Divarin
Veteran Member
I have this project I've been working on. I picked up a zx-81 (or was it a timex sinclair 1000?) from a flea market for $7 along with a 16k ram expansion.
The computer seems to work but the keypad doesn't (typical). Since I already have a working zx-81 I decided to use this as a project and have a little fun with it.
Right now it's kind of a big mess-o-wires on my bench but I plan on building an all-in-one case for the system out of pallet wood with a small portable monitor integrated into it, maybe an iron carry handle.
First, I made a mechanical keyboard for it because I had switches and caps left over from a previous project. The next thing I wanted to do was to attach the ram expansion. I actually had two ram expansion units and both were not usable for the same reason, broken edge connectors. Given that these were always flaky edge connectors anyway I got the idea that I'd cut the edge connector off and solder wires between the computer and the ram expansion.
The edge connector is 44 pins (a group of 40, a gap where the edge connector's "key" goes, then a group of 4.
For the group of 40 I used a repurposed an IDE cable and for the group of 4 I used dupont wires.
I can't get this to work however and I suspect the reason is that the length of the wires has affected the timing of the signals. I have triple-checked that each pin/wire has connectivity, they are connected to the correct pin, and there's no cross-talk between neighboring wires (except the ground wires on the end as expected). I also tried both ram expansion units.
The next most logical step is probably to de-solder the pin headers that I put onto the main board and try connecting a working ram expansion normally (I have one more that works and I use it with my other working zx-81) just to double check that there isn't an issue with the computer itself which is causing it to not work with ram expansion units. If that seems to work either I can spend the money to get another ram expansion unit or I can try to find a way to connect one of these "broken" ones to the board in a way that will function. I'd rather do the later because a) I don't want to waste old hardware that would function if I can hook it up correctly, b) I don't want to spend money if I don't have to, and c) I liked the idea of a soldered-on ram expansion so I don't have to worry about it coming loose as they do with those card-edge connectors.
Does anyone else have any experience with "alternative" and "non-traditional" ways to connect ram expansion units to ZX-81's and can you maybe confirm or discount my suspicion that my issue is caused by wire length/timing?
The computer seems to work but the keypad doesn't (typical). Since I already have a working zx-81 I decided to use this as a project and have a little fun with it.
Right now it's kind of a big mess-o-wires on my bench but I plan on building an all-in-one case for the system out of pallet wood with a small portable monitor integrated into it, maybe an iron carry handle.
First, I made a mechanical keyboard for it because I had switches and caps left over from a previous project. The next thing I wanted to do was to attach the ram expansion. I actually had two ram expansion units and both were not usable for the same reason, broken edge connectors. Given that these were always flaky edge connectors anyway I got the idea that I'd cut the edge connector off and solder wires between the computer and the ram expansion.
The edge connector is 44 pins (a group of 40, a gap where the edge connector's "key" goes, then a group of 4.
For the group of 40 I used a repurposed an IDE cable and for the group of 4 I used dupont wires.
I can't get this to work however and I suspect the reason is that the length of the wires has affected the timing of the signals. I have triple-checked that each pin/wire has connectivity, they are connected to the correct pin, and there's no cross-talk between neighboring wires (except the ground wires on the end as expected). I also tried both ram expansion units.
The next most logical step is probably to de-solder the pin headers that I put onto the main board and try connecting a working ram expansion normally (I have one more that works and I use it with my other working zx-81) just to double check that there isn't an issue with the computer itself which is causing it to not work with ram expansion units. If that seems to work either I can spend the money to get another ram expansion unit or I can try to find a way to connect one of these "broken" ones to the board in a way that will function. I'd rather do the later because a) I don't want to waste old hardware that would function if I can hook it up correctly, b) I don't want to spend money if I don't have to, and c) I liked the idea of a soldered-on ram expansion so I don't have to worry about it coming loose as they do with those card-edge connectors.
Does anyone else have any experience with "alternative" and "non-traditional" ways to connect ram expansion units to ZX-81's and can you maybe confirm or discount my suspicion that my issue is caused by wire length/timing?
