rack fault - cards in racks often have error indicator lights that turn on when a fault has occurred. This allows fast replacement.

radar () - radio waves are transmitted and reflected. The time between emission and detection determines the distance to an object.

radiation - the transfer of energy or small particles (e.g., neutrons) directly through space.

radiation pyrometry - a technique for measuring temperature by detecting radiated heat.

radix - the base value of a numbering system. For example the radix of binary is 2.

RAID (Redundant Array of Inexpensive Disks) - a method for robust disk storage that would allow removal of any disk drive without the interruption of service, or loss of data.

RAM (Random Access Memory) - Computer memory that can be read from, and written to. This memory is the main memory type in computers. The most common types are volatile - they lose their contents when power is removed.

random noise - there are no periodic waveforms, frequency and magnitude vary randomly.

random-scan devices - draw an image by refreshing one line or vector at a time; hence they are also called vector-scan or calligraphic devices. The image is subjected to flicker if there are more lines in the scene that can be refreshed at the refresh rate.

Rankine - A temperature system that uses absolute 0 as the base, and the scale is the same as the Fahrenheit scale.

raster devices - process pictures in parallel line scans. The picture is created by determining parts of the scene on each scan line and painting the picture in scan-line order, usually from top to bottom. Raster devices are not subject to flicker because they always scan the complete display on each refresh, independent of the number of lines in the scene.

rated - this will be used with other terms to indicate suggested target/maximum/minimum values for successful and safe operation.

RBOC (Regional Bell Operating Company) - A regional telephone company. These were originally created after a US federal court split up the phone company into smaller units.

Read/Write (R/W) - a digital device that can store and retrieve data, such as RAM.

reagent - an chemical used in one or more chemical reactions. these are often used for identifying other chemicals.

real-time - suggests a system must be able to respond to events that are occurring outside the computer in a reasonable amount of time.

redundancy - 1. added data for checking accuracy. 2. extra system components or mechanisms added to decrease the chance of total system failure.

refreshing - is required of a computer screen to maintain the screen image. Phosphors, which glow to show the image, decay at a fast rate, requiring the screen to be redrawn or refreshed several times a second to prevent the image from fading.

regenerative braking - the motor windings are reverse, and in effect return power to the power source. This is highly efficient when done properly.

register - a high speed storage area that can typically store a binary word for fast calculation. Registers are often part of the CPU.

regulator - a device to maintain power output conditions (such as voltage) regardless of the load.

relay - an electrical switch that comes in may different forms. The switch is activated by a magnetic coil that causes the switch to open or close.

relay - a magnetic coil driven switch. The input goes to a coil. When power is applied, the coil generates a magnetic field, and pulls a metal contact, overcoming a spring, and making contact with a terminal. The contact and terminal are separately wired to provide an output that is isolated from the input.

relief valve - designed to open when a pressure is exceeded. In a hydraulic system this will dump fluid back in the reservoir and keep the system pressure constant.

repeatability - the ability of a system to return to the same value time after time. This can be measured with a standard deviation.

repeater - added into networks to boost signals, or reduce noise problems. In effect one can be added to the end of one wire, and by repeating the signals into another network, the second network wire has a full strength signal.

resistance - this is a measurable resistance to energy or mass transfer.

resistance heating - heat is generated by passing a current through a resistive material.

resonant frequency - the frequency at which the material will have the greatest response to an applied vibration or signal. This will often be the most likely frequency of self destruction.

response time - the time required for a system to respond to a directed change.

return - at the end of a subroutine, or interrupt, the program execution will return to where it branched.

reverberation - when a sound wave hits a surface, part is reflected, and part is absorbed. The reflected part will add to the general (reverberant) sound levels in the room.

Reynolds number - a dimensionless flow value based on fluid density and viscosity, flow rate and pipe diameter.

RF (Radio Frequency) - the frequency at which a magnetic field oscillates when it is used to transmit a signal. Normally this range is from about 1MHz up to the GHz.

RFI (Radio Frequency Interference) - radio and other changing magnetic fields can generate unwanted currents (and voltages) in wires. The resulting currents and voltages can interfere with the normal operation of an electrical device. Filters are often used to block these signals.

RFS (Remote File System) - allows shared file systems (similar to NFS), and has been developed for System V UNIX.

RGB (Red Green Blue) - three additive colors that can be used to simulate the other colors of the spectrum. This is the most popular scheme for specifying colors on computers. The alternate is to use Cyan-Magenta-Yellow for the subtractive color scheme.

ripple voltage - when an AC voltage is converted to DC it is passed through diodes that rectify it, and then through capacitors that smooth it out. A small ripple still remains.

RISC (Reduced Instruction Set Computer) - the more standard computer chips were CISC (Complete Instruction Set Computers) but these had architecture problems that limited speed. To overcome this the total number of instructions were reduced, allowing RISC computers to execute faster, but at the cost of larger programs.

rlogin - allows a text based connection to a remote computer system in UNIX.

robustness - the ability of a system to deal with and recover from unexpected input conditions.

ROM (Read Only Memory) - a permanent form of computer memory with contents that cannot be overwritten. All computers contain some ROM to store the basic operating system - often called the BIOS in personal computers.

router - as network packets travel through a network, a router will direct them towards their destinations using algorithms.

RPC (Remote Procedure Call) - a connection to a specific port on a remote computer will request that a specific program be run. Typical examples are ping, mail, etc.

RS-232C - a serial communication standard for low speed voltage based signals, this is very common on most computers. But, it has a low noise immunity that suggests other standards in harsh environments.

RS-422 - a current loop based serial communication protocol that tends to perform well in noisy environments.

RTC (Real-Time Clock) - A clock that can be used to generate interrupts to keep a computer process or operating system running at regular intervals.

RTD (Resistance Temperature Detector) - as temperature is changed the resistance of many materials will also change. We can measure the resistance to determine the temperature.

RTS (Request To Send) - A data handshaking line that is used to indicate when a signal is ready for transmission, and clearance is requested.

R/W (Read/Write) - A digital line that is used to indicate if data on a bus is to be written to, or read from memory.

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