If a variable is declared in VAR CONSTANT, does it occupy memory or is it like #define in 'C'? I have an array in my function block and I would like to give the indeces sensible names so the reader can see what's going on in the code but it seems wasteful to allocate memory varaibles.
Perhaps there is another way to do this?
Thanks
Tony
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See also addressing bits in variables
see below taken from help file.
Addressing bits in variables
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.
Please regard: 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 ''! </n>
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"</type>
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.
Please regard: 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.
Attention: 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:
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Here is a NOOB question.
If a variable is declared in VAR CONSTANT, does it occupy memory or is it like #define in 'C'? I have an array in my function block and I would like to give the indeces sensible names so the reader can see what's going on in the code but it seems wasteful to allocate memory varaibles.
Perhaps there is another way to do this?
Thanks
Tony
Search in the help for "replace constants" with this setting your VAR CONSTANT do no longer more allocate memory.
br
Ralph
See also addressing bits in variables
see below taken from help file.
Addressing bits in variables
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.
Please regard: 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 ''! </n>
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"</type>
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.
Please regard: 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.
Attention: 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: