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Programmable logic controllers ( plc) combined glossary of terms.



galvonometer - a simple device used to measure currents. This device is similar to a simple DC motor

gamma rays - high energy electromagnetic waves resulting from atomic fission or fusion

gate - 1. a circuit that performs on of the Boolean algebra function (i.e., and, or, not, etc.) 2. a connection between a runner and a part, this can be seen on most injection molded parts as a small bump where the material entered the main mold cavity

gateway - translates and routes packets between dissimilar networks

Geiger-Mueller tube - a device that can detect ionizing particles (eg, atomic radiation) using a gas filled tube

global optimum - the absolute best solution to a problem. When found mathematically, the maximum or minimum cost/utility has been obtained

gpm (gallons per minute) - a flow rate

grafcet - a method for programming PLCs that is based on Petri nets. This is now known as SFCs and is part of the IEC 1131-3 standard

gray code - a modified binary code used for noisy environments. It is devised to only have one bit change at any time. Errors then become extremely obvious when counting up or down

ground - a buried conductor that acts to pull system neutral voltage values to a safe and common level. All electrical equipment should be connected to ground for safety purposes

GUI (Graphical User Interface) - the user interacts with a program through a graphical display, often using a mouse. This technology replaces the older systems that use menus to allow the user to select actions.


half cell - a probe that will generate a voltage proportional to the hydrogen content in a solution

half duplex - see HDX handshake - electrical lines used to establish and control communications

hard copy - a paper based printout

hardware - a mechanical or electrical system. The ‘functionality’ is ‘frozen’ in hardware, and often difficult to change

HDLC (High-level Data Link Control) - an ISO standard for communications

HDX (Half Duplex) - a two way serial connection between two computer. Unlike FDX, characters that are sent are not reflected back to the sender

head - pressure in a liquid that is the result of gravity

hermetic seal - an airtight seal

hertz - a measure of frequency in cycles per second. The unit is Hz

hex - see hexadecimal

hexadecimal - a base 16 number system where the digits are 0 to 9 then A to F, to give a total of 16 digits

This is commonly used when providing numbers to computers

high - another term used to describe a Boolean true, logical positive, or one

high level language - a language that uses very powerful commands to increase programming productivity

These days almost all applications use some form of high level language (i.e., basic, Fortran, Pascal, C, C++, etc.)

horsepower - a unit for measuring power host - a networked (fully functional) computer

hot backup - a system on-line that can quickly replace a failed system

hydraulic - 1. a study of water 2. systems that use fluids to transmit power

hydrocarbon - a class of molecules that contain carbon and hydrogen. Examples are propane, octane

hysteresis - a sticking or lagging phenomenon that occurs in many systems. For example, in magnetic systems this is a small amount of magnetic repolarization in a reversing field, and in friction this is an effect based on coulomb friction that reverses sticking force

Hz - see hertz


IAB (internet Activities Board) - the developer of internet standards

IC (Integrated Circuit) - a microscopic circuit placed on a thin wafer of semiconductor

IEC (International Electrical Commission) - IEEE (Institute of Electrical and Electronics Engineers) - IEEE802 - a set of standards for LANs and MANs

IGES (Initial Graphics Exchange Specification) - a standard for moving data between various CAD systems

In particular the format can handle basic geometric entities, such as NURBS, but it is expected to be replaced by PDES/STEP in the near future

impact instrument - measurements are made based by striking an object. This generally creates an impulse function

impedance - In electrical systems this is both reactive and real resistance combined. This also applies to power transmission and flows in other types of systems

impulse Noise - a short duration, high intensity noise. This type of noise is often associated with explosions

increment - increase a numeric value

inductance - current flowing through a coil will store energy in a magnetic field

inductive heating - a metal part is placed inside a coil. A high frequency AC signal is passed through the coil and the resulting magnetic field melts the metal

infrared - light that has a frequency below the visible spectrum

inertia - a property where stored energy will keep something in motion unless there is energy added or released

inference - to make a decision using indirect logic. For example if you are wearing shoes, we can infer that you had to put them on. Deduction is the complementary concept

inference engine - the part of an expert system that processes rules and facts using forward or backward chaining

