Glossary

Accuracy: Closeness of a reading or indication of a measurement device to the actual value of the quantity being measured.

ActiveX Control (OLE Control): Custom control applet based on Microsoft's OLE architecture. Can run in any OLE-enabled container application or web browser. Address: Label or number identifying the memory location of a unit of information.

Alias: A false lower frequency component that appears in sampled data acquired at too low a sampling rate.

Alphanumeric: A character set containing both letters and digits.

Alternating current (ac): An electric current that reverses its direction at regularly recurring intervals.

Ambient compensation: The design of an instrument such that changes in ambient temperature do not affect the readings of the instrument.

Ambient temperature: The average or mean temperature of the surrounding air in contact with equipment or instruments under test.

American National Standards Institute (ANSI): The United States standards body responsible for designating standards developed by other organizations as national standards.

American Standard Code for Information Interchange (ASCII): Standard seven or eight-bit code used to represent alphanumeric characters for communication among computers.

Ampere (amp): A unit used to define the rate of flow of electricity (current) in an electrical circuit; units are one coulomb (6.25x1018 electrons) per second. Symbolized by A.

Amplifier: A device that magnifies an input or output signal using power drawn from a source other than the signal itself.

Amplitude flatness: A measure of gain consistency of a circuit over a range of frequencies.

Amplitude: A measurement of the distance from highest to lowest excursion of a variable or physical motion. Often used with reference to waveforms.

Analog trigger: A trigger that occurs at a user-selected point on an incoming analog signal. Can be set to occur at a specific level on either an increasing or a decreasing signal (positive or negative slope).

Analog-to-digital (A/D): Generic term referring to the conversion of analog information to the digital language of computers.

Analog-to-digital converter (ADC): An electronic device that converts analog signals to an equivalent digital form.

Application program: Computer program designed to accomplish a specific task, such as word processing.

Application-specific integrated circuit (ASIC): Custom semiconductor component designed and manufactured to perform a set of specific functions, typically for a single application.

ARCnet: Token-passing network technology in which nodes Òtake turnsÓ talking according to which node has the token.

Arithmetic logic unit (ALU): The part of a digital processing system where binary data is operated upon mathematically.

Asynchronous: An event that occurs at an arbitrary time, without synchronization to a reference clock. May refer to network messaging, in which a node sends a message without waiting for the receiver to signal that it is ready to receive.

Backbone: High capacity network designed to provide high bandwidth for corporate communication requirements.

Backplane: Multi-conductor assembly into which computer-based boards are inserted. Typically supplies power and allows boards to communicate at high speeds.

Bolometer: Infrared thermometer detector consisting of a resistance thermometer arranged for response to radiation.

Baud rate: Serial communications data transmission rate expressed in bits per second (bps).

Bipolar: A signal range that includes both positive and negative values (i.e., -10 to +10 V).

Bit (b): A single binary digit, valued as either 0 or 1.

Block: A high-speed data transfer in which the address of the data is sent followed by a specified number of back-to-back data words.

Breakdown voltage: Threshold voltage at which circuit components begin to be damaged. See also "working voltage."

Burst mode: High-speed data acquisition mode in which data points are measured as quickly as possible then held for subsequent conversion. Helps reduce skew.

Bus master: A type of a plug-in board or controller with the ability to read and write to devices on the computer bus.

Bus: Group of conductors that interconnect individual circuitry in a computer. Examples of PC buses include ISA, EISA, and PCI.

Byte (B): Eight related bits of data, an eight-bit binary number. Also used to denote the amount of memory required to store one byte of data.

Cache: High-speed processor memory that buffers commonly used instructions or data to increase processing throughput.

Calibration: The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured.

Central processing unit (CPU): Microprocessor that functions as the primary computational ÒbrainÓ of a PC or other computer-based device.

Code width: Smallest voltage an A/D converter can detect; a function of resolution, gain, and range.

Cold junction compensation (CJC): The referencing of thermocouple voltage outputs to ambient temperature in a thermocouple measurement circuit.

Common-mode range: Input range over which a circuit can handle a common-mode signal.

Common-mode rejection ratio (CMRR): Measure of an instrument's ability to reject interference from a common-mode signal, usually expressed in decibels (dB).

