A monitoring system includes a controller, a number of input and output devices, a number of sensors, and a number of electronic devices. Each input and output device includes a switching circuit, a first input circuit, a second input circuit, and a connector. The connector is connected to a sensor or an electronic device. The controller controls the switching circuit to receive a detecting signal of the sensor or supply power to the electronic device. The first input circuit and the second input circuit are connected between the switching circuit and the controller.