Power management for a computer system and device is accomplished by implementing two separate power modes in a serially-connected device, and selecting from the two power modes depending on the state of a receiver in the device. A signal detector within the receiver is connected to a serial interface port of the device to detect the presence of an input signal. A power controller selects a first power mode for the device when the signal detector detects the input signal and a second power mode for the device when the input signal is not detected by the signal detector.