clock circuit and its two 74123 one-shots. Fortunately, it’s very easy to measure the CPU clocks and adjust the one-shot resistor value(s) to bring the clock into spec.

Mike


NOW ya tell me! :) The 8080a is cheap enough so that my shotgun approach is OK this time. It's good to know, though. Thanks!
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