| 摘要 |
A system for monitoring conditions of instrumentalities at subscriber premises. A central control unit, scanner (18), is connected across a set of subscriber loops (14-16).At each subscriber's premises, an individual subscriber terminal unit STU (22-24) is connected across the particular subscriber's loop (14-16). To each STU (22-24) are connected the various instrumentalities which are to be monitored by the system, e.g., signal sources (41-43) such as fire alarms, burglar alarms, etc. A signal below the audible range (low tone) is produced at the STU (22-24), and transmitted to the scanner (18), over the telephone loop if, but only if, all the instrumentalities at the particular subscriber premises are in a given state (e.g. non-alarm). If not, low tone is stopped. Such stoppage is sensed by the scanner (18), and causes immediate transmission of FSK modulated signals in the upper part of the audible range from the scanner (18) to the corresponding STU (22-24), whether the associated telephone is on-hook or off-hook at the time.The reply from the STU (22-24), to such transmission provides information about the reason for the stoppage, i.e. what is the source of the alarm. This system provides efficient monitoring of subscriber instrumentalities without using signals audible during normal telephone usage or special filtering in the telephone network. |
