Introduction to Structured Text

Structured Text: A Key Language for Programmable Logic Controllers (PLCs)

Structured Text (ST), also known as STX, is one of the five languages defined by the IEC 61131-3 standard for use in programmable logic controllers (PLCs). The IEC 61131-3 standard plays a crucial role in industrial automation, providing a standardized way of programming PLCs across different hardware platforms. With the increasing complexity of industrial control systems, it is vital that programmers have robust tools for developing reliable and efficient control logic. ST is one of those tools, offering a high-level, block-structured language that is both versatile and powerful.

This article delves into the characteristics of Structured Text, its significance within the IEC 61131-3 standard, and how it compares to other programming languages used in industrial control systems. We will explore its syntax, features, advantages, and how it supports modern industrial automation solutions.

Overview of Structured Text

Structured Text is designed to be a high-level, textual programming language, similar in structure and syntax to Pascal. It supports a wide variety of control structures, including conditional statements, loops, and functions, making it ideal for handling complex operations in automation systems. The language’s syntax is clear and concise, which aids in reducing the complexity of programming tasks and improving the maintainability of code.

In terms of its relationship with other languages within the IEC 61131-3 standard, ST shares many common elements, such as variable definitions, function calls, and data types. This allows for interoperability between different languages in the standard. For instance, a control system might use a combination of Ladder Diagram (LD), Function Block Diagram (FBD), and Structured Text in a single program, making use of each language’s strengths to solve different parts of the problem efficiently.

Key Features of Structured Text

1. Block-Structured Syntax
   ST’s block-structured syntax allows for organizing code in clear, logical sections, making it easier for programmers to follow and manage. This approach also promotes modularity, enabling the reuse of code blocks across different parts of a program or in different programs.

2. Support for Complex Control Structures
   Structured Text allows the use of complex control structures, such as iteration loops and conditional execution. These include:

   • REPEAT-UNTIL and WHILE-DO loops, which allow for repetitive execution of code under certain conditions.
   • IF-THEN-ELSE and CASE statements, which provide mechanisms for conditional logic, allowing different actions to be taken based on specific criteria.

   These features make it easier to implement sophisticated control algorithms, which are often required in industrial automation.

3. Mathematical and Logical Functions
   ST supports a wide range of built-in functions, including mathematical functions such as SQRT() (square root), SIN() (sine), and other trigonometric, logarithmic, and arithmetic functions. These capabilities are essential for implementing control logic that involves calculations or data processing, which is a common requirement in automated systems.

4. Support for Custom Functions and Function Blocks
   Structured Text allows users to define custom functions and function blocks. A function block in ST is similar to a subroutine or a module, which encapsulates specific functionality and can be reused across the program. This modular approach improves code readability and maintainability, reducing the likelihood of errors and making it easier to debug and extend the program.

5. Variable Declaration and Data Types
   ST uses a variety of data types, including integers, real numbers, booleans, and strings, among others. The language supports the declaration of variables at the beginning of the program or within specific sections. This helps in organizing the program’s memory management and enhances its efficiency.

6. Error Handling and Diagnostics
   One of the challenges in industrial automation is the need for reliable error detection and handling mechanisms. ST provides robust tools for implementing error handling logic, ensuring that the system can recover from or report errors effectively. This is especially important in real-time control systems, where the consequences of an error could be severe.

Comparison with Other IEC 61131-3 Languages

IEC 61131-3 includes a total of five programming languages: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC). Each of these languages has its strengths and is suited to different types of control tasks.

• Ladder Diagram (LD): Often used by electricians and technicians, Ladder Diagram resembles electrical relay logic, making it intuitive for those familiar with circuit diagrams. It is ideal for simple control tasks but can become cumbersome for complex algorithms.

• Function Block Diagram (FBD): FBD is graphical in nature and uses blocks to represent various functions. It is particularly useful for modeling control systems where the relationship between functions can be visualized as interconnected blocks. However, it may not be as efficient as ST for handling large-scale complex algorithms.

