PLC programming languages overview (IEC 61131-3) - Deep Dive

Overview - PLC programming languages overview (IEC 61131-3)

What is it?

PLC programming languages are special ways to write instructions for Programmable Logic Controllers (PLCs), which control machines and processes in factories. IEC 61131-3 is an international standard that defines five main languages to program PLCs, making it easier to design, understand, and maintain automation systems. These languages include graphical and text-based styles to suit different needs and skills. They help translate human ideas into machine actions reliably and clearly.

Why it matters

Without standardized PLC programming languages, every machine maker might use their own way to program controllers, causing confusion and errors. This would make fixing, upgrading, or sharing programs very hard and slow down industrial automation. IEC 61131-3 solves this by providing a common language set, so engineers worldwide can work together, reuse code, and keep machines running smoothly. It makes factories safer, more efficient, and easier to manage.

Where it fits

Before learning these languages, you should understand basic automation concepts and what a PLC does. After mastering these languages, you can explore advanced PLC programming techniques, integration with SCADA systems, and industrial communication protocols. This topic sits at the core of industrial automation programming knowledge.

Mental Model

Core Idea

IEC 61131-3 defines a set of standard languages that let people write clear, consistent instructions for PLCs to control machines and processes.

Think of it like...

Think of IEC 61131-3 languages like different ways to write a recipe: some use pictures (like flowcharts), some use step-by-step instructions (like a cookbook), but all describe how to make the same dish so anyone can follow it.

┌───────────────────────────────┐
│        IEC 61131-3 Standard    │
├──────────────┬───────────────┤
│ Graphical    │ Text-based    │
├──────────────┼───────────────┤
│ Ladder Logic │ Structured Text│
│ Function     │ Instruction    │
│ Block Diagram│ List          │
└──────────────┴───────────────┘

Build-Up - 7 Steps

1

Foundation

What is a PLC and Why Use It

Concept: Introduce the basic idea of a PLC and its role in automation.

A Programmable Logic Controller (PLC) is a small computer used to control machines like conveyor belts or robotic arms. It reads inputs (like sensors), makes decisions based on a program, and controls outputs (like motors). PLCs are built to work reliably in factories where conditions can be tough.

Result

You understand that PLCs are the brains behind many automated machines, making decisions quickly and safely.

Knowing what a PLC does helps you see why special programming languages are needed to communicate with it clearly and reliably.

2

Foundation

Why Standard Languages Matter

Concept: Explain the need for a common set of programming languages for PLCs.

Before IEC 61131-3, every PLC brand had its own programming style, making it hard to learn or fix programs from different machines. The IEC 61131-3 standard created five languages so engineers everywhere can use the same methods, making programs easier to share and maintain.

Result

You realize that standard languages reduce confusion and speed up automation work.

Understanding the problem IEC 61131-3 solves helps you appreciate why learning these languages is valuable.

3

Intermediate

Graphical Languages: Ladder Logic and Function Block Diagram

🤔Before reading on: do you think graphical languages are easier or harder to understand than text-based ones? Commit to your answer.

Concept: Introduce the two main graphical languages and their visual style.

Ladder Logic looks like electrical relay diagrams with rungs and contacts, making it familiar to electricians. Function Block Diagram uses blocks connected by lines to show data flow, good for complex functions. Both let you 'see' the logic visually, which helps debugging and understanding.

Result

You can identify Ladder Logic and Function Block Diagram as visual ways to program PLCs.

Knowing graphical languages helps you choose the right style for your background and task, improving clarity and teamwork.

4

Intermediate

Text-Based Languages: Structured Text and Instruction List

🤔Before reading on: do you think text-based languages are more flexible or less flexible than graphical ones? Commit to your answer.

Concept: Explain the two main text-based languages and their syntax style.

Structured Text is like a simple programming language with variables, loops, and conditions, making it powerful for complex logic. Instruction List is a low-level, assembly-like language that is compact but harder to read. Text languages allow precise control and are preferred by programmers familiar with coding.

Result

You understand the strengths and uses of Structured Text and Instruction List.

Recognizing text-based languages expands your ability to write complex automation programs efficiently.

5

Intermediate

Sequential Function Chart for Process Flow

Concept: Introduce Sequential Function Chart (SFC) as a way to organize steps and transitions.

SFC breaks a process into steps and transitions, like a flowchart showing stages of a machine cycle. It helps manage complex sequences by clearly showing what happens next and when. SFC is often combined with other languages to control overall process flow.

Result

You see how SFC helps organize and visualize step-by-step machine operations.

Understanding SFC gives you a powerful tool to design clear, maintainable process control programs.

6

Advanced

Combining Languages in Real Projects

🤔Before reading on: do you think one language is enough for all PLC tasks, or do projects usually mix languages? Commit to your answer.

Concept: Explain how different IEC 61131-3 languages work together in practice.

In real automation projects, engineers often use multiple languages together. For example, SFC can control the process flow, Ladder Logic handles simple input/output control, and Structured Text manages complex calculations. PLC software supports mixing these languages to use each one's strengths.

Result

You understand that mastering multiple languages and how they interact is key to professional PLC programming.

Knowing that languages complement each other helps you design flexible and efficient automation solutions.

7

Expert

IEC 61131-3 Standard Internals and Extensions

🤔Before reading on: do you think IEC 61131-3 languages are fixed forever, or do they evolve with new features? Commit to your answer.

Concept: Explore the standard's structure, data types, and how vendors extend it.

IEC 61131-3 defines data types, program organization units (POUs), and libraries to build reusable code. Vendors add proprietary extensions for special hardware or functions, but keep core compatibility. Understanding these internals helps you write portable, maintainable code and use advanced features like user-defined types and function blocks.

