| 摘要 |
PROBLEM TO BE SOLVED: To provide a data transmitter-receiver maintaining a connected communication line by periodically transmitting dummy data for connection maintenance from a centralized monitor center side to a terminal device in a non- communication state. SOLUTION: A terminal device 17 has a function that disconnects a connected communication line when received data do not exist for a prescribed time while the line is connected. The centralized monitor center is connected to a plurality of terminal devices 17 through the communication line 16. The data transmitter- receiver 10 is used by the centralized monitor center side and transmits and receives data to/from the plurality of terminal devices 17 through the line 16. The maintenance data transmitting means 108a of the transmitter-receiver 10 transmits line maintenance data for maintaining the connected line with a terminal device 17 in a cycle shorter than that of a monitoring time on the terminal device 17 side.
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