In our previous issue, we said a digital output module could be tested by using the PLC logic to force the output on and off then and observing the module’s response. If the module doesn't respond, replace it.

But what if the replacement module doesn't respond? Test both modules in a known working loop. If either one works there but not in the loop that's faulty, then the problem obviously isn't the module. It's in the backplane, cabling, or some other area between the module and the PLC logic.

Let's say the module was faulty, and now you have a replacement, but the loop still isn't working even though the module lights change as expected. Disconnect the field wiring, connect a simulated load on the module, and test again. If you don't have a loop simulator capable of providing the simulated load this loop requires, you could use a known good field device (e.g., a solenoid) connected in place of the field side of the loop. Just be sure you're using something appropriate to the module in question, and you aren't removing loop power.

If the module responds properly with the simulated load, test in the other direction by connecting a simulated module output to the field wiring. You should see, in this direction, the same failure, because you have ruled out the output module. Now, turn your attention to the field wiring.