ZX81 simple 16K RAM mod

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RetroTechie
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ZX81 simple 16K RAM mod

Post by RetroTechie »

Inspired by this thread, I put up a web page detailing how I did my last few internal 16K mods.

http://bitcycle.org/retro/zx81/internal_RAM/

Pictures included, that should make it easy to locate solder points etc. Description is complete and verified for accuracy.
Highlights:
  • RAM chip goes into a socket, without first bending pins sideways. So very easy to swap RAM chips. All changes are made on the board / IC socket itself.
  • 10 cm. wire in total does the job. 8-)
Only 4 pins on the socket are changed:
1: /RFSH -> A11
2: +5V -> A12
21: +5V -> A10 (LK1 -> LK2)
23: /WR -> A13

Most 16 or 32K RAM mods show a pull-up resistor on pin 27 (/WR). I have long suspected, and recently verified it's not needed for a basic 16K mod, so it's not included here. All hi-res software I've thrown at it, works fine regardless. What would it do for a basic 16K mod anyway? The Z80 drives the /WR signal always afaik, so a pull-up on there serves no purpose. Unless power is failing and you want to keep the SRAM bits as they are... :P (read: battery backup).

I believe this to be the 'cleanest' 16K mod I've seen to date. And for the 32K RAM modders out there: no reason you couldn't use this as a starting point. Pin 26 is where it's at... ;)
sirmorris
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Re: ZX81 simple 16K RAM mod

Post by sirmorris »

Nice!
TheStoryteller
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Re: ZX81 simple 16K RAM mod

Post by TheStoryteller »

The link is dead unfortunately... is this info still around somewhere? Or is there a better way to do it? :)
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1024MAK
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Re: ZX81 simple 16K RAM mod

Post by 1024MAK »

I showed how I did an issue 1 board here ;)
Note that my mod is designed to allow a further modification to 32k bytes. Hence the cutting of the +5V track, then fitting a wire link across it.

Dave Curran of Tynemouth Software shows how he does an upgrade on an issue 1 board here.

RetroTechie's site shows how to do the mod on an issue 3 PCB. See the archived web page here.

So take your pick. They all do the same. Expand a ZX81 to have 16k bytes of internal RAM.

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