PNP vs NPN: match the sensor to the PLC input

Understand the input circuit

A PNP output typically sources current toward the load from the positive supply. An NPN output typically sinks current toward the negative reference. The PLC input and wiring arrangement must be compatible. Identify the input module model and consult its diagram instead of relying on a cable color convention.

Do not assume a device is field-switchable between PNP and NPN. The supplied Lisen and legacy LBFS references describe model or ordering choices that require confirmation.

Check load and voltage levels

The PLC input current must be within the switching output limit. Also consider residual voltage, leakage current and the input’s switching thresholds where relevant. A nominally compatible connection can still behave unexpectedly if the interface is not matched.

Commission with a truth table

Record the expected input state for covered, uncovered and power-loss conditions. Test through the installed controller and indication, not only at the sensor LED. Electrical installation and testing should be performed by qualified personnel using the approved wiring documentation and isolation procedures.

What to confirm for your application

• PLC input module and wiring diagram
• PNP or NPN order code
• Input current and voltage thresholds
• Output logic for each process state
• Power-loss behavior