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Programmable logic controllers ( plc) combined glossary of terms.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

D

daisy chain - allows serial communication of devices to transfer data through each (and every) device between two points

Darlington coupled - two transistors are ganged together by connecting collectors to bases to increase the gain. These increase the input impedance, and reduce the back propagation of noise from loads

DARPA (Defense Advanced Research Projects Agency) - replaced ARPA. This is a branch of the US department of defense that has participated in a large number of research projects

data acquisition - refers to the automated collection of information collected from a process or system

data highway - a term for a communication bus between two separated computers, or peripherals. This term is mainly used for PLC’s

data link layer - an OSI model layer data logger - a dedicated system for data acquisition

data register - stores data values temporarily in a CPU

database - a software program that stores and recalls data in an organized way

DARPA (Defense Advanced Research Projects Agency) - DC (Direct Current) - a current that flows only in one direction. The alternative is AC

DCA (Defense Communications Agency) - developed DDN

DCD (Data Carrier Detect) - used as a handshake in asynchronous communication

DCE (Data Communications Equipment) - A term used when describing unintelligent serial communications clients. An example of this equipment is a modem. The complement to this is DTE

DCE (Distributed Computing Environment) - applications can be distributed over a number of computers because of the use of standards interfaces, functions, and procedures

DDN (Defense Data Network) - a group of DoD networks, including MILNET

dead band - a region for a device when it no longer operates

dead time - a delay between an event occurring and the resulting action

debounce - a switch may not make sudden and complete contact as it is closes, circuitry can be added to remove a few on-off transitions as the switch mechanically bounces

debug - after a program has been written it undergoes a testing stage called debugging that involves trying to locate and eliminate logic and other errors. This is also a time when most engineers deeply regret not spending more time on the initial design

decibel (dB) - a logarithmic compression of values that makes them more suited to human perception (for both scalability and reference) decision support - the use of on-line data, and decision analysis tools are used when making decisions. One example is the selection of electronic components based on specifications, projected costs, etc

DECnet (Digital Equipment Corporation net) - a proprietary network architecture developed by DEC

decrement - to decrease a numeric value

dedicated computer - a computer with only one task

default - a standard condition

demorgan’s laws - Boolean laws great for simplifying equations ~(AB) = ~A + ~B, or ~(A+B) = ~A~B

density - a mass per unit volume

depth first search - an artificial intelligence technique that follows a single line of reasoning first

derivative control - a control technique that uses changes in the system of setpoint to drive the system. This control approach gives fast response to change

design - creation of a new part/product based on perceived needs. Design implies a few steps that are ill defined, but generally include, rough conceptual design, detailed design, analysis, redesign, and testing

design capture - the process of formally describing a design, either through drafted drawings, schematic drawings, etc

design cycle - the steps of the design. The use of the word cycle implies that it never ends, although we must at some point decide to release a design

design Variables - are the parameters in the design that describe the part. Design variables usually include geometric dimensions, material type, tolerances, and engineering notes

detector - a device to determine when a certain condition has been met

device driver - controls a hardware device with a piece of modular software

DFA (Design For Assembly) - a method that guides product design/redesign to ease assembly times and difficulties

DFT (Design for Testability) - a set of design axioms that generally calls for the reduction of test steps, with the greatest coverage for failure modes in each test step

diagnostic - a system or set of procedures that may be followed to identify where systems may have failed

These are most often done for mission critical systems, or industrial machines where the user may not have the technical capability to evaluate the system

diaphragm - used to separate two materials, while allowing pressure to be transmitted

differential - refers to a relative difference between two values. Also used to describe a calculus derivative operator

differential amplifier - an amplifier that will subtract two or more input voltages

diffuse field - multiple reflections result in a uniform and high sound pressure level

digital - a system based on binary on-off values

diode - a semiconductor device that will allow current to flow in one direction

DIP switches - small banks of switches designed to have the same footprint as an integrated circuit

distributed - suggests that computer programs are split into parts or functions and run on different computers distributed system - a system can be split into parts. Typical components split are mechanical, computer, sensors, software, etc

DLE (Data Link Escape) - An RS-232 communications interface line

DMA (Direct Memory Access) - used as a method of transferring memory in and out of a computer without slowing down the CPU

DNS (Domain Name System) - an internet method for name and address tracking

documentation - (don’t buy equipment without it) - one or more documents that instruct in the use, installation, setup, maintenance, troubleshooting, etc. for software or machinery. A poor design supported by good documentation can often be more useful than a good design unsupported by poor documentation

domain - the basic name for a small or large network. For example (unc.edu) is the general extension for the University on North Carolina

