A restrictive access control system includes a 3-state switch for controlling the operating mode of the system and limiting tuning access to only designated channels. Tuning information is provided to a microprocessor via a control panel keyboard for storage in a nonvolatile memory and for subsequent...http://www.google.es/patents/US4768229?utm_source=gb-gplus-sharePatente US4768229 - Restrictive access control system