Insertion Loss - barriers, hoods, enclosures, etc. can be placed between a sound source, and listener, their presence increases reverberant sound levels and decreases direct sound energy. The increase in the reverberant sound is the insertion loss

instruction set - a list of all of the commands that available in a programmable system. This could be a list of PLC programming mnemonics, or a list of all of the commands in BASIC

instrument - a device that will read values from external sensors or probes, and might make control decision

intake stroke - in a piston cylinder arrangement this is the cycle where gas or liquid is drawn into the cylinder

integral control - a control method that looks at the system error over a long period of time. These controllers are relatively immune to noise and reduce the steady state error, but the do not respond quickly

integrate - to combine two components with clearly separable functions to obtain a new single component capable of more complex functions

intelligence - systems will often be able to do simple reasoning or adapt. This can mimic some aspects of human intelligence. These techniques are known as artificial intelligence

intelligent device - a device that contains some ability to control itself. This reduces the number of tasks that a main computer must perform. This is a form of distributed system

interface - a connection between a computer and another electrical device, or the real world

interlock - a device that will inhibit system operation until certain conditions are met. These are often required for safety on industrial equipment to protect workers

intermittent noise - when sounds change level fluctuate significantly over a measurement time period

internet - an ad-hoc collection of networks that has evolved over a number of years to now include millions of computers in every continent, and by now every country. This network will continue to be the defacto standard for personal users. (commentary: The information revolution has begun already, and the internet has played a role previously unheard of by overcoming censorship and misinformation, such as that of Intel about the Pentium bug, a military coup in Russia failed because they were not able to cut off the flow of information via the internet, the Tiananmen square massacre and related events were widely reported via internet, etc. The last stage to a popular acceptance of the internet will be the World Wide Web accessed via Mosaic/Netscape.) internet address - the unique identifier assigned to each machine on the internet. The address is a 32 bit binary identifier commonly described with the dotted decimal notation

interlacing - is a technique for saving memory and time in displaying a raster image. Each pass alternately displays the odd and then the even raster lines. In order to save memory, the odd and even lines may also contain the same information

interlock - a flag that ensures that concurrent streams of execution do not conflict, or that they cooperate

interpreter - programs that are not converted to machine language, but slowly examined one instruction at a time as they are executed

interrupt - a computer mechanism for temporarily stopping a program, and running another

inverter - a logic gate that will reverse logic levels from TRUE to/from FALSE

I/O (Input/Output) - a term describing anything that goes into or out of a computer

IOR (Inclusive OR) - a normal OR that will be true when any of the inputs are true in any combinations. also see Exclusive OR (EOR)

ion - an atom, molecule or subatomic particle that has a positive or negative charge

IP (internet Protocol) - the network layer (OSI model) definitions that allow internet use

IP datagram - a standard unit of information on the internet

ISDN (Integrated Services Digital Network) - a combined protocol to carry voice, data and video over 56KB lines

ISO (International Standards Organization) - a group that develops international standards in a wide variety of areas

isolation - electrically isolated systems have no direct connection between two halves of the isolating device

Sound isolation uses barriers to physically separate rooms

isolation transformer - a transformer for isolating AC systems to reduce electrical noise.


