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A tutorial on PC-based data acquisition This tutorial will cover the basics of instrumentation using plug-in data acquisition boards and personal computers. We will talk about transducers, signal conditioning and software. By DAVID POTTER & ASH RAZDAN Data acquisition hardware can be categorised into four distinct types: plug-in data acquisition boards, IEEE 488.2 (GPIB) controlled instruments, VXIbus instruments and RS-232 controlled instruments. This tutorial will mainly discuss plug-in data acquisi16 SILICON CHIP tion boards and will also compare the different computers used for data acquisition: XT /AT/EISA and IBM PS/2 computers (DOS and Microsoft Windows), Macintosh computers, and workstations. Let's first talk about the compo- nents of a Data Acquisition System. The fundamental task of such a system is the measurement or g~neration of real-world physical signals. Before a signal can be measured by a computer-based system, a sensor or transducer must be used to convert it into an electrical signal, such as voltage or current. A signal is measured because it conveys useful information. This information is extracted from the signal by a data acquisition system. Any signal can generally be classified as analog or digital. A digital, or binary, signal has only two possible discrete levels - a high (on) level and a low (off) level. An analog signal, on the other hand, can vary continuously in voltage level and frequency. Transducers Transducers convert physical phenomena such as temperature, force, sound, pressure, light, position and fluid flow into a measurable electrical quantity, such as voltage or current. Examples of transducers include thermocouples (temperature to voltage}, thermistors (temperature to electrical resistance), strain gauges (strain to voltage) and microphones (sound pressure to voltage). Table 1 lists different phenomena and the transducers used to convert them into measurable quantities. Signal conditioning The signals produced by these transducers are often not well conditioned or well suited for measurement by data acquisition boards. Therefore, signal conditioning circuits or peripherals are used to amplify, linearise, isolate and filter the signal so that it can be properly and accurately digitised. For example, low-level thermocouple signals must be amplified and linearised so that a simple scaling function can be used to convert the signal voltage to temperature. The system being monitored may contain high-voltage transients that could damage the computer or harm the operator, hence the need for signal isolation. In another example, con- Fig.I (facing page): PC-based data acquisition systems include external signal conditioners, plug-in AID boards, a computer and application software. Also depicted here is National Instruments' SCXI signal conditioning hardware which is optimised for low level analog signals. National Instruments has data acquisition hardware for PC/XT/AT, EISA, PS/2 and Macintosh computers. These boards feature various combinations of analog, digital and timing inputs and outputs, and can be easily integrated with SCXI signal conditioning products for isolating, amplifying and multiplexing signals. ditioning is often used to filter unwanted signals; the signal conditioning peripherals can contain low-pass filters that eliminate high frequencies that could otherwise produce erroneous data. Signal conditioning products Signal conditioning products can be split into four classes. The first class, plug-in board conditioning, puts the signal conditioning circuitry directly onto the data acquisition board. While this is low-priced and requires the smallest amount of space, it has several disadvantages: signals are not fully isolated because they are plugged directly into the board connector which is located on the back of the computer; signals must be conditioned in the noisy environment of the computer. Only a few signals can be han- TABLE 1 Phenomena Temperature Light Force Position (Displacement) Fluid Flow pH Transducer Thermocouples, thermistors Photodiodes, phototransistors, light dependent resistors (LDRs) Strain gauges, piezoelectric transducers Potentiometers, linear voltage differential transformers (LVDT), optical encoders Head meters, rotational flowmeters, ultrasonic flowmeters pH electrodes dled because there is not enough room at the back of the computer to connect a large number of cables. The second class, digitising, conditions and digitises the signals in an external box and sends the data to the computer via a serial bus such as RS232. This class is capable of a very large channel count and being used for distributed I/O whereby large numbers of sensors are fed into the external boxes, and the data sent by multiple RS-232 lines back to the computer. The main disadvantage is in the serial data bus which limits the amount of information which can be transmitted. The third class , digitising and processing, conditions, digitises and analyses the signals in an external box. Like the digitising class, the digitising and processing class is capable of a very large channel count and can be used for distributed I/0. This class also overcomes the problem of low througliput to the processor of the previous class. Unfortunately, this class has a major disadvantage in that the developer must program a processor that is not part of a