| 摘要 |
A security system for a protected premise has a controller which receives data, over a pair of line conductors, from the addressed transponder of a series of addressable transponders located within the protected premise and connected across the line. To enhance the security of the system and to prevent compromising, the data is sent to the controller in coded form that changes from time to time in accordance with a secret code schedule. A decoding or decrypting system in the controller operates in accordance and in step with the same secret code schedule to decode the received coded data to recover the original information. The coded data may represent an encrypted signature of the addressed transponder which signature is decrypted to check the validity of the replying transponder. As another example, the coded information sent back on the line conductors may relate to a particular condition monitored by the transponder, such as the state of a transducer that detects or indicates that a burglary or robbery is occurring, so that an unauthorized person cannot determine that particular condition merely by reading the data appearing on the line. Without the secret code schedule, the coded data on the line is useless.
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