PURPOSE:To reduce the communication time even for a long text and to decrease the access time up to the communication start by providing a line supervisory circuit having a priority to a terminal device unit so as to apply a transmission request positively. CONSTITUTION:Plural terminal units connected to a center unit used for the CAI system applying 2-wire semi-duplex data communication gives a reception control signal from a reception switching signal terminal 31' of a CPU control section 9' to await the signal input. In case of the priority communication mode, a read signal is sent from a terminal number read control terminal 40', after the time set in a terminal address setting switch circuit 34' is delayed for the start of line supervisory time, the absence of the input signal for a prescribed time is confirmed to allow the transmission. During the transmission, an interruption signal from a data collision detection signal terminal 30' is checked to prevent generation of dead lock at communication.
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