ZX81 ULA-in-a-CPLD

[mrtinb]

Can you select between white and black background?

[Paul]

If you haven't ordered loads of pcbs already you could go for a smaller jumper like 2mm instead of 2.54mm. Or even 1.27mm jumpers are available.
Or just leave it empty so anybody could solder it if required. Like with the m1not where you have no jumper.
Kind regards Paul

[RetroTechie]

Great progress Andy! Looking good!

oh my the link in the middle is mighty close to touching the case ... the easiest thing would be to fit a right angle header. (might have to bend it up slightly )

Personal opinion: put feature jumper(s) on a side end of the board. One pin solders to top side, other pin solders to bottom of pcb. Board thickness could be adjusted if necessary. Function chosen such that board works like a 'vanilla' ULA if jumpers are open or left out.

Optimal: such jumpers near the back of the ZX81's case (ULA pin 1/40), such that users could cut a hole there to make quick/easy changes. JTAG pads then on the opposite side (ULA pin 20/21).

the link is there to add a 75 ohm load should you be unlucky enough to own a monitor / tv that has a high impedance input.....

Is that really needed? I know of some of the crazy monitors seen among ZX81 users, but composite input without 75 Ohm load on the input must be pretty rare these days. I could think of quite a few settings that are more useful to have than a high impedance <-> 75 Ohm monitor choice.

And FWIW: personally I'd prefer re-purposing the ULA's OSC pin to feed in A13, and use that to enable RAM in 2000-3FFFh rather than a ROM mirror. Quite a bit of ZX81 software uses RAM in this area.

[Andy Rea]

Great progress Andy! Looking good!

Thanks

oh my the link in the middle is mighty close to touching the case ... the easiest thing would be to fit a right angle header. (might have to bend it up slightly )

Personal opinion: put feature jumper(s) on a side end of the board. One pin solders to top side, other pin solders to bottom of pcb. Board thickness could be adjusted if necessary. Function chosen such that board works like a 'vanilla' ULA if jumpers are open or left out.

Optimal: such jumpers near the back of the ZX81's case (ULA pin 1/40), such that users could cut a hole there to make quick/easy changes. JTAG pads then on the opposite side (ULA pin 20/21).

great idea i never thought about straddling the pcb.

the link is there to add a 75 ohm load should you be unlucky enough to own a monitor / tv that has a high impedance input.....

Is that really needed? I know of some of the crazy monitors seen among ZX81 users, but composite input without 75 Ohm load on the input must be pretty rare these days. I could think of quite a few settings that are more useful to have than a high impedance <-> 75 Ohm monitor choice.

well all those years ago ( the altera based ula ) there were a few users that had such crazy monitors.

And FWIW: personally I'd prefer re-purposing the ULA's OSC pin to feed in A13, and use that to enable RAM in 2000-3FFFh rather than a ROM mirror. Quite a bit of ZX81 software uses RAM in this area.

what ram would i be enabling ?

regards Andy

[Andy Rea]

Can you select between white and black background?

simple answer NO...

But i could make 2 version of the code one for black on white the other for white on black ( you would have to choose at the time of buying )

regards Andy

[Paul]

No room left for your poke goodies?

[RetroTechie]

what ram would i be enabling ?

A 32K*8 SRAM used in a simple internal 16K RAM mod? Say, enabled in 2000-9FFFh range. Or mirrored all over the 64K address space except the lower 8KB occupied by the ROM? (that would also be nice when using a 128K*8 SRAM to fill the entire 2000-FFFFh range with unique RAM locations ).

With a 32K*8 SRAM (mirrored to fill 56KB with RAM), that would give software 32KB RAM to use, with some choice of where in the address map to use that RAM.

[McKlaud]

Yeah! It is possible, I did it my ULA, mapped 32K in 8K-40K area. It works in principles but have not tested extensively.

[Andy Rea]

@Paul not for this ULA it was supposed to be a plain vanilla hopefully cheap-ish... ( I have. Another idea for a more advanced plug in solution the would plug into both the CPU and ULA sockets )

@retrotechie I feel that if people are savvy enough to fit internal ram then they can do there own decoding schemes. My favourite easily achievable ( and I feel zxpand should have this also ) is the 32k ram mapped 16 - 48k with the top 8k also mapped 8-16k.

@mcklaud see above.

Regards Andy

[iturbez]

Congratulations, Andy.
Very very very good job!