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Programmable logic controllers ( plc) combined glossary of terms.



abort - the disruption of normal operation

absolute pressure - a pressure measured relative to zero pressure

absorption loss - when sound or vibration energy is lost in a transmitting or reflecting medium. This is the result of generation of other forms of energy such as heat

absorptive law - a special case of Boolean algebra where A(A+B) becomes A

AC (Alternating Current) - most commonly an electrical current and voltage that changes in a sinusoidal pattern as a function of time. It is also used for voltages and currents that are not steady (DC)

Electrical power is normally distributed at 60Hz or 50Hz

AC contactor - a contactor designed for AC power

acceptance test - a test for evaluating a newly purchased system’s performance, capabilities, and conformity to specifications, before accepting, and paying the supplier

accumulator - a temporary data register in a computer CPU

accuracy - the difference between an ideal value and a physically realizable value. The companion to accuracy is repeatability

acidity - a solution that has an excessive number of hydrogen atoms. Acids are normally corrosive

acoustic - another term for sound

acknowledgement (ACK) - a response that indicates that data has been transmitted correctly

actuator - a device that when activated will result in a mechanical motion. For example a motor, a solenoid valve, etc

A/D - Analog to digital converter (see ADC)

ADC (Analog to Digital Converter) - a circuit that will convert an analog voltage to a digital value, also referred to as A/D

ADCCP (Advanced Data Communications Procedure) - ANSI standard for synchronous communication links with primary and secondary functions

address - a code (often a number) that specifies a location in a computers memory

address register - a pointer to memory locations

adsorption - the ability of a material or apparatus to adsorb energy

agitator - causes fluids or gases to mix

AI (Artificial Intelligence) - the use of computer software to mimic some of the cognitive human processes

algorithms - a software procedure to solve a particular problem

aliasing - in digital systems there are natural limits to resolution and time that can be exceeded, thus aliasing the data. For example. an event may happen too fast to be noticed, or a point may be too small to be displayed on a monitor

alkaline - a solution that has an excess of HO pairs will be a base. This is the compliment to an acid

alpha rays - ions that are emitted as the result of atomic fission or fusion

alphanumeric - a sequence of characters that contains both numbers and letters

ALU (Arithmetic Logic Unit) - a part of a computer that is dedicated to mathematical operations

AM (Amplitude Modulation) - a fixed frequency carrier signal that is changed in amplitude to encode a change in a signal

ambient - normal or current environmental conditions

ambient noise - a sort of background noise that is difficult to isolate, and tends to be present throughout the volume of interest

ambient temperature - the normal temperature of the design environment

analog signal - a signal that has continuous values, typically voltage

analysis - the process of review to measure some quality

and - a Boolean operation that requires all arguments to be true before the result is true

annealing - heating of metal to relieve internal stresses. In many cases this may soften the material

annotation - a special note added to a design for explanatory purposes

ANSI (American National Standards Institute) - a developer of standards, and a member of ISO

APF (All Plastic Fiber cable) - fiber optic cable that is made of plastic, instead of glass

API (Application Program Interface) - a set of functions, and procedures that describes how a program will use another service/library/program/etc

APT (Automatically Programmed Tools) - a language used for directing computer controlled machine tools

application - the task which a tool is put to, This normally suggests some level of user or real world interaction

application layer - the top layer in the OSI model that includes programs the user would run, such as a mail reader

arc - when the electric field strength exceeds the dielectric breakdown voltage, electrons will flow

architecture - they general layout or design at a higher level

armature - the central rotating portion of a DC motor or generator, or a moving part of a relay

ARPA (Advanced Research Projects Agency) - now DARPA. Originally funded ARPANET

ARPANET - originally sponsored by ARPA. A packet switching network that was in service from the early 1970s, until 1990

