SWAPPED CARD FOR THE KEYBOARD CONTROLLER BUT MACHINE FAILED
I had spares of the simple SLT card that implemented the three latches - Keyboard Response, Manual Interrupt and Keyboard select. I had pulled the suspect card to do some bench testing, but I thought I would stick in a spare just to see quickly whether this was indeed a card fault or something off card.
When I turned on the machine, the Run lamp came on except when in Single Step mode, the same symptom that the machine had initially. The one that was caused by a failed trace on a backplane which I jumpered over with wire-wrap. This was triggered at the end of the failure chain by the signal -Prog Ld Not SRP or PT Resp.
This signal is forced to low (active) during a program load until the peripheral being booted ends the process. If paper tape is used, the PT Response is activated, while SRP response is from a card reader if that is the boot device. This forces the CPU to run until the boot data is entered into memory.
BEGAN TRACING SIGNALS THAT ARE INVOLVED IN THE ERROR - MULTIPLE ARE OPEN
I found the AND gate that activates the signal, then looked at its inputs which are, not surprisingly, +Program Load, -PT Response and -Level 0 Response (reader end of card interrupt). The levels of the inputs were floating, not a valid 1 or 0 but high enough that the AND gate was triggered.
This tells me I have connectivity faults between the source of these signals and this gate, once again, I suspect that the simple act of unplugging and replugging a card was enough to worsen hidden cracking in the the backplane PCB.
THIS IS WHY I PREPARED THE DATABASE BUT IT WILL BE A SLOW PROCESS
With the database, I have the entire route and connectivity of every signal on the system that crosses from ALD pages to others. I am missing some signals that run solely on one backplane between card pins and are only involved in one or two ALD pages but I definitely have every signal that transits from one backplane to another.
First up is to trace all the signals involved in this latest manifest fault, correct those issues and verify that the system is back to normal operation. My suspicion is that that the cracks are near edges, as that would be where the flex was most when cards were pushed in or pulled out, but I don't know for sure. The two faults I found and corrected were both between edge connectors and an interior card.
If I have a fault in an edge connector, I can produce a list of all the signals running to a given connector, then trace out the full connectivity for each on the backplane. A sort to give me all the signals by card slot (edge connector), then sort back by signal and look at all the paths for those identified signals. A database sure would be handy here, rather than sorting a spreadsheet, writing down signal names, resorting the spreadsheet and then looking up each signal for its paths.