Direct memory accessed is used to perform database operations between two or more machines. Accordingly, power of the displaying apparatus is controlled to be on or off in response to a power-on or off state of the computer main body, and allows the power thereof to be consumed at a minimum level under the condition of a stand-by power mode. When a variation of the response current signal or a power variation is stable, the control module sets up the stable test voltage signal as an input voltage to the post-stage power-receiving device.