====== Structured Text ======

**Structured Text (ST)** is a high-level, **PASCAL-like textual programming language** designed for implementing complex control algorithms, mathematical calculations, and state machines within a controller.

It is the preferred choice for software developers because it uses familiar programming constructs, including conditional statements, iterative loops, and explicit variable assignments. ST allows you to express complex functions in just a handful of lines of code, making it ideal for extensive projects. It is also the primary language used to create **custom function blocks**.

{{ en:mervis-ide:40-st:st.png?600&direct |Structured Text Editor Example }}

=== Glossary: Key ST Terms ===

  * **Assignment Operator**: The operator used to **assign** the result of an expression or value to a specific variable (e.g., `Variable := Value;`).
  * **Control Structures**: Keywords like **`IF-THEN-ELSE`**, **`CASE`**, **`FOR`**, and **`WHILE`** that define the non-sequential flow of the program execution.
  * **Expression**: A combination of variables, constants, and operators that is evaluated by the controller to produce a **single result value**.
  * **Variable Declaration**: The mandatory process of defining a variable's **name**, its **data type** (e.g., BOOL, INT, REAL), and its scope before use.
  * **Program Organization Unit (POU)**: A common term for a modular, reusable code block, which can be either a **Function** or a **Function Block**.

=== Example Code: Control Structures ===

The following examples demonstrate how ST uses control structures for logic and calculations.

=== Conditional Logic (IF...THEN) ===

This is used to execute a block of code only if a specified Boolean condition is **TRUE**.

<code>
IF Temperature > 100.0 THEN
    Heater_Status := TRUE;
    Alarm_Light := TRUE;
END_IF;
</code>

=== Selection Logic (CASE) ===

The `CASE` statement executes a specific block of code based on the value of an integer expression.

<code>
CASE Input_Mode OF
    1: // Mode 1: Manual
        Motor_Speed := 50;
    2: // Mode 2: Automatic
        Motor_Speed := Auto_Setpoint;
    ELSE // Default for any other value
        Motor_Speed := 0;
END_CASE;
</code>

=== Iterative Loop (FOR) ===

The `FOR` loop repeats a block of code a fixed number of times, using a counter variable.

<code>
FOR i := 1 TO 10 DO
    Result_Array[i] := Result_Array[i] * 2;
END_FOR;
</code>