Why structured text is used in PLC Programming - Why It Works This Way

Overview - Why structured text is used

What is it?

Structured Text is a high-level programming language used to write instructions for programmable logic controllers (PLCs). It looks similar to regular programming languages with clear commands and logic, making it easier to read and write complex control tasks. Instead of wiring physical relays, you write code that tells machines what to do step-by-step. This language helps automate industrial processes by providing a clear and organized way to describe machine behavior.

Why it matters

Without Structured Text, programming PLCs would rely heavily on graphical or relay-based methods that are harder to scale and maintain. Complex logic would be difficult to express and prone to errors. Structured Text allows engineers to write clear, reusable, and testable code, speeding up development and reducing mistakes. This means safer, more reliable machines and faster production lines in factories.

Where it fits

Before learning Structured Text, you should understand basic PLC concepts and simple ladder logic programming. After mastering Structured Text, you can explore advanced PLC programming techniques, integration with other automation systems, and optimization of control algorithms.

Mental Model

Core Idea

Structured Text is like writing a clear recipe for machines to follow, using simple, step-by-step instructions that computers can easily understand and execute.

Think of it like...

Imagine giving someone a detailed cooking recipe instead of just showing pictures of ingredients. The recipe tells exactly what to do, in what order, and how long to do it, making the cooking process clear and repeatable.

┌─────────────────────────────┐
│ Structured Text Program      │
├─────────────────────────────┤
│ IF condition THEN            │
│    action1;                 │
│ ELSE                        │
│    action2;                 │
│ END_IF;                     │
│                             │
│ FOR i := 1 TO 10 DO         │
│    action;                  │
│ END_FOR;                    │
└─────────────────────────────┘

Build-Up - 6 Steps

1

Foundation

What is Structured Text Language

Concept: Introduction to Structured Text as a programming language for PLCs.

Structured Text is a text-based language used to program PLCs. It uses commands like IF, FOR, and variables to control machines. Unlike graphical languages, it looks like normal code with clear instructions.

Result

You understand that Structured Text is a readable, code-like way to tell machines what to do.

Knowing that Structured Text is a language similar to common programming languages helps you see PLC programming as writing instructions, not just wiring.

2

Foundation

Basic Syntax and Commands

Concept: Learn the simple commands and structure of Structured Text programs.

Structured Text uses commands like IF...THEN...ELSE for decisions, FOR loops for repeating actions, and variables to store data. Each instruction ends with a semicolon. This structure makes programs easy to follow.

Result

You can write simple programs that make decisions and repeat actions.

Understanding basic syntax lets you start writing clear, logical instructions for machines.

3

Intermediate

Advantages Over Ladder Logic

🤔Before reading on: Do you think Structured Text is harder or easier to write than Ladder Logic for complex tasks? Commit to your answer.

Concept: Structured Text handles complex logic more clearly than graphical Ladder Logic.

While Ladder Logic is visual and good for simple tasks, Structured Text shines when logic gets complicated. It allows nested conditions, loops, and functions that are harder to represent visually. This makes programs shorter and easier to maintain.

Result

You see why engineers choose Structured Text for complex automation tasks.

Knowing Structured Text's clarity for complex logic helps you pick the right tool for the job.

4

Intermediate

Modularity and Reusability

🤔Before reading on: Do you think Structured Text supports breaking code into reusable parts? Commit to your answer.

Concept: Structured Text allows creating functions and function blocks to reuse code.

You can write functions and function blocks in Structured Text to group instructions. This means you write code once and use it many times, making programs easier to manage and less error-prone.

Result

You can organize programs into smaller, reusable pieces.

Understanding modularity in Structured Text leads to cleaner, scalable automation projects.

5

Advanced

Integration with PLC Systems

🤔Before reading on: Do you think Structured Text can interact with hardware inputs and outputs directly? Commit to your answer.

Concept: Structured Text programs can read sensors and control actuators through PLC hardware interfaces.

Structured Text code runs inside PLCs and can read inputs like buttons or sensors, and control outputs like motors or lights. It uses variables linked to hardware addresses, allowing real-time control of machines.

Result

You understand how code translates to real machine actions.

Knowing how Structured Text connects to hardware bridges the gap between code and physical automation.

6

Expert

Optimizing Structured Text for Performance

🤔Before reading on: Do you think writing more lines of code always slows down PLC execution? Commit to your answer.

Concept: Efficient Structured Text coding improves PLC scan times and system responsiveness.

Experienced programmers write Structured Text to minimize unnecessary calculations and use efficient data types. They avoid complex nested loops or heavy computations inside time-critical parts to keep PLCs fast and reliable.

Result

You learn how to write Structured Text that runs smoothly in demanding industrial environments.

Understanding performance optimization prevents slowdowns and failures in real-world automation.

