Structured text vs ladder logic decision in PLC Programming - When to Use Which

The Big Idea

Discover how switching from drawing ladders to writing code can save hours and headaches in PLC programming!

The Scenario

Imagine you are controlling a factory machine using a PLC. You try to write the control logic by drawing many ladder diagrams by hand on paper or using basic software. Every time you want to change the logic, you must erase and redraw complex rungs, making it hard to keep track.

The Problem

This manual ladder logic drawing is slow and confusing. It's easy to make mistakes like missing connections or wrong contacts. Debugging is painful because you can't quickly see the flow or reuse parts. Changes take a long time and can cause downtime.

The Solution

Using structured text lets you write the control logic as clear, simple code. You can use variables, conditions, and loops just like writing instructions in a language. This makes it easier to read, change, and test your logic without redrawing diagrams.

Before vs After

✗ Before

----| |----|/|----( )  (ladder logic rung for AND NOT condition)

✓ After

IF input1 AND NOT input2 THEN output := TRUE; END_IF;

What It Enables

Structured text enables faster, clearer, and more flexible PLC programming that scales well for complex automation tasks.

Real Life Example

A factory upgrades from simple ladder diagrams to structured text to control a robotic arm. They quickly add new safety checks and motion sequences by editing code instead of redrawing ladders.

Key Takeaways

Manual ladder logic drawing is slow and error-prone.

Structured text uses clear code for easier logic writing and changes.

This decision improves automation reliability and development speed.

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