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Variables and Assignment

Variable Naming​

In Structured Text (ST), variables are the building blocks of your logic. They represent inputs, outputs, and internal states within your PLC program.

Naming Rules​

  • Must begin with a letter (A–Z or a–z)

  • Can contain letters, digits, and underscores ( _ )

  • No spaces, symbols (except _ ), or starting digits

  • Case-insensitive in most environments (but using consistent case is recommended)

Good Naming Conventions​

  • Use descriptive names that reflect the purpose of the variable.

  • Use camelCase or PascalCase consistently (or whatever style your team/project uses).

  • Prefixes can be helpful (e.g., is, has, num, val, btn, etc.)

Examples:


Motor_Start      // Boolean to start motor
Counter_Value    // Integer counter
Temperature_Set  // Real number (e.g., 23.5)
Valve_Open       // Output signal to open valve

Avoid names like:


x1, val, temp2, a123  // Not meaningful

Assignment Operator​

In Structured Text, assignment is done using :=, not =.

Variable := Value;

  • := assigns the value on the right to the variable on the left.

  • A semicolon ( ; ) must follow the assignment.

Examples:

Motor_On := 1;
Counter := Counter + 1;

Note: The = symbol is used only in comparisons, such as IF x = 10 THEN.

Tag References​

variables (also called tags) are categorized into:

1. Input Tags​

  • Represent physical or logical input signals from sensors, buttons, etc.

  • Usually read-only inside logic.

Start_Button, Temp_Sensor, Limit_Switch

2. Output Tags​

  • Represent signals that go out to actuators, motors, alarms, etc.

  • Controlled by your logic.

Motor_Start, Buzzer, Valve_Open

3. Internal Tags​

  • Variables used for intermediate logic, memory, and calculations.

  • Not tied to physical I/O.

Counter_Value, Temp_Average, isRunning

Each tag type can be used in ST just like normal variables, but their source (hardware or logic) determines how they behave in the program.

Examples​

// Start the motor when the start button is pressed
Motor_Start := Start_Button;

// Increment the counter value
Counter_Value := Counter_Value + 1;

// Turn on the fan if the temperature exceeds the setpoint
IF Temp_Sensor > Temp_Setpoint THEN
    Fan_On := 1;
END_IF;

// Reset the counter when the stop button is pressed
IF Stop_Button THEN
    Counter_Value := 0;
END_IF;