| 摘要 |
A constant DC current generator powers a communication link including a pair of telephone lines coupled between a protected premises and a central monitor station. The communications link includes means for reversing the DC polarity of the constant current from a predetermined polarity indicative of a normal condition to an opposite polarity in response to a detected alarm condition which occurs in response to a change in the operating parameters of a separate supervisory circuit located at the protected premises. The change in polarity is sensed at the central station and an indication of the alarm condition is presented. DC signalling of the alarm condition as well as the normal condition is thus provided by means of constant current control and DC polarity sensing. Signalling back to the protected premises from the central station is achievable by interrupting the continuity of the communication link and causing the constant current DC generator to sense the open circuit condition. Also when desirable an audio signal is adapted to be transmitted over the communication link from the protected premises to the central station.
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