Constant Variables

The constant variable (VAR_CONSTANT acc. to the IEC 61131-3) has a value that cannot be changed by the running application (read-only access). Constant variables can be declared in the variables table or data list by selecting the 'Constant' attribute of a variable (see also the following rules) or in data type worksheets using a VAR CONSTANT...END_VAR declaration block (see below).

The following rules apply for constant variables:

To be specified as literal acc. to the IEC 61131-3 standard. Expressions are not supported.
Allowed in the machine controller Data List for 'Global' declarations. The global constant variable must also be defined in the POU as 'External' variable with the 'Constant' attribute set. Otherwise a compiler error is generated.
Allowed in local variables table of POUs for 'Local', 'External', and 'Program' variables.
('Program' variables with set data direction ('I/O' property) cannot be 'Constant' variables.)
When declaring an 'External' variable which relates to an already existing constant global variable, the 'Constant' attribute is set automatically in the local variables table.
Can be used in data type worksheets (see below) as:
- boundaries and initialization values in arrays.
- initialization values in structures.
- initialization values and predefined values in enumerations.
Can be used as initialization values in variables tables and data lists. Observe the following:
- The constant, that is used as initial value of a variable in a POU, must be declared in the POU variables table as 'Local' or 'External'.
- The constant, that is used as initial value of a global variable in the resource's data list (PLC node), must be declared in the data list as 'Global'.
Not allowed for global input variables (mapped to an input process datum in the Data List): While 'Constant' is selected, no process datum can be selected, and if an input process datum is already assigned, the 'CONSTANT' checkbox is inactive.
The 'Constant' attribute is not allowed in combination with the variable's 'Retain' attribute and vice versa.
Not allowed in the Safety PLC Data List and in safety-related variables tables.
Not allowed in the machine controller Data List for exchange variables (as they are connected to a standard Safety PLC variable).
Not allowed for ports.
Supported by role picker and Intellisense.
Allows partial access to ANY_BIT data types in code worksheets.
Example: myBool := MY_CONST_WORD.X0;
See the topic "Partial Access to ANY_BIT Variables" for details.
In online mode:
- The initialization value is displayed.
- Constant variables cannot be edited or added to the WATCHES window or LOGIC ANALYZER window.
- Overwriting or forcing is not possible.

Declaration in data type worksheet (local constant)

Constant variables are declared in the data type worksheet using a VAR CONSTANT...END_VAR declaration block (see the following example). The initialization value of a constant variable must be a literal (expressions for the initialization value are not supported).

Note
Constants can only be used in the data type worksheet where they are declared.