eBook: Automating Manufacturing Systems; with PLCs


A.4 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 defence 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 scaleability 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.
DIN (the Deutsches Institut for Normung) - a German standards institute.
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 `'.
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

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