JEC (Japanese Electrotechnical Committee) - A regional standards group

JIC (Joint International Congress) - an international standards group that focuses on electrical standards

They drafted the relay logic standards

JIT (Just in Time) - a philosophy when setting up and operating a manufacturing system such that materials required arrive at the worksite just in time to be used. This cuts work in process, storage space, and a number of other logistical problems, but requires very dependable supplies and methods

jog - a mode where a motor will be advanced while a button is held, but not latched on. It is often used for clearing jams, and loading new material

jump - a forced branch in a program jumper - a short wire, or connector to make a permanent setting of hardware parameters.


k, K - specifies magnitudes. 1K = 1024, 1k = 1000 for computers, otherwise 1K = 1k = 1000. Note - this is not universal, so double check the meanings when presented

Kelvin - temperature units that place 0 degrees at absolute zero. The magnitude of one degree is the same as the Celsius scale

KiloBaud, KBaud, KB, Baud - a transmission rate for serial communications (e.g. RS-232C, TTY, RS-422)

A baud = 1bit/second, 1 Kilobaud = 1KBaud = 1KB = 1000 bits/second. In serial communication each byte typically requires 11 bits, so the transmission rate is about 1Kbaud/11 = 91 Bytes per second when using a 1KB transmission

Karnaugh maps - a method of graphically simplifying logic

kermit - a popular tool for transmitting binary and text files over text oriented connections, such as modems or telnet sessions

keying - small tabs, prongs, or fillers are used to stop connectors from mating when they are improperly oriented

kinematics/kinetics - is the measure of motion and forces of an object. This analysis is used to measure the performance of objects under load and/or in motion.


label - a name associated with some point in a program to be used by branch instructions

ladder diagram - a form of circuit diagram normally used for electrical control systems

ladder logic - a programming language for PLCs that has been developed to look like relay diagrams from the preceding technology of relay based controls

laminar flow - all of the particles of a fluid or gas are travelling in parallel. The complement to this is turbulent flow

laptop - a small computer that can be used on your lap. It contains a monitor ad keyboard

LAN (Local Area Network) - a network that is typically less than 1km in distance. Transmission rates tend to be high, and costs tend to be low

latch - an element that can have a certain input or output lock in. In PLCs these can hold an output on after an initial pulse, such as a stop button

LCD (Liquid Crystal Display) - a fluid between two sheets of light can be polarized to block light. These are commonly used in low power displays, but they require backlighting

leakage current - a small amount of current that will be present when a device is off

LED (Light Emitting Diode) - a semiconductor light that is based on a diode

LIFO (Last In First Out) - similar to FIFO, but the last item pushed onto the stack is the first pulled off

limit switch - a mechanical switch actuated by motion in a process

line printer - an old printer style that prints single lines of text. Most people will be familiar with dot matrix style of line printers

linear - describes a mathematical characteristic of a system where the differential equations are simple linear equations with coefficients

little-endian - transmission or storage of data when the least significant byte/bit comes first

load - In electrical system a load is an output that draws current and consumes power. In mechanical systems it is a mass, or a device that consumes power, such as a turbine

load cell - a device for measuring large forces

logic - 1. the ability to make decisions based on given values. 2. digital circuitry

loop - part of a program that is executed repeatedly, or a cable that connects back to itself

low - a logic negative, or zero

LRC (Linear Redundancy Check) - a block check character LSB (Least Significant Bit) - This is the bit with the smallest value in a binary number. for example if the number 10 is converted to binary the result is 1010. The most significant bit is on the left side, with a value of 8, and the least significant bit is on the right with a value of 1 - but it is not set in this example

LSD (Least Significant Digit) - This is the least significant digit in a number, found on the right side of a number when written out. For example, in the number $1,234,567 the digit 7 is the least significant

LSI (Large Scale Integration) - an integrated circuit that contains thousands of elements

LVDT (Linear Variable Differential Transformer) - a device that can detect linear displacement of a central sliding core in the transformer.


machine language - CPU instructions in numerical form

macro - a set of commands grouped for convenience

magnetic field - a field near flowing electrons that will induce other electrons nearby to flow in the opposite direction

MAN (Metropolitan Area Network) - a network designed for municipal scale connections

manifold - 1. a connectors that splits the flow of fluid or gas. These are used commonly in hydraulic and pneumatic systems. 2. a description for a geometry that does not have any infinitely small points or lines of contact or separation. Most solid modelers deal only with manifold geometry

