Abstract: A tamper-resistant security system for and method of operating and installing the same are disclosed for electronically controlling an access device such as a garage door. A manual keyboard encoder mounted exteriorly on a garage door jamb is operative for generating coded electrical signals, including a predetermined sequence of coded electrical signals required to move the garage door from a closed to an open position. A control-processor unit mounted within the interior garage area is operative for detecting the coded signals, and for moving the garage door from the closed to the open position in response to detection of the predetermined coded signal sequence. A single pair of electrical conductors interconnects the encoder and the control-processor unit. The transmission of the predetermined coded signal sequence over the single pair of conductors deters intruders from compromising system security, and also simplifies the installation.