Abstract: A plurality of air conditioning system control units are connected to each other through a communication line (20). Discharge resistors (R1, R2) are provided for discharging residual charges of the communication line (20). The control unit has a power superposition circuit (50) for applying a direct-current voltage and outputs an identification number signal from the power superposition circuit (50) to the communication line (20). The control unit further has a polarity select circuit (60) having a negative-polarity resistance characteristic that current increasingly flows as an applied voltage is lowered, and a voltage discriminating circuit (70) for detecting the terminal voltage of the polarity select circuit (60). When detecting the voltage of the communication line (20), the control unit determines whether its own control unit is a master unit or a slave unit according to output signals from the voltage discriminating circuit (70) and a polarity discriminating circuit (3C).

Abstract: A signal transmission/reception device of an air conditioner is provided with a plurality of control groups (8A, 8B, . . . ). The control groups (8A, 8B, . . . ) are each composed so that an outdoor control unit (5) and an indoor control unit (6) are connected to each other through a group communication path (7) so as to be capable of signal transmission and reception therebetween. The outdoor control units (5,5, . . . ) are connected to each other through a centralized communication path (10) so as to be capable of signal transmission and reception among them. The respective group communication paths (7) are connected at the respective outdoor control units (5,5, . . . ) to the centralized communication path (10) thereby forming a single signal transmission/reception network (NW). Further, the signal transmission/reception network (NW) is connected to a centralized controller (9) for subjecting the outdoor units (3,3, . . . ) and the indoor units (4,4, . . . ) to centralized control.