ASCII (American Standard Code for Information Interchange) - a set of numerical codes that correspond to numbers, letters, special characters, and control codes. The most popular standard ASIC (Application Specific Integrated Circuit) - a specially designed and programmed logic circuit. Used for medium to low level production of complex functions

aspirator - a device that moves materials with suction

assembler - converts assembly language into machine code

assembly language - a mnemonic set of commands that can be directly converted into commands for a CPU

associative dimensioning - a method for linking dimension elements to elements in a drawing

associative laws - Boolean algebra laws A+(B+C) = (A+B)+C or A(BC) = (AB)C asynchronous - events that happen on an irregular basis, and are not predictable

asynchronous communications (serial) - strings of characters (often ASCII) are broken down into a series of on/off bits. These are framed with start/stop bits, and parity checks for error detection, and then send out one character at a time. The use of start bits allows the characters to be sent out at irregular times

attenuation - to decrease the magnitude of a signal

attenuation - as the sound/vibration energy propagates, it will undergo losses. The losses are known as attenuation, and are often measured in dB. For general specifications, the attenuation may be tied to units of dB/ft

attribute - a nongraphical feature of a part, such as color

audible range - the range of frequencies that the human ear can normally detect from 16 to 20,000 Hz

automatic control - a feedback of a system state is compared to a desired value and the control value for the system is adjusted by electronics, mechanics and/or computer to compensate for differences

automated - a process that operates without human intervention

auxiliary power - secondary power supplies for remote or isolated systems

AWG (American Wire Gauge) - specifies conductor size. As the number gets larger, the conductors get smaller


B-spline - a fitted curve/surface that is commonly used in CAD and graphic systems

backbone - a central network line that ties together distributed networks

background - in multitasking systems, processes may be running in the background while the user is working in the foreground, giving the user the impression that they are the only user of the machine (except when the background job is computationally intensive)

background suppression - the ability of a sensing system to discriminate between the signal of interest, and background noise or signals

backplane - a circuit board located at the back of a circuit board cabinet. The backplane has connectors that boards are plugged into as they are added

backup - a redundant system to replace a system that has failed

backward chaining - an expert system looks at the results and looks at the rules to see logically how to get there

band pressure Level - when measuring the spectrum of a sound, it is generally done by looking at frequencies in a certain bandwidth. This bandwidth will have a certain pressure value that is an aggregate for whatever frequencies are in the bandwidth

base - 1. a substance that will have an excess of HO ions in solution form. This will react with an acid. 2. the base numbering system used. For example base 10 is decimal, base 2 is binary baseband - a network strategy in which there is a single carrier frequency, that all connected machines must watch continually, and participate in each transaction

BASIC (Beginner’s All-purpose Symbolic Instruction Code) - a computer language designed to allow easy use of the computer

batch processing - an outdated method involving running only one program on a computer at once, sequentially. The only practical use is for very intensive jobs on a supercomputer

battery backup - a battery based power supply that keeps a computer (or only memory) on when the master power is off

BAUD - The maximum number of bits that may be transmitted through a serial line in one second. This also includes some overhead bits

baudot code - an old code similar to ASCII for teleprinter machines

BCC (Block Check Character) - a character that can check the validity of the data in a block

BCD (Binary Coded Decimal) - numerical digits (0 to 9) are encoded using 4 bits. This allows two numerical digits to each byte

beam - a wave of energy waves such as light or sound. A beam implies that it is not radiating in all directions, and covers an arc or cone of a few degrees

bearing - a mechanical support between two moving surfaces. Common types are ball bearings (light weight) and roller bearings (heavy weight), journal bearings (rotating shafts)

beats - if two different sound frequencies are mixed, they will generate other frequencies. if a 1000Hz and 1001Hz sound are heard, a 1Hz (=1000-1001) sound will be perceived

benchmark - a figure to compare with. If talking about computers, these are often some numbers that can be use to do relative rankings of speeds, etc. If talking about design, we can benchmark our products against our competitors to determine our weaknesses

Bernoulli’s principle - a higher fluid flow rate will result in a lower pressure

beta ratio - a ratio of pipe diameter to orifice diameter

beta rays - electrons are emitted from a fission or fusion reaction

beta site - a software tester who is actually using the software for practical applications, while looking for bugs. After this stage, software will be released commercially

big-endian - a strategy for storing or transmitting the most significant byte first

BIOS (Basic Input Output System) - a set of basic system calls for accessing hardware, or software services in a computer. This is typically a level lower than the operating system

binary - a base 2 numbering system with the digits 0 and 1

bit - a single binary digit. Typically the symbols 0 and 1 are used to represent the bit value

bit/nibble/byte/word - binary numbers use a 2 value number system (as opposed to the decimal 0-9, binary uses 0-1). A bit refers to a single binary digit, and as we add digits we get larger numbers. A bit is 1 digit, a nibble is 4 digits, a byte is 8 digits, and a word is 16 digits

