What you'll learn
**Symptom:** the line operator reports the conveyor isn't indexing — the PLC isn't reacting to the photoeye on the infeed. You walk over to the panel.
Lab time: ~8 minutes.
Lesson briefing
Broken Signal Wire Between Photoeye and PLC
Symptom: the line operator reports the conveyor isn't indexing — the PLC isn't reacting to the photoeye on the infeed. You walk over to the panel.
The photoeye's own status LED is on. There's a target in front of it; the sensor is detecting and its output is driving. But the PLC's X0 input LED is dark — the input the sensor feeds into is reading low. The sensor works; the PLC's input circuit works; something between them doesn't.
Reach for the multimeter on the toolbar. Voltage mode is your friend here, but the trick is to probe BOTH SIDES of the signal wire. A signal wire that's broken in the middle has a clean diagnostic asymmetry: the sensor end is still energised (the PNP output is internally driving +V to its signal pin), but the PLC end is electrically floating. Put the black probe on the panel 0 V rail and walk the red probe along the signal path: the reading you get tells you which side of the break you're on.
Continuity mode is the secondary check — once you suspect the signal wire, probe its two ends for a beep that should be there but isn't.
Hints
Start at the PLC. With the multimeter in voltage mode, put the black probe on the panel 0 V rail (PSU 0V or PLC 0V — same node) and red on PLC X0. On a healthy panel with the sensor detecting, you'd read +24 V there. What do you actually read?
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This lesson uses 6 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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