10 PLC Programming Best Practices for Maximum Reliability and Maintainability

Programmable Logic Controllers (PLCs) form the backbone of industrial automation systems worldwide. While getting a PLC program to function correctly is one thing, creating robust, maintainable, and efficient code that will serve reliably for years is quite another. In my years as an automation engineer, I’ve developed and refined these best practices that separate exceptional PLC implementations from merely adequate ones.

1. Implement a Consistent Program Structure

A well-structured PLC program is easier to troubleshoot, maintain, and modify. I always organize my programs into logical sections:

• Initialization routines: Execute once at startup
• Input processing: Scale and condition raw inputs
• Main control logic: Organized by machine function or process step
• Alarm handling: Centralized alarm detection and management
• Output processing: Final conditioning before physical outputs
• Communication routines: Data exchange with HMIs, SCADA, or other systems

Practical Tip: Use state machines for sequential processes. They’re easier to understand, troubleshoot, and modify than complex nested branching logic.

2. Create Comprehensive Documentation

Documentation shouldn’t be an afterthought—it’s an integral part of the development process:

• Document the overall system architecture before writing any code
• Add descriptive comments for every rung or function block
• Use clear, descriptive variable names that indicate purpose and source
• Create detailed I/O lists with physical locations and signal types
• Document all communication parameters and data mappings
• Include a revision history that tracks who changed what and why

Practical Tip: When implementing, imagine a technician with minimal PLC knowledge attempting to troubleshoot your system at 3 AM during a critical production run. What documentation would they need?

3. Standardize Tag Naming Conventions

Consistent naming makes programs instantly more understandable:

[Location]_[DeviceType]_[Function]_[Number]

For example:

• MIXER1_MTR_START_CMD (Command to start Mixer 1 motor)
• TANK3_LVL_HI_ALM (High level alarm for Tank 3)
• OVEN2_TMP_PV (Process value for Oven 2 temperature)

Practical Tip: Create a standards document for your organization or client that defines naming conventions and ensure everyone follows it rigorously.

4. Use Structured Programming Techniques

Break down complex automation tasks into manageable, reusable components:

• Create standard function blocks for common operations like motor control, PID loops, or valve sequencing
• Test each function block thoroughly before integration
• Use parameters rather than hardcoded values for maximum flexibility
• Include error handling in each function block
• Document the expected inputs, outputs, and behavior

Practical Tip: Develop a personal library of proven, tested function blocks that you can reuse across projects to save time and ensure reliability.

5. Implement Robust Fault Handling

Never assume everything will work perfectly:

• Add timeout monitoring for operations that should complete in a specific timeframe
• Implement “watchdog” timers for critical processes
• Create comprehensive alarm systems that identify the specific fault condition
• Design for graceful degradation when possible (continue limited operation during partial failures)
• Include recovery procedures that don’t require engineering intervention

Practical Tip: Simulate various fault conditions during testing, including sensor failures, communication interruptions, and power fluctuations to verify proper handling.

6. Optimize for Maintainability

Someone will eventually need to modify or troubleshoot your code:

• Break complex logic into smaller, more manageable routines
• Avoid “clever” programming tricks that save a few lines but obscure functionality
• Keep network/rung sizes manageable—consider 7 contacts maximum per rung
• Use logical grouping of related functionality
• Implement indirect addressing carefully and with thorough documentation

Practical Tip: Have a colleague review your code. If they can’t understand a section in a few minutes of examination, it probably needs restructuring.

7. Consider Performance Impacts

While modern PLCs are powerful, inefficient programming can still cause issues:

• Be mindful of scan time impacts, particularly with complex calculations
• Understand the execution order of your controller
• Use appropriate data types to avoid unnecessary type conversions
• Consider segregating fast and slow logic into different tasks when supported
• Be especially careful with loops that could potentially run away

Practical Tip: Use the PLC’s diagnostic tools to monitor scan time and memory usage during development, paying special attention after adding complex functionality.

8. Secure Your Systems

Security is increasingly critical in industrial systems:

• Implement proper access controls with appropriate privilege levels
• Use secure communication protocols when available
• Never hardcode passwords in the program
• Keep controller firmware updated after appropriate testing
• Maintain offline backups of all program versions

Practical Tip: Conduct a security review of each system design, identifying potential vulnerabilities and mitigation strategies.

9. Design for the Future

Anticipate future needs and changes:

• Reserve I/O capacity for future expansion (typically 15-20%)
• Make parameter values adjustable through HMI rather than hardcoded constants
• Document assumptions and design decisions that might affect future modifications
• Create scalable architectures that can accommodate additional stations or functionality
• Use modular programming approaches that allow for easy expansion

Practical Tip: Discuss likely future requirements with stakeholders before finalizing the design to ensure your architecture can accommodate them.

10. Test Thoroughly

Comprehensive testing is non-negotiable:

• Create detailed test plans that verify each function against specifications
• Test boundary conditions and edge cases, not just the “happy path”
• Verify all alarm conditions and recovery procedures
• Test at full production speeds, not just slow manual operation
• Involve operators in testing to identify usability issues

Practical Tip: Develop simulation routines that can exercise your code without physical hardware for more comprehensive testing before commissioning.

Conclusion

Following these best practices requires additional effort during the development phase, but this investment pays tremendous dividends over the lifecycle of an automation system. Well-structured, properly documented, and thoughtfully designed PLC programs reduce downtime, simplify troubleshooting, and make future modifications safer and more efficient.

The most successful automation engineers think beyond simply making the machine work today—they create systems that remain reliable, maintainable, and adaptable for years to come. These best practices represent lessons learned through years of industrial experience, often the hard way through troubleshooting poorly implemented systems.

If you’re looking to improve your PLC programming practices or need assistance with industrial automation projects that prioritize long-term reliability and maintainability, I’m here to help. Contact me to discuss your specific automation challenges.