In our previous issue, we noted that the first step in troubleshooting digital input modules is to verify the power source of each loop. If input power is missing, the causes can be such things as blown fuses, broken wiring, or broken connections.

Once you've fixed any power problems, you're ready to simulate input changes. Connect a DMM across the input (at the module). Then, actuate the field device and watch for the voltage to change. If it doesn't, you have a problem in the loop between the input module and the field device. It's probably a wiring issue. If the voltage changes, you've just verified the integrity of the loop between the input module and the field device. So where is the problem?

Check the module's power and logic indicators. They should change state as you vary that input.

  • If they don't, the module isn't responding. This used to mean an automatic replacement, but it may be possible on your module to reboot and solve this problem.
  • If they do, the problem is on the processor side of the input module.
Next, watch the PLC logic for a corresponding change. If it doesn't happen, then the signal isn't making it from the input module to the processor. Methodically check the system (from the assembly and wiring diagrams) to see where exactly that signal is getting lost.