Structured text vs ladder logic decision in PLC Programming - Quick Revision & Key Differences

Recall & Review

beginner

What is Structured Text (ST) in PLC programming?

Structured Text is a high-level, text-based programming language used in PLCs. It looks like regular programming languages and is good for complex calculations and algorithms.

beginner

What is Ladder Logic (LD) in PLC programming?

Ladder Logic is a graphical programming language that looks like electrical relay diagrams. It is easy to understand and is often used for simple control and switching tasks.

intermediate

When should you choose Structured Text over Ladder Logic?

Choose Structured Text when you need to write complex math, loops, or algorithms that are hard to show in Ladder Logic. It is better for advanced control and data handling.

beginner

What are the advantages of Ladder Logic?

Ladder Logic is easy to learn and visualize because it looks like electrical circuits. It is great for simple on/off control and is widely used by electricians and technicians.

intermediate

How does readability affect the choice between Structured Text and Ladder Logic?

If the team is more familiar with electrical diagrams, Ladder Logic is easier to read and maintain. For programmers comfortable with code, Structured Text is clearer and more efficient for complex tasks.

Which PLC programming language is best for complex calculations?

A Function Block Diagram

B Ladder Logic

C Structured Text

D Instruction List

Structured Text is designed for complex calculations and algorithms, unlike Ladder Logic which is more visual and simple.

Which language looks like electrical relay diagrams?

A Structured Text

B Ladder Logic

C Sequential Function Chart

D Structured Query Language

Ladder Logic visually resembles electrical relay diagrams, making it easy for electricians to understand.

When is Ladder Logic preferred over Structured Text?

A For simple on/off control

B For complex math operations

C For advanced data handling

D For writing loops

Ladder Logic is best for simple control tasks like turning devices on or off.

Which factor is important when deciding between Structured Text and Ladder Logic?

A Team familiarity with programming style

B Color of the PLC hardware

C Brand of the PLC

D Size of the PLC cabinet

Choosing the language depends on what the team understands better: code or electrical diagrams.

Structured Text is NOT ideal for:

A Complex algorithms

B Looping structures

C Data processing

D Simple relay control

Simple relay control is easier and more intuitive in Ladder Logic than in Structured Text.

Explain the main differences between Structured Text and Ladder Logic in PLC programming.

Think about how each language looks and what tasks they handle best.

Describe a scenario where you would choose Ladder Logic over Structured Text.

Imagine a factory with technicians who know wiring diagrams well.

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