ZX80 Core - new ZX80 motherboards

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Sat May 17, 2014 8:29 pm

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.

XorA
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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Sat May 17, 2014 10:16 pm

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.

XorA
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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Sat May 17, 2014 11:54 pm

Found it, got confused with the SMD caps and fitted C97 with a 10uF, replaced with the 1nF and I have the magic K :-)

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Sun May 18, 2014 12:28 am

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.

XorA
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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Sun May 18, 2014 12:47 am

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 :-)

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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Sun May 18, 2014 12:50 am

As a bonus this was the first time I had a chance to use logic analiser properly since I bought it years ago :-)

poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Sun May 18, 2014 3:41 pm

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.

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Sun May 18, 2014 3:49 pm

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.

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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Sun May 18, 2014 6:56 pm

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

poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Sun May 18, 2014 7:00 pm

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.

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