RD is rotating detection. A fan with rotating detection function has a separate output signal to detect if the fan is working properly. If it is rotating normally, the output signal appears LOW. If it stops rotating or malfunctions, the output signal appears HIGH. This function allows the computer or electric equipment to detect the fan rotation status. Also, the high output signal can help conduct malfunction alarm. A correct warning and self-protected message are the essential elements for the secured system of the wise products.