The Down Counter (CTD) is an instruction that counts false-to-true rung transitions in ladder logics.
Down Counter (CTD)
Rung transitions can be caused by events occurring in the program such as parts traveling past a detector or actuating a limit switch.
When rung conditions for a CTD instruction have made a false-to-true transition, the accumulated value is decremented by one count, provided that the rung containing the CTD instruction is evaluated between these transitions.
The accumulated counts are retained when the rung conditions again become false.
The accumulated count is retained until cleared by a reset (RES) instruction.
Down Counter Example
Accumulated count is reset only by the (RES) instruction.
The counter will decrement the accumulator value even after a 0 count is reached.
