Glossary
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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.
RS-422 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. The scan also includes peripheral processing,
I/O refreshing, etc. The scan is called the cycle with CV-series PCs.
scan time The time required for a single scan of a ladder-diagram program.
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.
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. In Link Sys-
tems wired through Link Adapters, the Units are still functionally wired in series,
even though Units are placed on branch lines.
servicing The process whereby the PC provides data to or receives data from external de-
vices or remote I/O Units, or otherwise handles data transactions for Link Sys-
tems.
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 register One or more words in which data is shifted a specified number of units to the right
or left in bit, digit, or word units. In a rotate register, data shifted out one end is
shifted back into the other end. In other shift registers, new data (either specified
data, zero(s) or one(s)) is shifted into one end and the data shifted out at the oth-
er end is lost.
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