A circuit for use in a first display device to facilitate communication with a second display device is provided. The first display device and the second display device each include a connector compliant with a digital display interface standard. The circuit includes a digital display interface circuit and a control unit. The digital display interface circuit is used for transmission or receiving of video data according to the digital display interface standard, the digital display interface circuit for being connected to the connector of the first display device. The control unit is configured to transmit at least one first communication signal through at least one first pin of the connector of the first display device to communicate with the second display device, wherein the at least one first communication signal is non-standard with respect to the digital display interface standard.