Testing ZX81 ROM

Discussions about Sinclair ZX80 and ZX81 Hardware
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Luc
Posts: 7
Joined: Sun Aug 01, 2021 5:01 pm

Testing ZX81 ROM

Post by Luc »

I still can't get my old ZX81 to work. I think the problem is with my monitor's composite video input, but I don't have another one to try.
Meanwhile I plugged my Z80 into another Z80 computer and it works.
My ULA also gives a signal, but it is still the type without a back porch.
To make sure it's not my ROM I tested it with an Arduino UNO for which I made a shield with 2 MCP23017 ICs (=2X I2C to 16bit parallel
I use 1 MCP as output for the address bus (16 bit of which I use 13)
1/2 MCP as input for the data bus (8 bit which I all use)
1/2 MCP as output for control lines (8 bit of which I use 1)
My ROM works and gives the correct data.
So it can only be the ULA and the composite video output or monitor.
My next step is to install a composite video with a back porch and then go to an old monitor with a composite video input.
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Paul
Posts: 1511
Joined: Thu May 27, 2010 8:15 am
Location: Germanys west end

Re: Testing ZX81 ROM

Post by Paul »

I didn't find why you are shure the problem can't be with the RAM.
Without (working) the Zeddy won't display anything.
The easiest way is alway to try with a working ZX81 and swapping parts.
My preference is finding someone near you who has a working setup. This way you can be shure wether it's your screen, keyboard, powersupply or what ever.
And it's always good to meet someone who is interested in the same Things. Not just beer and meat.
In theory, there is no difference between theory and practice. But, in practice, there is.
Neurogate
Posts: 24
Joined: Tue Aug 31, 2021 6:46 pm

Re: Testing ZX81 ROM

Post by Neurogate »

How did you test the video signal? Do you have an oscilloscope or did you see something wrong on the monitor?
If you think that it is problem with monitor, where you have a composite input, I can advise to use a simple AV to VGA converter with usual LCD monitor, at me such solution work with ZX81 without any problem.
Sinclair ZX81, Sinclair QL, Sinclair ZX Spectrum +2, Atari 130XE, Commodore 64C, Commodore C128, Yamaha YIS503IIIR MSX2 & etc
Neurogate
Posts: 24
Joined: Tue Aug 31, 2021 6:46 pm

Re: Testing ZX81 ROM

Post by Neurogate »

Paul wrote: Mon Sep 06, 2021 6:22 pm I didn't find why you are shure the problem can't be with the RAM.
Some time ago in similar situation I was thinking that I have problem with ULA or RAM and even has ordered new RAM chips... but after ordering I found that resistor between emitter and video output in composite mode was wrong and after installing of variable resistor 100ohm and some tuning I got good result. As for me better to check everything before ordering some parts, better to start from output - to check by oscilloscope video signal, maybe problem is more simple as we think.
Sinclair ZX81, Sinclair QL, Sinclair ZX Spectrum +2, Atari 130XE, Commodore 64C, Commodore C128, Yamaha YIS503IIIR MSX2 & etc
Luc
Posts: 7
Joined: Sun Aug 01, 2021 5:01 pm

Re: Testing ZX81 ROM

Post by Luc »

Thank You all,


Regarding the video signal, I did some tests a few weeks ago, see https://sinclairzxworld.com/viewtopic.php?f=7&t=4306
After that post I connected my 'oscilloscope' again and got a much better image.
I think I,m going to change the composite electronics with one with a 555 timer, then I will get a back porch and then I'm going to test again on a reliable monitor.

Replacing the Ram (now 2K) with a 16K is also a good idea.

rgs

Luc
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