Array and Enumeration in Structured Text
What is an array:
An array can be defined as an ordered collection of items indexed by contiguous integers. An array is a data structure that can store a number of variables of the same data type in sequence. These similar elements could be of type int, float, double, char, Function Block, etc.
Arrays are used when there is a need to use many variables of the same type. It can be defined as a sequence of objects which are of the same data type. It is used to store a collection of data, and it is more useful to think of an array as a collection of variables of the same type. Arrays can be declared and used. A programmer has to specify the types of elements and the number of elements that are required by an array. This is called a single-dimensional array. The array size should be an integer constant and greater than zero.
One-, two-, and three-dimensional fields (arrays) are supported as elementary data types. Arrays can be defined both in the declaration part of a POU and in the global variable lists.
In the following statement, we declare an array of 5 integers and initialized them with 1, 2, 3, 4, and 5 respectively.
ArrayInt : ARRAY [1..5] OF INT := [1,2,3,4,5];
Look at the syntax ARRAY[lower..upper] OF allows us to define any lower or upper bounds. As compared to C#, our lower bound doesn’t have to be zero. The total number of elements in the array is upper – lower + 1. We could have declared the above statement as the following as well
ArrayInt : ARRAY [2..6] OF INT := [1,2,3,4,5];
In this case, we have accessed first number by by the following way.
firstOne := ArrayInt[2]; // 1
Now we shall describe, how we can declare an array of FUNCTION_BLOCK
Sample function block
FUNCTION_BLOCK MotorAC VAR_INPUT machineName : WSTRING :="Type A"; END_VAR VAR_OUTPUT machineAlarm : BOOL := FALSE; END_VAR VAR maxRotationValue : REAL := 0.0; rotationPerSeconds : INT := 0; myTimer : TON ; startMachine : BOOL := FALSE; END_VAR
We declare and initialize those array of 3 instances in the following way.
ArrayMotorAc : ARRAY [1..3] OF MotorAC := [ (machineName:= "Motor_one"), (machineName:= "Motor_two"), (machineName:= "Motor_three") ];
Initialization can be done in the following way.
FOR index := 1 TO number_of_motor DO
ArrayMotorAc[index].InitMachine();
END_FOR;
Arrays with variable length
We can pass pointer of data to a method and calculate the index as shown in the following example.
FUNCTION FuncArrayTestByPointer : INT
VAR_INPUT
pData : POINTER TO INT;
nSize : UDINT;
END_VAR
VAR
pIndex : POINTER TO INT;
nUpperIndex : UDINT;
nIndex : UDINT;
END_VAR
FuncArrayTestByPointer := 0;
nUpperIndex := nSize / SIZEOF(pIndex^);
IF (nUpperIndex > 0) THEN
FOR nIndex := 0 TO (nUpperIndex - 1) DO
pIndex := pData + (nIndex * SIZEOF(pIndex^));
FuncArrayTestByPointer := FuncArrayTestByPointer + pIndex^;
END_FOR
END_IF
Example usage
array : ARRAY[2..6] OF INT := [16, 34, 4, 43, 35];
Sum : INT;
Sum := FuncArrayTestByPointer(ADR(array), SIZEOF(array));
Newer way use the variable array as defined in 3rd Edition of IEC 61131-3
In function blocks, functions or methods, arrays with variable length can be declared in the declaration section VAR_IN_OUT. The operators LOWER_BOUND and UPPER_BOUND can be used to determine the index limits of the array that is actually used at runtime. LOWER_BOUND returns the lower limit, UPPER_BOUND returns the upper limit.
FUNCTION FuncArrayTestByPointerNewWay : INT
VAR_IN_OUT
arrData : ARRAY[*] OF INT;
END_VAR
VAR
nIndex : DINT;
END_VAR
FuncArrayTestByPointerNewWay := 0;
FOR nIndex := LOWER_BOUND(arrData, 1) TO UPPER_BOUND(arrData, 1) DO
FuncArrayTestByPointerNewWay := FuncArrayTestByPointerNewWay + arrData[nIndex];
END_FOR
Example usage
array : ARRAY[2..8] OF INT := [36, 34, 4, 43, 35, 2, 65];
Sum : INT;
Sum := FuncArrayTestByPointerNewWay(array);
PLC Array index and HMI array index
We declare an array like the following.
ArrayInt : ARRAY [5..9] OF INT := [10,2,3,4,5];
myvar : INT := -1;
END_VAR
//body
myvar := ArrayInt[5];
We have mapped those variables and want to display the ArrayInt and myvar on the HMI, see the mapping in the following picture.
Figure 01: ArrayInt[5] is mapped as ArrayInt[0] in HMI
As we see in the HMI it is mapped from 0 to 4 and in the PLC it is declared as 5 to 0. This is the default behavior. We can read the first item by the following code. Note that we are using ArrayInt[0] not 5
async function asyncFunctionCall() {
try {
const myvar = await ReadPLCVariable('%s%PLC1.MAIN.ArrayInt[0]%/s%');
var myTextBlockControl = TcHmi.Controls.get('TcHmiTextblock_1');
myTextBlockControl.setText(myvar);
}
catch(error) {
console.log(`ErrorCode = ${error.ErrorCode}`);
console.log(`ErrorText = ${error.ErrorText}`);
}
}
asyncFunctionCall(); 
Figure 02: Second text block shows the number as read by the above code
Tips
=> Changing index in HMI
Index in PLC can have any index, we can adjust the index HMI in the following way.
Some info related to variable length in array.
LOWER_BOUND(arrData, 1) returns as lower bound of the supplied array, and its type is DINT. We can know the number of items by substracting lower bound from the upper bound.
References:
Download the sample from the link given above.
Next, let’s try to understand PLC data type at https://www.hemelix.com/plc/data-type-used-in-plc-programming/
Ask questions related to Hemelix sample code and design at Google group https://groups.google.com/g/hemelix