MAP (Manufacturers Automation Protocol) - a network type designed for the factory floor that was widely promoted in the 1980s, but was never widely implemented due to high costs and complexity

mask - one binary word (or byte, etc) is used to block out, or add in digits to another binary number

mass flow rate - instead of measuring flow in terms of volume per unit of time we use mass per unit time

mass spectrometer - an instrument that identifies materials and relative proportions at the atomic level. This is done by observing their deflection as passed through a magnetic field

master/slave - a control scheme where one computer will control one or more slaves. This scheme is used in interfaces such as GPIB, but is increasingly being replaced with peer-to-peer and client/server networks

mathematical models - of an object or system predict the performance variable values based upon certain input conditions. Mathematical models are used during analysis and optimization procedures

matrix - an array of numbers MB MByte, KB, KByte - a unit of memory commonly used for computers. 1 KiloByte = 1 KByte = 1 KB = 1024 bytes. 1 MegaByte = 1 MByte = 1MB = 1024*1024 bytes

MCR (Master Control Relay) - a relay that will shut down all power to a system

memory - binary numbers are often stored in memory for fast recall by computers. Inexpensive memory can be purchased in a wide variety of configurations, and is often directly connected to the CPU

memory - memory stores binary (0,1) patterns that a computer can read or write as program or data. Various types of memories can only be read, some memories lose their contents when power is off

RAM (Random Access Memory) - can be written to and read from quickly

It requires power to preserve the contents, and is often coupled with a battery or capacitor when long term storage is required. Storage available is over 1MByte ROM (Read Only Memory) - Programs and data are permanently written on this low cost ship. Storage available is over 1 MByte

EPROM (ELECTRICALLY Programmable Read Only Memory) - A program can be written to this memory using a special programmer, and erased with ultraviolet light. Storage available over 1MByte. After a program is written, it does not require power for storage. These chips have small windows for ultraviolet light

EEPROM/E2PROM (Electronically Erasable Programmable Read Only Memory) - These chips can be erased and programmed while in use with a computer, and store memory that is not sensitive to power. These can be slower, more expensive and with lower capacity (measured in Kbytes) than other memories. But, their permanent storage allows system configurations/ data to be stored indefinitely after a computer is turned off

memory map - a listing of the addresses of different locations in a computer memory. Very useful when programming

menu - a multiple choice method of selecting program options

message - a short sequence of data passed between processes

microbar - a pressure unit (1 dyne per sq. cm) microphone - an audio transducer (sensor) used for sound measurements

microprocessor - the central control chip in a computer. This chip will execute program instructions to direct the computer

MILNET (MILitary NETwork) - began as part of ARPANET

MMI (Man Machine Interface) - a user interface terminal

mnemonic - a few characters that describe an operation. These allow a user to write programs in an intuitive manner, and have them easily converted to CPU instructions

MODEM (MOdulator/DEModulator) - a device for bidirectional serial communications over phone lines, etc

module - a part o a larger system that can be interchanged with others

monitor - an operation mode where the computer can be watched in detail from step to step. This can also refer to a computer screen

motion detect flow meter - a fluid flow induces measurement

MRP (Material Requirements Planning) - a method for matching material required by jobs, to the equipment available in the factory

MSD (Most Significant Digit) - the largest valued digit in a number (eg. 6 is the MSD in 63422). This is often used for binary numbers

MTBF (Mean Time Between Failure) - the average time (hours usually) between the last repair of a product, and the next expected failure

MTTR (Mean Time To Repair) - The average time that a device will out of use after failure before it is repaired. This is related to the MTBF

multicast - a broadcast to some, but not necessarily all, hosts on a network

multiplexing - a way to efficiently use transmission media by having many signals run through one conductor, or one signal split to run through multiple conductors and rejoined at the receiving end

multiprocessor - a computer or system that uses more than one computer. Normally this term means a single computer with more than one CPU. This scheme can be used to increase processing speed, or increase reliability

multivibrator - a digital oscillator producing square or rectangular waveforms.







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