Peter,

Perhaps, I built one way back when; from a PCB provided by a fellow who would make Elector PCBs and sell them at "swap meets".

It worked ok, I used a "real" Basic-52 Chip, and i gave it away (as i am not really a "basic" kind of guy).

As i recall it had buffers on everything (DATA,ADDRS,) and had series resistors in case BUS Direction and Port Direction would result in conflict.

Also it had a MOS-FET switch for programming power.

Is this the design you are refering to?

regards,
p