Introduction:

As industrial automation evolves, so do the tools and languages used to program PLCs. One language that’s gaining popularity for its power and flexibility is Structured Text (ST). Unlike Ladder Logic, ST is closer to traditional programming languages like Pascal or C, making it ideal for complex tasks, math operations, and logic-heavy functions.

Whether you’re just transitioning from Ladder Logic or starting fresh, this guide introduces you to the world of Structured Text — and how it can level up your PLC programming skills.

1. What Is Structured Text (ST)?

Structured Text is one of the five IEC 61131-3 standard PLC programming languages. It looks like high-level code and uses control structures such as IF, FOR, WHILE, and CASE blocks.

Example:

IF SensorValue > 50 THEN
   Motor := TRUE;
ELSE
   Motor := FALSE;
END_IF;

Why ST?

1. Easier to scale and maintain
2. Great for mathematical operations and loops
3. Readable by engineers from software backgrounds

2. When to Use Structured Text

While Ladder Logic shines for basic control and relay logic, ST is ideal for:

• Complex math (PID control, kinematics, etc.)
• Batch processing
• State machines
• Data handling and arrays
• Repetitive code via loops or functions

3. Getting Started with ST in Popular Platforms

Most major PLC vendors support ST:

• Siemens (TIA Portal) – Program blocks in ST
• Allen-Bradley (Studio 5000) – Use ST for advanced routines
• Codesys – Fully supports ST with simulation tools
• Schneider Electric (EcoStruxure) – Offers a clean ST interface

4. Common Structured Text Syntax Elements

Here are key language structures you’ll use often:

• IF…THEN…ELSE
• FOR loops
• WHILE loops
• CASE statements
• FUNCTION and FUNCTION_BLOCK definitions

Example: Looping through an arrayray

FOR i := 1 TO 10 DO
   Sum := Sum + MyArray[i];
END_FOR;

5. Tips for Writing Efficient ST Code

• Break your code into FUNCTION_BLOCKS for modularity
• Use comments generously for clarity
• Watch out for infinite loops or uninitialized variables
• Use simulation mode before deploying to hardware

6. Combining ST with Other Languages

One great feature of IEC 61131-3 is that you can mix languages. For example:

• Use Ladder Logic for basic I/O
• Use ST for logic-heavy tasks like temperature control or motor sequencing

This hybrid approach gives you flexibility while maintaining readability for technicians.aintaining readability for technicians.

Conclusion

Structured Text is the gateway to high-level PLC programming. It brings precision, scalability, and professional coding practices to the industrial world. If you’re serious about growing as a modern automation engineer, investing in Structured Text mastery is a no-brainer.

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