2) It's better.
3) Yes, and No !
Only the Zx81 interfaces based on IO data cards.
The easy way to know if a card is compatible, you had to have a look to the socket BUS.
A 2x23 pins connectons are compatible with the Spectrum and Zx81/TS1000, but you need to check the connector key to avoid to kill your power supply or mother card components.
Zx81 & Spectrum:
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There are some (but not many) joystick interfaces that are specific to the ZX81 (ZX80, TS1000 etc...) (some details of the interfaces available in the 1980s are here).
In terms of modern hardware, there are two that I know of:
The Chroma 81 Interface supports the ‘cursor’ standard.
And the ZXpand/ZXpand+ interfaces can also support joysticks.
However, there is no common standard that is supported by games.
Quite handy as it meant you can map any keyboard commands.
Can't remember if this version of the interface was zx80/81 only though
Just count the pins on the connector, 23x2 for the Zx81 and 25x2 for the Spectrum.
If the card is ok for both computer, the socket is a 23x2 (zx81) socket.
Have a look to the KEY (the little peace of plastic in the socket!), count the pin column.
2 for the ZX81, and 4 for the Spectrum.
If a Spectrum card is a Zx81 socket (2 pins at the left of the socket key), it will work on both computers !
Note: An7805, your PM is disabled !
https://forum.tlienhard.com/phpBB3/view ... 739#p43739
It is designed for the ZX Spectrum, but it would be easy to adapt it to the ZX81. The advantage: It is programmable, so you can play even all games which are designed only for keyboard use.