Glossary
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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 bit of a group of bits, generally of an entire word, or the
lowest numbered word of a group of words. This bit/word is often called the
least-significant bit/word.
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
execution of specific subroutines that can be used for instructions that must be
executed 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
warning 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 pro-
cessing is required.
set
The process of turning a bit or signal ON.
set value
The value from which a decrementing counter starts counting down or to which
an incrementing counter counts up (i.e., the maximum count), or the time from
which or for which a timer starts timing. Set value is abbreviated SV.
shift input signal
An input signal whose OFF to ON transition causes data to be shifted one bit.
