Allen Bradley PLC Subroutines: A Comprehensive Guide

Allen Bradley PLC Subroutines: A Comprehensive Guide

Subroutines are powerful tools in Allen Bradley PLC programming that allow you to break down complex tasks into smaller, more manageable modules. This modular approach enhances code readability, reusability, and overall efficiency. In this comprehensive guide, we’ll delve into the intricacies of Allen Bradley PLC subroutines, exploring their fundamental concepts, creation, calling, and practical applications.

Understanding Subroutines

Imagine a PLC program as a recipe for a complex dish. Instead of listing all the ingredients and steps in one long sequence, you can organize the recipe into smaller sub-recipes, each focusing on a specific ingredient or preparation stage. Subroutines in PLC programming work similarly. They are self-contained blocks of code that perform a specific function, just like a sub-recipe in a larger recipe.

Benefits of Using Subroutines

Utilizing subroutines in your PLC programming offers numerous advantages:

• Improved Code Readability: By breaking down complex tasks into smaller subroutines, your program becomes more structured and easier to understand. This simplifies debugging and maintenance.
• Enhanced Reusability: Subroutines can be called multiple times within your program or even in different programs, saving you time and effort by eliminating redundant code.
• Increased Efficiency: Subroutines allow you to optimize specific tasks, leading to faster execution and improved overall program performance.
• Reduced Development Time: The modular nature of subroutines facilitates faster program development, as you can work on individual modules independently.

Creating Subroutines

To create a subroutine in an Allen Bradley PLC, you’ll need to follow these steps:

1. Define the Subroutine: Use the “SUBROUTINE” instruction in your program to declare the subroutine. Provide a descriptive name for the subroutine.
2. Write Subroutine Code: Inside the subroutine, write the code that performs the desired function. This code may include logic instructions, data manipulation, and I/O operations.
3. End the Subroutine: Use the “END_SUBROUTINE” instruction to mark the end of the subroutine.

Calling Subroutines

Once you’ve created a subroutine, you can call it from other parts of your program using the “CALL” instruction. The CALL instruction specifies the name of the subroutine to be executed. When the CALL instruction is encountered, the PLC executes the code within the subroutine and then returns control to the next instruction after the CALL.

Passing Data to Subroutines

Subroutines can accept input data and return output data. This data exchange is handled using parameters. Parameters are variables that are passed to the subroutine when it’s called. The subroutine can then use these parameters to perform its calculations or operations.

Example: Controlling a Conveyor Belt

Let’s consider a scenario where you need to control a conveyor belt based on sensor readings. You can use subroutines to streamline the program logic:

1. Subroutine 1: “ReadSensorData”: This subroutine reads the sensor readings and stores them in variables.
2. Subroutine 2: “ConveyorControl”: This subroutine takes the sensor data as input and determines the appropriate conveyor belt speed based on the readings. It then sends the speed command to the conveyor motor.

By using these subroutines, your program becomes more organized and easier to understand. You can modify the sensor readings and conveyor control logic in their respective subroutines without affecting the rest of the program.

Conclusion

Subroutines are an essential part of efficient and maintainable Allen Bradley PLC programming. By using them, you can improve code readability, reusability, and overall program performance. The concepts discussed in this guide provide a solid foundation for understanding and utilizing subroutines in your PLC projects.