This is designed to be a simple set of questions and answers - please no general chit chat in here - keep it focused to help new users!
So can I get my ZX81 to act exactly as a ZX80 by just changing the ROM?
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
It does work but will need several reboot because the ZX81 ULA has no default for the startup state of the NMI generator.
The ZX81 ROM sets it off with the very first command.
The ZX80 ROM lacks this feature (because when it was designed there was no NMI-Generator).
User Holmatic of the German Forum has designed a "harness" for the CPU (four resistors if I remember right) that makes the ULA turn off the NMI-Generator before the CPU starts working.
VLA81 and Andy Rea ULAs Default to NMI OFF so these work good without the "harness"
https://www.sinclairzxworld.com/viewtop ... ness#p2837
Kind regards Paul
I burned an EPROM with the ZX80 image then added the resistors to the CPU and all is good.
Paul mentions a reset problem which I overcome by adding a reset switch across the reset capacitor.
If I didn't get the "K" cursor on power up a quick press of the switch always gives for me at least a perfect power up.