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Glossary
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 in-
struction can vary with the execution conditions for the instruction and the op-
erands 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 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 re-
quired. An interlocked program section is executed normally for an ON execu-
tion 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 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 ap-
pearance 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 software con-
nection created to data existing at another location (i.e., data links).
load The processes of copying data either from an external device or from a storage
area to an active portion of the system such as a display buffer. Also, an output
device connected to the PC is called a load.
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