Result

You gain insight into the standard's design and how to leverage its full power in complex systems.

Understanding the standard's internals and extensions prepares you to work effectively across different PLC platforms and future-proofs your skills.

Under the Hood

PLC programs written in IEC 61131-3 languages are compiled or interpreted into machine code that runs on the PLC's processor. The PLC continuously scans inputs, executes the program logic step-by-step, and updates outputs in a fast loop called the scan cycle. The languages map closely to this cycle, with clear timing and execution order to ensure predictable control.

Why designed this way?

The standard was created to unify diverse programming methods, improve portability, and reduce errors. It balances graphical ease for electricians with text power for programmers. The modular design with POUs and data types supports reuse and scalability. Extensions allow vendors to innovate without breaking compatibility.

┌───────────────┐
│ IEC 61131-3  │
│ Program Code │
└──────┬────────┘
       │ Compile/Interpret
       ▼
┌───────────────┐
│ PLC Runtime   │
│ Scan Cycle:   │
│ 1. Read Inputs│
│ 2. Execute    │
│    Program    │
│ 3. Update     │
│    Outputs    │
└───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Is Ladder Logic just a graphical way to write any program, or is it limited to simple logic? Commit to your answer.

Common Belief: Ladder Logic can handle any complex programming task just like a full programming language.

Reality: Ladder Logic is best suited for simple, discrete control tasks and can become unwieldy for complex algorithms, where Structured Text is better.

Why it matters: Using Ladder Logic for complex tasks can lead to confusing, hard-to-maintain programs and bugs.

Quick: Do you think Instruction List is widely used today or mostly obsolete? Commit to your answer.

Common Belief: Instruction List is a common and modern way to program PLCs.

Reality: Instruction List is considered legacy and is rarely used in new projects, replaced by Structured Text for better readability and power.

Why it matters: Relying on Instruction List limits your ability to work with modern PLCs and maintain code effectively.

Quick: Can you write a PLC program in just one IEC 61131-3 language for all tasks? Commit to your answer.

Common Belief: One language is enough to program any PLC task.

Reality: Most real-world projects combine multiple languages to use each one's strengths effectively.

Why it matters: Trying to use only one language can make programs inefficient or hard to understand.

Quick: Do you think IEC 61131-3 languages are the same across all PLC brands? Commit to your answer.

Common Belief: All PLCs implement IEC 61131-3 languages exactly the same way.

Reality: Vendors add proprietary extensions and variations, so programs may need adjustments between brands.

Why it matters: Assuming full compatibility can cause errors when moving programs between different PLCs.

Expert Zone

1

Some vendors extend IEC 61131-3 with custom data types and functions that can improve performance but reduce portability.

2

The scan cycle timing affects how quickly inputs are read and outputs updated, which can cause subtle bugs if not considered.

3

Function blocks can maintain internal state, enabling object-like behavior in PLC programs, a powerful but often overlooked feature.

When NOT to use

IEC 61131-3 languages are not ideal for very high-speed or complex data processing tasks better suited to specialized embedded systems or real-time operating systems. For such cases, using dedicated motion controllers or industrial PCs with advanced programming languages like C++ or Python is preferable.

Production Patterns

In industry, engineers often use Structured Text for complex math and data handling, Ladder Logic for straightforward I/O control, and SFC to organize process steps. Reusable function blocks and libraries are common to standardize control across machines. Version control and simulation tools integrate with IEC 61131-3 environments for safer deployments.

Connections

State Machines

SFC in IEC 61131-3 is a direct application of state machine concepts to control process flow.

Understanding state machines helps grasp how SFC organizes steps and transitions in automation.

Software Engineering Modularization

IEC 61131-3's Program Organization Units (POUs) mirror modular programming principles in software engineering.

Knowing modular design improves how you structure PLC programs for reuse and clarity.

Electrical Relay Logic

Ladder Logic visually resembles relay wiring diagrams used in electrical control panels.

Familiarity with relay logic helps electricians transition smoothly to PLC programming.

Common Pitfalls

#1 Trying to write complex algorithms entirely in Ladder Logic.

Wrong approach:

Using many rungs with nested contacts and coils to implement math calculations.

Correct approach:

Use Structured Text for complex calculations and Ladder Logic only for simple control.

Root cause:

Misunderstanding the strengths and limits of Ladder Logic leads to overly complicated and fragile programs.

#2 Assuming IEC 61131-3 programs run identically on all PLC brands.

Wrong approach:

Copying code from one vendor's PLC to another without testing or modification.

Correct approach:

Review vendor-specific extensions and test thoroughly when porting programs.

Root cause:

Overlooking vendor differences causes unexpected behavior and bugs.

#3 Ignoring scan cycle timing effects in program design.

Wrong approach:

Writing code that assumes inputs update instantly and outputs change immediately.

Correct approach:

Design programs considering the scan cycle delay and use synchronization techniques.

Root cause:

Lack of awareness about PLC runtime behavior leads to timing-related bugs.

Key Takeaways

IEC 61131-3 standardizes five programming languages to make PLC programming clear, consistent, and portable.

Graphical languages like Ladder Logic and Function Block Diagram help visualize control logic, while text languages like Structured Text offer power for complex tasks.

Real-world PLC programs often combine multiple languages to use their strengths effectively.

Understanding the PLC scan cycle and vendor-specific extensions is crucial for reliable and maintainable automation.

Avoid common pitfalls by choosing the right language for the task and testing programs across different PLC platforms.

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