BITNET (Because It’s Time NET) - An academic network that has been merged with CSNET

blackboard - a computer architecture when different computers share a common memory area (each has its own private area) for sharing/passing information

block - a group of bytes or words

block diagrams - a special diagram for illustrating a control system design

binary - specifies a number system that has 2 digits, or two states

binary number - a collection of binary values that allows numbers to be constructed. A binary number is base 2, whereas normal numbering systems are base 10

blast furnace - a furnace that generates high temperatures by blowing air into the combustion

bleed nozzle - a valve or nozzle for releasing pressure from a system

block diagram - a symbolic diagram that illustrates a system layout and connection. This can be ued for analysis, planning and/or programming

BOC (Bell Operating Company) - there are a total of 7 regional telephone companies in the U.S.A

boiler - a device that will boil water into steam by burning fuel

BOM (Bills Of Materials) - list of materials needed in the production of parts, assemblies, etc. These lists are used to ensure all required materials are available before starting an operation

Boolean - a system of numbers based on logic, instead of real numbers. There are many similarities to normal mathematics and algebra, but a separate set of operators, axioms, etc. are used

bottom-up design - the opposite of top-down design. In this methodology the most simple/basic functions are designed first. These simple elements are then combined into more complex elements. This continues until all of the hierarchical design elements are complete

bounce - switch contacts may not make absolute contact when switching. They make and break contact a few times as they are coming into contact

Bourdon tube - a pressure tube that converts pressure to displacement

BPS (Bits Per Second) - the total number of bits that can be passed between a sender and listener in one second. This is also known as the BAUD rate

branch - a command in a program that can cause it to start running elsewhere

bread board - a term used to describe a temporary electronic mounting board. This is used to prototype a circuit before doing final construction. The main purpose is to verify the basic design

breadth first search - an AI search technique that examines all possible decisions before making the next move

breakaway torque - the start-up torque. The value is typically high, and is a function of friction, inertia, deflection, etc

breakdown torque - the maximum torque that an AC motor can produce at the rated voltage and frequency

bridge - 1. an arrangement of (typically 4) balanced resistors used for measurement. 2. A network device that connects two different networks, and sorts out packets to pass across

broadband networks - multiple frequencies are used with multiplexing to increase the transmission rates in networks

broad-band noise - the noise spectrum for a particular noise source is spread over a large range of frequencies

broadcast - a network term that describes a general broadcast that should be delivered to all clients on a network. For example this is how Ethernet sends all of its packets

brush - a sliding electrical conductor that conducts power to/from a rotor

BSC (Binary Synchronous Communication) - a byte oriented synchronous communication protocol developed by IBM

BSD (Berkeley Software Distribution) - one of the major versions of UNIX

buffer - a temporary area in which data is stored on its way from one place to another. Used for communication bottlenecks and asynchronous connections

bugs - hardware or software problems that prevent desired components operation

burn-in - a high temperature pre-operation to expose system problems

burner - a term often used for a device that programs EPROMs, PALs, etc. or a bad cook

bus - a computer has buses (collections of conductors) to move data, addresses, and control signals between components. For example to get a memory value, the address value provided the binary memory address, the control bus instructs all the devices to read/write, and to examine the address. If the address is valid for one part of the computer, it will put a value on the data bus that the CPU can then read

byte - an 8 bit binary number. The most common unit for modern computers.


C - A programming language that followed B (which followed A). It has been widely used in software development in the 80s and 90s. It has grown up to become C++ and Java

CAA (Computer Aided Analysis) - allows the user to input the definition of a part and calculate the performance variables

cable - a communication wire with electrical and mechanical shielding for harsh environments

CAD (Computer Aided Design) - is the creation and optimization of the design itself using the computer as a productivity tool. Components of CAD include computer graphics, a user interface, and geometric modeling

CAD (Computer Aided Drafting) - is one component of CAD which allows the user to input engineering drawings on the computer screen and print them out to a plotter or other device

CADD (Computer Aided Design Drafting) - the earliest forms of CAD systems were simple electronic versions of manual drafting, and thus are called CADD

CAE (Computer Aided Engineering) - the use of computers to assist in engineering. One example is the use of Finite Element Analysis (FEA) to verify the strength of a design

