Interrogation device

摘要

An interrogation unit (1) for connecting into an installation to be monitored via sensing elements (sensors 1-16) is activatable by a remote signal e.g. over a telephone line 23 to supply tone signals indicative of the condition of each of various sensors in the installation, in turn. The signals may be coded. e.g. using pairs of tones, to indicate which sensor is responding and the system may initiate an alarm signal if any sensor changes its condition.