IEC 61131

IEC 61131
Title	IEC 61131
Status	Published
Year started	1992
First published	1992
Latest version	IEC 61131-3:2013
Organization	International Electrotechnical Commission
Committee	IEC Technical Committee 65
Related standards	IEC 61499, IEC 61508, ISO/IEC 14977
Domain	Industrial automation, Programmable logic controller

IEC 61131. It is a series of international standards developed by the International Electrotechnical Commission for programmable logic controllers and related industrial automation systems. The standards, particularly the widely adopted third part, define a suite of programming languages, data types, and software models to unify development practices across the industry. This harmonization aims to improve software portability, reusability, and maintainability for control engineers working with equipment from diverse manufacturers like Siemens, Rockwell Automation, and Mitsubishi Electric.

Overview

The standard originated from the work of IEC Technical Committee 65, which focuses on industrial-process measurement, control, and automation. Prior to its publication, the programmable logic controller market was fragmented, with each major vendor like Allen-Bradley or Schneider Electric using proprietary and incompatible software tools. The initial release in 1992 consolidated earlier regional efforts, including those from Germany and the United States, into a global framework. The series is structured into several parts, covering general information, equipment requirements, programming languages, user guidelines, and communications, with the core programming concepts defined in IEC 61131-3.

Programming languages

This part of the standard specifies two textual and two graphical languages to accommodate different engineering backgrounds and application complexities. The textual languages are Structured Text, resembling Pascal or C (programming language), and Instruction List, which is mnemonic-based and similar to assembly language. The graphical languages are Ladder Diagram, which evolved from electromechanical relay logic diagrams familiar to electricians, and Function Block Diagram, used for depicting signal and data flow between reusable function blocks. This multi-language approach allows projects within the same system, such as a Ford Motor Company assembly line, to be developed using the most appropriate paradigm.

Data types and variables

It defines a comprehensive and extensible type system to ensure data consistency and integrity across program components. This includes elementary data types like BOOL, INT, and REAL, along with derived types such as arrays, structures, and enumerated types. Variables are characterized by specific attributes, including a name, data type, and a crucial memory location qualifier like `VAR`, `VAR_INPUT`, or `VAR_GLOBAL`. This rigorous typing model, supported by tools from CODESYS or Beckhoff Automation, helps prevent common programming errors and facilitates the integration of safety-related software compliant with IEC 61508.

Program organization units

Software is constructed from standardized, reusable components known as program organization units. The primary POU types are functions, which always produce a single result, function blocks, which have internal memory and represent the cornerstone of reusable object-oriented design, and programs, which form the main executable unit. These POUs can call each other and are typically stored in libraries, promoting modular design. This structure enables the creation of complex applications, from a Boeing manufacturing cell to a Siemens SCADA system, by assembling tested, vendor-independent software modules.

Conformance and compliance

For a programmable logic controller or its development software to claim compliance, it must implement the standard's features as specified. The International Electrotechnical Commission defines conformance classes and a standardized XML-based exchange format in IEC 61131-10 to facilitate the porting of code between different systems. Independent organizations may offer testing and certification. While full compliance ensures maximum portability, many implementations from Omron or ABB offer extensions for proprietary hardware features, requiring careful engineering to maintain interoperability goals across platforms like Microsoft Windows or VxWorks.

Applications and industry impact

The standard's influence extends far beyond traditional programmable logic controller programming. Its principles are foundational in distributed control systems, motion control, and robotics. The model has been extended by subsequent standards like IEC 61499 for distributed systems. It is taught in engineering curricula worldwide and is supported by a vast ecosystem of software tool vendors, including Siemens TIA Portal, Rockwell Studio 5000, and open-source platforms. This widespread adoption by industries from Toyota to ExxonMobil has solidified it as the dominant global paradigm for industrial control software engineering.

Category:International Electrotechnical Commission standards Category:Automation Category:Industrial computing Category:Programming languages