PLC Glossary.pdf.PDF

Glossary

Glossary of Terms used in

Programmable Controller-based

Systems

From

Industrial Text and Video Co.

the leader in Electrical, Motor

Control and PLCs

Video Training Programs

www.industrialtext.com

1-800-752-8398

Glossary

G LOSSARY

AC/DC I/O interface. A discrete interface that converts alternating current (AC)

voltages from field devices into direct current (DC) signals that the processor can

use. It can also convert DC signals into proportional AC voltages.

action. A set of control instructions prompting a PLC to perform a certain control

function during the execution of a sequential function chart step.

acyclic message. An unscheduled message transmission.

A/D. See analog-to-digital converter .

address. (1) The location in a computer’s memory where particular information is

stored. (2) The alphanumeric value used to identify a specific I/O rack, module

group, and terminal location.

addressability. The total number of devices that can be connected to a network.

address field. The sequence of eight (or any multiple of eight) bits immediately

following the opening flag sequence of a frame, which identifies the secondary

station that is sending (or is designated to receive) the frame.

AI. See artificial intelligence .

algorithm. A set of procedures used to solve a problem.

alphanumeric code. A character string consisting of a combination of letters,

numbers, and/or special characters used to represent text, commands, numbers,

and/or code groups.

ambient temperature. The temperature of the air surrounding a device.

American National Standards Institute (ANSI). A clearinghouse and coordinat-

ing agency for voluntary standards in the United States.

American Wire Gauge (AWG). A standard system used to designate the size of

electrical conductors. Gauge numbers have an inverse relationship to size; larger

gauges have a smaller diameter.

analog device. An apparatus that measures continuous information signals (i.e.,

signals that have an infinite number of values). The only limitation on resolution

is the accuracy of the measuring device.

analog input interface. An input circuit that uses an analog-to-digital converter to

translate a continuous analog signal, measured by an analog device, into a digital

value that can be used by the processor.

analog output interface. An output circuit that uses a digital-to-analog converter to

translate a digital value, sent from the processor, into an analog signal that can

control a connected analog device.

analog signal. A continuous signal that changes smoothly over a given range, rather

than switching suddenly between certain levels as discrete signals do.

analog-to-digital converter (A/D). A device that translates analog signals from

field devices into binary numbers that can be read by the processor.

AND. A logical operator that requires all input conditions to be logic 1 for the output

to be logic 1. If any input is logic 0, then the output will be logic 0.

ANSI. See American National Standards Institute.

www.industrialtext.com

1-800-752-8398

Glossary

application. (1) A machine or process monitored and controlled by a PLC. (2) The

use of computer or processor-based routines for specific purposes.

application memory. The part of the total system memory devoted to storing the

application program and its associated data.

application program. The set of instructions that provides control, data acquisition,

and report generation capabilities for a specific process.

arithmetic instructions. Computer programming codes that give a PLC the ability

to perform mathematical functions, such as addition, subtraction, multiplication,

division, and square root, on data.

artificial intelligence (AI). A subfield of computer science dealing with the

development of computer programs that solve tasks requiring extensive knowl-

edge.

ASCII. For American Standard Code for Information Interchange . A seven-bit code

with an optional parity bit used to represent alphanumeric, punctuation, and

control characters.

ASCII I/O interface. A special function interface that transmits alphanumeric data

between peripheral equipment and a PLC.

assembly language. A symbolic programming language that can be directly

translated into machine language instructions.

asynchronous. Recurrent or repeated operations that occur in unrelated patterns

over time.

AWG. See American Wire Gauge .

back plane. A printed circuit board, located in the back of a chassis, that contains

a data bus, power bus, and mating connectors for modules that will be inserted into

the chassis.

backup. A device or system that is kept on hand to replace a device or system that

fails.

backward chaining. A method of finding the causes of an outcome by analyzing its

consequents to obtain its antecedents.

bandwidth. The range of frequencies expressed in Hertz over which a system is

designed to operate.

base. The maximum number of digits used to represent values in a number system.

baseband coaxial cable. A communication medium that can send one transmission

signal at a time at its original frequency.

BASIC module. An intelligent I/O interface capable of performing computational

tasks without affecting the PLC processor’s computing time.

battery backup. A battery or set of batteries that will provide power to the

processor’s memory in the event of a power outage.

baud. (1) The reciprocal of the shortest pulse width in a data communication stream.

(2) The number of binary bits transmitted per second during a serial data

transmission.

Baye’s theorem. An equation that defines the probability of one event occurring

based on the fact that another event has already occurred.

