Allen Bradley PLC Subroutines: Mastering Control with Subroutines

In the realm of industrial automation, Allen Bradley PLCs (Programmable Logic Controllers) stand as the backbone of countless processes. These powerful devices, capable of executing complex sequences of instructions, rely on a variety of programming techniques to achieve optimal control. Among these techniques, subroutines emerge as a fundamental tool for enhancing code organization, efficiency, and reusability. This comprehensive guide delves into the world of Allen Bradley PLC subroutines, equipping you with the knowledge and skills to master their implementation and leverage their full potential.

The Essence of Subroutines

Imagine a complex recipe with multiple steps. Instead of writing out the entire recipe repeatedly for each batch, you could create a separate section for each step and simply call upon those sections as needed. Subroutines in Allen Bradley PLCs operate on a similar principle. They are essentially self-contained blocks of code that perform a specific task. By encapsulating repetitive or complex logic within subroutines, programmers can streamline their code, reduce redundancy, and improve maintainability.

Types of Subroutines

Allen Bradley PLCs offer two primary types of subroutines, each designed for specific purposes:

• Function Blocks (FBs): Function blocks are versatile subroutines that can accept input values (parameters) and return output values. They are particularly useful for implementing complex algorithms or calculations.
• Function (FUN): Functions are more streamlined subroutines that focus on performing a specific task without requiring input parameters. They are ideal for simplifying repetitive tasks or calculations.

Understanding the Structure

Subroutines are defined within a PLC program using the following structure:

SUBROUTINE SubroutineName
  // Code within the subroutine
END_SUBROUTINE

The subroutine name should be descriptive and reflect its purpose. The code within the subroutine block defines the specific logic or tasks it performs. Once defined, subroutines are called from the main program using the following syntax:

CALL SubroutineName

Mastering Subroutine Control with Master Controls

Master controls are essential elements for effectively managing and controlling subroutines. They provide a mechanism for initiating, stopping, or pausing subroutines as needed. Here’s a breakdown of common master control strategies:

• Enable/Disable Control: A simple and effective method for controlling subroutines is by using enable/disable signals. An enable signal allows the subroutine to execute, while a disable signal prevents it from running. This approach is ideal for situations where you want to activate or deactivate a subroutine based on specific conditions.
• Timer-Based Control: Using timers, you can set a specific duration for subroutine execution. This is useful for tasks that require a fixed time interval, such as periodic updates or data collection.
• Event-Triggered Control: Subroutines can be triggered by specific events, such as a change in input value, a timer expiring, or a specific condition being met. Event-triggered control ensures that subroutines are executed only when necessary, optimizing resource utilization.
• State-Based Control: For more complex scenarios, subroutines can be designed to operate in different states. Each state represents a specific stage of the subroutine’s execution, allowing for flexible and dynamic control. This approach is particularly useful for processes with multiple steps or phases.

Benefits of Subroutine Implementation

The use of subroutines offers numerous advantages in PLC programming:

• Code Organization: Subroutines promote a structured and modular approach to PLC programming, making code easier to understand, maintain, and debug.
• Reusability: Subroutines can be reused in different parts of the program or even in other PLC projects, reducing code duplication and development time.
• Efficiency: By encapsulating complex logic within subroutines, programmers can simplify the main program, improving execution speed and reducing memory usage.
• Maintainability: Subroutines make it easier to modify or update specific sections of code without affecting other parts of the program. This is crucial for maintaining PLC programs over time.

Practical Examples

Let’s consider some practical examples of how subroutines can be used in real-world applications:

• Conveyor Belt Control: A subroutine could be created to control the movement of a conveyor belt, including starting, stopping, and reversing. This subroutine could be reused for multiple conveyor belts in the same system.
• Temperature Control: A subroutine could be implemented to regulate the temperature of a process by controlling heating or cooling elements. The subroutine could adjust its operation based on sensor readings and setpoint values.
• Data Acquisition: A subroutine could collect data from various sensors and process it to generate reports or trigger alarms. This subroutine could be used to monitor system performance or detect potential issues.

Conclusion

Subroutines are essential tools in the arsenal of every Allen Bradley PLC programmer. By leveraging their power, you can create more organized, efficient, and maintainable PLC programs. Master the art of subroutine implementation and master control with master controls.