PLC Programming

Appendix C: Data types in CoDeSys
Example: In the case of a variable belonging to a signed subrange type (like i, above), the function
CheckRangeSigned is called; it could be programmed as follows to trim a value to the permissible
range:
FUNCTION CheckRangeSigned : DINT
VAR_INPUT
value, lower, upper: DINT;
END_VAR
IF (value < lower) THEN
CheckRangeSigned := lower;
ELSIF(value > upper) THEN
CheckRangeSigned := upper;
ELSE
CheckRangeSigned := value;
END_IF
In calling up the function automatically, the function name CheckRangeSigned is obligatory, as is the
interface specification: return value and three parameters of type DINT
When called, the function is parameterized as follows:
- value: the value to be assigned to the range type
- lower: the lower boundary of the range
- upper: the upper boundary of the range
- Return value: this is the value that is actually assigned to the range type
An assignment i:=10*y implicitly produces the following in this example:
i := CheckRangeSigned(10*y, -4095, 4095);
Even if y for example has the value 1000, then i still has only the value 4095 aft er this assignment.
The same applies to the function CheckRangeUnsigned: function name and interface must be correct.
FUNCTION CheckRangeUnsigned : UDINT
VAR_INPUT
value, lower, upper: UDINT;
END_VAR
Important: If neither of the functions CheckRangeSigned or CheckRangeUnsigned is present, no type checking
of subrange types occurs during runtime! The variable i could then take on any value between –
32768 and 32767 at any time!
Attention: If neither of the functions CheckRangeSigned or CheckRangeUnsigned is present like described
above, there can result an endless loop if a subrange type is used in a FOR loop. This will happen
when the range given for the FOR loop is as big or bigger than the range of the subrange type !
Example:
VAR
ui : UINT (0..10000);
END_VAR
FOR ui:=0 TO 10000 DO
...
END_FOR
The FOR loop will never be finished, because ui cannot get bigger than 10000.
Also take care of the definition of the CheckRange functions when you define the incremental value of
a FOR loop !
CoDeSys V2.3 10-37