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Glossary
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 lines 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
diagram. 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
usually involves changes in the way the communicated data is represented.
Interface 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 exe-
cution condition 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
program 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 dia-
gram and are called terminal instructions.
least-significant (bit/word) See rightmost (bit/word).
LED Acronym for light-emitting diode; a device used 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.
link A hardware or software connection formed between two Units. “Link” can
refer either to a part of the physical connection between two Units or a soft-
ware connection created to data existing at another location (i.e., data links).
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