m_thompson
Veteran Member
After lots of studying of the Maintenance Manual and schematics we learned that the CPU pauses when executing IOT instructions and lets the I/O controller handle the I/O functions. When the IOT instruction is complete, the I/O controller tells the CPU that it can resume executing instructions.
In our case when running at full speed, and after running for a few minutes, the IOF instruction would not turn interrupts off. We could see three IOT timing pulses for the ION instruction, then two of the three pulses for the IOF instruction, and then three pulses for the IORS instruction. There is a Gray Code counter in the I/O controller that counts the IOP states and after IOP 4 generates a IO RESTART signal to restart the CPU.
In the 'scope image below you can see the IO 0(1) flip-flop part of the Gray Code counter in the purple trace looks OK, but the IO 1(1) flip-flop has a weak transistor. The weak transistor looked OK when measuring the diode-drops and Beta with a DVM. In our Tektronix transistor curve tracer the weak transistor barely turned on. We replaced both 2N3639 transistors in that half of the S203 flipchip and now the system runs OK.
The Gray Code counter signals look much better with the new transistors installed.
In our case when running at full speed, and after running for a few minutes, the IOF instruction would not turn interrupts off. We could see three IOT timing pulses for the ION instruction, then two of the three pulses for the IOF instruction, and then three pulses for the IORS instruction. There is a Gray Code counter in the I/O controller that counts the IOP states and after IOP 4 generates a IO RESTART signal to restart the CPU.
In the 'scope image below you can see the IO 0(1) flip-flop part of the Gray Code counter in the purple trace looks OK, but the IO 1(1) flip-flop has a weak transistor. The weak transistor looked OK when measuring the diode-drops and Beta with a DVM. In our Tektronix transistor curve tracer the weak transistor barely turned on. We replaced both 2N3639 transistors in that half of the S203 flipchip and now the system runs OK.
The Gray Code counter signals look much better with the new transistors installed.