Structured text vs ladder logic decision in PLC Programming - Hands-On Comparison

Structured Text vs Ladder Logic Decision

📖 Scenario: You work in a factory automation team. You need to decide which programming style to use for a simple control task: Structured Text or Ladder Logic. Both styles can control machines, but they look and work differently.Structured Text is like writing code with clear instructions. Ladder Logic looks like electrical diagrams with switches and relays.

🎯 Goal: Build a small program that uses a decision variable to choose between Structured Text and Ladder Logic styles. The program will print which style is chosen based on a simple condition.

📋 What You'll Learn

Create a variable called use_structured_text with a boolean value.

Create a variable called style to hold the chosen programming style as a string.

Use an if statement to set style to 'Structured Text' if use_structured_text is true, otherwise set it to 'Ladder Logic'.

Print the chosen style.

💡 Why This Matters

🌍 Real World

Choosing the right programming style helps automation engineers write clear and maintainable control programs for machines.

💼 Career

Understanding Structured Text and Ladder Logic is essential for PLC programmers working in industrial automation.

1

Create the decision variable

Create a boolean variable called use_structured_text and set it to TRUE.

PLC Programming

# Create a boolean variable called use_structured_text and set it to TRUE
# Your code here

Hint

Use the syntax variable_name : BOOL := TRUE; to create a boolean variable in Structured Text.

2

Create the style variable

Create a string variable called style to hold the chosen programming style.

PLC Programming

use_structured_text : BOOL := TRUE;
# Create a string variable called style
# Your code here

Hint

Use the syntax variable_name : STRING; to declare a string variable in Structured Text.

3

Set the style based on the decision

Use an IF statement to set style to 'Structured Text' if use_structured_text is TRUE, otherwise set style to 'Ladder Logic'.

PLC Programming

use_structured_text : BOOL := TRUE;
style : STRING;
# Use IF statement to set style based on use_structured_text
# Your code here

Hint

Use IF condition THEN ... ELSE ... END_IF; to make decisions in Structured Text.

4

Print the chosen style

Write a statement to print the value of style.

PLC Programming

use_structured_text : BOOL := TRUE;
style : STRING;

IF use_structured_text THEN
    style := 'Structured Text';
ELSE
    style := 'Ladder Logic';
END_IF;

# Print the style variable
# Your code here

Hint

Since PLC Structured Text may not have a standard print function, imagine the output as a comment or use your system's print method.

Practice

1. Which programming style uses text commands like IF and ELSE to write logic in PLCs?

easy

A. Ladder Logic

B. Structured Text

C. Function Block Diagram

D. Sequential Function Chart

Solution

1. Step 1: Understand programming styles in PLCs

   Structured Text uses text-based commands like IF and ELSE to write logic clearly.

2. Step 2: Compare with Ladder Logic

   Ladder Logic uses graphical symbols resembling electrical circuits, not text commands.

3. Final Answer:

   Structured Text -> Option B

4. Quick Check:

   Text commands = Structured Text [OK]

Hint: Text commands like IF mean Structured Text [OK]

Common Mistakes:

• Confusing Ladder Logic with text commands
• Thinking graphical diagrams use IF/ELSE
• Mixing Function Block Diagram with text style

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

easy

A. MOVE input1 TO output;

B. LD input1; AND output;

C. SET input1; RESET output;

D. IF input1 THEN output := TRUE; END_IF;

Solution

1. Step 1: Identify Structured Text syntax

   Structured Text uses IF ... THEN ... END_IF; to write conditions.

2. Step 2: Check other options

   The other options use Ladder Logic or other instruction styles, not Structured Text.

3. Final Answer:

   IF input1 THEN output := TRUE; END_IF; -> Option D

4. Quick Check:

   IF ... THEN ... END_IF; = Structured Text condition [OK]

Hint: IF ... THEN ... END_IF; is Structured Text syntax [OK]

Common Mistakes:

• Using Ladder Logic commands in Structured Text
• Missing END_IF; in Structured Text
• Confusing assignment operator := with =

3. Given this Ladder Logic snippet:

--| |--| |--( )--

Which best describes its function compared to Structured Text?
IF input1 AND input2 THEN output := TRUE; ELSE output := FALSE; END_IF;

medium

A. Ladder Logic cannot represent conditional logic

B. Ladder Logic uses OR logic, Structured Text uses AND

C. Both represent the same AND logic controlling output

D. Structured Text cannot handle multiple inputs

Solution

1. Step 1: Understand Ladder Logic AND

   The two series contacts in Ladder Logic represent AND condition between inputs.

2. Step 2: Compare with Structured Text

   The Structured Text IF statement uses AND logic between input1 and input2 to set output TRUE or FALSE.

3. Final Answer:

   Both represent the same AND logic controlling output -> Option C

4. Quick Check:

   Series contacts = AND logic = Structured Text IF with AND [OK]

Hint: Series contacts in Ladder = AND condition [OK]

Common Mistakes:

• Thinking Ladder Logic only does OR logic
• Believing Structured Text can't handle multiple inputs
• Confusing parallel and series contacts

4. You wrote this Structured Text code but it does not compile:

IF input1 THEN output = TRUE; END_IF;

What is the error?

medium

A. Using = instead of := for assignment

B. Missing semicolon after TRUE

C. END_IF should be lowercase

D. IF statement cannot be used in Structured Text

Solution

1. Step 1: Check assignment operator

   Structured Text uses := for assignment, not = which is comparison.

2. Step 2: Verify other syntax

   Semicolon is present after TRUE; END_IF is case-insensitive.

3. Final Answer:

   Using = instead of := for assignment -> Option A

4. Quick Check:

   Assignment uses := not = in Structured Text [OK]

Hint: Use := for assignment, = is comparison [OK]

Common Mistakes:

• Using = instead of := for assignment
• Picking missing semicolon when it's present
• Thinking END_IF is case sensitive

5. You want to create a PLC program that is easy to read and modify by electricians familiar with circuit diagrams. Which programming style should you choose and why?

hard

A. Ladder Logic, because it visually resembles electrical circuits

B. Sequential Function Chart, because it shows steps

C. Function Block Diagram, because it uses blocks

D. Structured Text, because text is easier for electricians

Solution

1. Step 1: Consider electrician familiarity

   Electricians are used to reading electrical circuit diagrams.

2. Step 2: Match programming style to familiarity

   Ladder Logic visually looks like circuit diagrams, making it easier for electricians to read and modify.

3. Final Answer:

   Ladder Logic, because it visually resembles electrical circuits -> Option A

4. Quick Check:

   Electricians prefer Ladder Logic for circuit-like visuals [OK]

Hint: Choose Ladder Logic for circuit-like visuals [OK]

Common Mistakes:

• Assuming text is easier for electricians
• Confusing Function Block Diagram with Ladder Logic
• Ignoring user familiarity in choice