Abstract: A passive optical network (PON) comprising optical fibers and optical couplers. An central equipment 10 comprises a hub 16, plural modems 17, a mixer 18, and a splitter 19. Carrier waves of a predetermined frequency are assigned to the plural modems 17 and each customer equipment 60. In the downstream, the hub 16 selects each modem 17 (that is, each customer equipment 60) and converts the assigned carrier wave into modulated signals. The modulated signals are mixed in the mixture 18, converted into frequency-multiplexed signals, and transmitted through a first optical coupler 52 to the customer equipment 60. Each customer equipment 60 demodulates the signals by using carrier waves of the assigned frequency. In the upstream, a first optical coupler 53 converts signals from each customer equipment 60 into frequency-multiplexed signals and transmits to the central equipment 10.

Abstract: A computer network which uses a community wiring for TV or wired broadcast and which is hardly influenced by disconnection, short-circuit and noise. The network is a coaxial cable network (1) which can transmit a mixed signal of the received signal of a receiver and a digital signal modulated into an RF signal. This coaxial cable network is provided with a signal input unit (2) for inputting a received signal into the coaxial cable network, and receivers (3a to 3c) and computers (4a to 4c) are connected with the coaxial cable network. Between the coaxial cable network and the computers, there are connected computer connection units (5a to 5c) for modulating a digital signal into an RF signal and for demodulating the RF signal into the digital signal.

Abstract: To provide a vehicular air conditioning system for activating an electric heater with the output coming from an inverter by actuating a relay at a reasonable cost, a relay and an electric heater acting as electric heat source are connected in series between an output line of three output lines of an inverter and the negative side of a vehicular power supply. The electric heater employs a nichrome wire as its heating element and is used for warming the inside of a vehicle compartment. Moreover, the relay is turned ON/OFF after the output voltage of the inverter is set to zero. As a result, the durability of the relay can be improved, and this relay can be implemented by an existing one without newly manufacturing any dedicated relay so that the cost can be lowered.