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http://www.extron.com/company/article.aspx?id=uedi d&version=print

Hot plug detection is not supported for VGA, but is supported in digital interfaces including DVI, HDMI, and DisplayPort. For these interfaces, the display device will supply a voltage on an HPD - Hot Plug Detect pin, to signal to the video source device that it is connected. The absence of a voltage on the HPD pin indicates disconnection. The video source device monitors the voltage on the HPD pin and initiates EDID requests as it senses incoming voltage.