Signal Field Guide
TROUBLESHOOTING INDEX

A symptom is
a starting point.

A meter-first map for faults that look like bad sensors, bad PLCs, or bad luck.

Analog signal noise path diagramA clean signal path is compared with a noisy path affected by a motor cable.CLEAN PATH / NOISE PATHdesired signalmeasured signalCHECK FIRSTrouting + shield
5 field symptoms
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01

The loop reads 0 mA

Power, polarity, open circuit, or a transmitter that never entered the loop.

Trace the fault → 02

The PLC shows the wrong value

Scaling, input mode, reference, or a signal range that never matched the configuration.

Trace the fault → 03

A PNP sensor stays ON

Shorted output, wrong common, target hysteresis, or a floating input.

Trace the fault → 04

An M12 sensor drops out

A connector, vibration, water ingress, or a pin that only opens when the machine moves.

Trace the fault → 05

The analog value wanders

Cable routing, shield termination, common-mode voltage, or a nearby VFD.

Trace the fault →
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