PLC programming languages overview (IEC 61131-3) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is IEC 61131-3 in PLC programming?

IEC 61131-3 is an international standard that defines programming languages and software architecture for programmable logic controllers (PLCs). It helps programmers use common languages and methods.

beginner

Name the five programming languages defined by IEC 61131-3.

The five languages are: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC).

beginner

What type of language is Ladder Diagram (LD)?

Ladder Diagram is a graphical language that looks like electrical relay logic. It is easy to understand for electricians and shows logic as rungs on a ladder.

intermediate

How does Structured Text (ST) differ from Ladder Diagram (LD)?

Structured Text is a text-based language similar to programming languages like Pascal. It is good for complex calculations and algorithms, unlike LD which is graphical and simpler.

intermediate

What is the purpose of Sequential Function Chart (SFC) in PLC programming?

SFC is used to organize the control process into steps and transitions. It helps visualize and manage sequences of operations clearly.

Which IEC 61131-3 language is graphical and resembles electrical relay logic?

A Ladder Diagram (LD)

B Structured Text (ST)

C Instruction List (IL)

D Sequential Function Chart (SFC)

Ladder Diagram (LD) is designed to look like relay logic and is graphical.

Which language is best suited for complex calculations in IEC 61131-3?

A Ladder Diagram (LD)

B Instruction List (IL)

C Function Block Diagram (FBD)

D Structured Text (ST)

Structured Text (ST) is a text-based language ideal for complex calculations.

What does Sequential Function Chart (SFC) help you do?

A Organize control processes into steps and transitions

B Write low-level instructions

C Draw electrical circuits

D Create function blocks

SFC organizes control processes into clear steps and transitions.

Instruction List (IL) in IEC 61131-3 is:

A A graphical language

B A language for function blocks

C A text-based low-level language similar to assembly

D A language for sequencing

Instruction List (IL) is a low-level text language similar to assembly code.

Which IEC 61131-3 language uses blocks connected by lines to show data flow?

A Structured Text (ST)

B Function Block Diagram (FBD)

C Ladder Diagram (LD)

D Instruction List (IL)

Function Block Diagram (FBD) uses blocks connected by lines to represent data flow.

Explain the main differences between Ladder Diagram (LD) and Structured Text (ST) in IEC 61131-3.

Think about how each language looks and what tasks they are good for.

Describe the role of Sequential Function Chart (SFC) in organizing PLC programs.

Imagine controlling a machine with clear stages.

Practice

1. Which of the following is NOT one of the five main PLC programming languages defined by IEC 61131-3?

easy

A. Function Block Diagram

B. Python

C. Structured Text

D. Ladder Diagram

Solution

1. Step 1: Recall IEC 61131-3 languages

   IEC 61131-3 defines Ladder Diagram, Function Block Diagram, Structured Text, Instruction List, and Sequential Function Chart.

2. Step 2: Identify the language not listed

   Python is a general-purpose programming language, not part of IEC 61131-3.

3. Final Answer:

   Python -> Option B

4. Quick Check:

   IEC 61131-3 excludes Python [OK]

Hint: Remember IEC 61131-3 languages exclude general programming languages [OK]

Common Mistakes:

• Confusing general programming languages with PLC languages
• Assuming all text-based languages are included
• Mixing up graphical and text-based languages

2. Which syntax correctly represents a simple assignment in Structured Text (ST)?

easy

A. Output := Input + 5;

B. Output = Input + 5

C. Output == Input + 5;

D. Output <- Input + 5;

Solution

1. Step 1: Understand Structured Text assignment

   In ST, the assignment operator is ':=' and statements end with a semicolon.

2. Step 2: Check each option

   Output := Input + 5; uses ':=' and ends with ';' correctly. Others use wrong operators or miss semicolons.

3. Final Answer:

   Output := Input + 5; -> Option A

4. Quick Check:

   ST assignment uses ':=' and ';' [OK]

Hint: Look for ':=' operator and semicolon in Structured Text [OK]

Common Mistakes:

• Using '=' instead of ':=' for assignment
• Omitting semicolon at statement end
• Confusing equality '==' with assignment

3. Given the following Ladder Diagram logic:
--[ ]--( )--
where the input contact is normally open and connected to output coil, what is the output when the input is OFF?

medium

A. Output is undefined

B. Output is ON

C. Output toggles ON and OFF

D. Output is OFF

Solution

1. Step 1: Understand Ladder Diagram symbols

   The normally open contact [ ] closes when input is ON, allowing current to flow to the coil ( ).

2. Step 2: Analyze input OFF condition

   When input is OFF, the contact remains open, so no current flows to the coil, output stays OFF.

3. Final Answer:

   Output is OFF -> Option D

4. Quick Check:

   Open contact means no output current [OK]

Hint: Open contact means output coil is OFF [OK]

Common Mistakes:

• Assuming output is ON when input is OFF
• Confusing normally open with normally closed contacts
• Thinking output toggles without input change

4. Identify the error in this Structured Text snippet:

IF Input = TRUE THEN
  Output := TRUE
END_IF

medium

A. Missing semicolon after Output assignment

B. Incorrect IF syntax, should be IF (Input = TRUE) THEN

C. Assignment operator should be '=' not ':='

D. END_IF should be lowercase

Solution

1. Step 1: Check Structured Text syntax rules

   Each statement must end with a semicolon ';' in ST.

2. Step 2: Inspect the code snippet

   The line 'Output := TRUE' lacks a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after Output assignment -> Option A

4. Quick Check:

   Statements end with ';' in ST [OK]

Hint: Always end ST statements with semicolon [OK]

Common Mistakes:

• Forgetting semicolons after assignments
• Misusing '=' for assignment
• Assuming IF needs parentheses

5. You need to implement a control logic that turns ON an output only if two inputs are ON simultaneously, using Function Block Diagram (FBD). Which block combination correctly represents this?

hard

A. Two OR blocks connected in parallel

B. Two AND blocks connected in series

C. One AND block with both inputs connected to it

D. One NOT block with inputs connected

Solution

1. Step 1: Understand AND logic in FBD

   AND block outputs TRUE only if all inputs are TRUE simultaneously.

2. Step 2: Choose correct block setup

   One AND block with both inputs connected directly checks if both inputs are ON together.

3. Final Answer:

   One AND block with both inputs connected to it -> Option C

4. Quick Check:

   AND block with multiple inputs = simultaneous ON check [OK]

Hint: Use single AND block with all inputs for simultaneous ON [OK]

Common Mistakes:

• Using OR blocks instead of AND
• Connecting AND blocks in series unnecessarily
• Using NOT block incorrectly for AND logic