How to troubleshoot a “PLC communication down” error

How to troubleshoot a “PLC communication down” error

This article provides the recommended steps for troubleshooting a “PLC communication down” error. 


 â’¾ NOTICE   Only qualified personnel familiar with the construction and operation of this equipment and the hazards involved should install, adjust, operate, and/or service this machine. Read and understand the manual in its entirety before proceeding. Failure to observe this precaution could result in severe bodily injury or loss of life. By proceeding, you are indicating that you acknowledge and agree that Foltex USA is not responsible for any damage to property or persons while accessing and/or using this service. 

🔧 Tools you may need: ☐ multimeter      â˜ #2 Phillips screwdriver


The steps below should be followed in order. Confirm proper functioning of the specified parts at each step before proceeding to the next. 

Verify Cat5 communication on HMI and PLC

  1. Remove the Cat5 connector from the port on the front face of the PLC.
  2. Verify that the connections points are straight.
    Damaged ethernet port
  3. Verify that the port and connector are clean; if not, vacuum any debris before reconnecting.
  4. Remove the Cat5 connector from the port on the HMI.
  5. Verify that the connections points are straight.
  6. Verify that the port and connector are clean; if not, vacuum any debris before reconnecting.
  7. Verify that connectors are securely connected at the ports; if not, repair or replace the cable. 

Verify PLC has 24VDC supply voltage

  1. Using a multimeter set to DC, place the red lead on the positive terminal of the PLC.
  2. Place the black lead on the negative terminal of the PLC.
  3. Verify voltage supply reads 24VDC on the multimeter; if not, troubleshoot for 24VDC power loss.
    🗈 If LED indicator lights go out while the supply voltage of 24VDC is present, then the power supply in the PLC is faulty and the PLC will need to be replaced.

Verify connections of the supply voltage wiring

Verify that wires are properly inserted in the terminals of the PLC and all terminal screws are secure; if not then gently push the wires back into the terminals and tighten the terminal screws.

Troubleshoot Cat5 cable

  1. Run a temporary Cat5 cable around the outside of the machine.
  2. Connect one end of the temporary cable to the PLC port.
  3. Connect the other end of the temporary cable to the HMI port.
  4. Monitor the HMI for the fault.
    🗈 It may be necessary to use the temporary cable for multiple weeks to ensure the fault does not reoccur.
    🗈 The temporary cable can be less than 30’, but the permanent replacement must be at least 30’.

Verify proper ground on PLC

  1. Using a multimeter set to ohms, place one lead on the ground terminal of the PLC.
    🗈 The ground terminal will be the only one with a yellow/green wire running to it. 
  2.  Place the other lead on a non-painted metallic surface of the machine frame
  3. Verify that the multimeter reads 0 resistance (ohms); if not, then verify that the ground wire is properly inserted in the terminal and the terminal screw is secure.