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Glossary
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retrieve The processes of copying data either from an external device or from a storage
area to an active portion of the system such as a display buffer. Also, an output
device connected to the PC is called a load.
retry The process whereby a device will re-transmit data which has resulted in an er-
ror message from the receiving device.
return The process by which instruction execution shifts from a subroutine back to the
main program (usually the point from which the subroutine was called).
reversible counter A counter that can be both incremented and decremented depending on the
specified conditions.
reversible shift register A shift register that can shift data in either direction depending on the specified
conditions.
right-hand instruction See terminal instruction.
rightmost (bit/word) The lowest numbered bits of a group of bits, generally of an entire word, or the
lowest numbered words of a group of words. These bits/words are often called
least-significant bits/words.
rising edge The point where a signal actually changes from an OFF to an ON status.
ROM Read only memory; a type of digital storage that cannot be written to. A ROM
chip is manufactured with its program or data already stored in it and can never
be changed. However, the program or data can be read as many times as de-
sired.
rotate register A shift register in which the data moved out from one end is placed back into the
shift register at the other end.
RS-232C interface An industry standard for serial communications.
RUN mode The operating mode used by the PC for normal control operations.
rung See instruction line.
scan The process used to execute a ladder-diagram program. The program is ex-
amined sequentially from start to finish and each instruction is executed in turn
based on execution conditions.
scan time See cycle time.
scheduled interrupt An interrupt that is automatically generated by the system at a specific time or
program location specified by the operator. Scheduled interrupts result in the ex-
ecution of specific subroutines that can be used for instructions that must be ex-
ecuted repeatedly at a specified interval of time.
SCP See subtract count input.
seal See self-maintaining bit.
self diagnosis A process whereby the system checks its own operation and generates a warn-
ing or error if an abnormality is discovered.
self-maintaining bit A bit that is programmed to maintain either an OFF or ON status until set or reset
by specified conditions.
series A wiring method in which Units are wired consecutively in a string.
servicing The process whereby the PC checks a connector or Unit to see if special proces-
sing is required.