Common-mode signal: Mathematical average voltage, relative to a computer's ground, of the signals from a differential input.

Compiler: Software utility that converts code from a high-level programming language, such as C or Visual Basic, into a machine language. Compiled programs run 10 to 1,000 times faster than interpreted programs. See also Interpreter.

Component Object Model (COM): Underlying Microsoft software architecture that allows applications to be readily integrated.

Component software: Object-oriented software approach in which standalone objects, or components, can readily interact in a container environment without custom program development.

Controller Area Network (CAN): A serial bus developed by Bosch for use in automotive in-vehicle communications. With added communication layers, is the foundation of DeviceNet industrial automation protocol.

Conversion time: Time required to convert an analog or digital signal into its converse.

Counter/timer: Circuit that counts external or clock pulses.

Crosstalk: Unwanted signal on one channel due to interference from another channel.

Current drive capability: Amount of current a digital or analog output channel is capable of sourcing or sinking while still operating within voltage range specifications.

Current sink capability: Ability of a data acquisition board to dissipate current for analog or digital output signals.

Current source capability: Ability of a data acquisition board to supply current for analog or digital output signals.

Current: The rate of flow of electricity. The unit is the ampere (A), which equals one coulomb per second.

Data acquisition (DAQ): The activity of measuring, transmitting, and recording electrical signals from sensors, switches, and transducers. Often implies the conversion of these signals into computer-compatible digital information.

Data communications equipment (DCE): Generic telecommunications term referring to the equipment used to actually communicate messages among data terminal equipment (DTE).

Data terminal equipment (DTE): Generic telecommunications term referring to a computer-based device that needs to communicate, also referred to as a generator or receiver. See also Data communications equipment.

Decibel (dB): Unit for expressing a logarithmic measure of the ratio of two signal levels.

Degree: An incremental value in a temperature scale.

Derivative control: Control action in which output correction is proportional to the rate of change of the error signal. Derivative control anticipates the magnitude difference between the process variable and the setpoint.

DeviceNet: Non-proprietary device-level networking protocol for industrial automation. Developed by Allen-Bradley based on Bosch's CAN chips, now the province of the non-profit Open DeviceNet Vendors Association. (ODVA).

Differential input: Analog input consisting of the voltage difference between two terminals, both of which are different from computer ground.

Digital input/output (DIO): Input or output points allowed only two discrete states, typically on or off, 1 or 0.

Digital signal processing (DSP): The manipulation of signal information while it exists in digital rather than analog form.

Digital-to-analog converter (D/A): Electronic device, often an integrated circuit, that converts a digital number into a corresponding analog voltage or current.

Digital-to-analog: Generic term referring to the conversion of digital data to a corresponding analog voltage or current.

DIN: German agency responsible for setting engineering and dimensional standards.

Direct memory access (DMA): Method by which data can be rapidly transferred to or from computer memory without the processor's intervention.

Distributed control system (DCS): Typically a large-scale process control system characterized by a distributed network of processors and I/O subsystems that encompass the functions of control, user interface, data collection, and system management. DCSs are commonly used in large industrial facilities, such as chemical plants, petroleum refineries, and paper mills.

Distributed Component Object Model (DCOM): Distributed version of Microsoft's COM architecture, allowing applications to be readily integrated across networks.

Drift: A change in an instrument's reading or setpoint value over extended periods due to factors such as time, line voltage, or ambient temperature effects.

Driver: Software that controls a specific hardware device such as a PC board or programmable controller.

Dynamic Data Exchange (DDE): Microsoft interprocess communication protocol for transferring data among Microsoft Windows-based applications. Predecessor to OLE.

Dynamic Link Library (DLL): Executable code and data that can be called or used by Microsoft Windows-based applications or other DLLs. Functions and data in a DLL are loaded and linked when referenced .

Dynamic range: Ratio of the largest to smallest signal level a circuit can handle, normally expressed in dB.

EEPROM: Electrically erasable programmable ROM; read-only memory that can be erased and reprogrammed.

Electromagnetic interference (EMI): electrical noise induced upon signal wires with the possible effect of obscuring the instrument signal.

Electromotive force (EMF): A measure of voltage in an electrical circuit. .

Encoder: Device that converts linear or rotary displacement into digital or pulse signals.

