PURPOSE:To reduce the load factor of a communication line, by switching the circuit state between the time of the start of data transmission from a station and the time, when the station receives data transmitted round on the communication line after completion of transmission, to prevent the transmission of frames transmitted round on the communication line. CONSTITUTION:When a station A is the transmission right control station and a station C is the data frame transmitting station, the station A is switched from a repeat mode MR to a transmission mode MT at a time t1 to transmit a transmission right frame TN and a GA pattern and is set to the transmission completion state, and a switching register is reset at a time t7 when the station A receives an end flag 57 of the frame TN transmitted round on the communication line, and the station A is switched to the mode MR again. Thus, frames fllowing the frame TN which is transmitted round on the communication line are transmitted, and the remainder of this frame TN is not transmitted onto the communication line.
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