Patent ID: 7504748

Claim:
A relay switch circuit, comprising: a first data port coupled to a first pair of wires carrying a first pair of differential signals; a second data port coupled to a second pair of wires carrying a second pair of differential signals; a power supply terminal receiving a power supply voltage signal; an energy detect circuit coupled to measure the energy of incoming signals on the first data port and the second data port, the energy detect circuit generating a first energy detect signal and a second energy detect signal indicative of an energy level of the incoming signals on the respective first and second data ports, the first and second energy detect signals having a first state when an energy level exceeding a predetermined threshold is measured on the incoming signals at the respective data port and a second state otherwise; a control signal generator circuit coupled to receive incoming signals from the first data port and the second data port, the power supply voltage signal, and the first and second energy detect signals and generate one or more control signals in response, the control signal generator deriving power for its operation from the incoming signals; and a switch circuit coupled to the first data port and the second data port and being controlled by the one or more control signals, the switch circuit being operated to open to isolate the first data port from the second data port or close to transmit differential signals onto and receive differential signals from the first and second data ports, wherein the control signal generator generates the one or more control signals to cause the switch circuit to open when the power supply voltage signal indicates a valid power supply voltage and to open when the power supply voltage signal indicates an invalid power supply voltage and one of the first and second energy detect signals has the first state, and the control signal generator generates the one or more control signals to cause the switch circuit to close when the power supply voltage signal indicates an invalid power supply voltage and both of the first and second energy detect signals have the second state.