BCC. See block check character .

www.industrialtext.com

1-800-752-8398

Glossary

BCD. See binary coded decimal .

binary coded decimal (BCD). A binary number system in which each decimal digit

from 0 to 9 is represented by four binary digits (bits). The four positions have a

weighted value of 1, 2, 4, and 8, respectively, starting from the least significant

(right-most) bit.

binary number system. A base 2 number system that uses only the numbers 0 and

1 to express all values. Each digit position of a binary number has a weighted value

of 1, 2, 4, 8, 16, 32, 64, and so on, starting with the least significant (right-most)

digit.

bit. For binary digit . The smallest unit of binary information. A bit can have a value

of 1 or 0.

bit rate. See baud .

bit-wide bus network. An I/O bus network that interfaces with discrete devices that

transmit less than 8 bits of data at a time.

blackboard architecture. The distribution of knowledge inferencing, as well as

global and knowledge databases, in a control system through the use of several

subsystems containing local, global, and knowledge databases that work indepen-

dently of each other.

block. A group of words transmitted as a unit.

block check character (BCC). A character, placed at the end of a data block, that

corresponds to the characteristics of the block.

block diagram. A schematic drawing.

block length. The total number of words transmitted at one time.

block transfer. A programming technique used to transfer up to 64 words of data to

or from an intelligent I/O module.

Boolean action. A set of control instructions that assigns a discrete value to a

variable during a sequential function chart step.

Boolean language. A PLC programming language, based primarily on the Boolean

logic operators, that implements all of the functions of the basic ladder diagram

instruction set.

Boolean operators. Logical operators, such as AND, OR, NAND, NOR, NOT, and

exclusive-OR, that can be used singly or in combination to form logical statements

that have output responses of TRUE or FALSE.

Boolean variable. A single-bit variable whose value is transmitted in the form of 1s

and 0s.

Bourdon tube. A pressure transducer available in spiral, helical, twisted, and C-tube

configurations that converts pressure measurements into displacement.

branch. A parallel logic path within a rung.

breadth-first search. A method of rule evaluation that evaluates each rule in the

same level of a decision tree before proceeding downward.

bridge circuit. A mechanism found in transducer circuits that uses resistors to

change the parameters (e.g., voltage and current) of an incoming signal.

broadband coaxial cable. A communication medium that can transmit two or more

transmission signals at one time via frequency division multiplexing.

burn-in procedure. The process of operating a device at an elevated temperature to

identify early-failing parts.

www.industrialtext.com

1-800-752-8398

Glossary

bus. (1) A group of lines used for data transmission or control. (2) Power distribution

conductors.

bus topology. A network configuration in which all stations are connected in parallel

with the communication medium and all stations can receive information from any

other station on the network.

bypass/control station. A device that allows a process to be switched to either PLC

or manual control.

byte. A group of eight adjacent bits that are operated on as a unit, such as when

moving data to or from memory.

byte-wide bus network. An I/O bus network, which interfaces with discrete and

small analog devices, that can transmit between 1 and 50 or more bytes of data at

a time.

cascade control. The use of two controllers to regulate a process so that the feedback

loop of one controller is the set point of the other controller.

center of gravity method. A method of calculating the final output value of a fuzzy

logic controller by finding the value that corresponds to the center of the mass

under the control output curve.

centralized control. A PLC control system organization in which a central PLC

controls several machines or processes.

central processing unit (CPU). The part of a programmable controller responsible

for reading inputs, executing the control program, and updating outputs. Some-

times referred to as the processor , the CPU consists of the arithmetic logic unit,

timing/control circuitry, accumulator, scratch pad memory, program counter,

address stack, and instruction register.

centroid. The point in a geometrical figure whose coordinates equal the average of

all the other points comprising the figure.

channel. A designated path for a signal.

channel capacity. The amount of information that can be transmitted per second on

a given communication channel depending on the medium, line length, and

modulation rate.

character. One symbol of a set of elementary symbols, such as a letter of the alphabet

or a number.

chassis. A hardware assembly that houses PLC devices, such as I/O modules,

adapter modules, processor modules, power supplies, and processors.

checksum. A transmission verification algorithm that adds the binary values of all

the characters in a data block and places the sum in the block check character

position.

chip. A very small piece of semiconductor material that holds electronic compo-

nents. Chips are normally made of silicon and are typically less than 1/4 inch

square and 1/100 inch thick.

closed loop. A control system that uses feedback from the process to maintain

outputs at a desired level.

coaxial cable. A transmission medium, consisting of a central conductor surrounded

by dielectric materials and an external conductor, that possesses a predictable

characteristic impedance.

www.industrialtext.com

1-800-752-8398

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: