PLC SINKING vs SOURCING INPUTS

PLC SINKING vs SOURCING INPUTS – One of the Most Confusing PLC Concepts Explained!

Many engineers can troubleshoot PLC logic, but when it comes to sinking and sourcing inputs, confusion often starts.

The good news?

Once you understand the direction of current flow, everything becomes much easier.

SINKING INPUT

A sinking PLC input is looking for +24V to turn ON.

Current flow:

+24V ➝ Sensor/Switch ➝ PLC Input ➝ 0V

PLC input receives +24V when the device is activated.

Used with Sourcing Devices (PNP Sensors)

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SOURCING INPUT

A sourcing PLC input is looking for a path to 0V to turn ON.

Current flow:

+24V ➝ PLC Input ➝ Sensor/Switch ➝ 0V

Device provides the return path to 0V when activated.

Used with Sinking Devices (NPN Sensors)

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Easy Way to Remember

Sourcing Device = Gives (+24V)

Sinking Device = Gives (0V Path)

Sinking Input = Expects +24V

Sourcing Input = Expects 0V

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Common Industrial Sensors

PNP Sensors → Most common in Europe and many process industries

NPN Sensors → Common in some Asian machinery and OEM equipment

Always check the sensor datasheet before wiring.

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What Happens If You Wire It Incorrectly?

PLC input will not detect the signal

Input remains OFF even when the sensor operates

Troubleshooting becomes difficult

In some cases, incorrect wiring can damage the module

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Practical Troubleshooting Tip

If a PLC input is not turning ON:

Measure voltage at the input terminal

Verify PLC input type (Sinking or Sourcing)

Check sensor output type (PNP or NPN)

Confirm common terminal wiring

Verify 24VDC supply and return path

Many “PLC faults” are actually wiring compatibility issues between sensors and input modules.

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Quick Rule

PNP Sensor ➝ Sinking PLC Input

NPN Sensor ➝ Sourcing PLC Input

This single rule can save hours of troubleshooting in the field.

Key Takeaway

The PLC doesn’t care about the sensor type—it only cares whether current can flow through the input circuit.

Understand current flow, and PLC input wiring becomes simple.

Which sensor type is most commonly used in your plant—PNP or NPN? Let us know in the comments.