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Glossary
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instruction A direction given in the program that tells the PC of the action to be carried out,
and the data to be used in carrying out the action. Instructions can be used to
simply turn a bit ON or OFF, or they can perform much more complex actions,
such as converting and/or transferring large blocks of data.
instruction block A group of instructions that is logically related in a ladder-diagram program. A
logic block includes all of the instruction lines that interconnect with each other
from one or more line connecting to the left bus bar to one or more right-hand
instructions connecting to the right bus bar.
instruction execution time The time required to execute an instruction. The execution time for any one
instruction can vary with the execution conditions for the instruction and the
operands used in it.
instruction line A group of conditions that lie together on the same horizontal line of a ladder dia-
gram. Instruction lines can branch apart or join together to form instruction
blocks. Also called a rung.
interface An interface is the conceptual boundary between systems or devices and usu-
ally involves changes in the way the communicated data is represented. Inter-
face devices perform operations like changing the coding, format, or speed of
the data.
interlock A programming method used to treat a number of instructions as a group so that
the entire group can be reset together when individual execution is not required.
An interlocked program section is executed normally for an ON execution condi-
tion and partially reset for an OFF execution condition.
interrupt (signal) A signal that stops normal program execution and causes a subroutine to be run
or other processing to take place.
interrupt program A program that is executed in response to an interrupt.
inverse condition See normally closed condition.
JIS An acronym for Japanese Industrial Standards.
jump A type of programming where execution moves directly from one point in a pro-
gram to another, without sequentially executing any instructions in between.
jump number A definer used with a jump that defines the points from and to which a jump is to
be made.
ladder diagram (program) A form of program arising out of relay-based control systems that uses circuit-
type diagrams to represent the logic flow of programming instructions. The
appearance of the program is similar to a ladder, and thus the name.
ladder diagram symbol A symbol used in drawing a ladder-diagram program.
ladder instruction An instruction that represents the conditions on a ladder-diagram program. The
other instructions in a ladder diagram fall along the right side of the diagram and
are called terminal instructions.
Ladder Support Software A software package installed on a IBM PC/AT or compatible computer to func-
tion as a Programming Device.
least-significant (bit/word) See rightmost (bit/word).
LED Acronym for light-emitting diode; a device used as for indicators or displays.
leftmost (bit/word) The highest numbered bits of a group of bits, generally of an entire word, or the
highest numbered words of a group of words. These bits/words are often called
most-significant bits/words.