Discrete sensors: PNP/NPN, NO/NC, 2-, 3- and 4-wire

PNP and NPN outputs (3-wire DC sensors)

Wires (IEC 60947-5-2 colour code): brown +, blue −, black output (white second output on 4-wire). A PNP sensor on an input card for NPN never switches it.

NO and NC

• NO (normally open, make): output active when the target is detected.
• NC (normally closed, break): output active when no target.

For safety-relevant or "must not fail unnoticed" signals use the logic where a broken wire gives the safe state: e.g. an "end position reached" for an interlock or a "door closed" signal is usually wired so that a wire break reads as "not OK". Note that this is not the same as a certified safety function.

2-wire sensors

Two-wire DC sensors are connected in series with the input: they need a small residual current when off and have a voltage drop when on. Check the input module's allowed quiescent current — otherwise the input may stay "on". NAMUR sensors (IEC 60947-5-6) are 2-wire sensors whose current changes (e.g. ≤ 1.2 mA / ≥ 2.1 mA), evaluated by an isolating amplifier — used in hazardous areas and allowing wire-break and short-circuit detection.

Practical checks

• Switching distance depends on target material (steel vs aluminium for inductive sensors) — use correction factors.
• LED on the sensor shows its state; compare with the PLC input LED to locate wiring problems.
• Mechanical limit switches bounce — see signal conditioning.
• IO-Link sensors can work as standard switching sensors (SIO) and add diagnostics — see IO-Link.