IEC 61131-3 is the third part of the IEC 61131 international standard for programmable logic controllers, defining the syntax and semantics of five programming languages used in industrial automation: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC). The standard provides a common programming model across PLC vendors, enabling portability of control logic and reducing the effort required to maintain industrial systems. It is the primary reference for software engineering practice in the industrial automation domain and forms the basis of IEC 61131-3-compliant toolchains and runtime environments used in manufacturing, process control, and infrastructure systems.
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- The IEC 61131 standard was first published in 1993, with Part 3 defining the programming languages gaining rapid adoption across the automation industry as it replaced the previous situation of every PLC vendor having its own proprietary language. The third edition, published in 2013, added object-oriented extensions including classes, inheritance, interfaces, and namespace support, reflecting the growing complexity of modern industrial control applications.
- Ladder Diagram (LD) is the most widely used language and is graphically modelled on the relay-ladder logic used in electromechanical control panels, making it accessible to engineers with an electrical background. Each rung of the ladder represents a logical condition (contacts) controlling an output (coil). Ladder Diagram remains the dominant language for discrete manufacturing automation and is supported in virtually every PLC programming environment.
- Function Block Diagram (FBD) represents programs as networks of interconnected function blocks, resembling electronic circuit diagrams. It is particularly suited to analogue signal processing and continuous control, making it popular in the process industries. Each function block encapsulates a standard or custom algorithm, and blocks are connected by data flow lines that represent variable values.
- Structured Text (ST) is the only text-based language in IEC 61131-3 and is syntactically similar to Pascal or Ada, supporting conditional statements, loops, and complex arithmetic expressions. ST has gained popularity for implementing complex algorithms, data transformations, and communications handling where graphical languages become unwieldy. The third edition’s object-oriented extensions are primarily expressed through ST.
- Sequential Function Chart (SFC) provides a graphical representation of sequential processes using states (steps), transitions, and actions, closely related to Petri nets. SFC is ideal for batch processes and machine sequencing where distinct operational phases must be clearly represented. IEC 61131-3 compliant runtimes, such as CODESYS and Siemens TIA Portal, provide full SFC execution environments that can coordinate multiple simultaneously active steps in parallel branches.