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Re: ZX80 Core - new ZX80 motherboards

Posted: Sat May 17, 2014 8:29 pm
by PokeMon
So this looks like the video output is generated by the CPU. ;)
Did you check all jumpers as referenced in the manual ?

You can see the 32 characters on A15 (looks like 34 due to two "extra" cycles with A15 high).

Please check IC19 (74HCT74) pin 5 which should give the sync signal (repeating every 64us, small pulse of about 5us).
This is also digital.
And you can check IC20 (74HCT86) pin 6 which should give pixel data (when startup only the "K") and you can trigger that data two which should give only a small signal of about 1.2us (one character).

This can be done with a logic analyzer.

Re: ZX80 Core - new ZX80 motherboards

Posted: Sat May 17, 2014 10:16 pm
by XorA
ZX80-Core-A15-SYNC-PIXEL.jpg
(169.55 KiB) Downloaded 347 times
A15
Sync
Pixel

I have the bottom two positions of J96 (NMI) jumpered, top of J98.

Am I supposed to do anything with J94? Instructions are a little confusing around that, I have fitted SMDs to back, TV seems happy with signal apart from it being all black.

Re: ZX80 Core - new ZX80 motherboards

Posted: Sat May 17, 2014 11:54 pm
by XorA
Found it, got confused with the SMD caps and fitted C97 with a 10uF, replaced with the 1nF and I have the magic K :-)

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 12:28 am
by PokeMon
Great, yes 10uF for C97 is pretty too much.
This gives the backporch of about 5us with 1nF resulting in silence (or black screen).
So with 10uF you would have a very looooooong backporch - or silence forever. ;)

About J94 - I left this option if anybody wants to use the ZX8-CCB module instead of soldering the SMD components.
You can use it but should then remove R88/R89 smd resistors.
Maybe it works with both drivers when leaving them resulting in a more powerful output - but this can be too much for TV (out of standard).
I didn't try this. :shock:

About the SMD capacitors - I gave one more 10uF just to have a different amount of each capacitor value to distinct them.
Unfortunately the values are not printed on.

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 12:47 am
by XorA
PokeMon wrote: About the SMD capacitors - I gave one more 10uF just to have a different amount of each capacitor value to distinct them.
Unfortunately the values are not printed on.
Yeah, I just got over eager and soldered the two, I went back and checked the parts list and realised what I had done.

Now I need to decide about the keyboard :-)

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 12:50 am
by XorA
As a bonus this was the first time I had a chance to use logic analiser properly since I bought it years ago :-)

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 3:41 pm
by poglad
PokeMon wrote:So I meant to measure the pin 27 of EPROM (/WR).
What voltage do you have here ?
That pin is high on my board - ROM position 6.

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 3:49 pm
by PokeMon
So - sorry, no guess now what it is and without a scope or logic analyzer hard to find.
Please check the other EEPROM I did sent to you and maybe I have an idea when I checked the ZX80CORE from the german fellow who had problems with startup.

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 6:56 pm
by XorA
The place where you put the v1.0 manuals seems to be down, I should really get my v1.0b board up and running as well :-D

Re: ZX80 Core - new ZX80 motherboards

Posted: Sun May 18, 2014 7:00 pm
by poglad
I've done some more checking - with the ROM chip removed. In the various switch positions, with J100 connected in the right position, I've found that pin 27 /WE is high all the time, while pin 1 A14 toggles between low and floating in various positions. A13 and A12 toggle between low and high as I would expect.

If I connect J100 the other way (against the advice of the user guide), I can get /WE to go low sometimes. So I think those two pins are definitely the root of my troubles, and will look on the schematic and try to understand what's up.