Radio Frequency (RF)
Signals with a repetition rate above audible range, but below the frequencies associated with heat and light.

RAM
Random Access Memory (the equivalent of normal computer memory).

Reader
A device a cardholder presents his access card to that will read the card's encoded data and transmit it to an access control unit (ACU). The ACU then make a decision as to what action to take as a result of that card read.  

Request to Exit (REX)
A device used to disable a door alarm, thus allowing valid exit through an access controlled door. Usually a motion detector but can also be a pushbutton. On each access door equipped with only one card reader (or only an entry reader will be used during certain hours), it is necessary to provide free exit through the monitored door without creating a Door Forced alarm. The request to exit button (usually when electro-magnets are used) or the request to exit motion detector (usually when electric strikes are used) sends a pre-warning signal to the door controller that the door is going too open and that this will be a normal operation of a person exiting. When the door is locked, the access controller supervises the door contact and will report a "Forced door" alarm if the door is opened without using a card. To provide free exit from the premises without causing an alarm, the access controller must know whether the door is opened by someone breaking in or someone exiting. To do this, an "Exit Motion Detector" is installed above the door on the inside. When an individual leaving the premises approaches the door, the detector will send that information to the access controller which will record the event as an authorized exit.

RF/DC
A method of communication without physical contact. Accomplished through the use of radio waves (1997, Auto ID Service Providers).

RFID
Radio Frequency Identification: a technology which allows an object or person to be identified at a distance, using radio waves to energise and communicate with some form of tag or card.
 

RS-232
Data transmission standard using +/- 12 volt transitions to transmit digital data.

RS-485
Data transmission standard using differential voltage levels to send and receive data on a shielded, twisted pair of wire in a multi-drop configuration up to four thousand feet long. Highly immune to electrical noise.

RTE
Request to Exit. A circuit controlled by a push button installed within a secured area which, when pressed, allows a user to exit the area. RTE's should always be redundant, that is, a second method should be used to allow an exit.

RTS
Request to Send. An RS-232 output from the system. It tells a device attached to it that its OK to send data. Generally connected to the CTS of the sending device.

RXD
Receive Data. An RS-232 input that receives data from a transmitting device. This Input must be connected to the TXD output of the transmitting device.