UNION

A UNION is a data structure that usually contains different data types. In a union all components have the same offset, which means they occupy the same memory space.

You declare an union in a DUT object that you create using the command Add > DUT in the context menu of the PLC project tree in the project.

Syntax:

TYPE <union name>:
UNION
    <Variable declaration 1>
    ...
    <Variable declaration n>
END_UNION
END_TYPE

Sample 1:

In the following sample declaration of a union, an assignment to uName.fA also affects uName.nB and uName.nC.

Declaration:

TYPE U_Name:
UNION
    fA : LREAL;
    nB : LINT;
    nC : WORD;
END_UNION
END_TYPE

Instantiation:

VAR
    uName : U_Name;
END_VAR

Assignment:

uName.fA := 1;

Result:

fA = 1
nB = 16#3FF0000000000000
nC = 0

Sample 2:

In the following sample declaration of a union, an assignment to u2Byte.nUINT also affects u2Byte.a2Byte and u2Byte.aBits.

Declaration of the structure ST_Bits:

TYPE ST_Bits :
STRUCT
    bBit1  : BIT;
    bBit2  : BIT;
    bBit3  : BIT;
    bBit4  : BIT;
    bBit5  : BIT;
    bBit6  : BIT;
    bBit7  : BIT;
    bBit8  : BIT;    
END_STRUCT
END_TYPE

Declaration of the union U_2Byte:

TYPE U_2Byte :
UNION
    nUINT  : UINT;
    a2Byte : ARRAY[1..2] OF BYTE;    
    aBits  : ARRAY[1..2] OF ST_Bits;
END_UNION
END_TYPE

Instantiation of the union:

VAR
    u2Byte : U_2Byte;
END_VAR

Assignment 1:

u2Byte.nUINT := 5;

Result 1:

Assignment 2:

u2Byte.nUINT := 255;

Result 2:

Assignment 3:

u2Byte.nUINT := 256;

Result 3:

See also:

• Object DUT