Data Acquisition

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A data acquisition and control system, built around the power and flexibility of the PC,may consist of a wide variety of diverse hardware building blocks from different equipmentmanufacturers. It is the task of the system integrator to bring together theseindividual components into a complete working system.The basic elements of a data acquisition system, as shown in the functional diagram ofFigure 1.1, are as follows:• Sensors and transducers• Field wiring• Signal conditioning• Data acquisition hardware• PC (operating system)• Data acquisition softwarePhysicalphenomenaTransducersField wiringThermocoupleTemperaturepressuremotionStrain gaugeNoisyelectricalsignalSignal conditioningHost ComputerFiltersandamplifiersData acquisitionsoftwareFiltered andamplified signalField wiring16 samplesper second12-bit resolutionData acquisitionhardwareFigure 1.1Functional diagram of a PC-based data acquisition system
Each element of the total system is important for the accurate measurement andcollection of data from the process or physical phenomena being monitored, and is discussedin the following sections.1.2.1 Transducers and sensorsTransducers and sensors provide the actual interface between the real world and the dataacquisition system by converting physical phenomena into electrical signals that thesignal conditioning and/or data acquisition hardware can accept.Transducers available can perform almost any physical measurement and provide acorresponding electrical output. For example, thermocouples, resistive temperature detectors(RTDs), thermistors, and IC sensors convert temperature into an analog signal,while flow meters produce digital pulse trains whose frequency depends on the speed offlow.Strain gauges and pressure transducers measure force and pressure respectively, whileother types of transducers are available to measure linear and angular displacement,velocity and acceleration, light, chemical properties (e.g. CO concentration, pH), voltages,currents, resistances or pulses. In each case, the electrical signals produced are proportionalto the physical quantity being measured according to some defined relationship.1.2.2 Field wiring and communications cablingField wiring represents the physical connection from the transducers and sensors to thesignal conditioning hardware and/or data acquisition hardware. When the signalconditioning and/or data acquisition hardware is remotely located from the PC, then thefield wiring provides the physical link between these hardware elements and the hostcomputer. If this physical link is an RS-232 or RS-485 communications interface, thenthis component of the field wiring is often referred to as communications cabling.Since field wiring and communications cabling often physically represents the largestcomponent of the total system, it is most susceptible to the effects of external noise,especially in harsh industrial environments. The correct earthing and shielding of fieldwires and communications cabling is of paramount importance in reducing the effects ofnoise. This passive component of the data acquisition and control system is often overlookedas an important integral component, resulting in an otherwise reliable systembecoming inaccurate or unreliable due to incorrect wiring techniques.1.2.3 Signal conditioningElectrical signals generated by transducers often need to be converted to a formacceptable to the data acquisition hardware, particularly the A/D converter which convertsthe signal data to the required digital format. In addition, many transducers requiresome form of excitation or bridge completion for proper and accurate operation.The principal tasks performed by signal conditioning are:• Filtering• Amplification• Linearization• Isolation• Excitation
Page 2 and 3: Practical Data Acquisition forInstr
Page 4 and 5: Practical Data Acquisition forInstr
Page 6 and 7: In less than a decade, the PC has b
Page 8 and 9: PrefaceContentsxvii1 Introduction 1
Page 10 and 11: Contents vii4.2.1 Hardware interrup
Page 12 and 13: Contents ix6.2.3 Functional descrip
Page 14 and 15: Contents xi9.1 Ethernet and fieldbu
Page 16 and 17: Contents xiii12.5.2 Pin assignments
Page 18 and 19: Contents xvType R thermocouple 384T
Page 22 and 23: FilteringIn noisy environments, it
Page 24 and 25: are capable of programmed I/O and i
Page 26 and 27: Plug-in expansion boards are common
Page 28 and 29: 50 mRS-232 Communication InterfaceS
Page 30 and 31: speeds are of the order of 1 Mbyte/
Page 32 and 33: 14 Practical Data Acquisition for I
Page 54 and 55: 3PC based data acquisition (DAQ) sy
Page 56 and 57: 3.2.3 FilteringIsolation performs s
Page 58 and 59: The transfer characteristics of a p
Page 60 and 61: Figure 3.7Ideal band pass filter tr
Page 62 and 63: Figure 3.11Two-stage Butterworth fi
Page 64 and 65: As individual signal conditioning m
Page 66 and 67: ThermocouplesDigitalTransmitterDigi
Page 68 and 69: Examples of floating signal sources
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espect to the measurement system gr
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Figure 3.26Opto-coupler isolation o
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• Using isolation amplifiers to i
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Figure 3.31Physical representation
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field or if the field is caused by
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Where the shield is grounded (i.e.
