PURPOSE: To reduce the number of parts by connecting the input terminal of a second reception means in each unit to the output terminal of a first transmission means in common and connecting the output terminal of a second transmission means in each unit to the input terminal of a first reception means.

CONSTITUTION: When an input T1 of a unit designating circuit 11 is set to the high level and the other inputs T2 to T4 are set to the low level in the case of sending of a control signal to a unit U1, positive power 10 is supplied to the input terminal of a reception circuit 25, When data is given to the input terminal of an inverter 16 in this state, the same data is outputted from the output terminal of the circuit 25. Thereafter, the input terminal of an inverter 27 is set to the high level and detection data is supplied to the other input terminal in the unit U1. Thus, a signal is transmitted to a reception circuit 17, and detection output is outputted to its output terminal. Inputs of inverters 27 in the other units U2 to U4 are in the low level during transmission of the unit U1, and the interference of transmission signals is avoided.