If you remember from my initial post at the start of the month, the first thing that I needed to do in order to make progress on the ZX81 Module was to find a donor for the ULA. Some things can’t be rushed though, so this has only just happened. Luckily I have several candidates to choose from.
I had some constraints though. Obviously the ULA had to come from a machine that worked. Ideally one that had undergone a composite mod. And preferably not the original one that my dads school friend bought back in the day and passed on to me a few years ago. (The ULA will only be borrowed to check that things work, but I would rather not risk damaging one that has sentimental value)
This one seemed to fit the bill. Apart from the fact that it is housed in a full size case with mechanical keyboard, there is nothing special about it. I think this may well have had various expansions or add-ons back in the day, but those are long gone now.
Luckily, it also has also been composite modded. In particular, this has been done via a transistor, rather than just sending the video signal out of the socket. This means that I can test that this works with my monitor before taking the ULA. I know that some modern monitors are fussy with ZX81 signals as there is no back-porch generated. Adding the circuitry to do that isn’t overly complex, but if I can keep things simple then all the better.
The TV image is actually really sharp, and certainly works with my monitor. So it looks like I have a donor chip!
Before removing it, however, there were a couple of checks that I wanted to make. Firstly, I have a file on my computer called ZX81.ROM which I think might be exactly what it sounds like. When opening it up in a hex editor, though, there is nothing readable in there at all. I would have expected to see something like the copyright message or the keywords listed. So I will take this opportunity to dump the first couple hundred bytes of the ROM to the screen so that I can compare it to the file I have.
I also want to take a quick look at the output signal on a scope.
Everything looks good and just what I would expect from a video signal (without a back porch)
So, with no further ado, out comes the ULA, ready for its new mission in an RC2014
Next up, I need to finish the schematic so that I know what value components I need to populate the rest of this board…
