Concepts

Ladder Logic vs. Structured Text: When to Use Each PLC Language

Should you use Ladder Logic or Structured Text? This comparison helps you choose the right IEC 61131-3 language for your project.

2026-01-22 · 8 min read

Ladder Logic
Structured Text
IEC 61131-3
Comparison

The two PLC languages you will use most

IEC 61131-3 has defined five languages (Instruction List was deprecated in 2013 and removed in the 2025 edition), and two of them dominate industrial automation: Ladder Diagram (LD) and Structured Text (ST). Most real projects use both, so the useful question is which parts of a program belong in which language.

Side by side

Feature	Ladder Logic (LD)	Structured Text (ST)
Style	Graphical (circuit-like)	Text-based (code)
Best for	Simple I/O logic, relay replacement	Complex math, data processing
Learning curve	Easy for electricians	Easy for programmers
Debugging	Visual power flow	Variable watch, breakpoints
Loops/Arrays	Limited or none	Full support
Maintenance	Easy for simple logic	Better for complex systems
Industry use	North America dominant	Growing worldwide

Where Ladder Diagram fits

Ladder is at its best with simple Boolean logic: start/stop circuits, interlocks and permissives. Electricians on the maintenance team can read LD like a wiring diagram, and power flow through the contacts is intuitive to follow on screen. Some sites and industries also prefer LD for safety-related logic because it is easy to audit. That logic still sits beside a real safety function, not in place of one: emergency stops and guard circuits belong in a safety relay or a safety PLC.

Ladder example: motor start/stop

A classic seal-in circuit in Ladder has three parts:

a start button contact in series with a stop button (normally closed);

a motor output contact in parallel with the start button, which seals the circuit in;

a layout that reads like an electrical schematic.

Where Structured Text fits

Structured Text is the better choice for:

Mathematical operations, such as PID calculations, unit conversions and statistical analysis.

Data processing: string manipulation, recipe management, data logging.

Complex state machines, meaning multi-step sequences with many conditions.

Array operations, for example batch processing and buffer management.

Code reuse, through functions and function blocks for modular programming.

Structured Text example: a PID-like calculation

// scanTime must be > 0 (in seconds, e.g. 0.1). Real PLCs fault on
// division by zero; declare it with a sensible default and never let
// it drift to zero in a tuning UI.
error := setpoint - processValue;
integral := integral + (error * scanTime);
derivative := (error - lastError) / scanTime;
output := (Kp * error) + (Ki * integral) + (Kd * derivative);
lastError := error;

// Clamp output
IF output > 100.0 THEN output := 100.0;
ELSIF output < 0.0 THEN output := 0.0;
END_IF;

The same calculation in Ladder would be far more cumbersome to build and to read.

Mixing both in one project

Most current PLC platforms (CODESYS, TIA Portal, RSLogix 5000) let you mix languages within a project. A common split looks like this:

Interlock and safety-related logic in Ladder Diagram, where it stays clear, auditable and readable by electricians.

Data processing in Structured Text, which is compact and easy to maintain.

Sequence control in either language, or in Sequential Function Chart (SFC).

Reusable blocks written as Structured Text function blocks and called from any language.

Try both in CodeSchool ST

You can practise Structured Text in the online editor and work through Ladder Logic concepts in the Ladder Logic environment, then compare how the same logic looks in each language.

Picking the language for each part

No single PLC language is best. Put simple contact logic and interlocks in Ladder, put maths, strings, arrays and long sequences in Structured Text, and wrap anything you will reuse in a function block. Engineers who read and write both can make that choice block by block within a project.