Addressing Bits in Variables

Overview

In integer variables, individual bits can be accessed. For this purpose, append the index of the bit to be addressed to the variable and separate it by a dot. You can give any constant to the bit index. Indexing is 0-based.

Syntax

<variablename>.<bitindex>

Example

a : INT;
b : BOOL;
...
a.2 := b;

The third bit of the variable a will be set to the value of the variable b, this means that variable a will equal 3.

If the index is greater than the bit width of the variable, the following message will be generated:

'Index '<n>' outside the valid range for variable '<var>'!'

Bit addressing is possible with variables of the following data types:

oSINT

oINT

oDINT

oUSINT

oUINT

oUDINT

oBYTE

oWORD

oDWORD

If the data type does not allow bit accessing, the following message will be generated:

'Invalid data type '<type>' for direct indexing'.

Do not assign bit access to a VAR_IN_OUT variable.

Bit Access Via a Global Constant

If you have declared a global constant defining the bit index, you can use this constant for a bit access.

Example for a bit access via a global constant and on a variable:

1. Declaration of the global constant in a global variable list

The variable enable defines the bit that is accessed:

VAR_GLOBAL CONSTANT
    enable:int:=2;
END_VAR

2. Bit access on an integer variable

Declaration in POU:

VAR
    xxx:int;
END_VAR

Bit access:

xxx.enable := true; (* -> the third bit in variable xxx will be set TRUE *)

Bit Access on BIT Data Types

The BIT data type is a special data type which is only allowed in structures. For further information, refer to Bit Access in Structures.

Example: Bit access on BIT data types

Declaration of structure

TYPE ControllerData :
STRUCT
    Status_OperationEnabled : BIT;
    Status_SwitchOnActive : BIT;
    Status_EnableOperation : BIT;
    Status_Error : BIT;
    Status_VoltageEnabled : BIT;
    Status_QuickStop : BIT;
    Status_SwitchOnLocked : BIT;
    Status_Warning : BIT;
END_STRUCT
END_TYPE

Declaration in POU

VAR
    ControllerDrive1:ControllerData;
END_VAR

Bit access

ControllerDrive1.Status_OperationEnabled := TRUE;