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Advanced PLC Programming: Using Function Blocks and User-Defined Data Types

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By Safar zitoun Zine eddine 6 min read

1. Introduction

As PLC (Programmable Logic Controller) systems become more complex, advanced programming techniques are required to develop efficient and maintainable code. This tutorial explores the use of function blocks and user-defined data types in PLC programming. By leveraging these powerful features, programmers can create reusable code blocks and structured data types, leading to more modular and scalable PLC programs. This article provides step-by-step examples and practical use cases to demonstrate the benefits and implementation of function blocks and user-defined data types in PLC programming.

2. Function Blocks

2.1. What are Function Blocks?

Function blocks are reusable software components that encapsulate specific functionality. They provide a structured approach to programming by combining inputs, outputs, and internal variables into a single entity. Function blocks can be created for common operations, such as PID control, motor control, or communication protocols. They promote code reuse, simplify program structure, and enhance maintainability.

2.2. Advantages of Function Blocks

Using function blocks offers several advantages in PLC programming:

  • Modularity: Function blocks allow you to break down complex tasks into smaller, more manageable units, improving code organization and readability.
  • Reusability: Once created, function blocks can be easily reused in multiple parts of a program or even in different projects, saving development time and effort.
  • Encapsulation: Function blocks encapsulate their internal variables and logic, providing a clear interface to the rest of the program and enhancing code abstraction.
  • Simplified Debugging: Function blocks can be tested and debugged independently, making troubleshooting and maintenance more efficient.
  • Collaborative Development: Function blocks enable collaborative programming by allowing different team members to work on specific blocks without affecting the rest of the program.

2.3. Creating Function Blocks

To create a function block, follow these steps:

  1. Identify the functionality or task that the function block will encapsulate.
  2. Determine the inputs, outputs, and internal variables required for the desired functionality.
  3. Define the data types and structure of the function block, including its input and output parameters.
  4. Implement the logic and algorithms specific to the function block’s purpose.
  5. Test and validate the function block independently before integrating it into the main program.

2.4. Function Block Instances

Function blocks are instantiated within the main PLC program to utilize their functionality. Each function block instance has its own memory space, input/output connections, and internal variables. By creating multiple instances of the same function block, you can handle multiple instances of a device or process within the PLC program.

3. User-Defined Data Types

3.1. Why Use User-Defined Data Types?

User-defined data types (UDTs) allow programmers to define custom data structures tailored to their specific needs. By grouping related variables together, UDTs enhance code organization, readability, and maintainability. UDTs provide a way to create structured data types beyond the built-in elementary data types provided by the PLC manufacturer.

3.2. Creating User-Defined Data Types

To create a user-defined data type, follow these steps:

  1. Identify the variables that need to be grouped together based on their relationship or purpose.
  2. Determine the data types for each variable within the UDT.
  3. Define the structure of the UDT by specifying the data types and names of each variable.
  4. Optionally, add methods or functions to the UDT to perform operations specific to the data type.
  5. Test the UDT by creating instances and accessing its variables or methods.

3.3. Using User-Defined Data Types

Once a user-defined data type is created, it can be used in PLC programming in the following ways:

  • Variable Declarations: Declare variables of the user-defined data type to store and manipulate structured data.
  • Parameter Passing: Pass user-defined data types as parameters to function blocks or subroutines, enabling structured data exchange.
  • Array Elements: Use user-defined data types as elements of arrays to store and access multiple instances of structured data.
  • Data Organization: Utilize user-defined data types to organize data within data blocks or data structures, improving code clarity and accessibility.

4. Combining Function Blocks and User-Defined Data Types

4.1. Benefits of Combining Function Blocks and User-Defined Data Types

When function blocks and user-defined data types are combined, PLC programmers can achieve even greater flexibility and code reuse. By encapsulating complex functionality within function blocks and utilizing structured data types, the following benefits can be realized:

  • Modularity and Reusability: Function blocks can be created using user-defined data types as input and output parameters, allowing for modular and reusable code blocks.
  • Code Structuring: By using function blocks and user-defined data types together, the program’s structure becomes more organized and easier to understand.
  • Data Consistency: User-defined data types ensure consistent data structures are used across multiple instances of function blocks, preventing errors and enhancing maintainability.
  • Standardization: Function blocks and user-defined data types enable standardization of programming practices within an organization, leading to code consistency and efficiency.

4.2. Practical Examples

To illustrate the power of combining function blocks and user-defined data types, consider the following practical examples:

  • Motor Control: Create a function block for motor control that accepts a user-defined data type representing motor parameters. This combination allows for easy configuration of motor parameters and simplifies the reuse of the motor control functionality in different parts of the program.
  • Communication Protocol: Develop a function block encapsulating a communication protocol, such as Modbus or Ethernet/IP, utilizing a user-defined data type to represent the protocol-specific data structure. This approach enables consistent communication handling and simplifies integration with different devices.

5. Best Practices for Advanced PLC Programming

5.1. Modularity and Reusability

When working with advanced PLC programming techniques like function blocks and user-defined data types, it’s essential to follow best practices for modularity and reusability:

  • Identify Common Functionality: Identify frequently used or repetitive tasks that can be encapsulated into function blocks for easy reuse.
  • Design for Flexibility: Create function blocks and user-defined data types that are configurable and adaptable to different scenarios or system configurations.
  • Documentation and Version Control: Document function blocks and user-defined data types thoroughly, including their purpose, inputs, outputs, and usage instructions. Maintain version control to track changes and ensure consistency.

5.2. Documentation and Comments

To enhance code readability and maintainability, proper documentation and comments are crucial:

  • Function Block Documentation: Document each function block, describing its purpose, inputs, outputs, and any specific usage instructions.
  • Commenting: Add comments within the code to explain complex logic, provide context, and clarify the programmer’s intentions.

5.3. Error Handling and Fault Diagnostics

When implementing advanced PLC programming techniques, consider error handling and fault diagnostics:

  • Error Handling: Implement error handling mechanisms within function blocks to handle exceptional situations gracefully and provide appropriate feedback or recovery actions.
  • Fault Diagnostics: Incorporate fault diagnostics within function blocks to aid in troubleshooting and identifying the root causes of system failures.

6. Conclusion

Advanced PLC programming techniques, such as utilizing function blocks and user-defined data types, offer significant benefits in terms of code organization, reusability, and maintainability. By leveraging these techniques, programmers can create modular and structured PLC programs that are easier to understand, maintain, and scale. This tutorial provided an overview of function blocks and user-defined data types, their advantages, and practical examples of their combined usage. By following best practices and applying these techniques in PLC programming projects, professionals can improve efficiency and productivity while developing robust and flexible automation solutions.

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