Why structured text is used in PLC Programming - Visual Breakdown

Concept Flow - Why structured text is used

Start PLC Program

↓

Write Logic in Structured Text

↓

Easier to Read & Maintain?

No→Rewrite or Debug

Yes↓

Program Executes Correctly

↓

Simplified Troubleshooting & Updates

↓

End

This flow shows how using structured text helps write clear logic, making programs easier to read, maintain, and troubleshoot.

Execution Sample

PLC Programming

IF temperature > 100 THEN
  alarm := TRUE;
ELSE
  alarm := FALSE;
END_IF;

This code checks if temperature is above 100 and sets an alarm on or off accordingly.

Execution Table

Step	Condition	Evaluation	Action	alarm Value
1	temperature > 100	True	Set alarm := TRUE	TRUE
2	temperature > 100	False	Set alarm := FALSE	FALSE

💡 Execution stops after setting alarm based on temperature condition.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
temperature	varies	varies	varies	varies
alarm	FALSE	TRUE	FALSE	FALSE or TRUE depending on condition

Key Moments - 2 Insights

Why do we use structured text instead of just wiring PLCs?

Structured text lets us write clear, easy-to-read logic like in the execution_table, making complex programs simpler to understand and fix.

What happens if the condition is false in the IF statement?

As shown in execution_table step 2, the alarm is set to FALSE, so the program handles both cases clearly.

Visual Quiz - 3 Questions

Look at the execution_table, what is the alarm value when temperature > 100 is true?

A FALSE

B TRUE

C Undefined

D Remains unchanged

💡 Hint

Check execution_table row 1 under 'alarm Value'

At which step does the alarm get set to FALSE?

A Step 2

B Step 1

C Never

D Both steps

💡 Hint

See execution_table row 2 'Action' and 'alarm Value'

If we add another condition to check temperature < 0, how would the execution_table change?

A No change needed

B Remove existing steps

C Add a new step with condition temperature < 0

D Change alarm to always TRUE

💡 Hint

Think about how conditions are checked step-by-step as in execution_table

Concept Snapshot

Structured Text (ST) is a high-level PLC language.
It uses IF-THEN-ELSE for clear logic.
Makes programs easier to read and maintain.
Helps with debugging and updating code.
Used for complex automation tasks.

Full Transcript

Structured Text is used in PLC programming because it allows writing clear and easy-to-understand logic. Instead of wiring physical connections, programmers write code like IF temperature > 100 THEN alarm := TRUE ELSE alarm := FALSE END_IF. This makes the program easier to read, maintain, and troubleshoot. The execution table shows how the alarm variable changes based on the temperature condition. Beginners often wonder why use structured text; the answer is it simplifies complex logic and helps avoid wiring mistakes. The visual quiz checks understanding of how conditions affect the alarm variable and how adding conditions changes the program flow.

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