In our previous issue, we noted that, for PLC-controlled systems, problems rarely occur inside the PLC. However, what if:

  1. You have the proper incoming signals for your control loop, and
  2. You can simulate outputs at the field terminations and get the correct control action?
This means the problem is in the input module, PLC, or output module.

In the previous newsletter, we discussed how to use the divide-and-conquer method to troubleshoot a PLC system. Now, use this method to determine if the problem is in the I/O modules (the most likely place).

You'll need your control loop and ladder logic diagrams. Use a signal simulator at the input module for the malfunctioning control loop. For example, you'll supply a 4mA to 20mA input signal to simulate the temperature transmitter signal. Following the ladder logic, you see a 16mA input causes contact 11B to open, which causes the "Valve Open" coil 26A to pick up.

You can watch the PLC program elements respond to the input change. At 16mA, you should see contact 11B open. If it doesn't, your problem is in the PLC program (unlikely, if this worked previously) or in the input module. If the contact changes but the valve signal doesn't, then the problem is in the PLC program or the output module.