Doppler shift - as objects move relative to each other, a frequency generated by one will be perceived at another frequency by the other

DOS (Disk Operating System) - the portion of an operating system that handles basic I/O operations. The most common example is Microsoft MS-DOS for IBM PCs

dotted decimal notation - the method for addressing computers on the internet with IP numbers such as ‘129.100.100.13’

double pole - a double pole switch will allow connection between two contacts. These are useful when making motor reversers. see also single pole

double precision - a real number is represented with 8 bytes (single precision is 4) to give more precision for calculations

double throw - a switch or relay that has two sets of contacts

download - to retrieve a program from a server or higher level computer

downtime - a system is removed from production for a given amount of downtime

drag - a force that is the result of a motion of an object in a viscous fluid

drop - a term describing a short connection to peripheral I/O

drum sequencer - a drum has raised/lowered sections and as it rotates it opens/closes contacts and will give sequential operation

dry contact - an isolated output, often a relay switched output

DSP (Digital Signal Processor) - a medium complexity microcontroller that has a build in floating point unit

These are very common in devices such as modems

DSR (Data Set Ready) - used as a data handshake in asynchronous communications

DTE (Data Terminal Equipment) - a serial communication line used in RS-232 DTR (Data Terminal Ready) - used as a data handshake in asynchronous communications to indicate a listener is ready to receive data

dump - a large block of memory is moved at once (as a sort of system snapshot)

duplex - serial communication that is in both directions between computers at the same time

dynamic braking - a motor is used as a brake by connecting the windings to resistors. In effect the motor becomes a generator, and the resistors dissipate the energy as heat

dynamic variable - a variable with a value that is constantly changing

dyne - a unit of force

E

EBCDIC (Extended Binary-Coded Decimal Information Code) - a code for representing keyboard and control characters

eccentric - two or more objects do not have a common center

echo - a reflected sound wave

ECMA (European Computer Manufacturer’s Associated) - eddy currents - small currents that circulate in metals as currents flow in nearby conductors. Generally unwanted

EDIF (Electronic Design Interchange Format) - a standard to allow the interchange of graphics and data between computers so that it may be changed, and modifications tracked

EEPROM (Electrically Erasable Programmable Read Only Memory) - effective sound pressure - the RMS pressure value gives the effective sound value for fluctuating pressure values. This value is some fraction of the peak pressure value

EIA (Electronic Industries Association) - A common industry standards group focusing on electrical standards

electro-optic isolator - uses optical emitter, and photo sensitive switches for electrical isolation

electromagnetic - a broad range term referring to magnetic waves. This goes from low frequency signals such as AM radio, up to very high frequency waves such as light and X-rays

electrostatic - devices that used trapped charge to apply forces and caused distribution. An example is droplets of paint that have been electrically charged can be caused to disperse evenly over a surface that is oppositely charged

electrostatics discharge - a sudden release of static electric charge (in nongrounded systems). This can lead to uncomfortable electrical shocks, or destruction of circuitry

email (electronic mail) - refers to messages passed between computers on networks, that are sent from one user to another. Almost any modern computer will support some for of email

EMI (ElectroMagnetic Interference) - transient magnetic fields cause noise in other systems

emulsify - to mix two materials that would not normally mix. for example an emulsifier can cause oil and water to mix

enable - a digital signal that allows a device to work

encoding - a conversion between different data forms

energize - to apply power to a circuit or component

energy - the result of work. This concept underlies all of engineering. Energy is shaped, directed and focused to perform tasks

engineering work stations - are self contained computer graphics systems with a local CPU which can be networked to larger computers if necessary. The engineering work station is capable of performing engineering synthesis, analysis, and optimization operations locally. Work stations typically have more than 1 MByte of RAM, and a high resolution screen greater than 512 by 512 pixels

EOH (End of Header) - A code in a message header that marks the end of the header block

EOT (End Of Transmission) - an ASCII code to indicate the end of a communications

EPROM (Erasable Programmable Read Only Memory) - a memory type that can be programmed with voltages, and erased with ultraviolet light

EPS (Encapsulated PostScript) - a high quality graphics description language understood by high end printers. Originally developed by Adobe Systems Limited. This standard is becoming very popular

error signal - a control signal that is the difference between a desired and actual position

ESD - see electrostatic discharge

esters - a chemical that was formed by a reaction between alcohol and an acid

ETX (End Of Text) - a marker to indicate the end of a text block in data transmission

even parity - a checksum bit used to verify data in other bits of a byte

execution - when a computer is under the control of a program, the program is said to be executing

expansion principle - when heat is applied a liquid will expand

expert systems - is a branch of artificial intelligence designed to emulate human expertise with software

Expert systems are in use in many arenas and are beginning to be seen in CAD systems. These systems use rules derived from human experts.

