Provided by standard library.
Function block decrementer:
The input variables CD and LOAD as well as the output variable Q are type BOOL, the input variable PV and the output variable CV are type WORD.
When LOAD_ is TRUE, the counter variable CV will be initialized with the upper limit PV. If CD has a rising edge from FALSE to TRUE, CV will be lowered by 1 provided CV is greater than 0 (i.e., it doesn't cause the value to fall below 0).
Q returns TRUE when CVis equal 0.
Declaration example
CTDInst : CTD ;
Example in Function Block Diagram
Example in Structured Text
CTDInst(CD:= VarBOOL1, LOAD:=VarBOOL2 , PV:= VarINT1);
VarBOOL3 := CTDInst.Q ;
VarINT2 := CTDInst.CV;