Glossary
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Programmable Controller A computerized device that can accept inputs from external devices and gen-
erate outputs to external devices according to a program held in memory.
Programmable Controllers are used to automate control of external devices.
Although single-component Programmable Controllers are available, build-
ing-block Programmable Controllers are constructed from separate compo-
nents. Such building-block Programmable Controllers are formed only when
enough of these separate components are assembled to form a functional
assembly, i.e., no one individual Unit is called a PC.
programmed alarm An alarm given as a result of execution of an instruction designed to gener-
ate the alarm in the program, as opposed to one generated by the system.
programmed error An error arising as a result of the execution of an instruction designed to gen-
erate the error in the program, as opposed to one generated by the system.
programmed message A message generated as a result of execution of an instruction designed to
generate the message in the program, as opposed to one generated by the
system.
Programming Console The simplest form or programming device available for a PC. Programming
Consoles are available both as hand-held models and, for larger PCs, as
CPU-mounting models.
PROGRAM mode A mode of operation that allows inputting and debugging of programs to be
carried out, but that does not permit normal execution of the program.
PV Acronym for present value.
refresh The process of updating output status sent to external devices so that it
agrees with the status of output bits held in memory and of updating input
bits in memory so that they agree with the status of inputs from external de-
vices.
relay-based control The forerunner of PCs. In relay-based control, groups of relays are intercon-
nected to form control circuits. In a PC, these are replaced by programmable
circuits.
reset The process of turning a bit or signal OFF or of changing the present value of
a timer or counter to its set value or to zero.
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.
right-hand instruction Another term for 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.
RUN mode The operating mode used by the PC for normal control operations.
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.
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