Constant values are values that remain the same each time the program executes, such as pi, time intervals, fixed text string etc. For example, if a program needs a point to change state after a 2 minute pause, a constant can be used to define the 2 minute time period. Although the time period does decrease from 120 seconds to 0 seconds as the program executes, it is still represented by a constant as the time value is fixed as 120 seconds when the program is executed.
Constant variables need to be separated from internal variables, direct variables and function blocks in the VAR lists. This means that if you have a program that uses constants, internal variables, direct variables, and function blocks, you will have at least 4 VAR lists—1 for constants, 1 for internal variables, 1 for direct variables, and 1 for function blocks.
To enter a constant value, you need to use this format:
VAR CONSTANT
<Constant Name> : <Value Type> = <Value including code characters>;
END_VAR
VAR CONSTANT
Timer:TIME := T#60s;
END_VAR
Where VAR CONSTANT instructs the program that the value is a constant, read only value. Timer is the name of the constant, and TIME is the value type. T# is the code for time values (see Time and Date Values) and 60s is 60 seconds. The s for seconds is lower case, as are time units. END_VAR is used to end the list of constants. If there are other VAR lists after the constant list, the END_VAR should not have a semi-colon at the end. However, if the constant list is the last list of variables for your program, you have to add a semi-colon to the end, like this: END_VAR;
Unlike internal and direct variables, constants can only be of a certain data type. Each constant value has to be one of the Built-In Data Types.