Sinclair ZX80 / ZX81 / Z88 Forums

excellent stuff, with the right sized CPLD it and the osccilator (could do that in SMD too ) should easily fit on a board barely bigger than the 40 pin socket it's going to plug into. It's probably best to leave things like ps/2 keyboard intefaces, and the like as external add-ons, as for driving a ...

Blimey thats fast work... do you make the PCB's yourself ? regards the force nop timing, in my gal design it extends from the begining of the last 1/4 of T2 to the end of the first 1/4 of T3 (just one 6.5Mhz cycle) as the Z80 samples the data bus on the rising edge of T3 as you have discovered :lol:...

Thats pretty cool, i've managed to learn enough Verilog in the last week or so that i can understand what's going on :D 2 weeks ago i would have been guessing at best. first thing, so away with the logic that inverts the video comming out of the shift register, it was well rubbish on the original bo...

updated schematic, to correct a couple of errors. https://picasaweb.google.com/lh/photo/KqCwx15uKYJfUlvWzGJVbv4n1pR8c5fUX2bh5UqEfeU?feat=directlink the 47K resitor connected between GND and Tape-in may need to be altered to suit your device, the Gal i used has a reasonable internal pullup this resis...

Hi Prime, the oscilator circuit is took directly from the great Bodo Wenzels design. oops.... :oops: missed a connection between base of the npn (bc548) and the junction of the 2 capacitors... that should get it working. ah the 6.5Mhz crystal :lol: when i built a zx80 clone i used a 6.5536Mhz crysta...

be interested to see how much this goes for, i paid quite alot for mine.... it was the full educational pack in a brief case plus shipping from the us. and then customs wanted a few quid too

regards Andy

I've made a little alteration... not much but... essensially when the back porch is active the signal level on the video output is the same as black pixels, so instead of forcing the video to white so it does'nt interfere with the mixing of Csync, video, porch, i have made the video black during bac...

ZX81 ULA Replacement - Version 2. By Andy Rea. March 2011. Disclaimer... this document may contain errors or ommissions and is used entirely at your (zx81's) risk :-) Way back in the early part of 2007 i created a ULA replacement using just 74XX logic IC's the details of which can be found at http:/...

Here is the shcematic, too big to upload to the forum.

https://picasaweb.google.com/lh/photo/K ... directlink


Whoops missed off a 47K resitor connected between GND and Tape-in

regards Andy.

Sounds like it could be curtains for that poor ULA, but before addmitting defeat i'd just do a careful check that the clk output isn't shorted to the +5v. Bodo Wenzel made a replacement using A MACH210 cpld here. http://members.fortunecity.com/bodo4all/zx/zx81vid.html i made a replacement a few year...