Power line communication system

1. A power line communication system comprising:

a master station for supplying electric power;

a plurality of slave stations driven by the electric power; and

a plurality of power-supply lines connecting the slave stations to the master station respectively, the power-supply lines including at least one first power-supply line on which a control signal is superimposed and at least one second power-supply line on which no control signal is superimposed,

wherein the control signal is superimposed on the first power-supply line by the master station and also communicated between the master station and at least one slave station connected to the first power-supply line, and

impedance per unit length of the first power-supply line is set smaller than impedance per unit length of the second power-supply line.

2. The power line communication system as claimed in claim 1 , wherein,

each of the power-supply lines comprises two wires forming a power line and a grounding line, and

the first power-supply line is wired so that an interval between the power line and the grounding line of the first power-supply line is smaller than an interval between the power line and the grounding line of the second power-supply line.

3. The power line communication system as claimed in claim 1 , wherein,

each of the power-supply lines comprises two wires forming a power line and a grounding line and interposes a connector arranged in at least one position in a wiring route between the master station and each of the slave stations, the connector having a number of connect pins, and

an interval between the connect pin for the power line and the connect pin for the grounding line in the connector interposed in the first power-supply line is smaller than an interval between the connect pin for the power line and the connect pin for the grounding line in the connector interposed in the second power-supply line.

4. The power line communication system as claimed in claim 1 , wherein,

each of the power-supply lines comprises two wires forming a power line and a grounding line,

the master station includes a base plate provided with wiring patterns of the power-supply lines connecting the master station with the slave stations, and

an interval between the power line and the grounding line in the wiring pattern associated with the first power-supply line is smaller than an interval between the power line and the grounding line in the wiring pattern associated with the second power-supply line.

5. The power line communication system as claimed in claim 1 , wherein,

each of the power-supply lines comprises two wires forming a power line and a grounding line, and

the grounding line of the second power-supply line is formed by a vehicle body.

6. The power line communication system as claimed in claim 1 , wherein,

the master station is formed by an in-vehicle electronic control unit, and

the slave stations are formed by either loads or load control units, which are driven by the electric power supplied from the in-vehicle electronic control unit through the power-supply lines.

7. The power line communication system as claimed in claim 6 , wherein,

the first power-supply lines comprises two wires forming a power line and a grounding line, and

the second power-supply line comprises a power line and a grounding line using a vehicle body.