Sinclair ZX80 / ZX81 / Z88 Forums

Hiya,

There are a number of 32k expansions possible on the '81 and I presume '80. One such design is here:

http://www.zx81.de/english/32k-rame.htm

This seems like a silly newbie question but here goes:

What would be required to make these work as external add-on cards?

I think I know the answer but let's see

i'm Still scratching my head wondering why the full 32K is 16K-32K and 48k-64K instead of 16K-48K surely that would make more sense ? As for making this external, i don't see anything particually hard, you would have course have to add a little more decoding for the /RAMCS and also to disable the internal ram at the same time. getting hold of germanium diodes can be a little troublesome these days but i am led to believe that using a schotkey (or however you spell it) diode is ok for this usage.

Regards Andy

Hi Andy and Sir Morris!

the memory areas 16-32 and 48-64K are a result of "low cost" design. No extra /RAMCS is needed, it works with the internal ZX81 /RAMCS which is low in these two areas only.

If you want to build an external RAM-extension, you must create your own /RAMCS with a LS139 or LS138 e.g. and you must pull the internal /RAMCS to Vpp, when the external RAM is active.

Hope that helps

Peter

Doh ! Thanks Peter i knew it would be something Blindingly obvious.

Andy

Brilliant, thanks!