In previous issues, we have discussed troubleshooting input modules. In the examples, the output modules checked out OK. But what if they don't?

As with input modules, output modules can be digital or analog. Typical digital outputs include solenoid control signals (energize/de-energize) and motor start/stop signals. They also include various indicating signals, such as valve opened/closed, temperature limit reached, and high level alarm.

The actual output signals may be AC or DC, but the power isn't usually supplied by the module. As with the input modules, your first step in troubleshooting is to determine what power should be present and where it comes from.

Your second step is to check the output module fuse indicator. It's possible the indicator is faulty, so check the fuse directly if there's no “fuse blown” indication. Then, use a DMM to verify that the module power is what it’s supposed to be (e.g., 120V) at the terminals.

Now you're ready to test the module. A simple way to do that is to use the PLC logic to force the output on and off. With each change of state in the PLC logic, you should see a corresponding change in the output module (usually by observing the module's status lights). If the module output doesn't change, then replace out the module. In Part 2, we'll discuss what to do if this doesn't solve the problem.