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Programmable logic controllers ( plc) combined glossary of terms.



safety margin - a factor of safety between calculated maximums and rated maximums

SCADA (Supervisory Control And Data Acquisition) - computer remote monitoring and control of processes

scan-time - the time required for a PLC to perform one pass of the ladder logic

schematic - an abstract drawing showing components in a design as simple figures. The figures drawn are often the essential functional elements that must be considered in engineering calculations

scintillation - when some materials are high by high energy particles visible light or electromagnetic radiation is produced SCR (Silicon Controlled Rectifier) - a semiconductor that can switch AC loads

SDLC (Synchronous Data-Link Control) - IBM oriented data flow protocol with error checking

self-diagnosis - a self check sequence performed by many operation critical devices

sensitivity - the ability of a system to detect a change

sensor - a device that is externally connected to survey electrical or mechanical phenomena, and convert them to electrical or digital values for control or monitoring of systems

serial communication - elements are sent one after another. This method reduces cabling costs, but typically also reduces speed, etc

serial design - is the traditional design method. The steps in the design are performed in serial sequence. For example, first the geometry is specified, then the analysis is performed, and finally the manufacturability is evaluated

servo - a device that will take a desired operation input and amplify the power

session layer - an OSI network model layer

setpoint - a desired value for a controlled system

shield - a grounded conducting barrier that steps the propagation of electromagnetic waves

Siemens - a measure of electrical conductivity

signal conditioning - to prepare an input signal for use in a device through filtering, amplification, integration, differentiation, etc

simplex - single direction communication at any one time

simulation - a model of the product/process/etc is used to estimate the performance. This step comes before the more costly implementation steps that must follow

single-discipline team - a team assembled for a single purpose

single pole - a switch or relay that can only be opened or closed. See also single pole

single throw - a switch that will only switch one line. This is the simplest configuration

sinking - using a device that when active will allow current to flow through it to ground. This is complimented by sourcing

SLIP (Serial Line internet Protocol) - a method to run the internet Protocol (IP) over serial lines, such as modem connections

slip-ring - a connector that allows indefinite rotations, but maintains electrical contacts for passing power and electrical signals

slurry - a liquid with suspended particles

SMTP (Simple Mail Transfer Protocol) - the basic connection protocol for passing mail on the internet

snubber - a circuit that suppresses a sudden spike in voltage or current so that it will not damage other devices

software - a program, often stored on non-permanent media

solenoid - an actuator that uses a magnetic coil, and a lump of ferrous material. When the coil is energized a linear motion will occur

solid state - circuitry constructed entirely of semiconductors, and passive devices. (i.e., no gas as in tubes) sonar - sound waves are emitted and travel through gas/liquid. they are reflected by solid objects, and then detects back at the source. The travel time determines the distance to the object

sound - vibrations in the air travel as waves. As these waves strike the human ear, or other surfaces, the compression, and rarefaction of the air induces vibrations. In humans these vibrations induce perceived sound, in mechanical devices they manifest as distributed forces

sound absorption - as sound energy travels through, or reflects off a surface it must induce motion of the propagating medium. This induced motion will result in losses, largely heat, that will reduce the amplitude of the sound

sound analyzer - measurements can be made by setting the instrument for a certain bandwidth, and centre frequency. The measurement then encompasses the values over that range

sound level - a legally useful measure of sound, weighted for the human ear. Use dBA, dBB, dBC values

sound level meter - an instrument for measuring sound exposure values

source - an element in a system that supplies energy

sourcing - an output that when active will allow current to flow from a voltage source out to a device. It is complimented by sinking

specific gravity - the ratio between the density of a liquid/solid and water or a gas and air

spectrometer - determines the index of refraction of materials

spectrophotometer - measures the intensities of light at different points in the spectrum

spectrum - any periodic (and random) signal can be described as a collection of frequencies using a spectrum. The spectrum uses signal power, or intensity, plotted against frequency

spherical wave - a wave travels outward as if on the surface of an expanding sphere, starting from a point source

SQL (Structured Query Language) - a standard language for interrogating relational databases

standing wave - if a wave travels from a source, and is reflected back such that it arrives back at the source in phase, it can undergo superposition, and effectively amplify the sound from the source

static head - the hydrostatic pressure at the bottom of a water tank

steady state - describes a system response after a long period of time. In other words the transient effects have had time to dissipate

