PLC programming languages overview (IEC 61131-3)

Introduction

PLC programming languages help control machines and processes in factories. They make it easy to tell machines what to do step-by-step.

You want to automate a factory machine safely and reliably.

You need to control a process like filling bottles or moving parts on a conveyor.

You want to create clear instructions that technicians can understand and fix easily.

You are working with industrial equipment that uses PLCs for control.

You want to use standard languages that work with many PLC brands.

Syntax

PLC Programming

IEC 61131-3 defines five main languages:

1. Ladder Diagram (LD)
2. Function Block Diagram (FBD)
3. Structured Text (ST)
4. Instruction List (IL) - deprecated
5. Sequential Function Chart (SFC)

Each language has its own way to write control logic.

Ladder Diagram looks like electrical relay logic, easy for electricians.

Structured Text is like simple programming with variables and conditions.

Examples

This shows a start button controlling a motor output.

PLC Programming

Ladder Diagram example:
--| |----( )--
| Start |  Motor
--| |--------

This code turns the motor on if the start button is pressed.

PLC Programming

Structured Text example:
IF StartButton THEN
  Motor := TRUE;
ELSE
  Motor := FALSE;
END_IF;

This shows a block that turns on the motor when the start button is pressed.

PLC Programming

Function Block Diagram example:
[StartButton] --> [AND] --> [Motor]

Sample Program

This program uses Structured Text to turn a motor on when the start button is pressed. It shows how simple conditions control outputs.

PLC Programming

(* Simple Structured Text program to control a motor *)
VAR
  StartButton : BOOL := FALSE;
  Motor : BOOL := FALSE;
END_VAR

IF StartButton THEN
  Motor := TRUE;
ELSE
  Motor := FALSE;
END_IF;

(* Simulate pressing the button *)
StartButton := TRUE;

IF StartButton THEN
  Motor := TRUE;
ELSE
  Motor := FALSE;
END_IF;

(* Output motor state *)
Motor;

Important Notes

Instruction List (IL) is deprecated and not recommended for new projects.

Sequential Function Chart (SFC) helps organize complex sequences step-by-step.

Choosing the right language depends on your background and the task complexity.

Summary

IEC 61131-3 defines five standard PLC programming languages.

Ladder Diagram and Function Block Diagram are graphical and easy to visualize.

Structured Text is a text-based language similar to simple programming.

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