Sourcing vs Sinking: Why Your PLC Inputs or Outputs Aren’t Working

If your sensor is on but the PLC input never changes — or your output turns on in logic but the device does nothing — this is usually a sourcing vs sinking mismatch.

Fix Input Issues Fix Output Issues

This page helps with:

  • PNP vs NPN confusion
  • Inputs not registering
  • Outputs not energizing
  • Wiring mismatches

Where Should You Start?

Start based on the real symptom — not the term.

Sensor is ON but PLC input is OFF

Inputs Not Working PLC Troubleshooter

Output is ON but device is OFF

Outputs Not Working

Not sure yet

Start Troubleshooter

Common Workflows

Input Troubleshooting Flow

Troubleshooter → Inputs → Sourcing vs Sinking

Output Troubleshooting Flow

Outputs → Sourcing vs Sinking → Voltage Drop

Quick Explanation (What Actually Matters)

Sourcing

Sends +24V to the device (PNP)

Sinking

Sends path to 0V/common (NPN)

Rule: sourcing connects to sinking

Related Tools

Guide

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Guide

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Tool

PLC Troubleshooter

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Voltage Drop

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Reference

Reference Charts

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