STEP (Standard for the Exchange of Product model data) - a standard that will allow transfer of solid model data (as well as others) between dissimilar CAD systems

step response - a typical test of system behavior that uses a sudden step input change with a measured response

stoichiometry - the general field that deals with balancing chemical equations

strain gauge - a wire mounted on a surface that will be stretched as the surface is strained. As the wire is stretched, the cross section is reduced, and the proportional change in resistance can be measured to estimate strain

strut - a two force structural member

subroutine - a reusable segment of a program that is called repeatedly

substrate - the base piece of a semiconductor that the layers are added to

switching - refers to devices that are purely on or off. Clearly this calls for discrete state devices

synchronous - two or more events happen at predictable times

synchronous motor - an AC motor. These motors tend to keep a near constant speed regardless of load

syntax error - an error that is fundamentally wrong in a language

synthesis - is the specification of values for the design variables. The engineer synthesizes a design and then evaluates its performance using analysis

system - a complex collection of components that performs a set of functions.


T1 - a 1.54 Mbps network data link

T3 - a 45 Mbps network data link. This can be done with parallel T1 lines and packet switching

tap - a connection to a power line

tare - the ratio between unloaded and loaded weights

TCP (Transmission Control Protocol) - a transport layer protocol that ensures reliable data communication when using IP communications. The protocol is connection oriented, with full duplex streams

tee - a tap into a larger line that does not add any special compensation, or conditioning. These connectors often have a T-shape

telnet - a standard method for logging into remote computers and having access if connect by a dumb terminal

temperature - the heat stored in an object. The relationship between temperature and energy content is specific to a material and is called the specific heat

temperature dependence - as temperature varies, so do physical properties of materials. This makes many devices sensitive to temperatures

thermal conductivity - the ability of a material to transfer heat energy

thermal gradient - the change in temperature as we move through a material

thermal lag - a delay between the time heat energy is applied and the time it arrives at the load

thermistor - a resistance based temperature measurement device

thermocouple - a device using joined metals that will generate a junction potential at different temperatures, used for temperature measurement

thermopiles - a series of thermocouples in series

thermoresistors - a category including RTDs and thermistors

throughput - the speed that actual data is transmitted/processed, etc

through beam - a beam is projected over an opening. If the beam is broken the sensor is activated

thumbwheel - a mechanical switch with multiple positions that allow digits to be entered directly

TIFF (Tagged Image File Format) - an image format best suited to scanned pictures, such as Fax transmissions

time-division multiplex - a circuit is switched between different devices for communication

time-proportional control - the amount of power delivered to an AC device is varied by changing the number of cycles delivered in a fixed period of time

timer - a device that can be set to have events happen at predetermined times

titration - a procedure for determining the strength of a solution using a reagent for detection. A chemical is added at a slow rate until the reagent detects a change

toggle switch - a switch with a large lever used for easy reviews of switch settings, and easy grasping

token - an indicator of control. Often when a process receives a token it can operate, when it is done it gives it up

TOP (Technical Office Protocol) - a network protocol designed for offices. It was promoted in conjunction with MAP in the 1980s, but never became widely used

top-down design - a design is done by first laying out the most abstract functions, and then filling in more of the details as they are required

topology - 1. The layout of a network. 2. a mathematical topic describing the connection of geometric entities. This is used for B-Rep models

torque - a moment or twisting action about an axis

torus - a donut shape toroidal core - a torus shaped magnetic core to increase magnetic conductivity

TPDDI (Twisted Pair Distributed Data Interface) - counter rotating token ring network connected with twisted pair medium

TQC (Total Quality Control) - a philosophical approach to developing quality methods that reach all levels and aspects of a company

transceiver (transmitter receiver) - a device to electrically interface between the computer network card, and the physical network medium. Packet collision hardware is present in these devices

transducer - a device that will convert energy from one form to another at proportional levels

transformations - include translation, rotation, and scaling of objects mathematically using matrix algebra

Transformations are used to move objects around in a scene

transformer - two separate coils wound about a common magnetic coil. Used for changing voltage, current and resistance levels

transient - a system response that occurs because of a change. These effects dissipate quickly and we are left with a steady state response

transmission path - a system component that is used for transmitting energy

transport layer - an OSI network model layer

TRIAC (TRIode Alternating Current) - a semiconductor switch suited to AC power

true - a logic positive, high, or 1

truth table - an exhaustive list of all possible logical input states, and the logical results

TTL (Transistor Transistor Logic) - a high speed for of transistor logic

TTY - a teletype terminal

turbine - a device that generates a rotational motion using gas or fluid pressure on fan blades or vanes

turbulent flow - fluids moving past an object, or changing direction will start to flow unevenly. This will occur when the Reynold’s number exceeds 4000

twisted pair - a scheme where wires are twisted to reduce the effects of EMI so that they may be used at higher frequencies. This is casually used to refer to 10b2 ethernet

TXD (Transmitted Data) - an output line for serial data transmission. It will be connected to an RXD input on a receiving station.


