When the diag fails by leaving the PIE enabled after an IOF instruction, we see an IO RESTART pulse at the end of the fetch cycle of the IOF instruction, and 2 mS later we see another IO RESTART. We should not see the IO RESTART pulse after the fetch and we should see an IO RESTART pulse 4 mS after the beginning of the IOF instruction. The early IO RESTART pulse could terminate the IOF instruction execution just after the fetch cycle by restarting the Control Memory, so the IOT parts of the IOF instruction are not processed and the PIE is not disabled.
After running the toggle-in PIE test for a few minutes the processor halted with PIE on, so the fault can be reproduced.
We had temporarily replaced the S202 flipflop in I/O slot J07 with an R202 flipflop. The only difference between the S202 and the R202 is that the S202 has stronger pull-down resistors for faster edges, or heavier loads. We had replaced transistors Q3 & Q4 in the S202 with new 2N3639 parts, but it made no difference in the earlier testing, When we put the S202 back in the processor would not run the PIE test at full speed. It ran OK at REPEAT SPEED 5. We put the R202 back in, and the processor ran OK, but halted with the PIE On.
The other half of the S202 flipchip is the flipflop for CLK REQ. Maybe that part of the S202 is broken? All of the transistors and diodes measure OK when diode-tested with a DVM. We replaced Q1 & Q2 with new 2N3639 parts. We looked at the behavior of the original transistors on the Tektronix transistor curve tracer and saw that one transistor was really non-functional. We replaced the S202 in I/O slot J07 and the processor is back to halting with PIE on. Now we can look at the IO RESTART pulses, and why there are too many of them.
We looked at the signals that are used to create the IO RESTART signal and eventually got to the S202 flipchip in slot J18 that implements the IO 0 and IO 1 flipflops that make the Gray Code counter. We noticed that the signal on pin S did not have a sharp rise time, so maybe it had a weak Q3 transistor. We replaced Q3 & Q4 with 2N3639 parts, and reinstalled the S202 in I/O slot J18. The processor now runs the toggle-in ION/IOF test. The 'scope traces of the behavior of the Gray Code counter look good.