| 摘要 |
A change in state is detected at an output port terminal of a video graphics adapter card when an analog monitor is connected or disconnected. A negative current source causes a negative voltage to be developed at the output port terminal during a blanking interval of a color drive signal. A voltage comparator having one input coupled to the output port terminal and another input coupled to a voltage reference provides an output during the blanking interval that serves as an indication of the state (i.e., of whether the monitor is connected), to an operating system executing on a computing system in which the video adaptor card is installed. The negative voltage input to the comparator changes due to the input impedance of the monitor, enabling the comparator to detect the change in state.
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