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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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