HOME
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
N
NAK (Negative AKnowledgement) - an ASCII control code
NAMUR - A European standards organization
NAND (Not AND) - a Boolean AND operation with the result inverted
narrowband - uses a small data transmission rate to reduce spectral requirements
NC - see normally opened/closed
NC (Numerical Control) - a method for controlling machine tools, such as mills, using simple programs
negative logic - a 0 is a high voltage, and 1 is a low voltage. In Boolean terms it is a duality
NEMA (National Electrical Manufacturers Association) - this group publishes numerous standards for
electrical equipment
nephelometry - a technique for determining the amount of solids suspended in water using light
nesting - a term that describes loops (such as FOR-NEXT loops) within loops in programs
network - a connection of typically more than two computers so that data, email, messages, resources and
files may be shared. The term network implies, software, hardware, wires, etc
NFS (Network File System) - a protocol developed by Sun Microsystems to allow dissimilar computers to
share files. The effect is that the various mounted remote disk drives act as a single local disk
NIC (Network Interface Card) - a computer card that allows a computer to communicate on a network, such
as Ethernet
NIH (Not Invented Here) - a short-lived and expensive corporate philosophy in which employees believe
that if idea or technology was not developed in-house, it is somehow inferior
NIST (National Institute of Standards and Technology) - formerly NBS
NO - see normally opened
node - one computer connected to a network
noise - 1. electrical noise is generated mainly by magnetic fields (also electric fields) that induce currents
and voltages in other conductors, thereby decreasing the signals present. 2. a sound of high
intensity that can be perceived by the human ear
non-fatal error - a minor error that might indicate a problem, but it does not seriously interfere with the
program execution
nonpositive displacement pump - a pump that does not displace a fixed volume of fluid or gas
nonretentive - when power is lost values will be set back to 0
NOR (Not OR) - a Boolean function OR that has the results negated
normally opened/closed - refers to switch types. when in their normal states (not actuated) the normally open
(NO) switch will not conduct current. When not actuated the normally closed (NC) switch will
conduct current
NOT - a Boolean function that inverts values. A 1 will become a 0, and a 0 will become a 1
NOVRAM (NOn Volatile Random Access Memory) - memory that does not lose its contents when turned
off
NPN - a bipolar junction transistor type. When referring to switching, these can be used to sink current to
ground
NPSM - American national standard straight pipe thread for mechanical parts
NPT - American national standard taper pipe thread
NSF (National Science Foundation) - a large funder of science projects in USA
NSFNET (National Science Foundation NETwork) - funded a large network(s) in USA, including a high
speed backbone, and connection to a number of super computers
NTSC (National Television Standards Committee) - a Red-Green-Blue based transmission standard for
video, and audio signals. Very popular in North America, Competes with other standards
internationally, such as PAL
null modem - a cable that connects two RS-232C devices.
O OCR (Optical Character Recognition) - Images of text are scanned in, and the computer will try to interpret it, much as a human who is reading a page would. These systems are not perfect, and often rely on spell checkers, and other tricks to achieve reliabilities up to 99% octal - a base 8 numbering system that uses the digits 0 to 7
Octave - a doubling of frequency
odd parity - a bit is set during communication to indicate when the data should have an odd number of bits
OEM (Original Equipment Manufacturer) - a term for a manufacturer that builds equipment for consumers,
but uses major components from other manufacturers
off-line - two devices are connected, but not communicating
offset - a value is shifted away or towards some target value
one-shot - a switch that will turn on for one cycle
on-line - two devices are put into communications, and will stay in constant contact to pass information as
required
opcode (operation code) - a single computer instruction. Typically followed by one or more operands
open collector - this refers to using transistors for current sourcing or sicking
open loop - a system that does monitor the result. open loop control systems are common when the process
is well behaved
open-system - a computer architecture designed to encourage interconnection between various vendors
hardware and software
operand - an operation has an argument (operand) with the mnemonic command
operating system - software that existing on a computer to allow a user to load/execute/develop their own
programs, to interact with peripherals, etc. Good examples of this is UNIX, MS-DOS, OS/2
optimization - occurs after synthesis and after a satisfactory design is created. The design is optimized by
iteratively proposing a design and using calculated design criteria to propose a better design
optoisolators - devices that use a light emitter to control a photoswitch. The effect is that inputs and outputs
are electrically separate, but connected. These are of particular interest when an interface between
very noisy environments are required
OR - the Boolean OR function
orifice - a small hole. Typically this is places in a fluid/gas flow to create a pressure difference and slow the
flow. It will increase the flow resistance in the system
oscillator - a device that produces a sinusoidal output
oscilloscope - a device that can read and display voltages as a function for time
OSF (Open Software Foundation) - a consortium of large corporations (IBM, DEC, HP) that are promoting
DCE. They have put forth a number of popular standards, such as the Motif Widget set for XWindows
programming
OSHA (Occupational safety and Health Act) - these direct what is safe in industrial and commercial
operations
OSI (Open System Interconnect) - an international standards program to promote computer connectivity,
regardless of computer type, or manufacturer
overshoot - the inertia of a controlled system will cause it to pass a target value and then return
overflow - the result of a mathematical operation passes by the numerical limitations of the hardware logic,
or algorithm.
