Had issues with the signal input selection monitor bits for the PUM. A test bit controls where the drive signal for each channel comes from. The PUM has two DB9 connectors, "DRIVE IN" or "TEST IN". See Jeff Kissel Alog - QUAD BIO I/O Simulink/MEDM Upgrade for further details.

This test bit controls a relay that selects from "DRIVE IN" or "TEST IN". Attached to this circuit are two transistors that output the state of the relay. When the relay was activated, it would respond by switching. The problem came from the rest of the circuit, where the input voltage on this bit was around 0.8V. This was enough voltage to keep the base of the transistor "on" regardless of the state. Jay Heefner recommended we switched out R111 on all four channels from 10K to 100K. Did modifications on PUM Chassis SN S102650. Unit is back in SUS rack and monitor channels are all working.

F. Clara, J. Heefner, R. McCarthy