In integer variables individual bits can be accessed. For this, the index of the bit to be addressed is appended to the variable, separated by a dot. The bit-index can be given by any constant. Indexing is 0-based.
Bitaccess in Direct variables is not allowed.
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.
If the index is greater than the bit width of the variable, the following error message is issued: Index '<n>' outside the valid range for variable '<var>'!
Bit addressing is possible with the following variable types: SINT, INT, DINT, USINT, UINT, UDINT, BYTE, WORD, DWORD.
If the variable type does not allow it, the following error message is issued: "Invalid data type '<type>' for direct indexing"
A bit access must not be assigned to a VAR_IN_OUT variable!
Bitaccess via a global constant:
If you have declared a global constant, which defines the bit-index, you can use this constant for a bitaccess.
The project option 'Replace constants' (category Build) must be activated.
See in the following examples for such a bitaccess on a variable resp. a structure variable:
Declaration in global variables list for both examples:
Variable enable defines which bit should be accessed:
VAR_GLOBAL CONSTANT enable:int:=2; END_VAR
Example 1, Bitaccess on an integer variable:
Declaration in POU:
VAR xxx:int; END_VAR
Bitaccess:
xxx.enable:=true;
The third bit in variable xxx will be set TRUE.
Example 2, Bitaccess on an integer structure component:
Declaration of structure stru1:
TYPE stru1 :
STRUCT
bvar:BOOL;
rvar:REAL;
wvar:WORD;
{bitaccess enable 42 'Start drive'}
END_STRUCT
END_TYPE
Declaration in POU:
VAR x:stru1; END_VAR
Bitaccess:
x.enable:=true;
This will set TRUE the 42. bit in variable x. Since bvar has 8 bits and rvar has 32 bits, the bitaccess will be done on the second bit of variable wvar, which as a result will get value 4.
If a variable, which does a bitaccess on a structure variable with the aid of a global constant, should be displayed correctly in the input assistant, at monitoring in the declaration window and in the intellisense function. Please use pragma {bitaccess} which is shown in the example. Then in addition you get displayed the global constant beyond the respective structure variable during monitoring in the declaration window.
⮫ “Pragma instructions for initialization, monitoring, creation of symbols, bitaccess, linking”
