IEC 61131

Programmable Logic Controllers

IEC 61131 is the international standard for programmable logic controllers. Part 3 standardizes the 5 programming languages used in industrial automation (LD, FBD, SFC, ST, IL), enabling code portability between PLC platforms and a shared vocabulary for automation engineers worldwide.

Herausgeber: IEC
Family: Automation
Erstveröffentlichung: 1993
Letzte Revision: 2013 (Part 3 Edition 3)
Nächste Revision: Active maintenance, Edition 4 in development for Part 3
Status: current
Zugang: Paid (IEC webstore — ~250 CHF per part)

Dokumentstruktur

IEC 61131-1

General information

Definitions, scope, classification of PLCs.

IEC 61131-2

Equipment requirements and tests

Hardware requirements : environment, EMC, mechanical, electrical safety.

IEC 61131-3

Programming languages

THE famous part. Defines 5 programming languages (Ladder Diagram LD, Function Block Diagram FBD, Sequential Function Chart SFC, Structured Text ST, Instruction List IL — deprecated 2013). Plus data types, POUs (Program Organization Units), tasks.

IEC 61131-4

User guidelines

Selection and application of PLCs (informative).

IEC 61131-5

Communications

Cross-PLC messaging concepts (largely superseded by OPC UA in practice).

IEC 61131-6

Functional safety

Safety-related aspects when using PLCs in safety applications (links to IEC 61508).

IEC 61131-7

Fuzzy control programming

Niche : fuzzy logic standardization. Rarely used.

IEC 61131-8

Guidelines for the application and implementation of programming languages

Best practices for IEC 61131-3 language usage.

IEC 61131-9

Single-drop digital communication interface (IO-Link)

Standardizes IO-Link, the dominant smart sensor / actuator protocol in modern factories.

IEC 61131-10

PLCopen XML format for programs

PLCopenXML : standard XML schema for storing IEC 61131-3 programs. Enables vendor-neutral program exchange.

Schlüsselbegriffe

The 5 languages: Ladder Diagram (LD) : looks like electrical relay schematics, intuitive for electricians. Function Block Diagram (FBD) : looks like circuit blocks wired together, intuitive for process engineers. Sequential Function Chart (SFC) : state machines / GRAFCET. Structured Text (ST) : looks like Pascal, intuitive for programmers. Instruction List (IL) : assembly-like, deprecated in 2013.
Program Organization Unit(POU): Reusable code unit. Three types : Program (top-level, has memory state), Function Block (instance-based, has memory state), Function (stateless, like a math function). Composability built into the language.
Standard data types: BOOL, BYTE, WORD, DWORD, LWORD, SINT/INT/DINT/LINT, USINT/UINT/UDINT/ULINT, REAL, LREAL, TIME, DATE, TIME_OF_DAY, DATE_AND_TIME, STRING. Derived types (ARRAY, STRUCT, enumerations, type aliases) for engineering structure.
Tasks and execution model: Cyclic tasks (every N ms), event-driven tasks, freewheeling. Multi-tasking PLC with priority levels. Defined behavior under preemption.
PLCopen: Industry association (vendors + integrators) that publishes practical guidelines and reference libraries on top of IEC 61131-3. Notable : PLCopen Motion Control (PLCopen MC) standardized FBs for servo control, used by every modern motion controller.
Variable scoping: VAR / VAR_INPUT / VAR_OUTPUT / VAR_IN_OUT / VAR_GLOBAL / VAR_EXTERNAL. Strict typing, explicit declarations. Encourages disciplined code over the unstructured habits of early PLC programming.

Notes & guidance

Why language standardization in PLCs took 20 years

Industrial automation in the 1970-80s was a Tower of Babel : every PLC vendor (Modicon, Allen-Bradley, Siemens, Mitsubishi, Omron, Telemecanique…) had their own programming language, their own development environment, their own concept of memory addressing. A trained Siemens engineer could not work on an Allen-Bradley PLC without months of retraining.

IEC 61131-3 (1993, major revision 2013) settled this. The 5 languages it defines are now supported by every major PLC vendor in some form. Engineers can move between platforms with vocabulary intact. Code patterns are portable in principle (in practice, vendor-specific extensions still create lock-in).

Why 5 languages, not just one ?

Each addresses a different engineering tradition :

Electricians                      Process engineers              Programmers
                                                                            
        ╲                                ╱                              ╱
         ╲                              ╱                              ╱
          ╲                            ╱                              ╱
          [LD]   relay schematic   [FBD]   block diagram        [ST]   Pascal-like
                                                                            
                              Process engineers              
                                                                            
                                 [SFC]   state machine        
                                                                            
                              ↘ (IL was IL : assembly-like, deprecated 2013)

A modern industrial program typically MIXES :

ST for math-heavy logic (PID computation, alarms, recipe management)
FBD for control loop wiring (assembling PID + filter + scaler blocks)
LD for safety-critical interlocks (auditors and electricians can read it without training)
SFC for batch sequences and start-up procedures

This multilingual code is normal IEC 61131-3 practice, not a sign of confusion.

What is portable in practice (and what isn’t)

Portable : data types, basic algorithms, simple function blocks, standard motion FBs (PLCopen MC).

Not portable :

I/O addressing (every vendor has its own scheme)
Hardware configuration (PROFINET vs EtherCAT vs EthernetIP)
HMI / SCADA integration (proprietary)
Diagnostic features (extension blocks specific to vendor)
Performance characteristics (cycle time guarantees)

This means PLCopenXML import/export (Part 10) gives you ~70% of an automation project ; the rest still needs vendor-specific configuration. For a fully vendor-neutral codebase, CODESYS is the practical leader — its runtime is licensed by ~50 PLC manufacturers (Beckhoff, WAGO, Lenze, ABB, etc.) and provides genuine cross-platform portability.

Safety variant : IEC 61131-6 + IEC 61508

Standard PLCs are NOT safety-rated. For safety functions, you need a safety PLC (Pilz PNOZ, HIMA HIMax, Siemens S7-1500F, Allen-Bradley GuardLogix, Beckhoff TwinSAFE) that combines :

IEC 61131-3 programming for the standard logic
Dedicated safety language for safety logic (constraints, restricted set of FBs)
IEC 61508 certified runtime (SIL 2 or SIL 3 capable)
Hardware redundancy + diagnostic coverage internally

Modern safety PLCs are programmed in IEC 61131-3 with safety extensions, so the engineer’s experience is unified. But the safety code is separately certified at the toolchain level (TÜV / exida certification of the IDE + runtime + hardware platform).

The future : IEC 61131-3 vs object-oriented programming

Edition 3 (2013) added object-oriented features : METHODS on Function Blocks, INTERFACES, INHERITANCE. This brought PLC programming closer to modern software engineering practices, but adoption is uneven :

Beckhoff TwinCAT 3 : full OOP support, widely used
CODESYS : full OOP support
Siemens TIA Portal : partial OOP (improving each version)
Rockwell Studio 5000 : limited

For new projects, using OOP carefully (avoiding inheritance complexity but using methods + interfaces for testability) is now a recognized good practice. Older “all-global-state” PLC code is increasingly viewed as legacy.

Betroffene Branchen

Discrete manufacturing (automotive, electronics, packaging)
Process industries with PLC-based control (often hybrid with DCS)
Building automation
Energy management
Water and wastewater (frequent PLC-based)
Machine building (every CNC, robot, packaging machine)