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:

Start_Command    // BOOL (0 or 1) trigger
Counter_Value    // DINT counter
Temperature_Set  // REAL setpoint (e.g., 23.5)
Valve_Open       // BOOL output signal

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;