Glossary
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scan time
The time required for a single scan of the ladder-diagram program.
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.
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 (ei-
ther specified data, zero(s) or one(s)) is shifted into one end and the data
shifted out at the other end is lost.
slot
A position on a Rack (Backplane) to which a Unit can be mounted.
software error
An error that originates in a software program.
source
The location from which data is taken for use in an instruction, as opposed to
the location to which the result of an instruction is to be written. The latter is
called the destination.
SV
Abbreviation for set value.
switching capacity
The maximum voltage/current that a relay can safely switch on and off.
syntax error
An error in the way in which a program is written. Syntax errors can include
‘spelling’ mistakes (i.e., a function code that does not exist), mistakes in
specifying operands within acceptable parameters (e.g., specifying reserved
SR bits as a destination), and mistakes in actual application of instructions
(e.g., a call to a subroutine that does not exist).
system configuration
The arrangement in which Units in a system are connected.
system error
An error generated by the system, as opposed to one resulting from execu-
tion of an instruction designed to generate an error.
system error message
An error message generated by the system, as opposed to one resulting
from execution of an instruction designed to generate a message.
TC area
A data area that can be used only for timers and counters. Each bit in the TC
area serves as the access point for the SV, PV, and Completion Flag for the
timer or counter defined with that bit.
TC number
A definer that corresponds to a bit in the TC area and used to define the bit
as either a timer or a counter.
terminal instruction
An instruction placed on the right side of a ladder diagram that uses the final
execution conditions of an instruction line.