F

fail safe - a design concept where system failure will bring the system to an idle or safe state

false - a logical negative, or zero

Faraday’s electromagnetic induction law - if a conductor moves through a magnetic field a current will be induced. The angle between the motion and the magnetic field needs to be 90 deg for maximum current

Fahrenheit - a temperature system that has 180 degrees between the freezing and boiling point of water

fatal error - an error so significant that a software/hardware cannot continue to operate in a reliable manner

fault - a small error that may be recoverable, or may result in a fatal error

FAX (facsimile) - an image is scanned and transmitted over phone lines and reconstructed at the other end

FCS (Frame Check Sequence) - data check flag for communications

FDDI (Fiber Distributed Data Interface) - a fiber optic token ring network scheme in which the control tokens are counter rotating

FDX (Full Duplex) - all characters that are transmitted are reflected back to the sender

FEA (Finite Element Analysis) - is a numerical technique in which the analysis of a complex part is subdivided into the analysis of small simple subdivisions

feedback - a common engineering term for a system that examines the output of a system and uses is to tune the system. Common forms are negative feedback to make systems stable, and positive feedback to make systems unstable (e.g. oscillators)

fetch - when the CPU gets a data value from memory

fiber optics - data can be transmitted by switching light on/off, and transmitting the signal through an optical fiber. This is becoming the method of choice for most long distance data lines because of the low losses and immunity to EMI

FIFO (First In First Out) - items are pushed on a stack. The items can then be pulled back off last first

file - a concept of a serial sequence of bytes that the computer can store information in, normally on the disk

This is a ubiquitous concept, but file is also used by Allen Bradley to describe an array of data

filter - a device that will selectively pass matter or energy

firmware - software stored on ROM (or equivalent)

flag - a single binary bit that indicates that an event has/has not happened

flag - a single bit variable that is true or not. The concept is that if a flag is set, then some event has happened, or completed, and the flag should trigger some other event

flame - an email, or netnews item that is overtly critical of another user, or an opinion. These are common because of the ad-hoc nature of the networks

flange - a thick junction for joining two pipes

floating point - uses integer math to represent real numbers

flow chart - a schematic diagram for representing program flow. This can be used during design of software, or afterwards to explain its operation

flow meter - a device for measuring the flow rate of fluid

flow rate - the volume of fluid moving through an area in a fixed unit of time

fluorescence - incoming UV light or X-ray strike a material and cause the emission of a different frequency light

FM (Frequency Modulation) - transmits a signal using a carrier of constant magnitude but changing frequency. The frequency shift is proportional to the signal strength

force - a PLC output or input value can be set on artificially to test programs or hardware. This method is not suggested

format - 1. a physical and/or data structure that makes data rereadable, 2. the process of putting a structure on a disk or other media

forward chaining - an expert system approach to examine a set of facts and reason about the probable outcome

fragmentation - the splitting of an network data packet into smaller fragments to ease transmission

frame buffers - store the raster image in memory locations for each pixel. The number of colors or shades of gray for each pixel is determined by the number of bits of information for each pixel in the frame buffer

free field - a sound field where none of the sound energy is reflected. Generally there aren’t any nearby walls, or they are covered with sound absorbing materials

frequency - the number of cycles per second for a sinusoidally oscillating vibration/sound

friction - the force resulting from the mechanical contact between two masses

FSK (Frequency Shift Keying) - uses two different frequencies, shifting back and forth to transmit bits serially

FTP (File Transfer Protocol) - a popular internet protocol for moving files between computers

fudge factor - a number that is used to multiply or add to other values to make the experimental and theoretical values agree

full duplex - a two way serial communication channel can carry information both ways, and each character that is sent is reflected back to the sender for verification

fuse - a device that will destruct when excessive current flows. It is used to protect the electrical device, humans, and other devices when abnormally high currents are drawn. Note: fuses are essential devices and should never be bypassed, or replaced with fuses having higher current rating.

 

 

 

 

 

 

 

 
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