PURPOSE: To automate the turning on and off operation of the power source of peripheral equipment by detecting whether or not the electric power source of a host computer is turned on and controlling the output of an electric power source unit in accordance with the state detection result.

CONSTITUTION: A termination resistance part 71 is provided in a termination circuit, and connected to an AC switch SW by using a transistor Tr, a resistance R and a capacitor C. An electric power source control signal TERMPWR is a signal line supplying the power to this termination resistance 71. In general, the electric power source control signal TERMPWR is supplied from the host computer, each device can use the SCSI(Small Computer System Interface) as long as the electric power source of the host computer is turned on. The electric power source control signal TERMPWR becomes +5V when the power source of the host computer is turned on, the output control of the power supply unit is performed by detecting this level.