Beginner

Structured Text for Beginners: A Complete Guide to PLC Programming

Start your PLC programming journey with this beginner-friendly guide to Structured Text — the most versatile IEC 61131-3 language.

2025-12-15 · 12 min read

Structured Text
IEC 61131-3
Beginners
PLC Basics

What Is Structured Text?

Structured Text (ST) is a high-level programming language defined in the IEC 61131-3 standard for Programmable Logic Controllers (PLCs). Unlike Ladder Diagram or Function Block Diagram, ST uses text-based syntax similar to Pascal or C, making it powerful for complex logic, math-heavy operations, and data processing.

Why Learn Structured Text?

Universal: Works on Siemens S7, Allen-Bradley, CODESYS, Beckhoff TwinCAT, Schneider Electric, and more

Powerful: Handles loops, arrays, structs, and complex math that are tedious in graphical languages

Industry demand: Automation engineers with ST skills command higher salaries

Readable: Easier to document, version-control, and maintain than graphical programs

Your First Structured Text Program

Let's write a simple motor start/stop program:

PROGRAM MotorControl
VAR
    StartButton : BOOL := FALSE;
    StopButton  : BOOL := FALSE;
    MotorRunning : BOOL := FALSE;
END_VARIF StartButton AND NOT StopButton THEN
    MotorRunning := TRUE;
ELSIF StopButton THEN
    MotorRunning := FALSE;
END_IF;
END_PROGRAM

Breaking It Down

PROGRAM / END_PROGRAM: Defines a Program Organization Unit (POU)

VAR / END_VAR: Declares variables with types and optional initial values

BOOL: A Boolean data type (TRUE or FALSE)

IF / ELSIF / END_IF: Conditional logic — the backbone of PLC programming

Structured Text Data Types

Type	Description	Example
BOOL	Boolean (TRUE/FALSE)	`sensor_active : BOOL`
INT	16-bit integer	`count : INT := 0`
REAL	Floating-point number	`temperature : REAL := 22.5`
STRING	Text string	`name : STRING := 'Pump_01'`
TIME	Duration value	`delay : TIME := T#5s`

Control Structures

IF / THEN / ELSE

IF temperature > 80.0 THEN
    alarm := TRUE;
    coolingFan := TRUE;
ELSIF temperature > 60.0 THEN
    alarm := FALSE;
    coolingFan := TRUE;
ELSE
    alarm := FALSE;
    coolingFan := FALSE;
END_IF;

💡 Syntax fragment. To run this, declare temperature : REAL;, alarm : BOOL;, and coolingFan : BOOL; in the VAR panel.

FOR Loop

FOR i := 1 TO 10 DO
    total := total + sensorValues[i];
END_FOR;
average := total / 10.0;

💡 Syntax fragment. Declare i : INT;, total, average : REAL;, and sensorValues : ARRAY[1..10] OF REAL; in the VAR panel.

CASE Statement

CASE machineState OF
    0: status := 'IDLE';
    1: status := 'RUNNING';
    2: status := 'PAUSED';
    3: status := 'ERROR';
END_CASE;

💡 Syntax fragment. Declare machineState : INT; and status : STRING; in the VAR panel.

Next Steps

Now that you understand the basics, try our interactive ST editor to write and simulate your own programs. Start with Lesson 1 in our curriculum to build a traffic light controller!

Key Takeaways

Structured Text is the most versatile IEC 61131-3 PLC language

It uses familiar programming constructs: variables, conditionals, loops

ST skills transfer across all major PLC platforms

Practice in a simulator before deploying to real hardware