You are absolutely right David. My fault, I was testing the concept on the prototype board without the 8212’s. Thanks for pointing that out. Your suggestion is the solution. I will update the write-up on the site.
The moral of the story here is completely checkout prototypes before going to the final production runs. Back in the summer (when things were backed up), I thought I had completely checked out the board by outputting to the ports (8212’s and ST78C34) and watching the LED bars change. It never occurred to me that the strobe signal was missing.
Thanks again for suggestion
From: David Fry [mailto:dgf...@googlemail.com]
just looking at the schematic for the S-100 Parallel I/O board and correct me if I am wrong (its 8am in the UK and I've just got up :-) ) but if you jumper pin 1 of U10 to Pin 3 of U10 you will invert the bSLAVE_CLR* signal which as you say will correct the problem for the ST78C34 but will result in the 8212's being held in a clear state as pin 14 of each 8212 share the same bSLAVE_CLR* signal but are active low inputs.
I would think the solution would be to cut the trace leading to pin 24 of the ST78C34 and add a jumper from pin 24 of the ST78C34 to pin 2 of U10
please can you check what I have said and repost