Sinclair ZX80 / ZX81 / Z88 Forums

Why not use the first exisiting ROM Location, 3 (PRINT PEEK 3). This is in the beginning to initiate RAM Test ... ;; START L0000: OUT ($FD),A ; Turn off the NMI generator if this ROM is ; running in ZX81 hardware. This does nothing ; if this ROM is running within an upgraded ; ZX80. LD BC,$7FFF ; Se...

ceramic resonator ...

You can place a resonator on your "ULA replacement board" I think and forget the resonator on the issue one or issue 3 board ...
It's not used for something else, so why not build the oscillator on your ULA ?
The ULA is the boss of the clock ...

Yes and this product was called ZX Spectrum. :mrgreen: I dont think that this makes really sense because ZX81 was designed monochrome. So the DISPLAY_FILE don't has color information. You could maybe expand this via modification of ROM but this is definetely no standard ZX81. Even all programs don't...

Me too. It's 1k resistor connecting C6 / X1 (quarz oscillator) to ground.

however if the ULA was seeing the HALT signal as low all the time it should not generate forced NOPs on the data bus so a crash would happen very quickly when the cpu started to actually execute video bytes ! I don't think so. Because of an empty DFILE at startup there is only HALT executed and the...

I found that RAM must be okay because code is executed correct (A15 high). This is a good signal for measuring by the way. Can see a bit more than 40us activity and repeat every 64us. ZX81_A15.gif I found that HALT signal still there but there was a shortcut in the ULA at Pin17. I measured 3,5 ohms....

Hi sirmorris, No, the CPU is fine, because the ROM code is running successfully (VSYNCs are generated within the software). So, it is unfortunately pointing towards the ULA :( Grant Maybe - but maybe not. Today I made some new measurements in comparison of working and non working ZX81. I think with...

My plasma TV has no problem with missing back porch.
By the way this is dependend on ZX81 version.
The older ULA's don't have back porch (Issue One e.g.), the newer one's have it (ISS3 e.g.).

I think (or hope) it could be maybe the internal RAM. CPU seems running in general. Maybe a cleared Display File with nothing to show ... I will let you know further steps. I think if ULA is dead it would output nothing to PIN 16 but there are sync signals. Anyone with experience of what shows a dea...

VSync is working on the new computer (with missing Cursor K). :? ZX81_VID_Vsync_New.gif Next thing is maybe putting a programmable counter to clock and reset to see what's going on in phase of starting. So I can measure databus, addressbus and other signals when Z80 is starting up. I know that I cou...