IEC 61131-3 is the international standard that defines how PLCs are programmed. Before it existed, every controller vendor invented its own languages and conventions, and a program written for one brand was useless on another. The standard brought order by defining a common set of programming languages and data models that most modern PLCs now support.
IEC 61131-3 in one line: IEC 61131-3 is the international standard that defines the programming languages and software architecture for programmable logic controllers, specifying five languages - ladder diagram, function block diagram, structured text, sequential function chart, and instruction list.
The standard defines five languages, and engineers pick whichever suits the task. Ladder Diagram (LD) resembles relay wiring and is the default for discrete logic. Function Block Diagram (FBD) wires reusable blocks together and suits signal processing and analog control. Structured Text (ST) is a high-level, Pascal-like text language for complex algorithms and math. Sequential Function Chart (SFC) organizes a process into ordered steps and transitions. Instruction List (IL) is a low-level assembly-like language, now largely deprecated.
Crucially, these languages can be mixed within a single project. A batch sequence might be laid out in SFC, with each step calling function blocks defined in FBD and a tricky calculation written in ST. The standard also defines common data types, variables, and program organization units, so the structure of a program is portable in principle even when vendor implementations differ in detail.
The practical payoff is skills and portability. A controls engineer who knows structured text or function blocks can move between compliant platforms without relearning fundamentals, and code concepts transfer even when the exact toolchain changes. It also enables better software practices - reusable function blocks, structured programs, and cleaner logic - than the flat ladder programs of earlier eras.
In oil and gas, most PLCs used at skids, compressor packages, and metering stations are IEC 61131-3 compliant, which is why an engineer will describe a control scheme as "ladder" or "structured text" and expect to be understood. Whatever language runs inside, a SCADA platform reads the resulting tags over the PLC's protocol - Merobix polls Allen-Bradley, Siemens, and Modbus PLCs regardless of the IEC 61131-3 language their logic was written in.
Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Sequential Function Chart (SFC), and Instruction List (IL). SFC is often used to structure a process while the others implement its steps; Instruction List is now largely deprecated in newer editions.
No. Ladder logic (Ladder Diagram) is just one of the five languages the standard defines. IEC 61131-3 is the broader standard covering all of them plus common data types and program structure.
Most modern PLCs support it, but vendor implementations vary in the details, so programs are not always perfectly portable between brands. The standard provides common concepts and languages rather than guaranteeing drop-in code exchange.
This page references the standards, specifications, and official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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