RAID: Redundant Array of Independent Disks
Redundant Arrays of Independent Disks (RAID) is a type of disk drives with two or more drives in combination for increasing data integrity, fault tolerance, throughput or capacity and performance. RAID provides seceral methods of writing data across/to multiple disks at once. RAID is one of many ways to combine multiple hard drives into one single logical unit. Thus, instead of seeing several different hard drives, the operating system sees only one. RAID is typically used on server computers, and is usually implemented with identically-sized disk drives. With decreases in hard drive prices and wider availability of RAID options built into motherboard chipsets, RAID is also being found and offered as an option in higher-end end user computers, especially computers dedicated to storage-intensive tasks, such as video and audio editing.

RAM Cache
RAM cache, also known as Level 2 (L2) cache, is the cache memory external to the CPU. RAM cache memory resides on a separate chip from the microprocessor chip. RAM cache contains a subset of the contents of main memory. The design of the memory and L2 cache is a significant way designers differentiate their systems.

RAM Disk
A RAM disk, ramdisk or RAMdrive is a segment of active computer memory, RAM, which is being used as secondary storage, typically a role for a disk drive. Access times are greatly improved, because RAM disks are approximately a thousand times faster than hard disk drives. However, durability of data through power loss is completely absent, because they are made of normal volatile RAM. As a result, RAM disks lose their contents once the computer is turned off. RAM disks are great places to store temporary data or to hold uncompressed programs for short periods.

RAM: Random Access Memory
Random Access Memory (RAM) is a type of volatile computer storage whose contents can be accessed in any (i.e., random) order. This is in contrast to sequential memory devices such as magnetic tapes, discs and drums, in which the mechanical movement of the storage medium forces the computer to access data in a fixed order.

RAMDAC: Random Access Memory Digital-to-Analog Converter
Random Access Memory Digital-to-Analog Converter (RAMDAC ) is a single chip on video adapter cards that converts digitally encoded images into analog signals to be displayed by a monitor. A RAMDAC consists of four different components: SRAM to store the color map, three digital-to-analog converters (DACs) and one for each of the monitor's red, green, and blue electron guns.

RAS: Row Address Strobe
Row Address Strobe (RAS) is a signal sent by the processor to a DRAM circuit to tell it that an associated address is a row address. DRAM stores data in a series of rows and columns, and each cell where a data bit is stored has both a row and a column address. A processor uses both RAS and CAS (column address strobe) signals to retrieve data from DRAM.

Raster Graphics
A raster graphics image, also known as digital image or bitmap, is a data file or structure representing a generally rectangular grid of pixels, or points of color, on a computer monitor, paper, or other display device. The color of each pixel is individually defined; images in the RGB color space, for instance, often consist of colored pixels defined by three bytes—one byte each for red, green and blue. Less colorful images require less information per pixel; an image with only black and white pixels requires only a single bit for each pixel. Raster graphics are distinguished from vector graphics in that vector graphics represent an image through the use of geometric objects such as curves and polygons.

Raster Image Processor
A raster image processor (RIP) is a component used in a printing system which produces a bitmap. The bitmap is then sent to a printing device for output. The input may be a page description in a high-level page description language such as PostScript, Portable Document Format or another bitmap of higher or lower resolution than the output device. In the latter case, the RIP applies either smoothing or interpolation algorithms to the input bitmap to generate the output bitmap.

RDI: Remote Debug Interface
The Remote Debug Interface (RDI), an Application Programming Interface (API), defines a standard set of data structures and functions that abstract hardware for debugging purposes. The Remote Debug Interface has served as a useful and widely adopted interface between debuggers and debug target (simulators, emulators and monitors) for the ARM architecture.

RDMA: Remote Direct Memory Access
Remote Direct Memory Access (RDMA) is a communication technique whereby two or more computers communicate via Direct Memory Access directly from the main memory of one system to the main memory of another. As there is no CPU, cache, or context switching overhead needed to perform the transfer, and transfers can continue in parallel with other system operations, this is particularly useful in applications where high throughput, low latency networking is needed such as in massively parallel Linux clusters. The most common RDMA implementation is over InfiniBand.

