IEC 61131-3

IEC 61131-3 stands as the internationally recognized standard that defines programming languages for programmable logic controllers, establishing a unified framework that promotes portability, reusability, and consistency across different PLC manufacturers and platforms. Published by the International Electrotechnical Commission and continuously updated to reflect technological advances, this comprehensive standard specifies five programming languages—Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC)—along with common elements such as data types, variables, functions, and program organization units.

The standard defines not only syntax and semantics for each programming language but also establishes a complete software model encompassing data typing, variable declarations, program organization, and execution control. Strong data typing with elementary types and derived types ensures program reliability and portability. The software model organizes programs into hierarchical units including programs, function blocks, and functions, each with defined interfaces and execution rules. The five languages each serve specific purposes and complement each other within integrated automation projects, allowing engineers to choose the most appropriate tool for each aspect of their application.

The impact of IEC 61131-3 extends beyond technical standardization to influence the entire industrial automation ecosystem. PLC manufacturers have largely adopted the standard, third-party tools leverage the standard's formal definitions, and educational institutions use it as the foundation for automation curriculum. As Industry 4.0 drives convergence between operational and information technology, IEC 61131-3 continues evolving with object-oriented extensions and support for distributed automation architectures.