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Programmable logic controllers ( plc) combined glossary of terms.



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.


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


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.


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.






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