Is There Still a Need for Siemens' STL Language on Their Platform?

Introduction

In the field of industrial automation, Programmable Logic Controllers (PLCs) are among the core devices. As an important programming language for PLCs, Siemens' Statement List (STL) language was once the preferred choice for engineers. However, with the development of other programming languages and technologies, is STL still necessary? This article explores the characteristics, significance, comparisons with other languages, and application scenarios of STL.

1. What is STL Language?

STL (Statement List) is a low-level programming language, similar to assembly language, designed for direct interaction with PLC hardware. It uses an instruction list format, allowing engineers to control PLC operations with fine granularity. STL is widely used in Siemens PLCs, especially in scenarios requiring precise control and performance optimization.

2. Significance of STL Language

2.1 Precise Control
STL provides direct access to hardware, enabling engineers to precisely control each step of the PLC's operation. This is particularly important in systems with complex logic operations and high-performance requirements.

2.2 Performance Optimization
Due to its low-level nature, STL code is typically more efficient than higher-level languages (e.g., LAD, FBD), optimizing PLC execution speed and resource usage.

2.3 Rich Instruction Set
STL offers a comprehensive instruction set, capable of implementing complex functions, making it suitable for handling intricate control tasks.

3. Comparison of Advantages and Disadvantages with Other Languages

3.1 Advantages

• Real-Time Performance: STL directly controls hardware registers, reducing intermediate layer delays.
• Resource Usage: STL code is concise, optimizing memory and CPU utilization.
• Flexibility: Offers greater flexibility and control, ideal for complex control logic.
• Technical Barrier: Its complexity makes it difficult to understand, adding a layer of protection against code duplication for critical process control logic.

3.2 Disadvantages

• Difficult to Learn: Its syntax, similar to assembly language, has a steep learning curve.
• Poor Readability: Code readability is inferior to ladder logic (LAD) and function block diagrams (FBD).
• Poor Portability: Difficult to port to PLCs from other brands.

3.3 Comparison with Other Languages

• LAD (Ladder Logic): The most commonly used programming language, easy to learn, intuitive, and simple to debug, but cumbersome for complex logic.
• FBD (Function Block Diagram): Suitable for process control, easy to combine and reuse, but less flexible than STL.
• SCL (Structured Text): With hardware performance becoming surplus, SCL is currently the most promising language. It resembles modern high-level programming languages, suitable for complex algorithms, but its performance is inferior to STL.

4. Efficient Application Scenarios

4.1 Complex Algorithm Implementation
When precise algorithm control is required, STL offers higher efficiency and flexibility. For example, in a motion control system requiring real-time processing, STL enables more efficient control strate[gies.

4.2 Performance Optimiz
In resource-constrained PLC systems, STL improves system performance by streamlining code and optimizing resources.

4.3 Fine Hardware Control
In scenarios requiring direct manipulation of hardware registers, STL's low-level characteristics make it the optimal choice.

5. Case Study

5.1 Case Background
In a high-precision motion control system, the angle and speed of a robotic arm need real-time adjustments to ensure accurate operations. The system demands fast response and efficient resource utilization.

5.2 STL Code Example and Explanation

Sensor Input Check:

A     I0.0            // Check if sensor detects position
JC    ADJUST          // Jump to adjustment logic if true
JU    END             // End if no operation

• A I0.0: Checks if sensor input I0.0 is true (i.e., robotic arm has reached the designated position).
• JC ADJUST: If the sensor is true, jumps to the ADJUST label for adjustment logic.
• JU END: If the sensor is false, jumps to program end.

Adjustment Logic:

ADJUST: NOP 0          // Start of adjustment logic
        L     DB1.DBD0  // Load angle setpoint
        T     MD10      // Transfer to memory double word 10
        L     DB1.DBD4  // Load speed setpoint
        T     MD14      // Transfer to memory double word 14
        A     M0.0      // Check start flag
        JC    START_ARM // Jump to arm start if true
        JU    END       // Otherwise jump to end

• L DB1.DBD0: Loads angle setpoint from data block DB1 into the accumulator.
• T MD10: Transfers angle setpoint to memory address MD10.
• L DB1.DBD4: Loads speed setpoint.
• T MD14: Transfers speed setpoint to memory address MD14.
• A M0.0: Checks start flag M0.0.
• JC START_ARM: If start flag is true, jumps to START_ARM.
• JU END: Otherwise jumps to program end.

Robotic Arm Start:

START_ARM: NOP 0       // Start robotic arm
           L     MD10   // Load angle
           T     QW0    // Transfer to output word 0
           L     MD14   // Load speed
           T     QW2    // Transfer to output word 2
           S     M0.1   // Set running flag

• L MD10: Loads angle value from memory.
• T QW0: Transfers angle value to output word QW0, controlling arm angle.
• L MD14: Loads speed value.
• T QW2: Transfers speed value to output word QW2, controlling arm speed.
• S M0.1: Sets running flag, indicating the arm is operating.

Program End:

END: NOP 0             // End program

• END: Program ends, no operation.

Conclusion

Despite the availability of more modern and user-friendly programming languages, STL retains its place in specific industrial automation scenarios due to its efficiency, flexibility, and precise hardware control. For projects requiring performance optimization and complex control logic, STL remains an indispensable tool. In the future, as technology continues to advance, STL may evolve, but its core value will persist.