I would suggest using a digital camcorder to video tape the suspected input. Then play back the recording slowly to see if the suspected input LED dims out slightly. That would possibly be a faulty input device (proximity detector or photo eye). This method has never failed me on CNC's or PLC based equipment.
Best, Rick
When troubleshooting intermittent PLC input signal loss, a methodical and structured approach goes a long way—much like the PCB diagnostic process highlighted in the PCBWay blog. You start with a visual inspection, looking for loose contacts, damaged wires, or signs of corrosion—very much like how you'd carefully scan a PCB for broken traces or solder issues. That same discipline helps catch subtle wiring faults before moving deeper into diagnosis.
Moving on, measuring and verifying supply and signal voltages during both normal operation and the dropout event is essential. This mirrors the PCBWay’s recommendation to check voltage at regulator inputs with a multimeter to ensure the right levels are present. Similarly, you’d want to monitor the sensor’s supply line feeding your PLC inputs—especially under load—to see if voltage levels dip or become noisy.
Finally, adopting a step-by-step isolation strategy is key: swap suspect channels, test with known good signals, and use PLC input LEDs as diagnostics. Much like the PCBWay guide’s regimen of starting at the power input and working through the regulator's output, your path through physical checks, signal tests, and logical isolation creates a reliable troubleshooting workflow. For a solid reference, here’s the relevant PCBWay article for guidance: How to Troubleshoot a PCB
I’ve run into this a few times. What usually helps is checking the PLC’s diagnostic logs to see exactly when the signal drops, then gently moving the wiring harness while watching the input LED. If it flickers, it’s likely a loose crimp or ground. Swapping in a known good sensor is a quick way to confirm.
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