Anyone with a ZX80?

[nollkolltroll]

A thought: a zx80 rom and disabling nmi on a zx81, would that be 100% compatible? Or just a bit?

[Moggy]

Forgive my lack of photographic skills hope this helps.

The tips of the bars are just visible in the darker pics but don't actually move unlike your first attempt in which the screen was still dark but the bars moving.

[Paul]

A thought: a zx80 rom and disabling nmi on a zx81, would that be 100% compatible? Or just a bit?

It would be 100% compatible.

[nollkolltroll]

I found some encouraging artcles regarding downgrading a zx81 to something that is zx80-compatible, so that is my next step. That should help me debug what is going on.

[Paul]

I hope you try this:
viewtopic.php?f=7&t=347&p=2838&hilit=Harness#p2837

[nollkolltroll]

I've had a good look at the ZX80 schematic and understood more about its operation. Since it does not have a 207-counter, the hsync is not self-generated, but must be done in SW. My code does not do that, I assumed the hsync worked like on the ZX81.
So: I will try to do a SW-generated hsync and see if it works.
AND: a downgraded ZX81 is NOT 100% ZX80 compatible in this sense. The ULA will always generate hsyncs regardless of the NMI signal going to the CPU or not. EightyOne also generates hsyncs automatically, so its ZX80-mode is more like a downgraded ZX81. That's why my demo works in EO.

[Paul]

Very good point.
We could try to fix this in EO.

[Moggy]

I hope you try this:
viewtopic.php?f=7&t=347&p=2838&hilit=Harness#p2837

I too have a downgraded ZX81 using this method.
The only problem I had was that when powering up it took several "on and offs" of the power supply to get it to boot up. I solved it by fitting a reset switch to the CPU as per the ZX spectrum and now it boots perfectly after every press. possibly needing a fully discharged CPU capacitor to get the timing right I suppose?

[Paul]

This shouldn't be the case.
The resistors should deactivate the nmi on boot.
Is it still the original capacitor in the reset circuit? Maybe it's dry?
kind regards Paul

[Moggy]

This shouldn't be the case.
The resistors should deactivate the nmi on boot.
Is it still the original capacitor in the reset circuit? Maybe it's dry?
kind regards Paul

Thanks Paul I will replace capacitor. Why didn't I think of that?

I have to say it's inexpensive way to have a ZX80, a few resistors and, in my case, a 2764 eprom and instant ZX80.
One thing I haven't done is try it with the ZXpand so that will be my next project!