System monitor with innate line security ability

摘要

A system for monitoring a protected premises from a central station over telephone lines employs different audio tones sent back and forth over the lines. A random succession of audio tones from the central station interrogates the protected premises and is returned as a like succession of different tones to the central station where the latter tones are compared with the first to indicate a normal or a trouble condition at the protected premises or on the lines. When an alarm condition exists at the protected premises, still different tones are sent over the lines to cause an alarm alert at the central station. The entire system can be tested from the protected premises, including operation of the trouble and alarm circuits, without producing a trouble or an alarm alert at the central station.