The 5 Most Common PLC Programming Languages

The 5 Most Common PLC Programming Languages

The 5 Most Common PLC Programming Languages

Why Multiple PLC Programming Languages Exist

The IEC 61131-3 standard defines five programming languages for PLCs, each designed for different types of tasks and different user backgrounds. An electrician comfortable with relay schematics will prefer Ladder Logic. A software engineer will gravitate toward Structured Text. A process engineer will find Function Block Diagrams intuitive. Understanding all five makes you a more versatile automation professional.

1. Ladder Logic (LD) — The Industry Standard

What it looks like: Two vertical power rails with horizontal rungs containing contacts and coils — just like an electrical schematic.

Best for: Discrete control, simple sequencing, boolean logic, and any application where electricians need to troubleshoot the code.

Why it dominates: Ladder Logic was designed to replace relay panels, so maintenance electricians could transition without learning a new paradigm. It remains the most widely used PLC language worldwide.

Limitations: Becomes unwieldy for complex calculations, data manipulation, or large programs. Poor at structured programming and code reuse.

Example concept: A start button (NO contact) in series with a stop button (NC contact) energizes a motor coil. A parallel holding contact keeps the motor running after the start button is released.

2. Function Block Diagram (FBD) — Visual Process Control

What it looks like: Rectangular blocks representing functions (AND, OR, PID, timers, counters) connected by signal lines showing data flow.

Best for: Process control, PID loops, analog signal processing, and applications where the signal flow matters more than the execution order.

Why engineers love it: FBD maps directly to P and ID (Piping and Instrumentation Diagram) thinking. A process engineer can look at an FBD program and immediately understand the control strategy.

Limitations: Can become visually cluttered with complex logic. Not ideal for sequential operations or state machines.

3. Structured Text (ST) — The Programmer Choice

What it looks like: High-level text code similar to Pascal, C, or Python — with IF/THEN/ELSE, FOR/WHILE loops, CASE statements, and mathematical expressions.

Best for: Complex calculations, data processing, string manipulation, recipe management, and any task that would be painful in graphical languages.

Why it is growing: As PLCs become more powerful and handle more data, Structured Text is increasingly used for the complex parts of programs. It enables code reuse through functions and function blocks.

Limitations: Harder for electricians to troubleshoot. Requires programming discipline to write maintainable code. Different PLC brands have slightly different ST dialects.

4. Sequential Function Chart (SFC) — The Process Organizer

What it looks like: A flowchart with steps (rectangles) and transitions (horizontal lines). Each step contains actions; each transition has a condition.

Best for: Sequential processes with clear steps — batch processing, assembly sequences, startup/shutdown procedures.

Why it is powerful: SFC provides a high-level view of the process flow. It forces structured thinking about states and transitions, reducing logic errors in complex sequences.

Limitations: Not suitable for continuous control or non-sequential logic. Often used as a top-level organizer, with steps containing Ladder or Structured Text for detailed logic.

5. Instruction List (IL) — The Legacy Language

What it looks like: Low-level assembly-like instructions: LD (load), AND, OR, ST (store), one instruction per line, working with an accumulator.

Best for: Legacy systems. IL is being deprecated in the third edition of IEC 61131-3 and should not be used for new projects.

Why it is fading: IL is hardware-dependent, hard to read, and error-prone. Modern PLCs have enough processing power that the efficiency advantage of IL is negligible.

Limitations: Being phased out. If you encounter IL in a legacy system, consider migrating to Structured Text or Ladder Logic during your next upgrade cycle.

Which Language Should You Learn First?

Start with Ladder Logic — it is the most widely used and will let you work with the broadest range of PLCs. Then add Structured Text for complex tasks. Function Block Diagram is essential if you work in process industries. SFC is valuable for batch and sequential processes.

Programming Environments by Brand

  • Siemens TIA Portal: Supports all five languages. The industry most comprehensive IDE.
  • Rockwell Studio 5000: Ladder Logic, FBD, Structured Text, SFC.
  • ABB Automation Builder: All IEC 61131-3 languages with strong FBD support.
  • Omron Sysmac Studio: Ladder Logic and Structured Text with integrated motion control.
  • Beckhoff TwinCAT: Full IEC 61131-3 plus C++ and MATLAB/Simulink integration.

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