EPROM: Erasable programmable ROM; read only memory that can be erased (usually by ultraviolet light exposure) and reprogrammed.

Error: The difference between the correct or desired value and the actual read or value taken.

Ethernet: Popular local area networking technology used for office applications and, increasingly, control network and data acquisition applications.

External trigger: Voltage pulse from an external source that triggers an event such as A/D conversion.

Fiber Distributed Data Interface (FDDI): Standard defining a high capacity, dual-ring fiber optic communication protocol.

Fieldbus: All-digital communication network used to connect process instrumentation and control systems. Designed to replace existing 4-20 mA analog signals with bidirectional, multivariable data communications capability.

Fieldbus Foundation: Austin, Texas-based nonprofit consortium of instrumentation suppliers that is developing a standard digital communication network (fieldbus) for process control applications. The network developed by the Foundation is referred to as the Foundation Fieldbus.

First-in first-out (FIFO): Memory buffer in which the first data stored is the first data sent.

Flash ADC: High-speed ADC whose output code is determined in a single step by a bank of comparators and encoding logic.

Frequency: The number of cycles over a specified time period over which an event occurs. Normally expressed in cycles per second (hertz, Hz).

Function: A set of software instructions executed by a single line of code that may have input and/or output parameters and returns a value when executed.

Gain accuracy: Measure of deviation of the gain of an amplifier from the ideal gain.

Gain: The amount of amplification used in an electrical circuit, sometime expressed in decibels, dB.

General Purpose Interface Bus (GPIB): Synonymous with HP-IB (for Hewlett-Packard), the standard bus used for controlling electronic instruments with a computer. Also called IEEE 488 in reference to defining ANSI/IEEE standards.

Graphical user interface (GUI): An intuitive, graphical means of communicating information between computer-based devices and human users. GUIs can resemble the front panels of instruments or other objects associated with a computer program.

Ground: The electrical neutral line having the same potential as the surrounding earth; the negative side of a direct current power system; the reference point for an electrical system.

Hardware: Typically refers to the physical components of a data acquisition system, such as PCs, boards, data conversion devices, etc.

Hertz (Hz): Unit of frequency, defined as one cycle per second. Hierarchical: Method of organizing computer programs with a series of levels, each with further subdivisions.

Human-machine interface (HMI): The means by which a human operator interacts with an industrial machine or other piece of equipment. See also graphical user interface.

IEEE 488: See General Purpose Interface Bus.

Impedance: The total opposition to electrical flow in an electrical circuit. Input bias current: Current that flows into circuit inputs.

Input impedance: The measured resistance and capacitance between the input terminals of a circuit.

Input offset current: Difference in the input bias currents of the two inputs of an instrumentation amplifier.

Input/output (I/O):The transfer of data to and from a computer system involving communications channels, operator interface devices, and/or data acquisition and control interfaces.

Institute of Electrical and Electronic Engineers (IEEE): Professional and standards-writing organization of electrical and computer-related professionals.

Integral control: Control action that eliminates the offset inherent in proportional control. Offset from setpoint is ÒintegratedÓ over time resulting in an output correction.

Integral nonlinearity (INL): A measure in LSB of the worst-case deviation from the ideal A/D or D/A transfer characteristic of analog I/O circuitry.

Integrating ADC: ADC that works by integrating an unknown voltage over time. Time required is compared to the time required to integrate a known reference voltage.

Interchangeability error: A measurement error that can occur if two or more sensors are used to make the same measurement. Caused by slight variations from sensor to sensor.

Interpreter: Software utility that executes source code from a high-level language such as Visual Basic or C by reading one line at a time and executing the specified operation. See also Compiler.

Interrupt: Computer signal indicating that the CPU should suspend its current task to service a designated activity.

Intrinsically safe: An instrument in which electrical energy is limited such that it will not spark or otherwise ignite a flammable mixture.

ISA: Formerly the Instrument Society of America, now referred to as the International Society for Measurement & Control.

Isolation voltage: Voltage an isolated circuit can normally withstand, usually specified from input to input and/or from any input to the amplifier output, or to the computer bus.

Joint time-frequency analysis (JTFA): Technique for spectral analysis of rapidly changing waveforms.

Linearity: The deviation of an instrument's response from a straight line.

