1.  Field of the Invention
The present invention relates to a power supply controlling system having a receiving device, and a transmitting device connected to an information processing apparatus outputting a video signal and a power supply device supplying electric power to the information processing apparatus. 
2.  Description of the Related Art
Conventionally, there has been known a technique that superimposes a signal for power supply control other than a video signal on the video signal, and transmits the superimposed signal to a remote place (see Japanese Laid-Open Patent Publication Nos.  07-203491, 05-83689, and 11-340974, for example). 
In the technique, a controlling device is connected to a controlled device with a dedicated coaxial cable or a digital interface line. 
Since in the technique of Japanese Laid-Open Patent Publication Nos.  07-203491, 05-83689, and 11-340974, the controlling device is connected to the controlled device with the dedicated coaxial cable or the digital interface line, the connection between the controlling device and the controlled device suffices with one cable.  However, when the controlling device is connected to the controlled device with a cable to which standards have been decided, the connection between the controlling device and the controlled device may be insufficient in one cable. 
For example, when in the a power supply controlling system of FIG.  9, a local device is connected to a remote device with a Cat 5 (category 5) cable having a length of hundreds of meters, i.e., a LAN cable, the local device outputs RGB signals received from a personal computer as differential signals, and hence six signal lines are occupied by the RGB signals among eight signal lines included in one LAN cable.  A serial signal from a mouse, a keyboard, or a printer is output from the remote device to the local device as differential signals, so that two signal lines are occupied by the serial signal among the eight signal lines.  Therefore, an unused signal line is lost in the eight signal lines. 
In this state, to transmit an ON/OFF signal of electric power from the remote device to a remote power supply via the local device, it is necessary to connect a new LAN cable between the local device and remote device. 
As a result, the number of LAN cables connected between the local device and remote device increases, so that wiring work of the LAN cables is complicated, and cost performance deteriorates.