(57) Where _monitoring apparatus includes remote units intended to maintain surveillance
of an area for an event such as unauthorised entry into the area, a remote unit fails
to convey to a master unit any information if the remote unit ceases to function.
A master transmitter (11) therefore transmits, over mains electricity supply lines,
frequency-modulated bursts which stimulate the or each remote unit to reply with a
characteristic frequency-modulated burst for each burst from the master transmitter
(11). The master frequency-modulated burst resets a counter (38) in the remote unit.
If a remote unit fails, or detects an alarm situation, the remote unit does not reply,
the counter (38) being frozen. A master receiver and indicator (12) demodulates the
characteristic bursts from the remote unit or units and prevents an alarm (65) from
sounding provided that the characteristic bursts are received at the expected intervals.
Failure of a characteristic burst to arrive results in the alarm (65) being sounded
and a red LED (64, 76, 77 or 78) being energised to indicate which of four remote
units has failed to reply.
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