Abstract: A three-state switch includes a first input terminal, a first output terminal, a second output terminal, a pole, a first throw, a second throw, and a third throw. The first input terminal is for receiving a first signal. The first output terminal is coupled to the first input terminal, and for outputting a second signal. The second output terminal is for outputting a third signal. The first throw is for contacting with the pole to have the three-state switch output the second signal, and not output the third signal. The second throw is for contacting with the pole to have the three-state switch not output the second signal and the third signal. The third throw is for contacting with the pole to have the three-state switch output the second signal and the third signal. When the pole is suspended, the first output terminal outputs the second signal.