ZX80 Core - new ZX80 motherboards

[balford]

Thank you for the offer, that's very kind I'll persevere with it for the moment as I feel it's something very small, and I'll kick myself once I find the issue

I might take you up on it in the new year if I haven't gotten anywhere by then.

I've tried a number of CPU's (both CMOS and NMOS) BTW.

Thanks again!
B

[blittled]

I know what I'm going to use some of my Christmas bonus on, a ZX80 Core board. I've been looking for a discrete ZX81 motherboard for awhile for hacking so I can easily replace anything I blow up since I "killed" the ULA on about 3 TS1000's. I'll be looking with great anticipation to the release of next batch of boards in January!

I would also be interested in the ZX80NMIX board

[PokeMon]

@balford

There is one more test you could do, simply put IC1 and IC15 back to ZX80CORE and connect pin 25 of CPU (/BUSRQ) to GND with a small wire ?
This is a bit similar to my last suspect, that it could have to do with high impedance state of cpu when no clock is supplied.
So address bus, data bus and the main control signals of CPU will be either high impedance or just at logic high.
Then measure if the /RESET is still at 1.4V.

And could you please check, that R21 is really 22k and not 2k2 ?

Busrq.jpg (105.74 KiB) Viewed 4186 times

busrq2.jpg (56.04 KiB) Viewed 4186 times

[balford]

I attached a wire between GND and /BUSRQ, still measuring 1.4V at /RESET. I do have a good 3.25MHz signal at CLK though.

Also verified that R21 is indeed 22k.

Thanks,
B

[Kiwisek]

Count me in for a ZX80NMIX board!

[PokeMon]

I attached a wire between GND and /BUSRQ, still measuring 1.4V at /RESET. I do have a good 3.25MHz signal at CLK though.

Also verified that R21 is indeed 22k.

Thanks,
B

Okay - sorry then. No more idea. Would be interesting to test the board personally.
What happen if you just remove the reset capacitor C10 ? Same 1.4V ?

[1024MAK]

Hi Balford

Your ZX80CORE reset problem...

Are both the +5V and 0V connections to the Z80 CPU good (test on the chip pins)?

Have you tried a different Z80 CPU?

Alternatively, try temporarily piggybacking a 4.7k or similar value resistor across the 22k resistor in position R21 and see what voltage you now get.
If there are no circuit board short circuits, the problem may be a Z80 CPU with a damaged /Reset input or capacitor C10 (10uF) is leaky.

Mark

[balford]

Interesting...

I removed C10 and took a measurement of the /RESET pin...



Looks like there's a short with something carrying a clock signal. At least this narrows it down...

B

[1024MAK]

Is the 'scope waveform taken with the Z80 CPU trying to run, or with it held inactive by holding /BUSRQ low?
If the former, hold /BUSRQ low, as otherwise the Z80 may produce some confusing signals to the rest of the board as it will be constantly being reset...

Mark

[PokeMon]

Okay - this is a good hint.
Something is holding it partly down.
So every 1.5 us forabout 0.4 us.
This should result in a higher level than 1.4V.

Maybe it is because the CPU is running now.
So every time the reset go high to start the CPU it will give a new reset after 4 clock cyles / 1.2 us.
Funny, I have to think about it.

Does this signal look same when you hold /BUSRQ down as Mark stated ?
On the other hand you could wire /WAIT down (pin 24 IC1) to see if there is a difference (either /WAIT or /BUSRQ, not at the same time).
It looks like as soon as the CPU starts its work it is reset which confirms my suspicion that there is a connection of the CPU signals to reset or at least to one of the gates forming the control signals derived from CPU signals.