ZX81 Troubleshooting
Re: ZX81 Troubleshooting
Try a different CPU or look for shorts around pin17. (I use a Andy Rea's ULA).
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Re: ZX81 Troubleshooting
Check also the signal at CPU/18(=HALT) and ULA/17(=HALT) as well as the signals around TR1.
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Re: ZX81 Troubleshooting
I'm going to remove the Z80 (ordered) and try a different one
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Re: ZX81 Troubleshooting
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
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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.