Introduction

When an assignment is performed, the current value of a single or multi-element variable is replaced by the result of the evaluation of the expression.

An assignment consists of a variable specification on the left side, followed by the assignment operator :=, followed by the expression to be evaluated.

Both variables (left and right sides of the assignment operator) must have the same data type.

Arrays are a special case. After being explicitly enabled, assignment of two arrays with different lengths can be made.

Assigning the Value of a Variable to Another Variable

Assignments are used to assign the value of a variable to another variable.

The instruction 

A := B ;

is used, for example, to replace the value of the variable A with the current value of variable B. If A and B are elementary data types, the individual value of B is passed to A. If A and B are derived data types, the values of all B elements are passed to A.

Assigning the Value of a Literal to a Variable

Assignments are used to assign a literal to variables.

The instruction 

C := 25 ;

is used, for example, to assign the value 25 to the variable C.

Assigning the Value of an Operation to a Variable

Assignments are used to assign to a variable a value which is the result of an operation.

The instruction

X := (A+B-C)*D ;

is used, for example, to assign the result of the operation (A+B-C)*D to the variable X.

Assigning the Value of an FFB to a Variable

Assignments are used to assign a value returned by a function or a function block to a variable.

The instruction

B := MOD(C,A) ;

is used, for example, to call the MOD (Modulo) function and assign the result of the calculation to the variable B.

The instruction

A := MY_TON.Q ;

is used, for example, to assign the value of the Q output of the MY_TON function block (instance of the TON function block) to the variable A. (This is not a function block call) )

Multiple Assignments

Multiple assignments are a supplement to IEC 61131-3 and must be enabled explicitly.

Even after being enabled, multiple assignments are NOT allowed in the following cases:

  • in the parameter list for a function block call

  • in the element list to initialize structured variables

The instruction

X := Y := Z

is allowed.

The instructions

FB(in1 := 1, In2 := In3 := 2) ;

and

strucVar := (comp1 := 1, comp2 := comp3 := 2) ;

are not allowed.

Assignments between Arrays and WORD-/DWORD Variables

Assignments between arrays and WORD-/DWORD variables are only possible if a type conversion has previously been carried out, e.g.:

%Q3.0:16 := INT_TO_AR_BOOL(%MW20) ;

The following conversion functions are available (General Library, family Array):

  • MOVE_BOOL_AREBOOL

  • MOVE_WORD_ARWORD

  • MOVE_DWORD_ARDWORD

  • MOVE_INT_ARINT

  • MOVE_DINT_ARDINT

  • MOVE_REAL_ARREAL