CAM (Computer Aided Manufacturing) - a family of methods that involves computer supported manufacturing on the factory floor

capacitor - a device for storing energy or mass

capacitance - referring to the ability of a device to store energy. This is used for electrical capacitors, thermal masses, gas cylinders, etc

capacity - the ability to absorb something else

carrier - a high/low frequency signal that is used to transmit another signal

carry flag - an indication when a mathematical operator has gone past the limitations of the hardware/ software

cascade - a method for connecting devices to increase their range, or connecting things so that they operate in sequence. This is also called chaining

CASE (Computer Aided Software Engineering) - software tools are used by the developer/programmer to generate code, track changes, perform testing, and a number of other possible functions

cassette - a holder for audio and data tapes

CCITT (Consultative Committee for International Telegraph and Telephone) - recommended X25. A member of the ITU of the United Nations

CD-ROM (Compact Disc Read Only Memory) - originally developed for home entertainment, these have turned out to be high density storage media available for all platforms at very low prices (< $100 at the bottom end). The storage of these drives is well over 500 MB

CE (Concurrent Engineering) - an engineering method that involves people from all stages of a product design, from marketing to shipping

CE - a mark placed on products to indicate that they conform to the standards set by the European Common Union

Celsius - a temperature scale the uses 0 as the freezing point of water and 100 as the boiling point

centrifugal force - the force on an orbiting object the would cause it to accelerate outwards

centripetal force - the force that must be applied to an orbiting object so that it will not fly outwards

channel - an independent signal pathway

character - a single byte, that when displayed is some recognizable form, such as a letter in the alphabet, or a punctuation mark

checksum - when many bytes of data are transmitted, a checksum can be used to check the validity of the data. It is commonly the numerical sum of all of the bytes transmitted

chip - a loose term for an integrated circuit

chromatography - gases or liquids can be analyzed by how far their constituent parts can migrate through a porous material

CIM (Computer Integrated Manufacturing) - computers can be used at a higher level to track and guide products as they move through the facility. CIM may or may not include CAD/CAM

CL (Cutter Location) - an APT program is converted into a set of x-y-z locations stored in a CL file. In turn these are sent to the NC machine via tapes, etc

clear - a signal or operation to reset data and status values

client-server - a networking model that describes network services, and user programs

clipping - the automatic cutting of lines that project outside the viewing area on a computer screen

clock - a signal from a digital oscillator. This is used to make all of the devices in a digital system work synchronously

clock speed - the rate at which a computers main time clock works at. The CPU instruction speed is usually some multiple or fraction of this number, but true program execution speeds are loosely related at best

closed loop - a system that measures system performance and trims the operation. This is also known as feedback. If there is no feedback the system is called open loop

CMOS (Complimentary Metal Oxide Semi-conductor) - a low power microchip technology that has high noise immunity

CNC (Computer Numerical Control) - machine tools are equipped with a control computer, and will perform a task. The most popular is milling

coalescing - a process for filtering liquids suspended in air. The liquid condenses on glass fibers

coaxial cable - a central wire contains a signal conductor, and an outer shield provides noise immunity. This configuration is limited by its coaxial geometry, but it provides very high noise immunity

coax - see coaxial cable

cogging - a machine steps through motions in a jerking manner. The result may be low frequency vibration

coil - wire wound into a coil (tightly packed helix) used to create electromagnetic attraction. Used in relays, motors, solenoids, etc. These are also used alone as inductors

collisions - when more than one network client tries to send a packet at any one time, they will collide. Both of the packets will be corrupted, and as a result special algorithms and hardware are used to abort the write, wait for a random time, and retry the transmission. Collisions are a good measure of network overuse

colorimetry - a method for identifying chemicals using their colors

combustion - a burning process generating heat and light when certain chemicals are added

command - a computer term for a function that has an immediate effect, such as listing the files in a directory

communication - the transfer of data between computing systems

commutative laws - Booleans algebra laws A+B = B+A and AB=BA

compare - a computer program element that examines one or more variables, determines equality/inequality, and then performs some action, sometimes a branch

compatibility - a measure of the similarity of a design to a standard. This is often expressed as a percentage for software. Anything less than 100% is not desirable

compiler - a tool to change a high level language such as C into assembler

compliment - to take the logical negative. TRUE becomes false and vice versa

component - an interchangeable part of a larger system. Components can be used to cut down manufacturing and maintenance difficulties

compressor - a device that will decrease the volume of a gas - and increase the pressure

computer - a device constructed about a central instruction processor. In general the computer can be reconfigured (software/firmware/hardware) to perform alternate tasks