general-purpose computer. Therefore, the developer will not have the wide range of software which is available for PCs. The fourth class, direct-connect modular, is a two-port system that SEPTEMBER1992 17 older special purpose signal conditioning products are obsolete because of too-rigid architecture. • Signal conditioning systems must be robust enough to handle conditions on the factory floor. Rugged casing, secure cabling and rack mounting are all required to make a system long-lasting and reliable. Data acquisition boards NI-DAQ is a real-time, multifunction data acquisition driver for Microsoft Windows which controls National Instruments' plug-in data acquisition boards for PC/XT/AT/EISA and IBM PS/2 computers. Also available from National Instruments is DAQWare, a DOS-based software package which has menus for chart recording, temperature measurement and waveform generation. It outputs data in binary, ASCII or Lotus 1-2-3 formats. accepts unconditioned signals on one port and sends out conditioned signals on a second port to a plug-in data acquisition board in a PC. They can take advantage of the wide range of data acquisition boards and software available and the latest PCs can be used for analysis and presentation. They also can usually be configured for different transducer types for each channel and are capable of high throughput to computer memory. The disadvantage to this class is that it cannot generally be used for distributed I/O and can only condition a few channels. / What to look for in signal conditioning systems • High sensitivity and low noise requires extensive shielding from the . PC i...--~ Interface Circuitry PC Bus ADC electrical interference of the PC. • Power supplies and digital circuitry must be designed for optimum noise performance. The signal conditioning system should be expandable to be able to take advantage of the high sampling rate of today's plug-in data acquisition boards. • The fast data processing capabilities of a PC should not be reduced by the signal conditioning front end. Therefore, the signal conditioning system should have high performance with a high analog bandwidth. • A signal conditioning system must be flexible, offer a wide range of functions and interface with many types of computers and software packages. • The system must be able to be easily updated to keep up with the rapid changes in the PC industry. Many Analog Mux I/O Connector Fig.2: this diagram shows the analog input section of typical plug-in data acquisition board. 18 SILICON CHIP These boards plug directly into a slot in a PC and are available with analog, digital and timing inputs and outputs. The most versatile of the plug. in data acquisition boards is the multifunction I/O board. As the name implies, this board typically contains various combinations of analog-to-digital converters (ADCs) , digital-to-analog converters (DACs), digital I/O lines and counter/timers. ADCs and DACs are used to measure and generate analog voltage signals , respectively. The digital I/O lines sense and control digital signals. Counters/timers are used to measure pulse rates, widths, delays and generate timing signals. Multi-function boards are commonly used to measure· analog signals. This is done by the ADC which converts the analog voltage level into a digital number that the computer can interpret. The analog multiplexer (MUX), the instrumentation amplifier, the sample and hold circuitry and the ADC comprise the analog input sec- , tion of a multi-function board, as shown in Fig.2. Typically, multi-function data acquisition boards have one ADC. Multiplexing is a common technique for measuring multiple input channels (generally 16 single-ended or eight differential) with a single ADC. The analog multiplexer (MUX) switches between input channels and passes the signal to the instrumentation amplifier and the sample and hold circuitry. Instrumentation amplifiers typically provide a differential input and gain selectable by jumper or software. The output of the amplifier is sampled, or held at a constant voltage, by the sample and hold device at measurement time so that voltage does not change during short the digitisation period. The ADC digitises the analog signal into a digital value which is ultimately sent to computer memory. FLUKE AND PHILIPS - THE GLOBAL ALLIANCE IN TEST & MEASUREMENT e PHILIPS Flexible Data Acquisition? Fluke's Hydra Series has all the answers Whatever your questions about flexible, portable data acquisition, Fluke's Hydra Series has the answers. The economic 2620A Data Acquisition Untt is a compact front end for use wtth your PC or printer. Or for stand-alone or remote monitoring applications, the 2625A Data Logger stores over 2,000 scans in nonvolatile memory, Both systems are easy to set-up and reconfigure - at the touch of a buaon, or remotely via the computer interface. Check all the answers that Hydra offers: ■ Plug-in universal input module for up to 20 analog inputs without signal conditioning. ■ ■ ■ For further information please contact your local Philips Test & Measurement Organisation: SYDNEY (02} 888 0416 MELBOURNE /03} 8813666 PERTH (09} 277 4199 WELLINUTON (04} 88 9788 BRISBANE (0ll 8440191 ADELAIDE (08) 348 2888 AUCKLAND (09/ 89 4160 ■ Twelve digital 110 lines: four fixed alarm outputs and eight computerconfigurable alarm or general-purpose lines. Standard RS-232-C interface for printer hook-up or bidirectional