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Rack
PLC chassis.Modules are installed in the rack to meet the user's need.
Radio frequency (RF)
Communications technology in which there is a transmitter/receiver and tags. The transmitter/ receiver can read or write to the tags. There are active and passive tags available. Active tags are battery powered. Passive tags are powered from the RF emitted from the transmitter. Active tags have a much wider range of communication. Either tag can have several K of memory.
RAM (random access memory)
Normally considered user memory.
RDBMS (Relational DataBase Management System)
A standard designed to control interfacing with rigid relational database structures.
Register
Storage area. It is typically used to store bit states or values of items such as timers and counters.
Resolution
A measure of how closely a device can measure or divide a quantity. For example, in an encoder resolution would be defined as counts per turn. For an analog to digital card it would be the number of bits of resolution. For example, for a 12-bit card the resolution would be 4096.
Retentive coil
A coil that will remain in its last state, even though power was removed.
Retentive timer
Timer that retains the present count even if the input enable signal is lost. When the input enable is active again, the timer begins to count again from where it left off.
RTD (Resistance temperature detectors )
RTDs are made of coils or films of metals (usually platinum). When heated, the resistance of the metal increases; when cooled, the resistance decreases.
ROM (read-only memory)
This is operating system memory. ROM is nonvolatile. It is not lost when the power is turned off.
Redundancy
The capacity to switch from primary equipment to standby equipment automatically without affecting the process under control.
Relay Ladder Logic
This programming language expresses a program as a series of "coils" and "contacts", simulating the operation of electromechanical relays. The resultant program is the equivalent of a boolean equation, which is executed continuously in a combinatorial manner. The advantage of this language is the familiarity many electricians have with the simple operation of relays. Disadvantages include the complexity of large, cross-connected programs, and the difficulty of expressing such non-binary functions as motion control and analog I/O.
Ribbon Cable
A flat cable in which the conductors are arranged side-by-side. Also called flat-ribbon cable.
RS232
A standard for serial transmission of data between two devices. This defines the pin and plug in terms of size, shape and number of pins. An interface based on the RS232 standard.
RS-422 and RS-423
Standards for two types of serial communication. RS-422 is a balanced serial mode. This means that the transmit and receive lines have their own common instead of sharing one like RS-232. Balanced mode is more noise immune. This allows for higher data transmission rates and longer transmission distances. RS-423 uses the unbalanced mode. Its speeds and transmission distances are much greater than RS-232 but less than RS-422.
RS485
A standard for serial transmission of data between multiple devices. This defines the pin and plug in terms of size, shape and number of pins. Receivers have additional sensitivity which allows for longer distances and more communication drops. Includes some extra protection for receiver circuits.
RTOS (Real-Time Operating System)
An operating system designed specifically for use in real-time systems that respond to external events within a short and predictable time frame.
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