Backplane apparent issue was a loose cable!

USING MY DATABASE ENTRIES I TRACED OUT THE CONNECTIVITY OF SIGNALS

With the database containing my captured signal connections, I was able to spit out the list of connections that must exist across compartments and begin some signal testing. The signal did make it from the output of the Program Load latch in gate A, compartment C1 to the edge connector in slot N4. When I moved the other end of my VOM to the destination in gate B, compartment A1 where the other end of the cable is connected, there was no signal at all!

I was pretty sure that I didn't have a failure in the cable itself, so I opened up compartment C1 to look more closely. At once, I saw that the cable had popped out of the slot and was disconnected! The cable itself was in a position where this could happen when gate B snagged on the folded cable coming out of gate A.

I am so relieved that I don't have cascading failures from overly fragile PCBs in the compartments. I will come back after lunch and resume debugging the current problem, failure of the keyboard device controller to reset its interrupt level trigger.

The culprit - loose cable

Yes, I find the rusted out metal edges disgusting too and will deal with them once the system is running well. Likely I will get them to somewhere to be sandblasted and powder coated, or at least sand blasted to remove the corrosion.