ZX81 Troubleshooting

Discussions about Sinclair ZX80 and ZX81 Hardware
zx-heinz
Posts: 41
Joined: Thu Apr 27, 2017 8:56 pm

Re: ZX81 Troubleshooting

Post by zx-heinz »

Try a different CPU or look for shorts around pin17. (I use a Andy Rea's ULA).
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zx-heinz
Posts: 41
Joined: Thu Apr 27, 2017 8:56 pm

Re: ZX81 Troubleshooting

Post by zx-heinz »

Check also the signal at CPU/18(=HALT) and ULA/17(=HALT) as well as the signals around TR1.
citizenfish
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Joined: Sun Apr 11, 2021 5:31 pm

Re: ZX81 Troubleshooting

Post by citizenfish »

I'm going to remove the Z80 (ordered) and try a different one
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1024MAK
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Re: ZX81 Troubleshooting

Post by 1024MAK »

Pin 17 on the Z80A is the /NMI input. The non-maskable interrupt (/NMI) control is part of the ULA. The software has to write to the ULA to enable or disable it. This /NMI circuitry is the extra circuitry that ZX81/TS1000 etc. machines have compared to the ZX80 and clones, and it is this that provides the ‘display and compute’ or SLOW mode.

If the ROM software does not enable the /NMI generator in the ULA, or the ULA is faulty, then pin 17 on the Z80A will stay high.

Often if there is a RAM fault (or a ROM fault) it’s likely that the processor is executing garbage rather than running the intended ROM code.

So please test the other signals before doing anything else.

Also, do you have a working RAM pack?

Mark
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