Local area network (LAN): Network of computer-based devices, typically PCs and associated peripherals in a geographically limited location.

Logical address: Digital number that uniquely identifies each device in a system.

Loop resistance: The total resistance of a complete electrical circuit.

Least significant bit (LSB): Refers to the smallest increment of resolution in an A/D or D/A conversion.

Mainframe: Chassis that mechanically contains boards or modules inserted into a backplane. Provides environment conditioning and vibration and shock-resistant connections.

Man-machine interface (MMI): See Human-machine interface.

Media access control (MAC): Networking layer that determines which node can access the physical media.

Million floating-point operations per second (MFLOPS): Unit for expressing the computational power of a processor.

Million instructions per second (MIPS): Unit for expressing the speed of processor execution of machine code instructions.

Millivolt (mV): One thousandth of a volt.

Module: A board assembly and its associated mechanical parts. A module contains everything required to occupy one or more slots in a mainframe chassis.

Multiplexer (Mux): A switching device that sequentially connects multiple inputs or outputs in order to process several signal channels with a single A/D or D/A converter.

Multitasking: Property of an operating system in which several processes can be run simultaneously.

Noise: Any undesirable electrical signal, from external sources such as ac power lines, motors, electrical storms, and radio transmitters, as well as internal sources such as electrical components.

Normal-mode rejection ratio:The ability of an instrument to reject electrical interference across its input terminals, normally of line frequency (50-60 Hz).

Nyquist theorem: Law of sampling theory stating that data sampling frequency should be at least twice that of the highest frequency variations in the signal of interest. Must be observed to preserve patterns in data and to not introduce artificial, lower frequency patterns.

Object Linking and Embedding (OLE): Set of Microsoft system services that provides a means for application integration. Based on COM technology, OLE allows applications to dynamically identify and use the data and services of other applications.

Objects: Self-contained software modes that encapsulate both data and processing logic.

Ohmeter: A device used to measure electrical resistance.

OLE Control: See ActiveX Control.

Open Systems Interconnect (OSI): A seven-layer model for generically describing modern networking technologies.

Operating system: Base-level software that controls a computer, runs programs, interacts with users, and communicates with installed hardware or peripheral devices.

Optical isolation: Two networks or circuits in which an LED transmitter and receiver are used to maintain electrical discontinuity between the circuits.

Output settling time: Time required for the analog output voltage to reach its final value within specified limits.

Output slew rate: Maximum rate of change of analog output voltage from one level to another.

Overhead: Amount of computer processing resources required to accomplish a task.

Personal Computer Memory Card International Association (PCMCIA): An expansion card form factor popular in laptop and notebook computers.

Peripheral Component Interconnect (PCI): High-performance expansion bus architecture common in modern desktop PCs.

Phase: A time-based relationship between a periodic function and a reference.

Plug and Play ISA: Specification that allows PC ISA boards to be fully configured in software, without jumpers or switches on the boards.

Polarity: In electricity, the quality of having two charged poles, one positive and one negative.

Port: A communications connection on a computer-based device.

Postriggering: The acquisition of a programmed number of samples by a data acquisition board after trigger conditions are met.

Power supply: A separate unit or part of a circuit that provides power to the rest of a circuit.

Pretriggering: A data acquisition board that maintains a continuous buffer of data, so that when trigger conditions are met, data generated before the condition occurred can be acquired.

Profibus: Family of fieldbus, device-level, and cell controller protocols for industrial automation.

Programmable gain amplifier (PGA): Signal amplifier that can be programmed to apply a different signal gain depending on the input voltage. Effectively increases dynamic range and sensitivity of A/D converter.

Programmable logic controller (PLC): Special-purpose computer used in industrial monitoring and control applications.

Propagation delay: Amount of time required for a signal to pass through a circuit.

Proportional control: Control action in which output corrections are directly proportional to the process variable's deviation from setpoint.

Proportional-integral-derivative (PID) control:Three-term control algorithm combining proportional, integral, and derivative control actions.

Protocol: Defined sequence of bits, characters, and control codes used to transfer data among computer-based devices.

Quantization error: Inherent uncertainty in an A/D conversion due to the finite resolution of the conversion process.

Radio frequency interference (RFI):Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring the instrument signal.

Range: An area between two limits within which a quantity is measured, stated in terms of a lower and upper limit.