RDRAM: Rambus DRAM
Rambus DRAM (RDRAM) is a type of memory (DRAM) developed by Rambus, Inc. Whereas the fastest current memory technologies used by PCs (SDRAM) can deliver data at a maximum speed of about 100 MHz, RDRAM transfers data at up to 800 MHz.

Real-Time Computing
Real-time computing refers to the hardware and software systems which are subject to constraints in time. In particular, they are systems that are subject to deadlines from event to system response; that is, the computation can be said to fail if it does complete in the time-period after the event before the deadline relative to the event. These deadlines are independent of system load. Real-time computing can be illustrated by contrasting it with non-real-time computing.

Rectifier
Rectifier is an electrical device, comprising one or more diodes arranged for converting alternating current (AC) to direct current (DC). Rectifier allows current to pass in one direction yet blocks the flow of current in the other direction. A PC PSU typically contains a rectifier.

Register
In computing, register is a special high-speed storage area within the CPU for data be processed by the CPU. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register.

Registered Memory
Registered memory (also called buffered memory) contains registers that hold the data for one clock cycle before it is moved on to the motherboard. Registered memory is slightly slower than unregistered/unbuffered memory, and more expensive as well, so it is usually found only in applications where the need for scalability and stability outweighs the need for speed and a low price. Registered memory should not be confused with ECC memory; although many server-grade memory modules are both ECC and registered, there are registered non-ECC modules and non-registered ECC modules.

Removable Hard Disk
Removable hard disk is enclosed in plastic or metal cartridges so that they can be removed like floppy disks. Removable disk drives combine the best aspects of hard and floppy disks. They are nearly as capacious and fast as hard disks and have the portability of floppy disks.

Repeater
Repeater, also known as network repeater, is a type of network device that regenerates incoming electrical, wireless or optical signals. With physical media like Ethernet or Wi-Fi, data transmissions can only span a limited distance before the quality of the signal degrades. Repeaters attempt to preserve signal integrity and extend the distance over which data can safely travel. Active hubs are repeaters. In Wi-Fi, access points function as repeaters when operating in so-called "repeater mode."

Resolution
Resolution refers to the sharpness and clarity of an image in computer monitors, printers, etc. The unit of measurement of resolution is dpi (dpts per inch). For example, a 300-dpi printer is one that is capable of printing 300 distinct dots in a line 1 inch long. This means it can print 90,000 dots per square inch.

RGB monitor: Red, Green, Blue Monitor
Red, Green, Blue (RGB) monitor requires separate signals for each of the three colors. All color computer monitors are RGB monitors, which differs from color televisions which use composite video signals, in which all the colors are mixed together.

Ribbon Cable
Ribbon cable is a flat, thin cable containing many parallel wires used within a computer chassis to connect disk drives to the disk drive controllers.

RIMM: Rambus Inline Memory Module
RIMM, also called Rambus Inline Memory Module, is the memory module used with RDRAM chips. Rambus trademarked the term RIMM as an entire word. It is the term used for a module using Rambus technology. It is similar to a DIMM package but uses different pin settings.

RISC: Reduced Instruction Set Computer
Reduced instruction set computer (RISC) is a type of microprocessor that recognizes a relatively limited number of instructions. One advantage of reduced instruction set computers is that they can execute their instructions very fast because the instructions are so simple. Another, perhaps more important advantage, is that RISC chips require fewer transistors, which makes them cheaper to design and produce. Since the emergence of RISC computers, conventional computers have been referred to as CISCs (complex instruction set computers).

Riser Card
Riser card is a circuit board that connects directly into the computer motherboard and provides the ability for additional expansion cards to be added to the computer.

RJ: Registered Jack
Registered Jack (RJ) is a general term for electrical connector designs registered with the US Federal Communications Commission, including the RJ-11, RJ-14, RJ-25, RJ-48, RJ-61 and RJ-45 connectors. The most familiar registered jacks are the 4-conductor and the 6-conductor connectors known variously as RJ-11, RJ-12 and RJ-14, and the 8-conductor RJ-45, all sometimes simply called RJ connectors. These are commonly used in building wiring for telephone and local area networks. They were originally invented and patented by Bell Labs.

