Introduction to SCL: A PLC Programmer's Guide

Introduction to the Series

Welcome to the first article in my series on developing with Structured Control Language (SCL). Over the next ten articles, I'll delve into various aspects of SCL programming, providing PLC programmers, who are well-versed in ladder logic and function block diagrams, the knowledge they need to excel in SCL. My goal is to not only enhance your programming skills but also showcase your expertise to employers and customers, making you a sought-after professional in the field.

Which PLC Brands Support SCL

Several PLC brands support SCL, with Siemens, Emerson (formerly GE), and Allen-Bradley being notable examples, offering versatility across different platforms.

Overview of Structured Control Language (SCL) and Its Advantages

Structured Control Language (SCL) is a high-level programming language designed for PLCs, offering several advantages over traditional ladder logic:

• Text-Based: SCL is text-based, making it easier to use version control systems like GIT.
• Complex Calculations: Ideal for advanced mathematical calculations and interpolations.
• Modularity: Promotes modular programming, allowing for reusable and maintainable code.

Comparing SCL with Traditional Ladder Logic

While ladder logic is visual and intuitive, especially for those with an electrical background, SCL provides a more powerful and flexible approach for complex tasks:

• Ladder Logic: Visual, great for simple control tasks, easy to learn for beginners.
• SCL: Text-based, better for complex calculations and control algorithms, suitable for those familiar with programming concepts.

Basic Syntax and Structure of SCL Programs

SCL syntax is similar to other high-level programming languages. Here’s a basic example to get started:

   IF Start AND Permissive THEN

      MotorState := TRUE;

   ELSIF Stop OR Trip THEN

      MotorState := FALSE;

   END_IF;        

Setting Up an SCL Project in Your PLC Software

Setting up an SCL project typically involves:

1. Creating a New Project: In your PLC programming software (e.g., Siemens TIA Portal), create a new project.
2. Adding SCL Blocks: Add SCL function blocks to your project.
3. Configuring I/O: Define your input and output variables.
4. Writing Code: Implement your control logic in SCL.
5. Testing and Debugging: Use simulation tools to test and debug your SCL code.

Simple Example: Implementing a Basic On/Off Control Using SCL

Here's a straightforward example of implementing an on/off control using SCL:

   IF Start AND Permissive THEN

      MotorState := TRUE;

   ELSIF Stop OR Trip THEN

      MotorState := FALSE;

   END_IF;        

This basic function block turns a motor on or off based on the start and stop inputs. It’s a great starting point for understanding how SCL can be used in real-world applications.

Advanced Calculation Example: Converting Sensor Input to Position

Here's how to perform an advanced calculation to convert an input range of 0-32767 engineering units to an output range of 0-20 feet using SCL:

Position := (REAL(RawValue) / 32767.0) * 20.0;        

This single line of SCL code efficiently handles the entire conversion, demonstrating the power and simplicity of SCL for advanced calculations.

Function Block Diagram for Conversion

To achieve this conversion using FBD, you would typically use arithmetic function blocks. Here’s a step-by-step description:

1. Scale: First, convert the raw input value to a floating-point number and scale it to the 0-1 range.

DIV: Divide the raw input (`RawValue`) by 32767 to scale it to the 0-1 range.

2. Multiply: Then, multiply the scaled value by the desired range (20 feet).

MUL: Multiply the scaled value by 20.0 to get the position in feet.

Here's a visual representation of the FBD for this conversion (keep in mind that you will need to convert between FLOAT and INT data types with FBD with more functions, which makes this more complex):

Personal Insights and Practical Applications

Motivation for Learning SCL

I was motivated to explore SCL due to its increasing demand in programming jobs and its advantages in handling complex tasks. Transitioning to SCL allowed me to delve into more programming languages and leverage text-based version control systems like GIT, which are not always available for ladder logic.

Real-World Applications

One of my notable projects involved using SCL for interpolating a 10-15 point x-y curve for process values like conductivities and motor curves. SCL made it easy to provide a multi-curve response, which would have been cumbersome with ladder logic alone. Additionally, I used SCL to develop time-weighted averages of process signals, showcasing its capability for advanced calculations.

Overcoming Challenges

Switching from ladder logic to SCL was challenging, particularly with Siemens platforms due to their differences. However, using SCL within specific functions and gradually incorporating it into larger programs made the transition smoother. This approach can help other PLC programmers embrace SCL without feeling overwhelmed.

Final Thoughts

SCL offers numerous benefits for PLC programming, making it an invaluable tool for those looking to advance their careers. By mastering SCL, you can handle complex tasks more efficiently, showcase your skills to potential employers, and stay at the forefront of the industry.

Stay tuned for the next article in my series, where I’ll dive into getting started with timers and counters in SCL!

About the Author: Ken Jensen brings over 20 years of experience in electrical, automation, & controls engineering across diverse industries like nuclear, steel, and glass. He is a highly skilled problem-solver with expertise in electrical systems, PLCs, SCADA, and Industry 4.0 technologies. He is adept at leading projects, driving innovation, and fostering collaboration.