| 摘要 |
PURPOSE:To attain compactness and to improve reliability by outputting a carrier corresponding always 0 or 1 from a modulator-demodulator of each slave station in a polling system 1:N information transmission system and receiving a status change signal from a status signal generator of each slave station by a status change detection circuit to detect the presence of the carrier on a line and discriminate the disconnection of lines. CONSTITUTION:A switch 3 of a modulator-demodulator 1 of a master station is connected to a slave station A. A selection signal of the master station is transmitted to the slave station A. The slave station A receiving the station selection signal via a demodulation section 5b of a modulator-demodulator 5 starts transmitting data to the master station via a modulation section 5a. If a status change takes place in the slave station A other than station selection, an output of a status change signal generator 8 of the slave statioln is logical 1, an input of the modulation section 5a of the modulator-demodulator 5 is logical 1 and an output signal of the modulator-demodulator 5 is logical 1. A status change detection circuit 2 applying polling to each slave station A sequentially finds out a signal of logical 1 from the slave station. The master station transmits the signal of selection end to the slave station A transmitting the data at present to complete data transmission. The switch 3 of the modulator-demodulator 1 isconnected to the slave station A subjected to status change to transmit a station selection signal from the master station to the slave station A.
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