Under the Hood

Structured Text code is compiled or interpreted by the PLC runtime system into machine instructions that control hardware. The PLC reads inputs, executes the Structured Text logic in a scan cycle, and updates outputs accordingly. Variables in the code map to memory locations linked to physical devices or internal data. The scan cycle repeats continuously, allowing real-time control.

Why designed this way?

Structured Text was designed to bring the clarity and power of high-level programming languages to industrial control. Early PLC programming was graphical or relay-based, which limited complexity and maintainability. Structured Text allows engineers familiar with programming concepts to write precise, scalable control logic, improving productivity and reducing errors.

┌───────────────┐
│ Structured    │
│ Text Program  │
└──────┬────────┘
       │ Compiled or interpreted
       ▼
┌───────────────┐
│ PLC Runtime   │
│ System       │
└──────┬────────┘
       │ Executes in scan cycles
       ▼
┌───────────────┐
│ Hardware I/O  │
│ (Sensors,    │
│ Actuators)   │
└───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Is Structured Text only for very complex programs? Commit to yes or no.

Common Belief: Structured Text is only useful for very complex automation tasks.

Reality: Structured Text can be used for both simple and complex programs, offering clarity even for small tasks.

Why it matters: Avoiding Structured Text for simple tasks can lead to inconsistent programming styles and missed opportunities for code reuse.

Quick: Do you think Structured Text runs slower than Ladder Logic? Commit to yes or no.

Common Belief: Structured Text is slower because it is text-based and more complex.

Reality: Structured Text runs efficiently on PLCs and often executes faster than equivalent Ladder Logic due to optimized code paths.

Why it matters: Believing Structured Text is slow may prevent its use, limiting program efficiency and maintainability.

Quick: Can Structured Text directly control hardware without PLC? Commit to yes or no.

Common Belief: Structured Text can be used standalone to control hardware directly.

Reality: Structured Text runs inside PLCs and cannot control hardware without the PLC's runtime and interfaces.

Why it matters: Misunderstanding this can lead to wrong assumptions about system architecture and deployment.

Quick: Does Structured Text require deep programming knowledge to use? Commit to yes or no.

Common Belief: Only expert programmers can write Structured Text effectively.

Reality: Structured Text is designed to be accessible to engineers with basic programming skills and offers clear syntax for learning.

Why it matters: Thinking it is too hard may discourage learners from adopting a powerful tool.

Expert Zone

1

Structured Text allows conditional compilation and symbolic constants, enabling flexible code for different hardware setups.

2

The order of execution in Structured Text can affect program behavior, especially with side effects, requiring careful design.

3

Advanced use includes integrating Structured Text with other IEC 61131-3 languages in the same PLC program for optimal results.

When NOT to use

Structured Text is less suitable for very simple, fast-switching logic best handled by Ladder Logic or Function Block Diagram. For safety-critical timing tasks, specialized languages or hardware timers may be preferred.

Production Patterns

In industry, Structured Text is used to implement complex algorithms like PID control, data processing, and communication protocols. It is often combined with graphical languages for visualization and quick logic.

Connections

High-Level Programming Languages

Structured Text shares syntax and concepts with languages like Pascal and C.

Understanding general programming concepts helps learn Structured Text faster and write better PLC programs.

Industrial Automation Systems

Structured Text programs run inside PLCs that control physical machines.

Knowing how automation hardware works clarifies why Structured Text must be efficient and reliable.

Recipe Writing in Cooking

Both involve writing clear, step-by-step instructions to achieve a repeatable result.

Seeing programming as instruction writing helps non-technical learners grasp automation logic.

Common Pitfalls

#1 Writing unstructured, long blocks of code without functions.

Wrong approach:

IF sensor = TRUE THEN
motor := ON;
valve := OPEN;
timer := 0;
// many more lines
END_IF;

Correct approach:

FUNCTION ControlMotorValve : BOOL
IF sensor = TRUE THEN
motor := ON;
valve := OPEN;
timer := 0;
RETURN TRUE;
ELSE
RETURN FALSE;
END_IF;
END_FUNCTION

// Call ControlMotorValve() where needed

Root cause:

Lack of understanding of modular programming leads to hard-to-maintain code.

#2 Using complex nested loops inside time-critical PLC scan cycles.

Wrong approach:

FOR i := 1 TO 10000 DO
FOR j := 1 TO 10000 DO
// heavy computation
END_FOR;
END_FOR;

Correct approach:

// Break heavy computation into smaller steps or run externally
// Avoid deep nested loops in PLC scan

Root cause:

Not realizing PLC scan time limits causes slow or unresponsive control.

#3 Assuming variables automatically link to hardware inputs/outputs.

Wrong approach:

motor := TRUE; // expecting this to turn on physical motor without mapping

Correct approach:

motor AT %QX0.0 : BOOL; // variable linked to output address
motor := TRUE;

Root cause:

Misunderstanding how variables connect to physical devices in PLC.

Key Takeaways

Structured Text is a clear, text-based language designed to program PLCs with step-by-step instructions.

