I've borked it

[msknight]

3.7k does this...

[msknight]

1.8k does the same but, while on the ULA side the drop reaches 0v, on the RAM, the drop only reaches about 1.2V

[msknight]

Actually, the 3.7K-ish brought the RAM side down to about 0.8v and upping to 4.7K brought the ram side down to 0.7v

[msknight]

Reading up on the operation of the ram and rom select... wish I had a four channel scope!

[1024MAK]

1.8k does the same but, while on the ULA side the drop reaches 0v, on the RAM, the drop only reaches about 1.2V

That's due to R2 (680Ω).

If there are no unintended shorts to anything else, the circuit formed by the 1.8kΩ resistor, R2 and the ULA (when outputting a logic low, and assuming it's output is at or near 0V), the voltage at the RAM chip side of /RAMCS will be:

5V/(1.8k+680)*680=1.37V approx. or slightly lower due to the RAM chip drawing a little bit of current.

Mark

[1024MAK]

Does the ULA / vLA81 work okay in another ZX81?

Mark

[msknight]

Does the ULA / vLA81 work okay in another ZX81?

I don't have another socketed ZX81 - I'd have to socket one. However, I do have two vLA81 and neither of them work. Dip switches on-off-off-off ... I get the same display output.

[msknight]

Just for the giggles, I've tried a third Z80 processor... same result.

[msknight]

OK - first is with no resistor installed. Second two is with the 4.7K resistor installed. Top is ram select, bottom is rom select

I've been reading the three parter in Sincus News and it's supposed to be a straightforward circuit. But this looks very, very odd. I can't make much sense of what's going on here.

[msknight]

I took the processor and ULA out and used the signal generator to feed a square wave in to A14, /MREQ and Romsel and RAMsel. The signal ended up where it should, looking like it should (except a slight curve at the front of the two sels where presumably the resistor was playing a part) and I couldn't detect any shorts.

I also changed the RAM for another fresh MyRetroStore ram module and still the same result.

I've got a hunch that when the solution to this is found, that I'm going to kick myself so hard that I'll end up on the moon.