Page 420 - Programmable Logic Controllers, Fifth Edition - Mobile version

Line A component part of a system used to link various subsystems
located remotely from the processor; the source of power for operation; control relay zone must be delimited and controlled by master control
relay fence codes (master control relay instructions).
example: 120 V alternating current line. Matrix A logic network that is an intersection of input and output
Line-powered sensor Normally, three-wire sensors, although connection points.
four-wire sensors also exist. The line-powered sensor is powered Memory That part of the programmable logic controller in which
from the power supply. A separate wire (the third) is used for the data and instructions are stored either temporarily or semi-permanently.
output line. The control program is stored in memory.
Liquid-crystal display (LCD) A display device using reflected light Memory map A diagram showing a system’s memory addresses
from liquid crystals to form the segments of the displayed characters and what programs and data are assigned to each section of memory.
and numbers.
Menu A list of programming selections displayed on a programming
Load The power used by a machine or apparatus; to place data terminal.
into an internal register under program control; to place a program
from an external storage device into central memory under operator Metal-oxide semiconductor (MOS) A semiconductor device in
control. which an electric field controls the conductance of a channel under a
metal electrode called a gate.
Load-powered sensor A two-wire sensor. A small leakage current
flows through the sensor even when the output is off. The current is Metal-oxide varistor (MOV) Used for suppressing electrical
required to operate the sensor electronics. power surges.
Local area network (LAN) A system of hardware and software Microprocessor A central processing unit manufactured on a
designed to allow a group of intelligent devices to communicate single integrated-circuit chip (or several chips) by utilizing large-scale
within a fairly close proximity. integration technology.
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Local input/output (I/O) A programmable logic controller whose Microsecond One millionth of a second 5 1 3 10 second 5
input/output distance is physically limited. The PLC must be located 0.000001 second.
near the process; however, the PLC may still be mounted in a separate Millisecond One thousandth of a second 5 1 3 10 second 5
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enclosure. 0.001 second.
Local power supply The power supply used to provide power to the Mnemonic A term, usually an abbreviation, that is easy to remember
processor and a limited number of local input/output modules. and pronounce.
Location In reference to memory, a storage position or register Mnemonic code A code in which information is represented by
identified by a unique address. symbols or characters.
Logic A process of solving complex problems through the repeated Modbus A network that uses a master/slave communication technique.
use of simple functions that can be either true or false. The three basic Mode A term used to refer to the selected operating method, such as
logic functions are AND, OR, and NOT. automatic, manual, TEST, PROGRAM, or diagnostic.
Logic diagram A diagram that represents the logic elements and Module An interchangeable, plug-in item containing electronic
their interconnections.
components.
Logic level The voltage magnitude associated with signal pulses Module addressing A method of identifying the input/output modules
representing 1s and 0s in binary computation.
installed in a chassis.
Loop control A control of a process or machine that uses feedback. Most significant bit (MSB) The bit representing the greatest value
An output status indicator modifies the input signal effect on the of a byte or word.
process control.
Most significant digit (MSD) The digit representing the greatest
M value of a byte or word.
Motor controller or starter A device or group of devices that serve
Machine language A programmable language using the binary
form. to govern, in a predetermined manner, the electric power delivered to
a motor.
Major fault A fault condition that is severe enough for the controller
to shut down, unless the condition is cleared. Motor starter A special relay designed to provide power to motors;
it has both a contactor relay and an overload relay connected in
Manufacturing automation protocol (MAP) Standard developed series and prewired so that, if the overload operates, the contactor is
to make industrial devices communicate more easily. de-energized.
Masking A means of selectively screening out data. Masking allows Move instruction A programmable logic controller instruction that
unused bits in a specific instruction to be used independently. moves data from one location to another. Although a move instruction
Mass storage A means of storing large amounts of data on magnetic typically places the data in a new location, the original data still reside
tape, floppy disks, and so on. in their original location.
Master control relay (MCR) A mandatory hardwired relay that Multiplexing The time-shared scanning of a number of data lines
can be de-energized by any series-connected emergency stop switch. into a single channel, and only one data line is enabled at any time; the
Whenever the master control relay is de-energized, its contacts open to incorporation of two or more signals into a single wave from which
de-energize all application input and output devices. the individual signals can be recovered.
Master control relay (MCR) zones User program areas in which Multiply instruction A programmable logic controller instruction
all nonretentive outputs can be turned off simultaneously. Each master that provides for the mathematical multiplication of two numbers.

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