PLC Programming Best Practices

After hundreds of PLC projects—from small machine retrofits to greenfield plants—these best practices have become non-negotiable. They keep code understandable, safe, and ready for integration with modern analytics.

1. Start with Standards

• Adopt ISA-88/PackML state models where applicable.
• Use vendor guidelines (Siemens SiVArc, Rockwell PlantPAx) as templates.
• Maintain coding standards for tag naming, comments, and language usage.

2. Modular Architecture

• Break logic into reusable modules (equipment phases, routines) with clear interfaces.
• Create library function blocks for common equipment (motors, valves, conveyors).
• Version-control libraries and document change history.

3. Safety First

• Separate safety PLC programs or safety-rated routines from standard control logic.
• Document safety functions and validate during FAT/SAT.
• Include safe-state routines in case of communication loss or fault conditions.

4. Diagnostics and Visibility

• Expose key states, alarms, and counters for HMIs, historians, and MQTT payloads.
• Log operator interventions and mode changes.
• Provide troubleshooting guides aligned with program structure.

5. Data Handling and Integration

• Normalize units and scaling before exposing data to other systems.
• Use structured text or function blocks for recipe management and data parsing.
• Implement buffers or store-and-forward mechanisms when sending data to the cloud.

6. Testing and Simulation

• Use vendor simulators (FactoryTalk Logix Echo, Siemens PLCSIM) before deploying.
• Create test harnesses for functions and sequence routines.
• Record test cases and expected results as part of documentation.

7. Change Management

• Track revisions in source control with descriptive commit messages.
• Review changes with peers to catch logic errors and align with standards.
• Backup PLC programs automatically and verify restore procedures.

8. Continuous Improvement

• Hold post-commissioning reviews to capture lessons learned.
• Train maintenance teams on navigating code and using diagnostics.
• Update documentation whenever new features or process changes are introduced.

Great PLC code is more than rungs and blocks—it’s a disciplined process. Follow these practices to deliver systems that operators trust and IT teams can integrate without headaches.