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Signals and PLC · Glossary

What are Sinking and Sourcing?

digital I/O current directionSignals and PLC

Sinking and sourcing describe the direction of current in digital I/O: a sourcing device supplies current, a sinking device receives it.

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Sinking and Sourcing Explained

A PNP sensor sources current into a sinking PLC input; an NPN sensor sinks current from a sourcing input. Mismatched types do not work.

PNP sensors with sinking inputs are common in Europe, while NPN is common in Japanese and some other Asian equipment.

Sinking and Sourcing: Typical Values

PNP sensorsourcing, switches +24 V to the load
NPN sensorsinking, switches the load to 0 V
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Calculators for Sinking and Sourcing

CalculatorNAMUR Sensor CalculatorFree online calculator with formula and worked example.CalculatorPLC IO Count CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadIO List Template in Excel with Automatic I/O Count and SparesFree download in Excel and PDF.

Frequently Asked Questions

What are Sinking and Sourcing?

Sinking and sourcing describe the direction of current in digital I/O: a sourcing device supplies current, a sinking device receives it.

How do I tell PNP from NPN?

Check the datasheet or measure: a PNP output goes to +24 V when on.

Can a dry contact feed either type?

Yes, wired to suit the input.

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