Relative accuracy: Measure in LSB of the accuracy of an ADC. It includes all non-linearity and quantization errors.

Remote terminal unit: Industrial control and data collection device similar to a PLC, but designed for remote communication via wire-based or radio telemetry. Repeatability: The ability of an instrument to give the same output or reading under repeated, identical conditions.

Resistance temperature detector (RTD): A metallic probe that measures temperature based upon its coefficient of resistivity.

Resistance: The resistance to the flow of electric current, measured in ohms.

Resolution: The smallest signal increment that can be detected by a measurement system. Resolution can be expressed in bits, in proportions, or in percent of full scale. For example, a system has 12-bit resolution, one part in 4,096 resolution, and 0.0244 percent of full scale.

Ribbon cable: Flat cable in which the conductors are side by side.

Root-mean-square (RMS): Averaging algorithm that results from the square root of the arithmetical mean of the squares.

Sample-and-hold (S/H): Circuit that acquires and stores an analog voltage on a capacitor for subsequent conversion.

Self-calibrating: Data acquisition board that calibrates its own A/D and D/A circuits with reference to a stable onboard reference.

Sensitivity: The minimum change in a physical variable to which an instrument can respond.

Sequenced Packet Exchange/Internetwork Packet Exchange (SPE/IPE): Novell implementation of network addressing scheme.

Shielded twisted pair (STP): Cable construction that includes an external grounded shield, as well as twisting on a regular basis to help minimize noise interferences.

Signal-to-noise ratio (SNR): The ratio of the overall rms signal level to the rms noise level, expressed in dB.

Simultaneous sampling (SS): System in which each input or output channel is digitized or updated at the same time.

Single-ended (SE): An analog input that is measured with respect to a common ground.

Software trigger: A programmed event that triggers an event such as data acquisition.

Span: The difference between the upper and lower limits of a range, expressed in the same units as the range.

Stability: The ability of an instrument or sensor to maintain a consistent output when a constant input is applied.

Statistical process control (SPC): Analysis methodology in which characteristics of a process are measured or counted and tracked. Statistical rules are used to determined whether variations are random or need correction.

Strain gauge: Sensor whose resistance varies with applied force.

Structured Query Language (SQL): Computer language used to interact with databases.

Subroutine: Set of software instructions executed by a single line of code that may have input and/or output parameters.

Successive-approximation ADC:ADC that sequentially compares a series of binary-weighted values with an analog input to produce an output digital word in n steps, where n is the bit resolution of the ADC.

Supervisory control and data acquisition (SCADA): Automation and control architecture in which direct control and data acquisition are performed by PLCs and/or RTUs and supervised by another computer-based device, often a PC.

Synchronous: An event or action that is synchronized to a reference clock.

System noise: Measure of the amount of noise seen by an analog circuit or an ADC when the analog inputs are grounded.

Thermistor: A semiconductor sensor that exhibits a repeatable change in electrical resistance as a function of temperature. Most thermistors exhibit a negative temperature coefficient.

Thermocouple: The junction of two dissimilar metals through which a measurable current flows depending on the temperature difference between the two junctions.

Throughput rate:Speed of execution, in samples or bits per second, for a given continuous operation, calculated to include software overhead.

Transfer rate: Rate at which data is moved from source to destination.

Transistor-to-transistor logic (TTL): Voltage-level changes readily communicated and interpreted by microprocessors, typically in the 0-5 V range.

Transmission Control Protocol/Internet Protocol (TCP/IP): A set of standard protocols for communicating across a single network or interconnected set of networks.

Unipolar: A signal range that is always positive (for example, 1 to 5 V).

Unshielded twisted pair (UTP): Cable construction consisting of pairs of wires twisted at regular intervals (called the pitch) in order to reduce electrical noise interferences.

Volt (V): The electrical potential difference between two points in a circuit. One volt is the potential needed to move one coulomb of charge between two points while using one joule of energy.

Wide area network (WAN): Computer network typically designed to include and embrace multiple local area networks across multple geographic locations.

Word: Standard number of bits that a processor or memory manipulates at one time. Microprocessors typically use 8, 16, or 32-bit words.

Zero offset: The non-zero output of an instrument, expressed in units of measure, under conditions of true zero.

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