The current pcbs were not really ideal or practical to continue with that design. I still wanted modularity, but something to tie everything together without ribbon cables.
I started looking for passive 8-bit ISA backplanes. There are several commercial ones available, but none really fit my specifications. I thought about designing one and having the pcb made, but this was too advanced for my pcb skills currently.
I finally stumbled upon one for sale at 8bits4ever.net I bought this for $75
I didn't realize until I got it, that it was a design by Sergey Kiselev and available on github. ISA 8-bit Backplane
This nice design uses a standard pc ATX power supply. It has and on/off switch, power led, and an activity led. +/-5V and +/-12V are available on the bus.
There are seven ISA slots. I will need more than that, but for now this is a good place to start.
ISA bus pin usage
All power and ground lines remained the same, as they are controlled by the backplane. I kept most signals in the same configuration as a standard ISA bus, mostly removing unused signals and marking them with a no connect (NC).
The bus is tailored to the MC6802 cpu. 8 data lines, 16 address lines The MC6802 only has one IRQ, this is on B4 (IRQ 9), the NMI interrupt is A1 The other CPU signals (MR, VMA, BA, HALT, NMI) are present. The CLK is the E clock from the CPU. Pin B15, labeled E&VMA was added and supplied by the CPU card. This is simply the E clock ANDed with the VMA signal. All cards that do address decoding need this so I put it on the bus for convenience.
Now moving forward.
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