Why structured text is used in PLC Programming - See It in Action

Why Structured Text is Used in PLC Programming

📖 Scenario: You are learning how to program industrial machines using PLCs (Programmable Logic Controllers). Structured Text (ST) is one of the languages used to write these programs.Imagine you want to control a simple machine that turns on a motor when a button is pressed and turns it off when the button is released.

🎯 Goal: Understand why Structured Text is used by creating a simple program that reads a button input and controls a motor output using ST syntax.

📋 What You'll Learn

Create a variable to represent the button input

Create a variable to represent the motor output

Write a simple IF statement in Structured Text to turn the motor ON when the button is pressed

Print the motor output state

💡 Why This Matters

🌍 Real World

Structured Text is used in factories to program machines that assemble cars, package food, or control conveyor belts.

💼 Career

Learning Structured Text helps you become a PLC programmer, a valuable skill in automation and manufacturing industries.

1

Create Input and Output Variables

Create a variable called Button of type BOOL and set it to TRUE. Create another variable called Motor of type BOOL and set it to FALSE.

PLC Programming

// Declare variables for Button and Motor
// Your code here

Hint

Use VAR block to declare variables. BOOL means true or false.

2

Write the Control Logic

Write an IF statement that sets Motor to TRUE when Button is TRUE, otherwise sets Motor to FALSE.

PLC Programming

VAR
    Button : BOOL := TRUE;
    Motor : BOOL := FALSE;
END_VAR

// Write IF statement to control Motor based on Button
// Your code here

Hint

Use IF ... THEN ... ELSE ... END_IF; to control Motor.

3

Add a Configuration Variable

Create a variable called DebugMode of type BOOL and set it to TRUE. This variable will help us decide if we want to print the motor state.

PLC Programming

VAR
    Button : BOOL := TRUE;
    Motor : BOOL := FALSE;
    // Your code here
END_VAR

IF Button THEN
    Motor := TRUE;
ELSE
    Motor := FALSE;
END_IF;

Hint

Add DebugMode inside the VAR block.

4

Print the Motor Output State

Write code that prints 'Motor is ON' if Motor is TRUE and DebugMode is TRUE. Otherwise, print 'Motor is OFF' if DebugMode is TRUE.

PLC Programming

VAR
    Button : BOOL := TRUE;
    Motor : BOOL := FALSE;
    DebugMode : BOOL := TRUE;
END_VAR

IF Button THEN
    Motor := TRUE;
ELSE
    Motor := FALSE;
END_IF;

// Your code here to print motor state

Hint

Use nested IF statements to check DebugMode and Motor and print the correct message.

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