It simplifies writing complex automation logic compared to graphical methods, improving readability and maintenance.

Structured Text supports modular programming with functions and blocks, enabling reusable and organized code.

It runs inside PLCs, linking code variables to real hardware inputs and outputs for real-time control.

Expert use involves optimizing code for performance and combining Structured Text with other languages for best results.

Practice

1. Why is Structured Text commonly used in PLC programming?

easy

A. Because it does not support loops or conditions

B. Because it only supports simple on/off commands

C. Because it makes programs clearer and easier to maintain

D. Because it is a graphical programming language

Solution

1. Step 1: Understand the purpose of Structured Text

   Structured Text is designed to make PLC programs easier to read and maintain by using clear text-based code.

2. Step 2: Compare options with this purpose

   Options A, B, and C describe features that do not match Structured Text's capabilities or nature.

3. Final Answer:

   Because it makes programs clearer and easier to maintain -> Option C

4. Quick Check:

   Clarity and maintainability = D [OK]

Hint: Structured Text is clear and easy to maintain [OK]

Common Mistakes:

• Thinking Structured Text is graphical
• Believing it lacks loops or conditions
• Confusing it with simple on/off commands

2. Which of the following is the correct way to write a conditional statement in Structured Text?

easy

A. WHEN temperature > 100 DO alarm = TRUE

B. IF temperature > 100 THEN alarm := TRUE; END_IF;

C. if (temperature > 100) { alarm = true }

D. IF temperature > 100 alarm = TRUE END

Solution

1. Step 1: Identify Structured Text syntax for IF statements

   Structured Text uses IF ... THEN ... END_IF; with semicolons and uppercase keywords.

2. Step 2: Compare each option to this syntax

   IF temperature > 100 THEN alarm := TRUE; END_IF; matches the correct syntax. Options A, B, and D use syntax from other languages or are incomplete.

3. Final Answer:

   IF temperature > 100 THEN alarm := TRUE; END_IF; -> Option B

4. Quick Check:

   Correct IF syntax = C [OK]

Hint: Look for IF ... THEN ... END_IF; structure [OK]

Common Mistakes:

• Using curly braces instead of END_IF;
• Missing semicolons at line ends
• Using lowercase keywords without proper syntax

3. What will be the output of this Structured Text code snippet?

VAR count : INT := 0; END_VAR
FOR i := 1 TO 3 DO
  count := count + i;
END_FOR;

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Understand the FOR loop range and iterations

   The loop runs with i = 1, 2, 3.

2. Step 2: Calculate the sum of count after each iteration

   count starts at 0, then adds 1, then 2, then 3: 0+1=1, 1+2=3, 3+3=6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Sum numbers 1 to 3 in loop to find count [OK]

Common Mistakes:

• Confusing loop range (starting at 0 or 1)
• Forgetting to add all iterations
• Assuming syntax error due to VAR block

4. Identify the error in this Structured Text code:

IF sensorValue > 50 THEN
  alarm := TRUE
END_IF;

medium

A. Missing semicolon after alarm := TRUE

B. Incorrect keyword END_IF

C. sensorValue should be in quotes

D. THEN should be lowercase

Solution

1. Step 1: Check syntax for assignment statements

   Each statement must end with a semicolon in Structured Text.

2. Step 2: Verify the given code

   The line 'alarm := TRUE' is missing a semicolon at the end.

3. Final Answer:

   Missing semicolon after alarm := TRUE -> Option A

4. Quick Check:

   Statements end with semicolon = B [OK]

Hint: Check for missing semicolons after assignments [OK]

Common Mistakes:

• Thinking END_IF is wrong syntax
• Quoting variable names incorrectly
• Changing keyword case unnecessarily

5. You want to use Structured Text to check multiple sensor values and set an alarm if any exceed limits. Which feature of Structured Text helps you write this clearly and maintainably?

hard

A. Avoiding loops and case statements for simplicity

B. Using only simple ON/OFF commands without conditions

C. Writing all checks in one long line without structure

D. Using multiple nested IF statements with ELSE_IF

Solution

1. Step 1: Identify how to handle multiple conditions clearly

   Structured Text supports IF, ELSE_IF, and ELSE to handle multiple checks cleanly.

2. Step 2: Evaluate options for clarity and maintainability

   Using multiple nested IF statements with ELSE_IF uses nested IF and ELSE_IF, which is clear and maintainable. Options B, C, and D reduce clarity or ignore useful features.

3. Final Answer:

   Using multiple nested IF statements with ELSE_IF -> Option D

4. Quick Check:

   Multiple conditions = nested IF/ELSE_IF [OK]

Hint: Use nested IF and ELSE_IF for multiple checks [OK]

Common Mistakes:

• Ignoring ELSE_IF for multiple conditions
• Trying to write all logic in one line
• Avoiding loops or case statements unnecessarily