PLC5-SLC IO Scanner Fault

After adding another relay card to slot 02, and moving the outputs one at a time after each fault, the problem persisted. After all of slot 3 outputs were moved to slot 2, I removed the output card from slot 3. Well, the problem remained and still pointed at slot 3! Finally, I figured out that I could cause the fault by rapidly cycling a jog button which would turn off output 8 and turn on output 7. I removed the wires from these outputs and could not make the fault return. I replaced the wires, and was able to cause the fault again. I powered down and replaced the 1746-OW16 with a 1746-OA16. Now I can't manually cause the fault. In about a week, if the problem doesn't return, I'll add another post to summarize my findings.

Paul C.

Sounds like some kind of ground loop problem.

But, I've seen similar problems when a intermediate terminal (1492-F1) that was used to terminate the I/O point's common wasn't tightened down very well. The guy wiring the panel stuck the wire in and forgot to torque it down. Just a little vibration disconnected the common, forcing one of the other devices that shared the same common to become the new "common" and it faulted the rack. If I remember correctly it was a input card in a Modicon 984 system.

Found the problem!

After a lengthy and thorough troubleshooting process, I narrowed the problem down to two outputs. Both of them are solenoid valves. They are on outputs O:043/7 and O:043/8. I replaced the OW16 relay card with an OA16 (triac) card. The problem disappeared. On the 1746-OW16, the relays for outputs 7 and 8 are nearest the backplane. Apparently, after 7 years of trouble free operation, one of these solenoid valves began producing electrical noise or current spikes that caused the relay card to fault. In my application, there was no reason that a triac card could not be used, and swapping it was easier than getting to the solenoids.