Your factory has an automated shipping dock system with 10 bays. A conveyor system feeds these bays with a chute to each bay. When the chute for a bay is full, the indicator light outside the bay door turns from red to green so truck drivers know which bay to pull up to.

You come in on second shift to find a note that says the Bay 6 light never turns green when the bay is full. The tech tied back the position switch to simulate full, and the PLC console showed the appropriate response but no green light.

What’s your next step?

First, determine if the red light goes off during a simulation. If it does, then swap the red and green lamps to check the green lamp. If the green lamp doesn't light, replace it — problem solved.

If the red light stays on, then the change in the PLC isn't being communicated to the indicator lamps. This could be due to a problem in the output module, the wiring, or both. Check the module status lights to see if the output matches the PLC display (red light off, green light on).

What should you do until you find the problem? Here are the steps you need to take:

  1. Check the module fuse.
  2. Verify correct loop power is present.
  3. Disconnect the field wiring and simulate the loads (an indicating lamp or DMM will do, since the loads are indicating lamps) at the module.
  4. Ohm-check the field wiring, to check its integrity.