A modem interface device is coupled to a computer such that the device is powered via a power signal received from a bus port of the computer, such as a Universal Serial Bus (USB) port, a RS-232 port, or a PS-2 port. The modem interface device has a telephone base connector, a modem connector, a power connector, a two-to-four-wire converter circuit, and a power distribution circuit. The two-to-four-wire converter circuit bidirectionally couples signals between the telephone base connector and the modem connector. The power distribution circuit receives electrical power from the power connector and supplies power to the two-to-four-wire converter circuit.
A self-powered peripheral apparatus is connected upstream to another apparatus via a universal serial bus (USB), wherein one of the conductors of the USB provides a supply voltage to the self-powered peripheral apparatus. One of the two data conductors of the USB is connected to a voltage source of the self-powered peripheral apparatus. The self-powered peripheral apparatus includes a control device for controlling the data conductor supply for supplying the latter only if the supply voltage is present on the supply conductor. The control device includes a circuit for detecting the supply voltage and a logic circuit for controlling the regulator.
