IEC 61131-3 Tutorial for Beginners

IEC 61131-3 is the international standard for PLC programming. It defines 5 programming languages, data types, and program organization — making it possible to write PLC logic that works across Rockwell, Siemens, Schneider, Mitsubishi, and every other major vendor.

What is IEC 61131-3?

IEC 61131-3 is the third part of the IEC 61131 family of standards for programmable logic controllers (PLCs). Published by the International Electrotechnical Commission (IEC), it specifies:

• 5 programming languages — visual and text-based
• Common data types — BOOL, INT, REAL, TIME, STRING, etc.
• Program organization — Programs, Function Blocks, Functions
• Execution model — scan cycles, tasks, triggers

Think of it as the "JavaScript of PLCs" — a universal standard that runs on any hardware, though each vendor adds proprietary extensions.

The 5 IEC 61131-3 Languages

Unlike traditional programming where you pick one language, PLC programmers use multiple languages in the same project — each optimized for different tasks.

1. Ladder Diagram (LD)

Visual language mimicking electrical relay logic. Most popular for discrete on/off control.

|----[ StartButton ]----[ NOT StopButton ]----( Motor )----||

When StartButton is pressed AND StopButton is NOT pressed, energize Motor.

Best for: Simple logic, safety interlocks, sequential control

2. Function Block Diagram (FBD)

Visual language showing data flow through interconnected blocks. Great for complex logic.

[TempSensor] ──→ PID ──→ [HeaterOutput]
[Setpoint]   ──┘

A PID controller taking temperature sensor input and setpoint, outputting heater control signal.

Best for: Analog control, math operations, PID loops

3. Sequential Function Chart (SFC)

Visual language for state machines and sequential processes. Represents steps and transitions.

┌─────────┐
│  START  │ (initial step)
└────┬────┘
     │ [Button pressed]
┌────▼────┐
│  FILL   │ (open valve)
└────┬────┘
     │ [Level >= 100]
┌────▼────┐
│  HEAT   │ (enable heater)
└─────────┘

Best for: Multi-step batch processes, state machines

4. Structured Text (ST)

Text-based language similar to Pascal or C. High-level programming with loops, conditionals, functions.

IF Temperature > Setpoint THEN
    HeaterOutput := 0;  (* Turn off heater *)
ELSIF Temperature < (Setpoint - 5) THEN
    HeaterOutput := 100;  (* Full power *)
ELSE
    HeaterOutput := 50;  (* Half power *)
END_IF;

Best for: Complex algorithms, data manipulation, loops, calculations

5. Instruction List (IL)

Text-based assembly-like language. Low-level, rarely used in modern systems.

LD   StartButton
AND  StopButton
ST   Motor

Best for: Legacy systems, low-level optimization (mostly deprecated)

Common Data Types

IEC 61131-3 defines standard data types that work across all vendors:

Program Organization Units (POUs)

IEC 61131-3 structures code into three types of POUs:

1. Programs

Top-level containers. A PLC project has one or more Programs. Each Program runs in a Task (cyclic, periodic, or event-driven).

2. Function Blocks (FB)

Reusable code with internal state (memory). Examples: TON (timer), CTU (counter), PID controller.

MyTimer : TON;  (* Declare a timer instance *)

MyTimer(IN := StartButton, PT := T#5s);
Motor := MyTimer.Q;  (* Motor turns on after 5 seconds *)

3. Functions

Stateless operations that return a single value. Examples: ADD, MUL, MAX, SQRT.

Result := ADD(10, 20);  (* Result = 30 *)
MaxValue := MAX(Temp1, Temp2, Temp3);

Real-World Example: Conveyor Control

Let's build a simple conveyor belt controller using multiple IEC 61131-3 languages in one project.

Ladder Diagram (Start/Stop Logic)

|----[ StartButton ]----[ NOT EStop ]----[ NOT MotorRunning ]----( StartRelay )----||
|----[ StopButton ]-----------------------------------------------------( StopRelay )-----||
|----[ StartRelay ]----[ NOT StopRelay ]-----------------------------( MotorRunning )----||

Structured Text (Speed Control)

IF MotorRunning THEN
    IF ItemCount < 10 THEN
        ConveyorSpeed := 50;  (* Slow speed *)
    ELSIF ItemCount < 50 THEN
        ConveyorSpeed := 75;  (* Medium speed *)
    ELSE
        ConveyorSpeed := 100;  (* Full speed *)
    END_IF;
ELSE
    ConveyorSpeed := 0;  (* Stopped *)
END_IF;

Function Block (Item Counter)

ItemCounter : CTU;  (* Count-up counter *)

ItemCounter(CU := PhotoEye, RESET := ResetButton, PV := 999);
ItemCount := ItemCounter.CV;  (* Current count value *)

This example shows how different languages work together — LD for simple start/stop, ST for complex logic, FB for reusable counters.

Why IEC 61131-3 Matters

1. Vendor Portability

Write code once, export to multiple vendors. A Ladder Diagram program can be exported to Rockwell L5X, Siemens SimaticML, Schneider XEF, and PLCopen XML.

2. Skill Transferability

Learn IEC 61131-3, and you can program any PLC. No need to relearn for every vendor.

3. Future-Proofing

The standard is maintained and updated by the IEC. New PLC vendors must support it to be competitive.

Common IEC 61131-3 Function Blocks

Every IEC 61131-3 compliant PLC includes these standard function blocks:

Timers

• TON (Timer On-Delay): Output turns ON after a delay
• TOF (Timer Off-Delay): Output turns OFF after a delay
• TP (Pulse Timer): Generates a fixed-duration pulse

Counters

• CTU (Count Up): Increments on rising edge
• CTD (Count Down): Decrements on rising edge
• CTUD (Count Up/Down): Bidirectional counter

Bistable

• SR (Set-Reset): Set dominant latch
• RS (Reset-Set): Reset dominant latch

Edge Detection

• R_TRIG: Rising edge detector
• F_TRIG: Falling edge detector

Getting Started with IEC 61131-3

The best way to learn is by doing. Here's your roadmap:

1. Start with Ladder Diagram — easiest to visualize
2. Practice with standard function blocks — TON, CTU, SR
3. Add Structured Text for complex logic
4. Try Function Block Diagram for analog control
5. Use SFC for multi-step processes

Learn IEC 61131-3 with Plaxio

Plaxio is a free IEC 61131-3 compliant IDE with:

• Full support for LD, FBD, SFC, and ST
• All standard function blocks (TON, CTU, PID, etc.)
• Real-time simulation with live tag monitoring
• Export to Rockwell, Siemens, Schneider, PLCopen XML
• AI-assisted code generation

Download Plaxio for free →

Summary