Page 419 - Programmable Logic Controllers, Fifth Edition - Mobile version

from reading the inputs and processing the outputs according to the
control program. K
K 2 5 1K 5 1024; used to denote size of memory and can be
10
Input scan One of three parts of the PLC scan. During the input scan, expressed in bits, bytes, or words; example: 2K 5 2048.
input terminals are read and the input table is updated accordingly.
k Kilo; a prefix used with units of measurement to designate quantities
Instruction A command that causes a programmable logic controller 1000 times as great.
to perform one specific operation. The user enters a combination of
instructions into the programmable logic controller’s memory to form Keyboard The alphanumeric keypad on which the user types
a unique application program. instructions to the PLC.
Keying Bands installed on backplane connectors to ensure that only
Instruction set The set of general-purpose instructions available
with a given controller. In general, different machines have different one type of module can be inserted into a keyed connector.
instruction sets.
INT Two-byte integer. L
Integer A positive or negative whole number. Label instruction A programmable logic controller instruction
that assigns an alphanumeric designation to a particular location in a
Integrated circuit (IC) A circuit in which all components are
integrated on a single tiny silicon chip. program. This location is used as the target of a jump, skip, or jump to
subroutine instruction.
Intelligent field devices Microprocessor-based devices used to
provide process-variable, performance, and diagnostic information to Ladder diagram An industry standard for representing relay logic
the PLC processor. These devices are able to execute their assigned control systems. The diagram resembles a ladder because the vertical
control functions with little interaction, except communications, with supports of the ladder appear as power feed and return buses and the
their host processor. horizontal rungs of the ladder appear as series and/or parallel circuits
connected across the power lines.
Intelligent input/output module A microprocessor-based module that
performs processing or sophisticated closed-loop application functions. Ladder diagram programming A method of writing a user program
in a format similar to a relay ladder diagram.
Interface A circuit that permits communication between the central
processing unit and a field input or output device. Different devices Ladder matrix A rectangular array of programmed contacts that
require different interfaces. defines the number of contacts that can be programmed across a row
and the number of parallel branches allowed in a single ladder rung.
Interlock A system for preventing one element or device from
turning on while another device is on. Language A set of symbols and rules for representing and com-
municating information among people or between people and
Internal coil instruction A relay coil instruction used for internal machines; the method used to instruct a programmable device to
storage or buffering of an on/off logic state. An internal coil instruction perform various operations.
differs from an output coil instruction because the on/off status of the
internal coil is not passed to the input/output hardware for control of a Language module Enables the user to write programs in a high-level
field device. language. BASIC is the most popular language module. Other language
modules available include C, Forth, and PASCAL.
Inversion Conversion of a high level to a low level, or vice versa.
Inverter The digital circuit that performs inversion. Latching relay A relay that maintains a given position by mechanical
or electrical means until released mechanically or electrically.
I/O module A plug-in assembly, containing two or more identical
input or output circuits, that contain the connections between a processor Latch instruction One-half of an instruction pair (the second
and connected devices. instruction of the pair being the unlatch instruction) that emulates
the latching action of a latching relay. The latch instruction for a
Interrupt The act of redirecting a program’s execution to perform a programmable logic controller energizes a specified output point
more urgent task. or internal coil until it is de-energized by a corresponding unlatch
IP address A specified Internet protocol address, unique and assigned instruction.
by the manufacturer, for every Ethernet device. Leakage The small amount of current that flows in a semiconductor
Isolated input module A module that receives dry contacts as device when it is in the off state.
inputs, which the processor can recognize and change into two-state Least significant bit (LSB) The bit that represents the smallest
digital signals.
value in a byte or word.
Isolated input/output (I/O) circuits Input and output circuits that Least significant digit (LSD) The digit that represents the smallest
are electrically isolated from any and all other circuits of a module. value in a byte or word.
Isolated input/output circuits are designed to allow field devices that
are powered from different sources to be connected to one module. Light-emitting diode (LED) A semiconductor junction that emits
light when biased in the forward direction.
J Light-emitting diode (LED) display A display device incorporating
Jumper A short length of conduit used to make a connection between light-emitting diodes to form the segments of the displayed characters
terminals around a break in a circuit. and numbers.
Jump instruction An instruction that permits the bypassing of selected Limit switch An electric switch actuated by some part and/or
portions of the user program. Jump instructions are conditional whenever motion of a machine or equipment.
their operation is determined by a set of preconditions and unconditional Limit test A test that determines if a value is inside or outside a
whenever they are executed to occur every time they are programmed. specified range.

400 Glossary

pet73842_gls_395-406.indd 400 03/11/15 4:09 PM