P
parallel communication - bits are passed in parallel conductors, thus increasing the transmission rates dramatically
parallel design process - evaluates all aspects of the design simultaneously in each iteration. The design
itself is sent to all analysis modules including manufacturability, inspectibility, and engineering
analysis modules; redesign decisions are based on all results at once
parallel programs - theoretically, these computer programs do more than one thing simultaneously
parity - a parity bit is often added to bytes for error detection purposes. The two typical parity methods are
even and odd. Even parity bits are set when an even number of bits are present in the transmitted
data (often 1 byte = 8 bits)
particle velocity - the instantaneous velocity of a single molecule
Pascal - a basic unit of pressure
Pascal’s law - any force applied to a fluid will be transmitted through the fluid and act on all enclosing
surfaces
PC (Programmable Controller) - also called PLC
PCB (Printed Circuit Board) - alternate layers of insulating materials, with wire layout patterns are built up
(sometimes with several layers). Holes thought the layers are used to connect the conductors to
each other, and components inserted into the boards and soldered in place
PDES (Product Data Exchange using Step) - a new product design method that has attempted to include all
needed information for all stages of a products life, including full solids modeling, tolerances, etc
peak level - the maximum pressure level for a cyclic variation
peak-to-peak - the distance between the top and bottom of a sinusoidal variation
peer-to-peer - a communications form where connected devices to both read and write messages at any time
This is opposed to a master slave arrangement
performance variables - are parameters which define the operation of the part. Performance variables are
used by the designer to measure whether the part will perform satisfactorily
period - the time for a repeating pattern to go from beginning to end
peripheral - devices added to computers for additional I/O
permanent magnet - a magnet that retains a magnetic field when the original magnetizing force is removed
petri-net - an enhanced state space diagram that allows concurrent execution flows
pH - a scale for determining is a solution is an acid or a base. 0-7 is acid, 7-4 is a base
photocell - a device that will convert photons to electrical energy
photoconductive cell - a device that has a resistance that will change as the number of incident photons
changes
photoelectric cell - a device that will convert photons to electrical energy
photon - a single unit of light. Light is electromagnetic energy emitted as an electron orbit decays
physical layer - an OSI network model layer
PID (Proportional Integral Derivative) - a linear feedback control scheme that has gained popularity because
of it’s relative simplicity
piezoelectric - a material (crystals/ceramics) that will generate a charge when a force is applied. A common
transducer material
ping - an internet utility that makes a simple connection to a remote machine to see if it is reachable, and if it
is operating
pink noise - noise that has the same amount of energy for each octave
piston - it will move inside a cylinder to convert a pressure to a mechanical motion or vice versa
pitch - a perceptual term for describing frequency. Low pitch means low frequency, high pitch means a
higher frequency
pitot tube - a tube that is placed in a flow stream to measure flow pressure
pixels - are picture elements in a digitally generated and displayed picture. A pixel is the smallest
addressable dot on the display device
PLA (Programmable Logic Array) - an integrated circuit that can be programmed to perform different logic
functions
plane sound wave - the sound wave lies on a plane, not on a sphere
PLC (Programmable Logic Controller) - A rugged computer designs for control on the factory floor
pneumatics - a technique for control and actuation that uses air or gases
PNP - a bipolar junction transistor type. When referring to switching, these can be used to source current
from a voltage source
poise - a unit of dynamic viscosity
polling - various inputs are checked in sequence for waiting inputs
port - 1. an undedicated connector that peripherals may be connected to. 2. a definable connection number
for a machine, or a predefined value
positive displacement pump - a pump that displaces a fixed volume of fluid
positive logic - the normal method for logic implementation where 1 is a high voltage, and 0 is a low
voltage
potentiometer - displacement or rotation is measured by a change in resistance
potting - a process where an area is filled with a material to seal it. An example is a sensor that is filled with
epoxy to protect it from humidity
power level - the power of a sound, relative to a reference level
power rating - this is generally the maximum power that a device can supply, or that it will require. Never
exceed these values, as they may result in damaged equipment, fires, etc
power supply - a device that converts power to a usable form. A typical type uses 115Vac and outputs a DC
voltage to be used by circuitry
PPP (Point-to-Point Protocol) - allows router to router or host to network connections over other
