Variables & Data Types

Variables are the foundation of any Structured Text program. Understanding how to declare, initialize, and use different data types is essential for effective ST programming.

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Variable Declaration

Variables must be declared before use within VAR blocks. The basic syntax is:

Variable Declaration Syntax

VAR
    VariableName : DataType;
    AnotherVariable : DataType := InitialValue;
END_VAR

Basic Data Types

Boolean Types

VAR
    IsRunning : BOOL := FALSE;
    StartButton : BOOL;
    SafetyOK : BOOL := TRUE;
END_VAR

Values: TRUE, FALSE
Size: 1 bit
Use: Digital signals, flags, conditions

Integer Types

VAR
    Counter : INT := 0;
    LargeNumber : DINT := 100000;
    SmallValue : SINT := -50;
    PositiveOnly : UINT := 65000;
END_VAR

SINT: -128 to 127 (8-bit)

INT: -32,768 to 32,767 (16-bit)

DINT: -2³¹ to 2³¹-1 (32-bit)

UINT: 0 to 65,535 (16-bit unsigned)

Real (Floating Point) Types

VAR
    Temperature : REAL := 25.5;
    PreciseValue : LREAL := 3.14159265359;
    Percentage : REAL := 75.0;
END_VAR

REAL: 32-bit floating point

LREAL: 64-bit floating point

Range: ±1.175e-38 to ±3.402e+38 (REAL)

String Types

VAR
    Message : STRING := 'Hello World';
    PartNumber : STRING(20) := 'ABC-123';
    ErrorText : STRING(100);
END_VAR

STRING: Variable-length text

STRING(n): Fixed maximum length

Default length: Usually 80 characters

Time Data Types

VAR
    DelayTime : TIME := T#5s;           // 5 seconds
    CycleTime : TIME := T#100ms;        // 100 milliseconds
    LongDelay : TIME := T#2h30m15s;     // 2 hours, 30 minutes, 15 seconds
    
    CurrentTime : DATE_AND_TIME;
    StartDate : DATE := D#2024-01-15;
    StartTime : TIME_OF_DAY := TOD#08:30:00;
END_VAR

Time Format Examples

T#5s or TIME#5s - 5 seconds

T#1m30s - 1 minute 30 seconds

T#2h15m - 2 hours 15 minutes

T#100ms - 100 milliseconds

D#2024-12-25 - December 25, 2024

TOD#14:30:45 - 2:30:45 PM

Variable Initialization

✅ Good Practices

VAR
    // Initialize critical values
    SafetyEnabled : BOOL := TRUE;
    MaxTemp : REAL := 100.0;
    
    // Clear initial states
    AlarmActive : BOOL := FALSE;
    ErrorCount : INT := 0;
END_VAR

❌ Potential Issues

VAR
    // Uninitialized variables may have
    // unpredictable values
    Temperature : REAL;
    Counter : INT;
    Status : BOOL;
    Speed : REAL;
END_VAR

Arrays

Arrays allow you to store multiple values of the same type:

Array Declaration and Usage

VAR
    // One-dimensional arrays
    Temperatures : ARRAY[1..10] OF REAL;
    InputStates : ARRAY[0..15] OF BOOL;
    Messages : ARRAY[1..5] OF STRING(50);
    
    // Two-dimensional arrays
    Matrix : ARRAY[1..3, 1..3] OF INT;
    
    // Array with initialization
    DefaultValues : ARRAY[1..3] OF INT := [10, 20, 30];
END_VAR

// Using arrays
Temperatures[1] := 25.5;
InputStates[0] := TRUE;
Matrix[2,3] := 42;

Structures (User-Defined Types)

Structures allow you to group related data together:

Structure Definition and Usage

// Type definition (usually in a separate section)
TYPE
    MotorData : STRUCT
        Speed : REAL;
        Current : REAL;
        Temperature : REAL;
        IsRunning : BOOL;
        FaultCode : INT;
    END_STRUCT;
END_TYPE

// Variable declaration using the structure
VAR
    Motor1 : MotorData;
    Motor2 : MotorData;
END_VAR

// Using structure members
Motor1.Speed := 1500.0;
Motor1.IsRunning := TRUE;

IF Motor1.Temperature > 80.0 THEN
    Motor1.IsRunning := FALSE;
END_IF;

Variable Scope and Storage Classes

Declaration	Scope	Persistence
VAR	Local to program/function	Lost on power cycle
VAR_GLOBAL	Accessible from anywhere	Lost on power cycle
VAR_RETAIN	Local to program/function	Retained through power cycle
VAR_INPUT	Function/FB input parameter	Parameter passing
VAR_OUTPUT	Function/FB output parameter	Parameter passing

Example of Different Variable Scopes

Variable Scopes Example

VAR_GLOBAL
    SystemStatus : INT := 0;        // Available everywhere
END_VAR

VAR_GLOBAL RETAIN
    TotalRunTime : TIME := T#0s;    // Survives power cycles
END_VAR

PROGRAM MainProgram
VAR
    LocalCounter : INT := 0;        // Only in this program
END_VAR

VAR_RETAIN
    LastStopTime : DATE_AND_TIME;   // Local but retained
END_VAR

Type Conversion

ST provides explicit type conversion functions for safe data type changes:

Type Conversion Examples

VAR
    IntValue : INT := 42;
    RealValue : REAL;
    StringValue : STRING;
    BoolValue : BOOL;
END_VAR

// Numeric conversions
RealValue := INT_TO_REAL(IntValue);     // 42 → 42.0
IntValue := REAL_TO_INT(25.7);          // 25.7 → 25

// String conversions
StringValue := INT_TO_STRING(IntValue); // 42 → '42'
IntValue := STRING_TO_INT('123');       // '123' → 123

// Boolean conversions
BoolValue := INT_TO_BOOL(1);            // 1 → TRUE
IntValue := BOOL_TO_INT(TRUE);          // TRUE → 1

⚠️

Type Conversion Safety

Always use explicit type conversion functions rather than relying on implicit conversions:

• Check for potential data loss (e.g., REAL to INT truncates)
• Validate string-to-number conversions for invalid input
• Consider range limits when converting between integer types

Common Patterns and Best Practices

✅ Naming Conventions

// Clear, descriptive names
MotorSpeed : REAL;
IsSystemReady : BOOL;
ErrorMessage : STRING;

// Consistent prefixes
bStartButton : BOOL;    // Boolean
iCounter : INT;         // Integer  
rTemperature : REAL;    // Real
sPartNumber : STRING;   // String

💡 Initialization Tips

// Group related variables
VAR
    // Temperature control
    SetPoint : REAL := 25.0;
    Hysteresis : REAL := 2.0;
    
    // Safety limits
    MaxTemp : REAL := 100.0;
    MinTemp : REAL := -10.0;
END_VAR

🚀 Next Steps

Now that you understand variables and data types, explore:

→ Complete Data Types Reference (BYTE, WORD, DWORD, LWORD, POINTER, etc.) → Control Structures (IF, CASE, FOR, WHILE) → Standard Functions (Type Conversion, Math, String)