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Acronym for Random Access Memory, which is a type of device memory with read-write capabilities. This type of memory is volatile implying it loses contents
on power loss.




A relay is a electromechanical switch made up of an electromagnet and a set of contacts. Relays open and close under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. Simple relays consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a moveable iron armature, and a set, or sets, of contacts. When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energised, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open.


Example: LTR Relay (right)


  LTR Relay


Relay, Latching

A latching relay has two relaxed states (bistable). When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remnant core.


In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage.


Example: LTR Relay


Relay, Solid State (SSR)

A solid state relay (SSR), sometimes called electrically held relay, is a solid state electronic component that provides a similar function to an other relays but does not have any moving components. Compared to latching relays, they may be falsely triggered by transients.


Relay Panel

A relay panel is an enclosure used mount, separate and control lighting relays.



Retrofitting refers to the addition of new technology or features to older systems. Example: Upgrading a fixture, room or building by installing new lighting fixtures, parts or equipment.



Acronym for Read-Only Memory. A type of memory that maintains its contents after a power loss.


Rotary Dimmers

A dimming control that provides full-range, manual control of a light source. Some are equipped with a push-button operation that allows you to turn the light on and off and return to the previous lighting level.



Process of wiring a home and installing rough-in sections of the fixtures and the back boxes of electrical devices; precedes the finish work.


A router is a device used to join multiple networks or network types.

A BACnet router is a protocol message routing device that links dissimilar network types (i.e., Ethernet to EIA-485, or EIA-485 to EIA-232) and passes BACnet messages among the dissimilar network types without changing the message contents.



RS-232 is a standard for serial binary data interconnection between data terminal equipment (DTE) and a data communication equipment (DCE). It is commonly used in computer serial ports.


RS-485 (or EIA-485)

EIA-485 is use in programmable logic controllers in order to implement proprietary data communications. Since it is differential, it resists electromagnetic interference from motors and welding equipment.


Run on Request

A control strategy that optimizes the runtime of a source piece of equipment that supplies one or more receiving units - such as an air handler unit supplying zone terminal units with heating, cooling, ventilation, or similar service. Source equipment runs only when needed, not on a fixed schedule.

The source equipment runs when one or more receiving units request its services. An operator determines how many requests are required to start the source equipment.

For example, if all the zones in a building are unoccupied and the zone terminal units do not need heating or cooling, the AHU will shut down. However, if a zone becomes occupied or needs cooling, the terminal unit will send a run request to the AHU to initiate the start-up sequence. If this AHU depends on a central chiller, it can send a run request to the chiller.

The run on request algorithm also allows an operator to schedule occupancy for individual zones based on the needs of the occupants without having to adjust the schedules of related AHUs and chillers.