PLC Programming Best Practices

Understanding PLC Programming in Modern Industrial Automation

Programmable Logic Controllers (PLCs) form the backbone of industrial automation across Atlantic Canada's diverse manufacturing, processing, and resource extraction sectors. From fish processing plants along Nova Scotia's coastline to forestry operations in New Brunswick and offshore energy installations, PLCs control critical processes that demand reliability, efficiency, and safety. However, the difference between a PLC program that merely functions and one that excels in maintainability, performance, and longevity lies in adherence to established best practices.

For engineering teams across the Maritime provinces, implementing robust PLC programming standards isn't just about technical excellence—it's about ensuring that control systems can be maintained, troubleshot, and upgraded by personnel who may not have been involved in the original development. This comprehensive guide explores the essential best practices that Sangster Engineering Ltd. applies to every automation project we undertake.

Structured Programming and Code Organisation

The foundation of any maintainable PLC program begins with proper structure and organisation. Modern PLCs from manufacturers such as Allen-Bradley, Siemens, and Schneider Electric support modular programming approaches that should be leveraged fully.

Program Architecture Fundamentals

A well-organised PLC program typically follows a hierarchical structure that separates concerns and improves readability:

• Main Program Routine: Acts as the program coordinator, calling subroutines in a logical sequence without containing actual control logic

• Equipment Modules: Dedicated routines for each major piece of equipment (conveyors, pumps, valves, drives)

• Fault Handling Routines: Centralised fault detection, annunciation, and recovery logic

• Communication Routines: Handling of HMI exchanges, network communications, and data logging

• Initialisation Sequences: Startup procedures, mode selection, and system configuration

For a typical processing facility in Nova Scotia—whether it's a seafood plant in Lunenburg or a manufacturing facility in Truro—this modular approach allows maintenance technicians to quickly locate and understand specific sections of code without navigating through thousands of rungs of unorganised logic.

Naming Conventions and Documentation

Consistent naming conventions dramatically reduce troubleshooting time. We recommend adopting a standardised prefix system that immediately identifies tag types and locations:

• Input Tags: I_AreaName_DeviceName_Function (e.g., I_PKG_CONV01_RunFB)

• Output Tags: O_AreaName_DeviceName_Function (e.g., O_PKG_CONV01_Start)

• Internal Bits: B_AreaName_Function (e.g., B_PKG_AutoMode)

• Timers: T_AreaName_Function (e.g., T_PKG_ConvDelay)

• Counters: C_AreaName_Function (e.g., C_PKG_BoxCount)

Every rung should include a descriptive comment explaining its purpose, not simply restating what the logic does. Compare "This rung starts the conveyor" with "Conveyor 01 start permissives: Auto mode active, no downstream fault, and photoeye clear for minimum 2 seconds." The latter provides context that proves invaluable during 2:00 AM troubleshooting calls.

Safety-First Programming Methodology

In Canadian industrial environments, safety isn't optional—it's mandated by provincial occupational health and safety regulations and enforced through standards such as CSA Z432 and ISO 13849. PLC programming must integrate seamlessly with safety systems while maintaining clear boundaries between safety and standard control functions.

Fail-Safe Design Principles

Every output should be analysed to determine its safe state during power loss, communication failure, or processor fault conditions. Critical principles include:

• Normally Closed Logic: Safety-critical inputs (emergency stops, guard interlocks, limit switches) should be wired normally closed so that wire breaks result in a safe shutdown condition

• Watchdog Timers: Implement heartbeat monitoring between PLCs and safety systems, with typical timeout values of 100-500 milliseconds depending on process requirements

• Output Verification: For critical actuators, use feedback confirmation within defined time windows (typically 2-5 seconds for valve operations, 0.5-1 second for contactor feedback)

• Redundancy: Critical measurements should employ redundant sensors with discrepancy alarming when readings differ by more than acceptable tolerances

Safety PLC Integration

When projects require dedicated safety PLCs (such as Allen-Bradley GuardLogix or Siemens F-Series), maintain strict separation between safety and standard programs. Safety logic should be simple, auditable, and resistant to modification. The safety program should require no regular changes—if frequent modifications are needed, the safety architecture likely requires revision.

For Maritime industrial facilities, particularly those in fish processing or food manufacturing, machine guarding and emergency stop systems must comply with both federal and Nova Scotia provincial requirements. Our experience across dozens of facilities has shown that investing time in proper safety system design reduces commissioning delays and avoids costly retrofits.

Efficient Logic Design and Optimisation

While modern PLCs offer substantial processing power—with scan times often under 10 milliseconds even for programs exceeding 50,000 instructions—efficient logic design remains important for system responsiveness and maintainability.

