Pullup resistors

[blittled]

I've always wondered about the function of the 8 pull up resistors on the data bus of the ZX81. Are the outputs of the ULA open collector which would need them?

[PokeMon]

Yes - the ULA has open collector outputs for overdriving the databus with NOPs during video mode.

[RetroTechie]

I've always wondered about the function of the 8 pull up resistors on the data bus of the ZX81.

There's 2 data busses on the ZX81, fyi. But pull-up resistors on a data bus is pretty standard, even without OC outputs in the picture. It makes for defined logic levels when nothing responds to a memory or I/O read. Read: more predictable software behavior since for every "unused" bit a "1" will be read rather than "undefined". Imho, it's stupid that software exists where "undefined" bits can actually change a software's behavior. Or it relies on "undefined" to be a 'defined' level in reality. But as you know, stupid software is the rule rather than the exception...

Are the outputs of the ULA open collector which would need them?

If the ULA used tri-state outputs to put keyboard signals on the Z80 data bus, then external keyboards (attached to the edge connector) probably wouldn't work. I can't confirm personally since I don't own such keyboard(s), but this alone suggests the ULA has OC data outputs. And if so, pull-ups are pretty much a necessity. Outputting a Z80 NOP code doesn't conflict with OC outputs.