ZX80 Core - new ZX80 motherboards

[balford]

I took a reading with /BUSRQ held to GND and the resulting waveform is exactly the same as before.

Holding /WAIT down produces the following:



Thanks for the help so far guys, much appreciated

B

[1024MAK]

Best explore the rest of the board looking for the same signal elsewhere...

Mark

[PokeMon]

Looks like a charging curve of a capacitor of about 100pF with 22k - something like that.
Mysterious.

By the way - please remove ROM and RAM for further tests. This will reduce the components used.

[PokeMon]

Please reduce in a further step all components except the ICs you can see here now:

http://forum.tlienhard.com/phpBB3/viewt ... =175#p9094

This is my very first test of ZX80CORE and test only CPU.
This way the CPU is executing only LD B,B commands (4 clock cycles long) and counting address bus up.
So A15 will toggle every 40ms, A14 every 20ms, A13 every 10ms and so on.
This can be measured out easily and there are not much ICs which can disturb the RESET signal.

I think (and hope) you have all ICs socketed.

[balford]

Yes, luckily all IC's are socketed.

I removed all except the CPU, IC15, IC18 and IC20. I also reinstated C10 at this point.

Powering on the /RESET signal looks fine.

Measuring A10-A15 gives the following pattern though:



This pattern is identical and synchronised across A10-A15, and drops to 0v every couple of seconds for about half a second a time.

B

[PokeMon]

Okay - this looks good so far.
So please next step:

Put back IC6, IC7, IC8 (74HCT157) now and the EPROM IC2.
All other chips left removed except as in your last test.
So we will put them back step by step and find the problem, sure.
And put a wire for test only between A9 (IC1 pin 39) to /ROMCS (exactly now at pin 6 of IC6).

You should be able to measure the signal like in the scope picture in this posting which shows A9/ROMCS (so the wire you put) which shows a short phase of low (about 10 us) and a phase of toggling fast between low and high (440 us). This is a kind of loop and will show that the ROM is working. This works only with ZX81 rom, so the 28C256 and ROM switch to position 6.

http://forum.tlienhard.com/phpBB3/viewt ... =175#p9095


If you have this, you can remove the wire and put IC12 and IC13 on that board and the RAM (IC98).
If the RAM is working correctly, you should be able to measure A15 like in this posting shown scope picture.

http://forum.tlienhard.com/phpBB3/viewt ... =175#p9096

A15 is an indirect test if RAM access is working.
If that is in case, it should be active about 42 us and low for about 22 us showing a horizontal line of 64 us (15625 Hz).


In case you passed this test as well, you should put back all other chips in following order and measure the reset pin after every IC:
IC11
IC14
IC16
IC17
IC19
IC5
IC21
IC10
IC9

I think then are all ICs in the board back.
So after RAM check put IC after IC and measure the RESET pin after every insert.

[PokeMon]

This pattern is identical and synchronised across A10-A15, and drops to 0v every couple of seconds for about half a second a time.

B

I think the picture you posted show A10 (1.25 ms cycle time) and A11 must have half frequency (2.5ms) and so on till A15 (32*1.25 ms = 40ms).
Just to be sure.
By the way - this is a fine test and shows that the CPU is running perfectly (if not disturbed by any other IC ).
If you pass the ROM test this would be next step and the RAM test would be fine as well - showing ROM and RAM is running perfectly.
Anyway stop when your RESET pin is back again to 1.4 V and write what you did last.

[balford]

Ok, with a wire between pin 39 of IC1 and pin 6 of IC6, the /RESET signal is again held to 1.4v, and measuring pin 39 gives me the following signal with the ZX81 ROM image in place:



B

[PokeMon]

So looks we come closer to the problem.

1. Okay - then first remove this wire from pin 39 IC1 (A9) and connect it to GND instead (pin6 of IC6 to GND). Will that still give 1.4 V on reset ?

2. If still have 1.4V then remove IC2 (EEPROM). Will that still give 1.4 V on reset ?

3. If still 1.4V then remove IC 6,7,8 one by one and measure reset.

What is the condition to have reset normally ?

[PokeMon]

So I am in a short holiday by now, come back at 16th of decembre.
Hope you could find that issue now, otherwise could help you personally again in about one week or somebody here at the board could assist you to find it.