We will see in a few days if it is out there.
Does anybody know if Computer Digest was monthly or quarterly or bi monthly?
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
WILDONICS, Box 1763, Boise, ID 83701
Nothing current online.
I built such a thing, also using a 8255, because they where easier to program and cheap as sh*t compared to the fancy Z80PIO. EPROM-programming software is trivial. You put the address, you want to program into to the address pins and the contents to the data pins and then you pulse the programming pin. The exact nature of this pulse is described in the application sheet of the EPROM. I remember having done 2708, 2716 and 2732. The 2708 is rather complicated, even while in normal use. It is not content with +5V, but also needs -5V and 12V just to be read. You also need this voltages for programming. In addition you need high voltage pulses for the programming. So you need an amplifier to produce this voltage out of your 5V programming signal. You need to do about 100 pulses with a pause after each one so that you don't broil this one 8bit storage location. You can speed up programming by going through the address in the inner loop and the 100 pulses in the outer loop, but to be on the sure side it still took hours to get 1k into that chip.
It is much simpler with the 2716 and beyond. The are content with 5V. Only for programming you need a higher voltage, but this can be constant. The programming puls is a simple 5V signal directly out of one of the ports of the 8255. The amplifier seems to be integrated. You had to use fast mode of course to get the timing right.
What is new to me is that you could use IOREQ. I remember having read that a ZX81 was likely to crash on any IO-operation so I crammed the 8255 and everything else for that matter into the $2000 to $3FFF space with the help of one or more 74LS138.