Computer Graphics - is the use of the computer to draw pictures using an input device to specify geometry and other attributes and an output device to display a picture. It allows engineers to communicate with the computer through geometry

concentric - a shared center between two or more objects

concurrent - two or more activities occur at the same time, but are not necessarily the same

concurrent engineering - all phases of the products life are considered during design, and not later during design review stages

condenser - a system component that will convert steam to water. Typically used in power generators

conduction - the transfer of energy through some medium

configuration - a numbers of multifunction components can be connected in a variety of configurations

connection - a network term for communication that involves first establishing a connection, second data transmission, and third closing the connection. Connectionless networking does not require connection

constant - a number with a value that should not vary

constraints - are performance variables with limits. Constraints are used to specify when a design is feasible

If constraints are not met, the design is not feasible

contact - 1. metal pieces that when touched will allow current to pass, when separated will stop the flow of current. 2. in PLCs contacts are two vertical lines that represent an input, or internal memory location

contactor - a high current relay

continuous Noise - a noise that is ongoing, and present. This differentiates from instantaneous, or intermittent noise sources

continuous Spectrum - a noise has a set of components that are evenly distributed on a spectral graph

control relay - a relay that does not control any external devices directly. It is used like a variable in a high level programming language

control variable - a system parameter that we can set to change the system operation

controls - a system that is attached to a process. Its purpose is to direct the process to some set value

convection - the transfer of heat energy to liquid or gas that is moving past the surface of an object

cook’s constant - another name for the fudge factor

core memory - an outdated term describing memory made using small torii that could be polarized magnetically to store data bits. The term lives on when describing some concepts, for example a ‘core dump’ in UNIX. Believe it or not this has not been used for decades but still appears in many new textbooks

Coriolis force - a force that tends to cause spinning in moving frames of reference. Consider the direction of the water swirl down a drain pipe, it changes from the north to the south of the earth

correction factor - a formal version of the ‘fudge factor’. Typically a value used to multiply or add another value to account for hard to quantify values. This is the friend of the factor of safety

counter - a system to count events. This can be either software or hardware

cps (characters per second) - This can be a good measure of printing or data transmission speed, but it is not commonly used, instead the more confusing ‘baud’ is preferred

CPU (Central Processing Unit) - the main computer element that examines machine code instructions and executes results

CRC (Cyclic Redundancy Check) - used to check transmitted blocks of data for validity

criteria - are performance variables used to measure the quality of a design. Criteria are usually defined in terms of degree - for example, lowest cost or smallest volume or lowest stress. Criteria are used to optimize a design

crosstalk - signals in one conductor induce signals in other conductors, possibly creating false signals

CRT (Cathode Ray Tubes) - are the display device of choice today. A CRT consists of a phosphor-coated screen and one or more electron guns to draw the screen image

crucible - 1. a vessel for holding high temperature materials 2

CSA (Canadian Standards Association) - an association that develops standards and does some product testing

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) - a protocol that causes computers to use the same communication line by waiting for turns. This is used in networks such as Ethernet

CSNET (Computer+Science NETwork) - a large network that was merged with BITNET

CTS (Clear To Send) - used to prevent collisions in asynchronous serial communications

current loop - communications that use a full electronic loop to reduce the effects of induced noise. RS-422 uses this

current rating - this is typically the maximum current that a designer should expect from a system, or the maximum current that an input will draw. Although some devices will continue to work outside rated values, not all will, and thus this limit should be observed in a robust system. Note: exceeding these limits is unsafe, and should be done only under proper engineering conditions

current sink - a device that allow current to flow through to ground when activated

current source - a device that provides current from another source when activated

cursors - are movable trackers on a computer screen which indicate the currently addressed screen position, or the focus of user input. The cursor is usually represented by an arrow, a flashing character or cross-hair

customer requirements - the qualitative and quantitative minimums and maximums specified by a customer

These drive the product design process

cycle - one period of a periodic function

cylinder - a piston will be driven in a cylinder for a variety of purposes. The cylinder guides the piston, and provides a seal between the front and rear of the piston








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