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Glossary
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input bit A bit in the IR area that is allocated to hold the status of an input.
input device An external device that sends signal(s) into the PC System.
input point The point at which an input enters the PC System. An input point physically
corresponds to terminals or connector pin(s).
input signal A change in the status of a connection entering the PC. Generally an input
signal is said to exist when, for example, a connection point goes from low to
high voltage or from a nonconductive to a conductive state.
instruction A direction given in the program that tells the PC an action to be carried out
and the data to be used in carrying out the action. Instructions can 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.
Although any logically related group of instructions could be called an instruc-
tion block, the term is generally used to refer to blocks of instructions called
logic blocks that require logic block instructions to relate them to other in-
structions or logic blocks.
instruction execution time The time required to execution an instruction. The execution time for any one
instruction can vary with the execution condition 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.
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 as 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 ex-
ecution condition and partially reset for an OFF execution condition.
IR area A data area whose principal function is to hold the status of inputs coming
into the system and outputs that are to be set out of the system. Bits and
words in the IR are that are used this way are called I/O bits and I/O words.
The remaining bits in the IR area are work bits.
jump A type of programming where execution moves directly from one point in a
program to a separate point in the program without sequentially executing
the instruction in between. Jumps are usually conditional on an execution
condition.
jump number A definer used with a jump that defines the points from which 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 cir-
cuit-type diagrams to represent the logic flow of programming instructions.
The appearance of the program suggests a ladder, and thus the name.