When I began accumulating the boards for one system to document their schematics and dump the firmware as needed, there was one board I narrowly missed acquiring.
As I understand it, this company had an ongoing exchange program where end users could send back the old-style CPU board and its companion memory board in exchange for the new-style CPU board.
So, opportunities to acquire that board are now VERY few and far in between.
The main difference between the old board set and the replacement board (besides the firmware rev) is that the original eight 2516s were consolidated into a single 27128.
(my gut feeling is that an end user got the EPROMs mixed up when upgrading the firmware and likely caused a small industrial accident)
With that out of the way, the three boards in question are:
290009 / Old-style CPU
290051 / Old memory expander board
290077 / New style CPU
I managed to acquire examples of the 290051 and 290077 boards.
The one I missed out on was the 290009, and the picture I have of that board was from the reseller's site before they took the listing down.
I did ask the reseller if they had pictures of the back, but unfortunately, both the 290009 and 290077 boards have a backing plate, and their pictures of the back side are with that plate on.
Anyway, I have managed to document the schematic of the 290051 and 290077 boards.
For completeness on the schematic side, I would like to try and work out the last few details of the 290009 schematic...
I was able to work out the differences component-wise between the 290009 and 290077 boards:
The upper half of the ROM space, the timer, and the 25LS2521 (74LS688) address decode circuitry all relocated to the 290051 board.
The lower half of the ROM space remains on the 290009, and instead of the 74LS139, a 74LS138 is used for the address decode instead.
So far, so good.
The part where I run into trouble are a set of pads by each of the ROMs and the RAM, and also by the 74LS138 for the ROM chip select.
So, I'd like to see if the brain trust is able or willing to take a crack at this puzzle.