synchronous and asynchronous connections. For example a modem connection can be used to
connect to the internet using PPP
presentation layer - an OSI network model layer
pressure - a force that is distributed over some area. This can be applied to solids and gases
pressure based flow meter - uses difference in fluid pressures to measure speeds
pressure switch - activated above/below a preset pressure level
prioritized control - control operations are chosen on the basic of priorities
procedural language - a computer language where instructions happen one after the other in a clear
sequence
process - a purposeful set of steps for some purpose. In engineering a process is often a machine, but not
necessarily
processor - a loose term for the CPU
program - a sequential set of computer instructions designed to perform some task
programmable controller - another name for a PLC, it can also refer to a dedicated controller that uses a
custom programming language
PROM (Programmable Read Only Memory) -
protocol - conventions for communication to ensure compatibility between separated computers
proximity sensor - a sensor that will detect the presence of a mass nearby without contact. These use a
variety of physical techniques including capacitance and inductance
pull-up resistor - this is used to normally pull a voltage on a line to a positive value. A switch/circuit can be
used to pull it low. This is commonly needed in CMOS devices
pulse - a brief change in a digital signal
purge bubbling - a test to determine the pressure needed to force a gas into a liquid
PVC - poly vinyl chloride - a tough plastic commonly used in electrical and other applications
pyrometer - a device for measuring temperature
Q
QA (Quality Assurance) - a formal system that has been developed to improve the quality of a product
QFD (Quality Functional Deployment) - a matrix based method that focuses the designers on the significant
design problems
quality - a measure of how well a product meets its specifications. Keep in mind that a product that exceeds
its specifications may not be higher quality
quality circles - a team from all levels of a company that meets to discuss quality improvement. Each
members is expected to bring their own perspective to the meeting.
R
rack - a housing for holding electronics modules/cards
rack fault - cards in racks often have error indicator lights that turn on when a fault has occurred. This allows
fast replacement
radar () - radio waves are transmitted and reflected. The time between emission and detection determines the
distance to an object
radiation - the transfer of energy or small particles (e.g., neutrons) directly through space
radiation pyrometry - a technique for measuring temperature by detecting radiated heat
radix - the base value of a numbering system. For example the radix of binary is 2
RAID (Redundant Array of Inexpensive Disks) - a method for robust disk storage that would allow removal
of any disk drive without the interruption of service, or loss of data
RAM (Random Access Memory) - Computer memory that can be read from, and written to. This memory is
the main memory type in computers. The most common types are volatile - they lose their
contents when power is removed
random noise - there are no periodic waveforms, frequency and magnitude vary randomly
random-scan devices - draw an image by refreshing one line or vector at a time; hence they are also called
vector-scan or calligraphic devices. The image is subjected to flicker if there are more lines in the
scene that can be refreshed at the refresh rate
Rankine - A temperature system that uses absolute 0 as the base, and the scale is the same as the Fahrenheit
scale
raster devices - process pictures in parallel line scans. The picture is created by determining parts of the
scene on each scan line and painting the picture in scan-line order, usually from top to bottom
Raster devices are not subject to flicker because they always scan the complete display on each
refresh, independent of the number of lines in the scene
rated - this will be used with other terms to indicate suggested target/maximum/minimum values for
successful and safe operation
RBOC (Regional Bell Operating Company) - A regional telephone company. These were originally created
after a US federal court split up the phone company into smaller units
Read/Write (R/W) - a digital device that can store and retrieve data, such as RAM
reagent - an chemical used in one or more chemical reactions. these are often used for identifying other
chemicals
real-time - suggests a system must be able to respond to events that are occurring outside the computer in a
reasonable amount of time
reciprocating - an oscillating linear motion
redundancy - 1. added data for checking accuracy. 2. extra system components or mechanisms added to
decrease the chance of total system failure
refreshing - is required of a computer screen to maintain the screen image. Phosphors, which glow to show
the image, decay at a fast rate, requiring the screen to be redrawn or refreshed several times a
second to prevent the image from fading
regenerative braking - the motor windings are reverse, and in effect return power to the power source. This is
highly efficient when done properly
