Structured text vs ladder logic decision in PLC Programming - Visual Side-by-Side Comparison

Concept Flow - Structured text vs ladder logic decision

Start Decision

↓

Check Input Condition

↓

Structured Text: Evaluate Condition

↓

Set Output True

↓

End

↓

Ladder Logic: Evaluate Contacts

↓

Set Output False

↓

End

The decision process checks an input condition. If true, structured text runs to set output true; if false, ladder logic runs to set output false.

Execution Sample

PLC Programming

IF Input1 THEN
  Output := TRUE;
ELSE
  Output := FALSE;
END_IF;

This structured text code sets Output true if Input1 is true, otherwise false.

Execution Table

Step	Input1	Condition (Input1==TRUE?)	Branch Taken	Output Value
1	TRUE	TRUE	Structured Text branch	TRUE
2	FALSE	FALSE	Ladder Logic branch	FALSE

💡 Execution ends after setting Output based on Input1 condition.

Variable Tracker

Variable	Start	After Step 1	After Step 2
Input1	UNKNOWN	TRUE	FALSE
Output	UNKNOWN	TRUE	FALSE

Key Moments - 2 Insights

Why does the output change depending on the input condition?

Because the decision checks Input1; if true, structured text sets Output true (see execution_table step 1), else ladder logic sets Output false (step 2).

How do structured text and ladder logic differ in this decision?

Structured text uses a clear IF-ELSE statement to set Output, while ladder logic uses contacts and coils to represent the same logic visually (refer to execution_table branches).

Visual Quiz - 3 Questions

Look at the execution table, what is Output after step 1 when Input1 is TRUE?

A TRUE

B FALSE

C UNKNOWN

D Input1 value

💡 Hint

Check the Output Value column in execution_table row for step 1.

At which step does the condition Input1 == TRUE evaluate to FALSE?

A Step 1

B Step 2

C Never

D Both steps

💡 Hint

Look at the Condition column in execution_table for each step.

If Input1 was always TRUE, which branch would the decision always take?

A Neither branch

B Ladder Logic branch

C Structured Text branch

D Both branches

💡 Hint

Refer to the Branch Taken column in execution_table when Input1 is TRUE.

Concept Snapshot

Decision in PLC uses input condition check.
If TRUE, structured text sets output TRUE.
If FALSE, ladder logic sets output FALSE.
Structured text uses IF-ELSE syntax.
Ladder logic uses contacts and coils.
Both achieve same output control.

Full Transcript

This visual execution compares structured text and ladder logic for a decision based on an input condition. The flow starts by checking if Input1 is true. If yes, the structured text branch runs, setting Output to true. If no, the ladder logic branch runs, setting Output to false. The execution table shows two steps: step 1 with Input1 true sets Output true; step 2 with Input1 false sets Output false. Variables Input1 and Output change accordingly. Key moments clarify why output depends on input and how the two languages differ in expressing the decision. The quiz tests understanding of output values and branch choices. The snapshot summarizes that both methods control output based on input condition, using different syntax and visuals.

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