The Power of Modular PLC Code

Jan. 8, 2024
By breaking down a controller program into independent modules related to specific functions, engineers can avoid the all-too-common spaghetti code problem.

Everyone loves spaghetti! It's a universal comfort food, often bringing back fond memories of family gatherings and cozy dinners. However, while spaghetti on your dinner plate is a delightful experience, spaghetti in the context of coding is far less appetizing.

Spaghetti code is a term that brings a shiver down the spine of any programmer or system integrator. This kind of code, characterized by its tangled, unstructured and confusing nature, is the antithesis of what we strive for in automation software integration. It's hard to follow, harder to debug and a nightmare to modify. 

But here's the good news: With modular coding, one can avoid the tangle of spaghetti code and reap multiple benefits. Modular PLC code, with its structured and organized approach, not only enhances performance but also ensures reusability, flexibility and interoperability—essential ingredients for a successful automation recipe.

The benefits of a modular approach

Modular code refers to the practice of breaking down a program into separate, independent modules or components, each responsible for a specific function. There are several benefits to adopting a modular approach in coding:

  • Code reusability—Modular code allows you to encapsulate functionality and reuse code across different parts of an application.  Code is developed and tested once, increasing efficiency and quality.
  • Scalability is enhanced by allowing new functionality without restructuring the entire code base. 
  • Troubleshooting and maintenance—Modular components are easier to test and debug, ensuring that each component functions correctly before integration. 
  • Integration and interoperability can be achieved by exposing only necessary information through well-defined interfaces. 

Types of modules in ISA-88

Though the ISA-88 standard originated in PLC-based batch control, its modular principles extend well beyond this initial domain. Following are highlights of how ISA-88 modules empower automation programming across a vast landscape of discrete and process industries.

  • Control Module—Executes basic control actions, operating on inputs to execute commands ranging from simple binary actions to sophisticated PID algorithms.
  • Equipment Module—Coordinates the functions of control modules to manage specific unit operations like mixing or heating.
  • Phase—Defines discrete process-oriented tasks and can send commands to equipment and/or control modules ensuring consistent execution and quality control.
  • Operation—Combines phases into major process sequences, like transforming materials through chemical or physical changes. 
  • Procedure—Outlines strategies for executing processes by linking together multiple operations and serving as the master recipe for production. 

The modular design of the S88 standard, when used in conjunction with UDTs (user-defined types) and AOIs (add-on instructions), empowers complex manufacturing by providing a comprehensive framework for managing and optimizing every element, from individual equipment control to overall batch execution strategy. 

This is possible because:

  • UDTs allow programmers to create custom data types, bundling data into logical units.
  • AOIs offer reusable, standardized code blocks. 

This kind of modularity excels in recipe configuration, which can be abstracted from the PLC layer and managed within any HMI/SCADA platform. This frees process engineers to independently optimize processes without relying on controls engineers, fostering collaboration and efficiency across teams.

Modular coding in PLCs is akin to a well-prepared dish: organized, efficient and effective. It's like having a recipe that's easy to follow and adjust, always yielding a delightful result in performance, scalability or maintenance. 

Alex Donnell is vice president of operations at NeoMatrix Inc., an integrator member of the Control System Integrators Association (CSIA). For more information about NeoMatrix Inc., visit its profile on the Industrial Automation Exchange.