CTD

VAR_INPUT

VAR_INPUT
   CD   : BOOL; (* Count Down on rising edge *)
   LOAD : BOOL; (* Load Start Value *)
   PV   : WORD; (* Start Value *)
END_VAR

VAR_OUTPUT

VAR_OUTPUT
    Q  : BOOL; (* Counter reached 0 *)
    CV : WORD; (* Current Counter Value *)
END_VAR

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 CV is less than or equal to 0.

Requirements