Today we present sensor to PLC wiring methods — twenty wiring diagrams. Let us take a look.

1. Overview

PLC digital inputs are not complicated. To improve noise immunity, PLC input interfaces commonly use optocouplers to isolate external signals from internal circuits. The external input drives the optocoupler LED; the optocoupler photo-transistor receives it and reliably transmits the signal to internal processing.

Modern PLC inputs are typically single-ended common (common) or differential; users must select appropriate sensor wiring to match PLC input type for correct operation and stable programming.

2. Input circuit types

1) Input classification: by power supply (DC vs AC) and by interface (single-ended common vs differential). Single-ended common (COM) connects one side of all optocouplers to a common COM terminal; the other side connects to X0,X1,... This saves terminals while supporting N inputs with just N+1 terminals. SINK connections typically accept NPN sensors; SRCE accepts PNP.

2) Description of terms: SINK means current is pulled from the input to ground (NPN compatible). SRCE means current is sourced to the input from V+, compatible with PNP sensors. For two-wire sensors, polarity and leakage current must be considered.

3) DC input circuits: require passive dry contacts or DC active solid-state switch outputs. A resistor (R2) is often placed to bypass optocoupler LED current and avoid false triggering from sensor leakage.

4) AC input circuits: add a step-down and bridge rectifier ahead of the optocoupler to provide DC to the downstream circuit, suitable for harsh environments where wiring changes are limited.

Other sections cover SINK/SRCE options, line-drive for high-speed counters, 5VDC single-ended methods, and recommended resistor values for 24V sensors to prevent interface damage (e.g. R1=10Ω, R2=2.7KΩ recommended).