ZX81 ZXpand interfaces:
http://www.sellmyretro.com/offer/detail ... rface-2435
ZX80 ZXpand Interfaces:
http://www.sellmyretro.com/offer/detail ... rface-3754
http://www.sellmyretro.com/offer/detail ... odule-2331
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Internal RAM can be limited by using a smaller RAM.
So the ZX80CORE can be held exactly like a real ZX80.
The question is, what is important for the ZXpand to work on a ZX80 and how it is linked in the system (ROM) ?
To be sure: The ZX80CORE has /ROMCS on the connector (if J99 is set) - so if this signal is provided (like in the ZX81 manner with an appropriate resistor), there is no ROM change needed because the internal can be switched off with pulling ROMCS high. So this might be more easy than in a original ZX80, I think.
About the RAM - this can be switched off with the /RAMCS signal which is also available on original ZX80 in the same way.
I think this would work parallel with internal and external RAM together but might result in a higher currency consumption and would not propose it.
There is just one small mechanical problem with the video chinch socket which is in the same position as the RF socket on original ZX80 and might conflict when attaching ZXpand. So here might an extender be useful or providing a second chinch connector somewhere else or a cable with chinch connector coming out.
The issue 3 boards were slightly re-designed to make assembly easier and to get rid of those extra two pins on the PIC socket (which were used by the ZXpand-AY).
The ZXpand-AY was then modified with a wire to replace the necessity for those two extra pins - so if you plug a ZXpand-AY into an issue 3 board, you see the two pins dangling in mid air. You can still use these with the original issue 1 ZXpand, by snipping the small wire link on the back of the ZXpand-AY.