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mines the amount of noise in the ci
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For full-duplex systems using balan
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4.1.1 DOSusually consist of a small
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With the advent of Windows as a gra
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UNIX shellSimilar to the DOS comman
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available to the expansion bus. A l
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4.2.7 Interrupt service routinesIt
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Whichever CPU is being used, it mus
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• An I/O device requests a DMA tr
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Normal DMA using on-board FIFOThe D
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ferring samples until the counter r
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The read or write command signals (
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Shortened 3-BCLK 8-bit memory acces
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Extended 6-BCLK 16-bit memory acces
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• A cycle which the expansion boa
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Figure 4.13Timing chart of a shorte
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Figure 4.15Timing chart of an exten
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4.7.2 Expanded memory system (EMS)E
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The IBM PC and early versions of th
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Figure 4.17ISA signal mnemonics, si
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data bus. An example of this happen
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NOWSThe /NOWS (NO wait state) signa
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interrupt lines (1RQ13, 8, 2, 1 and
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one application at a time, it is ne
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The need for PCs to exchange data w
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Figure 4.198-bit 21-line I/O board4
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Address decoding on the 24-line pro
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The write cycle is considered in th
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data acquisition and control system
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Adjustable on-board fixed gain ampl
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level applied at the input. When a
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Each step effectively divides the r
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The operation of a dual slope integ
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Code widthThis is the fundamental q
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(a) Unipolar offset error(b) Bipola
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Changes in temperature result in a
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1 LSB. Therefore, an ideal A/D conv
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• The source and level of interru
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5.3.3 Differential inputsTrue diffe
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A/D board can divide the input rang
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a) DC alias caused by sampling at h
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Figure 5.15Frequency spectrum of or
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ate be a minimum of about five time
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Figure 5.20Many aliases combined wi
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initiated. The data is subsequently
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Figure 5.22Time skew between channe
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For each burst trigger, the A/D boa
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Analog output D/A boardsUnlike high
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Like the weighted-current source ne
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The generation of high frequency si
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• The plug-in connector, which pr
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Latched digital I/OFor applications
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y increasing the resistance of R x
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One advantage of this type of relay
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Figure 5.36Waveforms showing genera
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When a counter is configured to ena
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6The standardization of the RS-232
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A duplex system is designed for sen
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Some examples of the HEX and BIN va
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In summary, the optional settings f
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At the RS-232 receiver the followin
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110 850300 800600 7001200 5002400 2
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• Pin 7: Signal ground (common)Th
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6.2.5 Examples of RS-232 interfaces
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terminating resistors, approximatel
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Another commonly used technique, ba
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• Line controlThis applies to hal
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The calculation of the block checks
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The mechanism of operation of the C
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Figure 6.18, has appropriate intern
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77.1 IntroductionAs with other form
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inserted in the device. This is its
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How frequently logged data is uploa
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The following hardware components d
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In a typical stand-alone data acqui
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The maximum battery life that can b
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Input termination resistors, typica
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Figure 7.13 shows the standard conn
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interface, even at high speed, the
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• Use different data formats so t
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Command errors are reported immedia
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• Channel scalingThis automatical
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The channel scan for this type of s
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Data is logged in a fixed non-ASCII
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7.9 Stand-alone logger/controllers
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88.1 IntroductionThe communications
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Figure 8.2GPIB connector (IEEE 488)
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• TalkersA talker is a one-way co
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controller in charge (CIC). The IFC
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Each device connected to the GPIB h
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One of the additional features that
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• The controller temporarily conf
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Table 8.4IEEE 488.2 commandsThe SCP
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Figure 8.9SCPI instrument modelThe
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99.1 Ethernet and fieldbuses for da
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cabling tray etc and the transceive
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transceiver in the MAU and this is
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Advantages of the system include:
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of a cell as well, but these are ig
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Figure 9.8CSMA/CD collisionsAssume
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• PreambleThis field consists of
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of this figure. Some manufacturers
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Grounding has safety and noise conn
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Just as with any technology, each o
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information. The packet is then pla
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There are two types of connectors,
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HOSTClient SoftwareManages Interfac
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Figure 10.6USB connector pinsThere
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The idle states for low- and high-s
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The interrupt transfer type is used
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than in spending a lot of time and
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• The sensitivity of the output/i
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term error (m) in the system and ad
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11.2 Capturing high speed transient
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12IntroductionThe PCMCIA (PCMCIA st
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adapter on a full size computer has
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Pager cards are used in offices and
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Size 85.6 mm × 54.0 mmType I 3.3 m
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The memory only interface is conver
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The AIMS interface supports large d
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12.8 FutureThe card information str
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A/D conversion timeAddressAddress r
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Back-planeBand pass filterBandwidth
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BufferBusBWCache memoryCCDCCIRCCITT
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pixel is subjected to a mathematica
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data between the computer memory an
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FrameFrame grabberFringingFull dupl
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so that they interlock. The PAL sta
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Lux-secondSI unit of light exposure
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NTSCNull modemNumber of channelsNyq
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PortPPIPre-triggerProgram I/OProgra
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whereby the first gray level of eac
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allowing the user to control basic
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input.TrunkUARTUnipolar inputsUnloa
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Appendix BThe information in this s
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Table B.4Controller 2: 16-bit (AT o
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B.5 8253/8254 Counter/timer
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Table B.8Memory map for PC/XT/AT
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Table B.10BIOS data area
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Table B.15ROMTable B.16AT extended
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← ↔ ← ← ← ↔
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Figure B.1Card dimensions for PC/XT
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Appendix CThis section contains bri
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the control register of the 8255. T
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The bits A7 (MSB) down to A0 (LSB)
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Table C.6Instructions for reading o
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The program could also enable the I
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Group AConfiguration Informationto
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C.11 Single-bit set/resetAny of the
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Figure C.15Bi-directional bus (mode
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Figure D.18254 Block diagramFigure
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Figure D.3TCCTRL registerThe functi
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This is the data register of the fi
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• A simple read operation• A co
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The null count bit indicates if the
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For even counts: the output is init
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Appendix EThe IPTS-68 standard defi
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Number of ranges = 2Range #1 -270 t
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Range #2 0 to 1372°COrder of polyn
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Number of Ranges = 4Range #1 -50 to
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Appendix FF.1 IntroductionAll activ
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Table F.3 gives the conversion betw
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The conversion between binary and h
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F.8 Internal representation of info
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12 which is equivalent to: 1100-4 S
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DCDCASDCISDCLDDDIODTDTASDTISENDEOIE
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STBSTRSSWNST(T)TETACSTADSTAGTCATCST
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404 IndexDuplex:full duplex 178half
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406 IndexOpen loop control 285see a
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THIS BOOK WAS DEVELOPED BY IDC TECH
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Data Acquisition

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