Chapter 7: Testing and Troubleshooting 139
Digital Inputs
Symptom Expected Cause Troubleshooting
No external input power Supply power.
Low external input voltage
Make sure full voltage is being
supplied.
No inputs on the module will
turn On (LEDs are not
illuminated).
Terminal screw is loose/
Defective contact
Tighten screw/
Reconnect the module
Inputs will not turn to On state
(LEDs are illuminated).
Defective input circuit Replace the module.
Device connected to input module is
defective.
Replace the input device.
Loose input wiring Reconnect the input wiring.
External input time is too short. Adjust the input device.
One or more inputs on an I/O
module will not turn On.
Terminal screw is loose/
Defective contact
Tighten screw/
Reconnect module
One or more inputs on an I/O
module will not turn Off.
Defective input circuit Replace the module.
Low external input voltage Make sure full supply voltage is being
input.
Noise error Troubleshoot for noise.
Input changes On/Off state
erratically.
Terminal screw is loose/
Defective contact
Tighten screw/
Reconnect module
Input display LED will not
illuminate (input is On in PLC).
LED error Replace the module.
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