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An IEC 61131-3 programming language for industrial automation and PLC programming. Pascal-like syntax.
Structured Text (ST) is one of five programming languages defined in the IEC 61131-3 international standard for programmable logic controllers (PLCs), first published in 1993 and maintained by the PLCopen organization. Among the IEC 61131-3 languages — which also include Ladder Diagram, Function Block Diagram, Instruction List, and Sequential Function Chart — Structured Text most closely resembles a high-level textual language, with syntax influenced by Pascal and Ada. ST provides familiar constructs including IF/THEN/ELSE, CASE, FOR/WHILE/REPEAT loops, functions, and function blocks, making it accessible to software engineers transitioning from IT to operational technology. The language is used to program industrial automation systems in manufacturing, process control, building automation, power generation, water treatment, and transportation infrastructure.
Structured Text excels at implementing complex mathematical calculations, state machines, PID control algorithms, and data processing that would be unwieldy in graphical languages. Major PLC platforms supporting ST include Siemens TIA Portal, Rockwell Automation Studio 5000, Beckhoff TwinCAT, CODESYS, and Schneider Electric's EcoStruxure. In safety-critical applications, ST programs undergo formal verification and must comply with standards like IEC 61508 for functional safety. The language supports strong typing with BOOL, INT, REAL, TIME, DATE, and STRING types, as well as user-defined structures and enumerations.
Modern IEC 61131-3 extensions add object-oriented features including methods, interfaces, and inheritance. Structured Text programs typically run on dedicated real-time hardware with deterministic cycle times measured in milliseconds, controlling physical machinery where software errors can cause equipment damage or endanger human safety.
Structured Text controls physical machinery and industrial processes where code changes can cause equipment damage or safety incidents. Comparing ST files catches modified PID tuning parameters that affect process stability, altered safety interlock logic that protects equipment and personnel, changed timer and counter values that affect production timing, and updated I/O mappings that connect software logic to physical sensors and actuators.
Every change to PLC programs in regulated industries must be formally reviewed and approved before deployment to the factory floor.
UtraDiff loads Structured Text files into a diff editor with IEC 61131-3 syntax highlighting, distinguishing keywords like IF, THEN, CASE, and FUNCTION_BLOCK. Side-by-side and inline views reveal changes to PLC logic, variable declarations, and timer/counter configurations.
The whitespace-ignore toggle removes indentation noise from deeply nested control structures. Keyboard navigation jumps between changed regions, critical for verifying safety-related logic modifications in industrial automation code.
Supported extensions: .st .iecst .iecplc