Unit Testing for PLC Logic: Yes, It’s Possible

Software testing has long been part of IT, but OT engineers are catching up. With modern PLC frameworks, automated unit testing of Function Blocks and Structured Text code is now feasible — and it dramatically improves reliability.

Why Test PLC Logic?

• Detects logic errors before commissioning.
• Supports regression testing after firmware or library updates.
• Builds confidence in code reuse across multiple machines.

How It Works

• Simulation mode: Run FBs with test data offline in Codesys or TwinCAT.
• Test frameworks: Tools like PLCUnit or test automation via Python OPC UA clients.
• Assertions: Validate expected output values automatically.

Example Workflow

1. Write each Function Block with clear inputs/outputs.
2. Create test scripts for boundary conditions and edge cases.
3. Run automated tests during CI/CD pipeline before deployment.

Case Example

A machine builder adopted PLCUnit in Codesys. Test coverage reached 75%, reducing field bugs by 60% and improving documentation accuracy for ISO 13849 validations.

Related Articles

• Version Control for PLC Projects: Git without the Pain
• IEC 61131-3 Patterns for Maintainable Code
• From Ladder to Structured Text: When (and How) to Switch

Conclusion

Automated unit testing moves PLC programming into true software engineering. It brings predictability, documentation, and peace of mind — before a single output turns on.