An improved peripheral device for a computer wherein the improvement comprises circuitry which derives power for the peripheral device from the control and data signal lines interconnecting the peripheral device to the computer. Preferably, the peripheral device is a mouse which uses a microprocessor to translate signals received from a pair of encoders. Power for the microprocessor is derived from a pair of control lines through the use of a voltage reference setting zener diode, a polarity maintaining diode and a current limiting resistor. For an RS-232-type interface, the TXD signal is used for ground and the RTS signal is used for positive voltage. The computer is programmed to provide a positive signal on the RTS line and a negative signal on the TXD line.