| 摘要 |
In an alarm system utilizing a bidirectional wired television system, a large number of subscribers are combined in a plurality of group units, whereby the subscribers in each group unit are assigned with specified frequencies which are common to the respective group units but are different for the respective subscribers in the group unit, and the respective group units are assigned with another specified frequencies which are different for the respective group units. The group units are sequentially polled by interrogating signals comprising said another specified frequencies and applied from a central station, whereby when any subscriber in the polled group has an abnormal condition at its location, the subscriber answers to the polling by a signal of the specified frequency assigned to it. The central station discriminates and displays the answering subscriber in accordance with the combination of the frequency of the received answer signal and the specified frequency being generated for polling at the time that the answer signal was received.
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