I'm looking at the current arrangement of memory in my updated design, and realized something:

1) the RAM on the logic board is an MC6810, is only 128 bytes, and a long-obsolete and well-sought after part (apparently some pinball machines use this particular RAM chip as well).

2) the RAM on the interface board is a 6116, has a capacity of 2K bytes, of which only 128 bytes are used, and is still a 'common' part.

I would like to be able to tweak the partial address decoding such that I can use the 6116 for both address ranges without having to rewrite the firmware (if I can't find some NOS SC44125s, I may have to respin the board to use an HC11 variant instead... might as well address the RAM at the same time).

The attached picture contains all the existing 'glue logic' address decoding, address ranges called up by the firmware, etc.

I have a general idea what I need to do for the RAM equation (and which would involve adding a NAND and XOR gate to the RAM decode line), but want to see if the brain trust can see any equally workable approaches.

 

That's not a bad design- the logic looks correct on a quick look.. You might consider something like a 74LS138, it's a 3 to 8 decoder commonly used in memory decode apps- it could probably make things a little smaller rather than individual gates as you have here. A clean way to do this would be to use a PAL, almost any PAL could handle this in one chip. A nice versatile chip is the 22V10 that is still sold and supported for a couple of bucks. It would give you a lot of flexibility too.

 

A PAL was definitely on the table, but I am faced with the possibility the SC44125P microcontroller I'd been working with is dead in the water... it's an unobtainium automotive version of the MC6800 (with 6801 instruction set and hard-wired address / data bus pins).

I've got parts on the way that will let me patch my project boards into the original unit to test them and validate my findings (then again, it could be that I have the watchdog set up wrong).

The only other possible solution, the HC11, is now a legacy product, so I have to look at other options. I'm currently delving into the firmware and working on a rewrite from assembly into C.

 

Moot. The replacement HC11 MPU has on-board RAM.