ZX81 in four PLD chips

A while ago I was looking at connecting antique computers to modern TV sets with direct composite video or RGB inputs. I asked the Acorn newsgroup for details of the Acorn Atom colour interface boards. Atom enthusiast Ian Bradbury sent me the circuit diagrams. In return I sent him the Atom to BBC BASIC conversion MOS ROM for reading. He gave me a copy of the ROM contents so I can make my own if I need one.

I mentioned my interested in making a ZX81 in newer parts, and Ian said he'd done this with PLDs and sent me a photo to prove it. I've heard many people talk about doing this but as far as I know nobody has.

The German ZX Team have done some very impressive ZX81 clones in Xilinx logic cell arrays, but these are very different chips. For a start, the programs to turn logic equations into files for programming PLDs is relatively simple, and one can download them for free. The Xilinx chips are bigger and their function is usually specified in a high-level language such as VHDL or Verilog. These programs are more complex and thus expensive and less available to the hobbyist.

I thought Ian's project would be worth putting on the web because I haven't seen similar projects on the web. Ian kindly sent me the logic equations for the chips, and I have endeavoured to document the project here. Ian says the circuit is heavily based on the ZX97 design by Wilf Ritger, and this can be seen in the way that they both use series resistors in the A0-8 address lines to allow the video generator circuits to drive the memory address lines MA0-8 without level conflicts with the Z80 address lines.

Main Parts:

Circuit diagram details are available on request (this site's author, not Ian).

Note for Isidro (of Spain) who requested more info: I can't contact your e-mail address my server refuses to connect to yours, please write again with a different address.