• Structured Text (ST): As discussed, ST is text-based and ideal for more complex logic. It excels in situations requiring intricate calculations, advanced functions, and control loops. Programmers who are comfortable with high-level languages like Pascal or C will find ST to be a natural fit.

• Instruction List (IL): Instruction List is a low-level language similar to assembly. While it offers the highest level of control over the hardware, it is more difficult to work with and is becoming less common in modern control systems.

• Sequential Function Chart (SFC): SFC is used for defining sequential operations and workflows. It is typically used for processes that involve step-by-step operations, such as batch processes.

In comparison, Structured Text is best suited for situations that demand the handling of complex algorithms, mathematical operations, and programmatic logic. It is especially beneficial when precise control over the execution of algorithms is required, making it more versatile than graphical languages like FBD or LD.

Use Cases of Structured Text in Industrial Automation

Structured Text is widely used in various sectors of industrial automation due to its ability to handle complex control logic efficiently. Some of the prominent use cases include:

• Manufacturing and Process Control
  In manufacturing plants, ST is commonly used to control machinery, robots, and production lines. Its ability to handle loops, conditionals, and mathematical operations makes it ideal for applications that require precise control of production processes, such as controlling temperature, pressure, and other variables in chemical plants or automated assembly lines.

• Building Automation
  In building management systems, ST is employed to control lighting, HVAC (heating, ventilation, and air conditioning) systems, and security systems. These systems often require sophisticated logic for energy efficiency, and ST’s support for conditionals and calculations makes it suitable for these tasks.

• Water Treatment Plants
  Water treatment plants require complex control systems for monitoring and adjusting chemical dosing, flow rates, and filtration processes. Structured Text is used in these systems to create reliable control loops and manage sensor data effectively.

• Automated Testing Systems
  ST is used in automated testing applications to control testing rigs, measure parameters, and log data. The ability to integrate mathematical functions and control structures allows for the development of sophisticated test protocols.

Advantages of Using Structured Text

1. High-Level Abstraction
   Structured Text offers a high-level programming model, which makes it easier to develop complex control logic compared to lower-level programming languages like IL. This abstraction also helps in reducing the time spent on coding and debugging.

2. Improved Code Maintainability
   The block-structured nature of ST, combined with its modular design capabilities (such as functions and function blocks), leads to improved code readability and maintainability. This is crucial in industrial settings, where long-term system reliability is essential, and modifications are often required over time.

3. Interoperability with Other Languages
   ST’s compatibility with other IEC 61131-3 languages enables engineers to combine the strengths of different languages within a single project. For instance, a system might use Ladder Diagram for visual control logic and ST for complex computational tasks, all within the same program.

4. Rich Functionality
   The built-in mathematical and logical functions in ST provide a rich set of tools for developing a wide range of applications. From simple motor control to advanced data processing, ST is capable of handling diverse industrial automation tasks.

5. Portability Across Platforms
   Since ST is part of the IEC 61131-3 standard, programs written in this language can be ported between different PLC brands and models, offering significant flexibility in hardware selection.

Conclusion

Structured Text (ST) plays a critical role in modern industrial automation, offering a high-level programming approach that is well-suited to handle complex control logic and data processing tasks. Its syntax, inspired by Pascal, is easy to understand and maintain, making it ideal for industrial applications requiring sophisticated algorithms. While other IEC 61131-3 languages like Ladder Diagram and Function Block Diagram are more suitable for simpler tasks, ST excels in situations where precision, flexibility, and advanced functionality are needed.

As industrial control systems continue to grow in complexity, the demand for robust and efficient programming languages like Structured Text will only increase. Its ability to seamlessly integrate with other languages and handle a wide variety of tasks ensures that ST will remain a key player in the field of industrial automation for years to come.

For further reading, you can explore the official Structured Text Wikipedia page.