US Patent 7568127 Signal drive de-emphasis control for serial bus

Transmitting a transition between high and low states across a lengthy conductor with a main transmitter to transmit data, providing emphasis with an emphasis transmitter to strengthen the transmission of the transition, transmitting a low-to-high transition to test for the absence of an electronic device coupled to the lengthy conductor, and detecting an occurrence of an overvoltage level indicating the absence of such an electronic device.