UART (Universal Asynchronous Receiver/Transmitter) - UDP (User Datagram Protocol) - a connectionless method for transmitting packets to other hosts on the network. It is seen as a counterpart to TCP

ultrasonic - sound or vibration at a frequency above that of the ear (> 16KHz typ.) ultraviolet - light with a frequency above the visible spectrum

UNIX - a very powerful operating system used on most high end and mid-range computers. The predecessor was Multics. This operating system was developed at AT & T, and grew up in the academic environment. As a result a wealth of public domain software has been developed, and the operating system is very well debugged

UPS (Uninterruptable Power Supply) - user friendly - a design scheme that simplifies interaction so that no knowledge is needed to operate a device and errors are easy to recover from. It is also a marketing term that is badly misused

user interfaces - are the means of communicating with the computer. For CAD applications, a graphical interface is usually preferred. User friendliness is a measure of the ease of use of a program and implies a good user interface

UUCP (Unix to Unix Copy Program) - a common communication method between UNIX systems.


Vac - a voltage that is AC

vacuum - a pressure that is below another pressure

vane - a blade that can be extended to provide a good mechanical contact and/or seal

variable - a changeable location in memory

varistor - voltage applied changes resistance

valve - a system component for opening and closing mass/energy flow paths. An example is a water faucet or transistor

vapor - a gas

variable - it is typically a value that will change or can be changed. see also constant

VDT (Video Display Terminal) - also known as a dumb terminal velocity - a rate of change or speed

Venturi - an effect that uses an orifice in a flow to generate a differential pressure. These devices can generate small vacuums

viscosity - when moved a fluid will have some resistance proportional to internal friction. This determines how fast a liquid will flow

viscosity index - when heated fluid viscosity will decrease, this number is the relative rate of change with respect to temperature

VLSI (Very Large Scale Integration) - a measure of chip density. This indicates that there are over 100,000(?) transistors on a single integrated circuit. Modern microprocessors commonly have millions of transistors

volt - a unit of electrical potential

voltage rating - the range or a maximum/minimum limit that is required to prevent damage, and ensure normal operation. Some devices will work outside these ranges, but not all will, so the limits should be observed for good designs

volume - the size of a region of space or quantity of fluid

volatile memory - most memory will lose its contents when power is removed, making it volatile

vortex - a swirling pattern in fluid flow

vortex shedding - a solid object in a flow stream might cause vortices. These vortices will travel with the flow and appear to be shed.


watchdog timer - a timer that expects to receive a pulse every fraction of a second. If a pulse is not received, it assumes the system is not operating normally, and a shutdown procedure is activated

watt - a unit of power that is commonly used for electrical systems, but applies to all

wavelength - the physical distance occupied by one cycle of a wave in a propagating medium

word - 1. a unit of 16 bits or two bytes. 2. a term used to describe a binary number in a computer (not limited to 16 bits)

work - the transfer of energy

write - a digital value is stored in a memory location

WYSIWYG (What You See Is What You Get) - newer software allows users to review things on the screen before printing. In WYSIWYG mode, the layout on the screen matches the paper version exactly


X.25- a packet switching standard by the CCITT

X.400 - a message handling system standard by the CCITT

X.500 - a directory services standard by the CCITT

X rays - very high frequency electromagnetic waves

X Windows - a window driven interface system that works over networks. The system was developed at MIT, and is quickly becoming the standard windowed interface. Personal computer manufacturers are slowly evolving their windowed operating systems towards X-Windows like standards. This standard only specifies low level details, higher level standards have been developed: Motif, and Openlook

XFER - transfer

XMIT - transmit

xmodem - a popular protocol for transmitting files over text based connections. compression and error checking are included.


ymodem - a popular protocol for transmitting files over text based connections. compression and error checking are included.


zmodem - a protocol for transmitting data over text based connections.








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