Scan Time Optimisation

Understanding how your PLC executes code allows for intelligent optimisation:

• Conditional Subroutine Calls: Only execute routines when their associated equipment is active or their area is selected

• Event-Driven Logic: Use one-shot instructions for operations that only need to execute on state transitions rather than continuously

• Appropriate Data Types: Use BOOL for binary states, INT for small numbers (-32,768 to 32,767), DINT for larger integers, and REAL only when floating-point precision is genuinely required

• Array Processing: When working with repetitive data, use indexed addressing and loop structures rather than duplicating logic hundreds of times

Sequence Design Best Practices

Sequential operations—common in batch processing, packaging, and material handling—benefit from state machine architecture. Rather than using complex interlocking logic that becomes difficult to follow, implement explicit state variables that clearly indicate process phase:

• State 0: Idle/Ready

• State 10: Initialising

• State 20: Running - Phase A

• State 30: Running - Phase B

• State 100: Complete

• State 900: Faulted

Using non-consecutive state numbers (incrementing by 10) allows insertion of additional states during future modifications without renumbering the entire sequence—a small detail that saves significant time during system upgrades.

Communication and Integration Standards

Modern control systems rarely operate in isolation. PLCs must communicate with Human-Machine Interfaces (HMIs), Supervisory Control and Data Acquisition (SCADA) systems, Manufacturing Execution Systems (MES), and increasingly, cloud-based analytics platforms.

HMI Data Exchange Design

Structure your PLC data for efficient HMI communication by creating dedicated data structures (User-Defined Types in Allen-Bradley terminology, or Data Blocks in Siemens) that group all information needed for specific HMI screens or functions:

• Equipment Faceplates: Combine status bits, command bits, setpoints, and feedback values into single structures that can be passed to standardised HMI objects

• Alarm Arrays: Organise alarms in arrays with consistent indexing that maps directly to alarm management displays

• Trending Data: Pre-scale analogue values to engineering units within the PLC to reduce HMI processing load

Network Architecture Considerations

For facilities across Atlantic Canada, where many operations occur in harsh coastal environments with potential for electromagnetic interference, robust network design is essential:

• EtherNet/IP or PROFINET: Preferred for high-speed, deterministic communication between PLCs and drives

• Managed Switches: Enable network diagnostics, VLAN segmentation, and traffic prioritisation

• Ring Topologies: Provide redundancy for critical communication paths with sub-50-millisecond failover

• Cybersecurity: Segment control networks from business networks, implement firewall rules, and follow ISA/IEC 62443 guidelines

Testing, Commissioning, and Documentation

The value of a well-programmed PLC is only realised when the system is properly tested and documented. Rushing through commissioning to meet production deadlines inevitably creates problems that cost more to resolve than the time "saved."

Factory Acceptance Testing (FAT)

Before equipment ships to site—whether that's a facility in Amherst, Halifax, or anywhere across the Maritimes—conduct thorough FAT procedures:

• I/O Verification: Force every input and verify corresponding logic response; command every output and verify field device operation

• Sequence Testing: Run through all automatic sequences, including abnormal termination scenarios

• Fault Simulation: Deliberately create fault conditions (sensor failures, communication losses, safety device activation) and verify appropriate system response

• Performance Validation: Confirm scan times, communication response times, and data logging rates meet specifications

Documentation Requirements

Complete documentation packages should include:

• Functional Design Specification: Describing what the system does in plain language

• I/O Lists: Complete mapping of all physical and virtual I/O points with addresses, descriptions, and scaling information

• Network Diagrams: Showing all communication paths, IP addresses, and device configurations

• Program Printouts: Current versions of all PLC code with comments visible

• Backup Procedures: Instructions for program backup and restoration, including any required licenses or software versions

Continuous Improvement and Lifecycle Management

PLC programs are living documents that evolve throughout a system's operational life. Establishing proper change management procedures protects your investment and maintains system integrity.

Version Control and Change Documentation

Every program modification should be tracked through:

• Revision History: Maintained within the program file itself, documenting date, author, and description of each change

• Backup Protocols: Regular backups stored in secure, off-site locations with clear naming conventions indicating version and date

• Change Request Process: Formal documentation of why changes are needed, what modifications were made, and verification that changes achieved desired results without unintended consequences

Technology Refresh Planning

PLC platforms have typical lifecycles of 15-25 years, but software and communication technologies evolve more rapidly. Plan for periodic reviews to address:

• Obsolescence: Monitor manufacturer announcements for end-of-life notifications on processors, I/O modules, and communication cards

• Security Updates: Apply firmware updates that address identified vulnerabilities

• Performance Upgrades: Evaluate whether newer platforms offer capabilities that would benefit operations

• Documentation Currency: Ensure documentation reflects actual system configuration, particularly after modifications

Partner with Experienced Automation Professionals

Implementing these PLC programming best practices requires experience, discipline, and ongoing commitment to quality. For industrial facilities throughout Nova Scotia, New Brunswick, Prince Edward Island, and beyond, the investment in properly designed and documented control systems pays dividends through reduced downtime, faster troubleshooting, and systems that can be confidently maintained and upgraded over their operational lives.

Sangster Engineering Ltd. brings decades of combined experience in industrial automation to every project we undertake. Based in Amherst, Nova Scotia, our team provides PLC programming, system integration, and automation consulting services to clients across Atlantic Canada and beyond. Whether you're planning a new automation project, upgrading legacy systems, or seeking to improve the documentation and maintainability of existing installations, we deliver solutions built on the best practices outlined in this guide.

Contact Sangster Engineering Ltd. today to discuss how our automation expertise can support your operational excellence. Let us help you build control systems that perform reliably, maintain easily, and adapt to your evolving operational needs.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.