Patent ID: 7010703

Claim:
A bus power-supply device in a node for connection to a serial bus, said bus power-supply device structured to supply power from a power-supply voltage of a node of a proceeding stage to a node of a next stage through the serial bus connected to said node by a physical layer and a plurality of connectors conductive to each other of the node, comprising: a power-supply circuit which converts said power-supply voltage into a DC voltage for said serial bus and outputs said DC voltage; a converter which converts a DC voltage output from said power-supply circuit into a DC voltage for said physical layer; a voltage detection unit which detects said power-supply voltage being supplied or not being supplied to said power-supply circuit; a first means connected between said power-supply circuit and said converter for supplying a DC voltage outputted from said power-supply circuit to the serial bus, and for preventing application to said power-supply circuit of a DC voltage from the serial bus; a second means connected between said converter and the serial bus for supplying a DC voltage outputted from said power-supply circuit to the serial bus, and for preventing application to said converter and said power-supply circuit of a DC voltage from said serial bus; and a switch which is connected in parallel with said second means and controlled by an output signal outputted from said voltage detection unit; wherein when an output voltage of said power-supply circuit is detected, said switch is turned off, and when an output voltage of said power-supply circuit is not detected, said switch is turned on.