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 Most Popular PLC Languages

Out of the five languages defined in IEC 61131-3, Ladder Diagram (LD) and Structured Text (ST) dominate industrial automation. Understanding when to use each is a critical skill for any automation engineer.

Quick Comparison

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

When to Use Ladder Logic

Ladder Diagram excels in scenarios where:

Simple Boolean logic: Start/stop circuits, interlocks, safety logic

Electrical team maintenance: Electricians can read LD like a wiring diagram

Regulatory compliance: Some industries require LD for safety-critical logic

Quick visualization: Seeing power flow through contacts is intuitive

Ladder Logic Example: Motor Start/Stop

A classic seal-in circuit in Ladder:

Start button contact in series with a stop button (normally closed)

Motor output contact in parallel with start button (seal-in)

The circuit reads like an electrical schematic

When to Use Structured Text

Structured Text is the better choice when:

Mathematical operations: PID calculations, unit conversions, statistical analysis

Data processing: String manipulation, recipe management, data logging

Complex state machines: Multi-step sequences with many conditions

Array operations: Batch processing, buffer management

Code reuse: Functions and function blocks for modular programming

Structured Text Example: 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;

This would be extremely cumbersome in Ladder Logic!

The Best Approach: Use Both

Most modern PLC platforms (CODESYS, TIA Portal, RSLogix 5000) support mixing languages within a project:

Main safety logic: Ladder Diagram — clear, auditable, electrician-friendly

Data processing: Structured Text — powerful, compact, maintainable

Sequence control: Either, or Sequential Function Chart (SFC)

Reusable blocks: Structured Text function blocks called from any language

Try Both in CodeSchool ST

Practice Structured Text in our online editor and explore Ladder Logic concepts in our Ladder Logic environment. See how the same logic translates between languages.

Key Takeaway

There's no single "best" PLC language. The best automation engineers are fluent in both Ladder Logic and Structured Text, choosing the right tool for each task within a project.