register - a high speed storage area that can typically store a binary word for fast calculation. Registers are
often part of the CPU
regulator - a device to maintain power output conditions (such as voltage) regardless of the load
relay - an electrical switch that comes in may different forms. The switch is activated by a magnetic coil that
causes the switch to open or close
relay - a magnetic coil driven switch. The input goes to a coil. When power is applied, the coil generates a
magnetic field, and pulls a metal contact, overcoming a spring, and making contact with a
terminal. The contact and terminal are separately wired to provide an output that is isolated from
the input
reliability - the probability of failure of a device
relief valve - designed to open when a pressure is exceeded. In a hydraulic system this will dump fluid back
in the reservoir and keep the system pressure constant
repeatability - the ability of a system to return to the same value time after time. This can be measured with
a standard deviation
repeater - added into networks to boost signals, or reduce noise problems. In effect one can be added to the
end of one wire, and by repeating the signals into another network, the second network wire has a
full strength signal
reset - a signal to computers that restarts the processor
resistance - this is a measurable resistance to energy or mass transfer
resistance heating - heat is generated by passing a current through a resistive material
resolution - the smallest division or feature size in a system
resonant frequency - the frequency at which the material will have the greatest response to an applied
vibration or signal. This will often be the most likely frequency of self destruction
response time - the time required for a system to respond to a directed change
return - at the end of a subroutine, or interrupt, the program execution will return to where it branched
reverberation - when a sound wave hits a surface, part is reflected, and part is absorbed. The reflected part
will add to the general (reverberant) sound levels in the room
Reynolds number - a dimensionless flow value based on fluid density and viscosity, flow rate and pipe
diameter
RF (Radio Frequency) - the frequency at which a magnetic field oscillates when it is used to transmit a
signal. Normally this range is from about 1MHz up to the GHz
RFI (Radio Frequency Interference) - radio and other changing magnetic fields can generate unwanted
currents (and voltages) in wires. The resulting currents and voltages can interfere with the normal
operation of an electrical device. Filters are often used to block these signals
RFS (Remote File System) - allows shared file systems (similar to NFS), and has been developed for System
V UNIX
RGB (Red Green Blue) - three additive colors that can be used to simulate the other colors of the spectrum
This is the most popular scheme for specifying colors on computers. The alternate is to use Cyan-
Magenta-Yellow for the subtractive color scheme
ripple voltage - when an AC voltage is converted to DC it is passed through diodes that rectify it, and then
through capacitors that smooth it out. A small ripple still remains
RISC (Reduced Instruction Set Computer) - the more standard computer chips were CISC (Complete
Instruction Set Computers) but these had architecture problems that limited speed. To overcome
this the total number of instructions were reduced, allowing RISC computers to execute faster,
but at the cost of larger programs
rlogin - allows a text based connection to a remote computer system in UNIX
robustness - the ability of a system to deal with and recover from unexpected input conditions
ROM (Read Only Memory) - a permanent form of computer memory with contents that cannot be
overwritten. All computers contain some ROM to store the basic operating system - often called
the BIOS in personal computers
rotameter - for measuring flow rate with a plug inside a tapered tube
router - as network packets travel through a network, a router will direct them towards their destinations
using algorithms
RPC (Remote Procedure Call) - a connection to a specific port on a remote computer will request that a
specific program be run. Typical examples are ping, mail, etc
RS-232C - a serial communication standard for low speed voltage based signals, this is very common on
most computers. But, it has a low noise immunity that suggests other standards in harsh
environments
RS-422 - a current loop based serial communication protocol that tends to perform well in noisy
environments
RS-485 - uses two current loops for serial communications
RTC (Real-Time Clock) - A clock that can be used to generate interrupts to keep a computer process or
operating system running at regular intervals
RTD (Resistance Temperature Detector) - as temperature is changed the resistance of many materials will
also change. We can measure the resistance to determine the temperature
RTS (Request To Send) - A data handshaking line that is used to indicate when a signal is ready for
transmission, and clearance is requested
rung - one level of logic in a ladder logic program or ladder diagram
R/W (Read/Write) - A digital line that is used to indicate if data on a bus is to be written to, or read from
memory. |
