How to Use Arrays in Structured Text for PLC Programming

In Structured Text, use ARRAY to declare a collection of elements of the same type with a fixed size. Access elements by their index using square brackets, starting at 0 or 1 depending on the PLC system. Arrays help organize multiple values efficiently in your PLC programs.

Syntax

To declare an array in Structured Text, use the ARRAY keyword followed by the index range and the data type. You access elements using square brackets with the element's index.

• ARRAY[lower_bound..upper_bound] OF data_type: Declares the array size and type.
• variable[index]: Accesses an element at the given index.

structured-text

VAR
  myArray : ARRAY[1..5] OF INT;  // Array of 5 integers
END_VAR

// Access example
myArray[1] := 10;  // Set first element
value := myArray[1];  // Read first element

Example

This example shows how to declare an integer array, assign values to each element, and then sum all elements using a loop.

structured-text

PROGRAM ArrayExample
VAR
  numbers : ARRAY[1..4] OF INT := [5, 10, 15, 20];
  sum : INT := 0;
  i : INT;
END_VAR

FOR i := 1 TO 4 DO
  sum := sum + numbers[i];
END_FOR

// sum now holds 50
END_PROGRAM

Output

sum = 50

Common Pitfalls

Common mistakes when using arrays in Structured Text include:

• Using an index outside the declared range causes runtime errors.
• Forgetting that some PLCs start array indexes at 1, not 0.
• Not initializing arrays before use, leading to unpredictable values.

Always check your PLC documentation for index base and initialize arrays properly.

structured-text

VAR
  arr : ARRAY[1..3] OF INT;
  val : INT;
END_VAR

// Wrong: Accessing index 0 (out of range)
// val := arr[0];  // This causes error

// Right: Accessing valid index
val := arr[1];

Quick Reference

Concept	Syntax/Description
Declare array	VAR myArray : ARRAY[1..10] OF INT; END_VAR
Access element	myArray[3] := 100;
Initialize array	myArray : ARRAY[1..3] OF INT := [1, 2, 3];
Loop through array	FOR i := 1 TO 3 DO ... END_FOR
Index range	Usually starts at 1, check PLC specifics

Key Takeaways

Declare arrays with ARRAY and specify index range and type.

Access elements using square brackets with valid indexes.

Initialize arrays to avoid unpredictable values.

Check your PLC's index base (0 or 1) to prevent errors.

Use loops to process array elements efficiently.