Secure entry system with radio communications

DETAILED DESCRIPTION In the century since radio science was developed, radio signal transmission has found myriad applications.
Some have been in the field of electronic security systems.
A familiar example is a garage door opener.
A radio transmitter is used to relay instruction signals to a receiver unit, causing the receiver unit to activate a door opening mechanism.
More sophisticated access control devices operate in conjunction with a personal access card that includes a battery, a microprocessor, a receiver and a transmitter.
The access control device periodically broadcasts an interrogation signal, which the card receives when it is brought into close proximity thereto.
In response, the card transmits an RF reply signal to the access control device, authorizing access to the secured area.
French patent publication 2,604,808 and European patent publication EP 393,784 are illustrative of such systems.
Radio data transmission is sometimes used in home security systems to relay data from motion, continuity, vibration or other detectors to a central monitoring unit.
Radio frequency energy is sometimes used to convey operating power to an otherwise unpowered unit.
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4,851,652, for example, illustrates a secure entry system in which a passive lock unit is powered from RF energy radiated from an associated key card.
The lock also receives data from the key by modulation of the same RF energy.
French patent publication 2,542,792 shows such a system in which a passive key is powered by an RF signal that is coupled to it by an associated lock.
Radio is sometimes used outside the secure entry field to relay reprogramming instructions to remote units.
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4,525,865 and 4,910,510, for example, disclose pagers and other radios whose operational characteristics can be reprogrammed remotely by radio.
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5,020,135 discloses such a system that also features remote memory dumps and diagnostics from radio transceivers using radio signals.
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4,543,955 and 4,958,632 disclose cardiac pacemakers and other implantable devices that can be reprogrammed via use of radio