| 摘要 |
At a central location a telephone link is established betwen central and remote locations. A decoder at the remote location produces a verification signal in response to an action performed by the person whose presence is to be verified and transmits the verification signal through the telephone line to the central location. The remote telephone is uncoupled from the telephone line while the decoder is coupled to the latter. A normally open switch and a normally closed switch are used for the coupling and uncoupling. The decoder includes a power supply sourced from the telephone line to produce an output voltage when the normally open switch is closed, in response to the decoder times out for a set period. The switches are responsive to this time out period.
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