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8032 - No Chirp, No Screen

Bungo Pony

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I decided to bring out my two Commodore 8032s. One of them works (after I replaced a RAM chip) and one of them does not. I figured I could use the working one to help diagnose the one that doesn't work. However, I'm hitting a bit of a wall.

Here's what I've got figured out so far... The voltages are all good. Also, the socketed chips are all good as I was able to swap them between the working machine and the bad one. The monitor powers up and works, as I'm able to get the bright dot when I power it on with the connector unplugged. I have good clock signals, both 16MHz and 1 MHz.

Now, if I use my scope on the address lines, it appears that the CPU is trying to send signals on at least some of them while some of them just go high. My guess is it may be one of the ROM chips not functioning. Is there a way to tell which one may be dead?
 

daver2

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You have checked the power rails and clock.

Have you checked the /RESET, /IRQ, /NMI and SYNC signals on the CPU?

After that, yes a NOP generator, burn a copy of my PETTESTER ROM or use one of the available ROM/RAM replacement cards. Some of these (notably from VCFED members here) incorporate a NOP generator and my PETTESTER.

Any further questions, just ask.

Dave
 

daver2

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Do you get a continuous train of high-going short pulses on SYNC or do they stop after a while?

Each pulse of SYNC indicates one instruction being executed by the CPU.

If the pulses stop, this indicates that the CPU has given up executing instructions and has most likely gone into the weeds.

Dave
 

daver2

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It’s dying fairly quickly then!

Either you need to make a NOP generator, download and burn my PETTESTER firmware (to replace the EDIT ROM) or use a ROM/RAM replacement card with the above built in.

Dave
 

Bungo Pony

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Either you need to make a NOP generator, download and burn my PETTESTER firmware (to replace the EDIT ROM)
I could probably do both of those. I had to look up what an NOP generator was, and it just a little piece of hardware that needs to be soldered together; something I should have all the parts for. I can also probably program your PETTESTER onto an EPROM. Unfortunately, my programmer is kinda crap and I might need to use the one in my Apple II to make it.
 

daver2

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If you have all the bits for the NOP generator in your ‘bits box’ I would suggest making one of those first and giving it a try.

The CPU should then continuously execute instructions (even though they are just NOPs).

If you have an oscilloscope or logic analyser you can also check to see that the address bus (including address buffers) and the RAM/ROM decoding logic is working OK.

You can also look for ‘strange’ voltage levels on the data bus (potentially indicating bus contention or faulty ROMs - but not bad cells in the ROM).

After that, you will need my PETTESTER.

You may find that another VCFED member close by would program you an EPROM if you cover their costs.

Dave
 

dave_m

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I get exactly one small pulse and then nothing.
Assuming your Reset is giving a good half-second low pulse to initialize the CPU, that is a very rare scenario. One Sync pulse indicates one fetch then a hang up condition. That would indicate a bad 6502 CPU, but you say you swapped it so that can not be it.

The only way that would happen is if the first instruction fetched was an illegal 'kill' instruction. So I suppose a bad kernal ROM (UD6) could be the problem. Or maybe a bad CPU socket.

Keep that in mind if the Pettester code does not run.
-dave_m
 
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