Found one cause of flaky memory operation and finished reconfiguration without two CRs, LP and PT

FINISHED RECONFIGURATION OF DEVICE CONTROLLERS

I had pulled all the cards that implemented the device controllers for the 2501 and 1442 readers, the 1132 printer and the paper tape punch since the devices are not currently available to hook to the machine. I have the cards and can reenable them if/when the VCF museum restores one or more of the peripherals.

I found the machine triggering an interrupt on IL4, which was due to a signal line that was driven by the 2501 device controller. Adding a jumper blocked this spurious interrupt request. The main issues when certain peripherals are not configured in the machine is that certain requests like interrupts and cycle steal are positive logic signals - requesting when high - and SLT logic interprets an unconnected wire as logic high. Thus the jumper to pull that wire to logic low.

FOUND A LOOSE CARD IN B-B1 COMPARTMENT, CARD IN E7

I did a hand check of the seating of all the cards in the B gate, which is essentially the CPU plus memory. I did find the card in B-B1 E7 to be wiggling around, so I popped it back in place. 

DISCOVERED A LOOSE CABLE CONNECTOR INTO MEMORY COMPARTMENT

I was doing some load and display tests of memory locations and kept seeing odd results. I decided to set a word to FFFF - all bits on - which looked correct in the Storage Buffer Register as I did the load, but when I read back the word I saw that bits 1 through 9 were coming back as zero. The parity didn't add up so it causes a parity error on the first read. 

I realized that this looked like a failure of the B bits to drive the inhibit current and the fact that it was a range of bits was suspicious. The CPU would have sent all 16 B bits as 1, meaning no inhibit. It calculated the parity bits based on this - each half of the word had an even number of 1 bits thus the parity bit was also set to 1. When it read back as 803F, the left word had a single bit (bit 0) turned on but the parity P1 was also set - an error. The right half word had six bits set to 1, thus it did need parity P2 to be correct and thus was good parity. 

Thus the failure happened after the memory parity generation circuits saw the B register and before memory acted on them during the write. Looking at the ALD, the B bits come into the memory compartment (C1) as inverted logic, thus if they are to write a 1 in that position, the signal would be pulled to ground (notB is false therefore B is true/1). 

Further, the bits that were missing were all on one connector (T1) plugged into the top of the memory compartment. I visually inspected the connector and could see that it was loose. The card covers on the compartments fit into side mounting plates which block the A and N columns of the SLT card, so that any connectors on the left or right can't be directly accessed until those plates are removed. 

The top and bottom connectors are rows 1 and 8 are blocked by the frame itself, with no easy access. One can remove the adjacent row of cards (all cards in B7 through M for example) to get fingers down into the space for the row 8 connectors. For the top, however, there is the possibility of removing the grating at the top of the compartment that allows the air coming from fans below to be blown out of the card compartment at the top. 

Connector dangling out of its slot

I found that the connector was completely unattached. I plugged it back in and cured that problem. While I was doing that I saw that the plastic cable retainer, which keeps the cables to the side so they don't block the airflow out of the compartment, had come loose from the wall of the compartment because the foam double sided tape had disintegrated with age. The old foam looked like loose rust, which is very dangerous indeed in a compartment with electronics and cores. However, it was just gross nonconductive foam. 

I glued the cable retainer back where it belonged and closed up the compartment. I began loading code into the machine to do some tests, but the day came to an end so this will have to wait until tomorrow.

Grating back in place