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ZX-81 Project, ram expansion help

Divarin

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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?

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NeXT

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When I built my Speak & Spell interface my ribbon cable off the expansion interface was a bit longer but it did work. For a machine that simple and that slow timing on a cable like wouldn't be a problem. For a connector however I stole an ISA slot and cut it down to size. Now that you have soldered the edge connector that option isn't available, but a pin header should be fine. Is the ZX81 just crashing at random or failing to come to BASIC at all with the RAM pack plugged in?
 

Divarin

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I could desolder the pin headers if needed.

As long as I don't plug in the ram expansion the zx81 runs fine. If I try to power it up with it connected either I'll get a white screen (no cursor) or a rolling horizontal black bar.
 

Eudimorphodon

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You're absolutely positive that when soldering the cable you didn't make a mistake like flipping the wire pairs top to bottom? It's easy to do.
 

Divarin

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You're absolutely positive that when soldering the cable you didn't make a mistake like flipping the wire pairs top to bottom? It's easy to do.
Yeah that was my assumption as well. I went through each pin about 4 times with a multimeter checking to make sure the top rows are connected to top rows and bottoms to bottoms. I might just do it for a 5th time after I get off work though.
 

Divarin

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ah figured it out, turns out I had it right (eventually) and it was just that I wasn't waiting long enough on startup. I forgot it takes a few seconds to get the "K" cursor when the ram expansion is connected.
 

Divarin

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As long as you're soldering, you might want to consider a mod to put 16K RAM directly onto the motherboard, as a single chip. e.g. http://blog.tynemouthsoftware.co.uk/2017/10/zx81-internal-16k-ram-reversible-version.html
Not a bad idea, but there should be plenty of room in my wooden case for it as it is given the top needs to support one of those portable TVs (I'm hoping I can find one with a composite input or figure out how I can feed a composite source to it)
Right now I'm working on a video inverter circuit to try to make it white on black instead of black on white.
 

SiriusHardware

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Have to agree with wmcbrine above, you can't beat just putting a 62256 RAM in in place of the original 1K or 2K RAM and soldering a few extra wires to link up the higher address pins. The end result is compact, stable, and uses less power, you can even do it to a ZX81 in its original case so it combines the original neat looking (design award winning) outline of the base machine with 16K of internal RAM. Both of my ZX81s are modified that way.

As an original ZX81 user back in the day I wouldn't want to suffer external RAM packs again, ever, although connecting one via a flexy ribbon should be better than the 'hard' connections they all used to use back then.
 
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