RJ-11: Registered Jack-11
Registered Jack-11 (RJ-11), also called plug, is a four- or six-wire connector used to connect telephone equipment, modems, fax to a famle RJ-11 jack on the wall. It is occasionally used to connect some types of local-area networks (LANs) in some cases.

RJ-21: Registered Jack-21
Registered Jack-21 (RJ21 or RJ-21) is an Ethernet cable that uses a 50-pin Telco connector on one end and branches out to 12 RJ-45 connectors on the other.

RJ-22: Registered Jack-22
Registered Jack-22 (RJ-22) is a four wire modular jack used for connecting telephone handsets to telephone instruments.

RJ-25C: Registered Jack-25C
Registered Jack 25C (RJ-25C) is a standard for a modular connector using 6 conductors. It is usually used to implement a 3-line telephone connection.

RJ-45: Registered Jack-45
Registered Jack-45 (RJ-45) is an eight-wire connector used to connect computers onto a local-area networks (LAN), especially Ethernet. RJ-45 comes in two types: keyed and non-keyed.

RJ Connector
RJ Connector (Registered jack connector) is a standard connector originally used to connect telephone lines. RJ connectors are now used for telephone connections and for 10BaseT and other types of network connections. RJ-11, RJ-12, and RJ-45 are popular types of RJ connectors.

ROM Emulator
ROM emulator is a debugging tool that helps debug a ROM chip by simulating the ROM with RAM.Since RAM can be written over, whereas ROM cannot, programming changes can be made easily. ROM emulators link the target system to a host computer. A plug either inserts into the target ROM sockets, clips over SMT ROM chips, or is soldered down in place of the ROM parts. The emulator then links this connection to the host computer over RS-232, Ethernet, or USB.

ROM: Read Only Memory
Read-only memory (ROM) is a type of nonvolatile computer memory on which data has been prerecorded. Once data has been written onto a ROM chip, it cannot be removed and can only be read. The data are permenently there regardless if the power is on or off.

ROMB: RAID on Motherboard
RAID on motherboard (ROMB) refers to RAID circuits built on the motherboard rather than a plug-in host adapter. ROMB is a memory system in which the RAID component of the system is kept separate from the PCI buses so that the PCI buses do not interfere with RAID traffic and cause bottlenecking. ROMB saves a PCI slot and improves speed because it is not using a shared bus.

Router
A router is a device or a piece of software in a computer that forwards and routes data packets along networks. A router connects at least two networks, commonly two LANs or WANs or a LAN and its ISP network. A router is often included as part of a network switch. A router is located at any gateway where one network meets another, including each point-of-presence on the Internet.

Routing Switch
Routing switch is a switch that also performs routing functions. Usually a switch operates at layer 2 (the Data Link layer) of the OSI Reference Model while routers operate at layer 3 (the Network layer). Routing switches, performing both layer 2 and layer 3 functions, typically have the routing operations implemented in hardware rather than using software. Therefore it is faster than the software based routers. However, routing switching may not as powerful or as flexible as full-fledged routers.
Because they perform some layer 3 functions, routing switches are sometimes called layer-3 switches.

RTOS: Real Time Operating System
Real-time operating system (RTOS) is an operating system designed specifically for use in real-time systems and applications. Examples include embedded applications (programmable thermostats, household appliance controllers, mobile telephones), industrial robots, industrial control, and scientific research equipment. A real-time operating system does not necessarily have high throughput — the specialized scheduling algorithm and a high clock-interrupt rate can both interfere with throughput. That is to say, RTOS are valued more for how quickly they can respond to input than for the total amount of work they can do.

RTU: Remote Terminal Unit
Remote Terminal Unit (RTU) is a device located at a remote site to collect data and transmit the data back to a central station (or master). An RTU also collects information from the master device and implements processes that are directed by the master. RTUs are equipped with input channels for sensing or metering, output channels for control, indication or alarms and a communications port. RTU is usually part of the SCADA system.

Run-time Memory
Run-time Memory is the computer memory accessed while a program runs.