Structured text

Structured Text is a high level, block structured programming language defined as one of the five languages standardized by IEC 61131-3, the international standard covering programming of programmable logic controllers used throughout industrial automation. Its syntax closely resembles Pascal, on which it is directly based, giving it readable keywords, named variables and nested block structures rather than the ladder diagrams or instruction lists more traditionally associated with PLC programming. Structured Text follows an imperative programming style and supports iteration through REPEAT-UNTIL and WHILE-DO loops, conditional branching through IF-THEN-ELSE statements, and multi way branching through CASE statements, in which execution exits after the first matching condition rather than falling through to later cases as some other languages allow. Because it belongs to the shared IEC 61131-3 standard, Structured Text can be combined within the same automation program alongside the standard's other supported languages, such as ladder diagram or function block diagram, letting an engineer choose whichever language best expresses a given piece of control logic while keeping the overall program interoperable across compliant PLC programming environments from different vendors. Structured Text is widely used for writing more complex control algorithms, data handling routines and mathematical calculations within industrial control systems, where its higher level, textual syntax makes intricate logic easier to write and review than the standard's more graphical languages.