computer control; GPIB/IEEE-488 optional on 2620A. Fu/I front-panel control for alarm limits, scaling, min./max./last memory, auto-print outpuC real-time clock and selectable scan triggers. Free Hydra Starter Software Package and labtech ACQUIRE™ for quick, easy set-up. PHILIPS Lab VIEW - Windows Software for Data Acquisition Boards Top-Level Front Panel Top-Level Block Diagram Run Edit Free Single Execution Print Run Step Highlighting Indicators Fr~~! Panel Data Flow Formula Acquisition Analysis Terminal Wire Node and Control Icon Icon In the 1980s, plug-in data acquisition boards for personal computers became a widely accepted method for automating test and measurement applications. These instruments are very flexible because of the way in which the computer can process the data. However, suitable software was lacking, that could easily provide the analysis and user interface, in addition to the ability to program the data acquisition hardware and integrate existing programmable instrumentation. The introduction in 1986 of LabVIEW for the Macintosh revolu- The fundamental parameter of an ADC is the number of bits. For exam- f ple, many ADCs are 12-bit, so a voltage within the input range of the ADC will produce a binary value that has one of 212 = 4096 different values. The more bits that an ADC has, the higher the resolution of the measurement. The resolution of the AID conversion is also determined by the input range of the ADC and the gain of the instrumentation amplifier. The gain is applied to bring the signal up into the optimum range to suit the ADC and thereby give best resolution. For example, suppose you are measuring a low-level ±30mV signal with a 12-bit AID converter that has a ±5V input range. If the system includes an amplifier with a gain of 100, the re20 SILICON CHIP tionised PC-based instrumentation with the concept of graphical programming - developing block diagram programs rather than writing conventional, text-based code. Lab VIEW was the first graphical programming language that could integrate several popular classes of hardware for test and measurement applications. Now Lab VIEW is available for both Microsoft Windows-based IBM-compatible PCs and UNIX-based Sun SPARCstations running tMe X Window System or OpenWindows. To run Lab VIEW for Windows you need a 386 PC with a 387 co- sulting resolution of the measurement will be 10V/(100x4096) = ±0.0244mV. Finally, an important parameter of digitalisation is the sampling rate. This must be at least twice as fast as the highest frequency present in the signal (this is the Nyquist Criterion). In addition, the instrumentation amplifier must be able to settle to the required accuracy within a specified time. Types of ADC The most popular ADC on plug-in data acquisition boards is the successive approx;imation type because it offers high speed (up to 200 kilosamples/sec) and high resolution (12-bit) at modest cost. Subranging (also called half-flash) ADCs offer very high speed conversion with sampling processor, 8 megabytes RAM, 10 megabytes hard disk space, Microsoft Windows 3.1, and DOS 5.0. A Super VGA display and a Windows 3.1 compatible graphics accelerator are recommended. The minimum configuration for LabVIEW for Sun is a SPARCstation with 24 megabytes of main memory, 32 megabytes disk swap space, and 10 megabytes disk space (for the application and associated files) . LabVIEWforSunwill run under MIT's X Window System, Version 11, Release 4 or 5 or Open Windows Version 3. It does not require Motif or OPEN LOOK. speeds up to 1 megasamples/sec. Delta-sigma modulating ADCs are the state of the art. They sample at high rates, are able to achieve high resolution and offer the best linearity of all ADCs. Integrating and flash ADCs are older types still used on data acquisition boards today. Integrating ADCs are able to digitise w ith high resolution but must sacrifice sampling speed to obtain it. Flash ADCs are able to achieve the highest sampling rate (GHz) but are available only with low resolution. · Noise & wiring considerations Even after the correct signal conditioning and plug-in boards have been selected, care must be taken to avoid noise corruption. Power lines, computer monitors and electronic equip- I like the feeling of our new igital troubleshooting scope. Now there's a 100 MHz digital scope that handles just like analog. .instantly to the slightest control change. Digital oscilloscopes have certain advantages that are hard to overlook. But for troubleshooting, many engineers still prefer analog scopes. Simply because they like the way they handle. But when it comes to troubleshooting, the HP 54600's digital performance leaves analog and hybrid scopes far behind. At millisecond sweep speeds, the display doesn't even flicker. Low-rep-rate signals are easy to see without a hood. The HP 54600 changes that. It looks like a 100 MHz analog scope. All primary functions are controlled directly with dedicated knobs. And itfeels like one. The display responds It has all the advantages that only a true digital scope can provide. Like storage, high accuracy, pretrigger viewing, hard copy output, and programming. And since it's one of HP's basic instruments the HP 54600 gives you all this performance at a very affordable price. So if you like the feel of analog control, you'll like the way our new digital scope handles troubleshooting. To find out more call the Customer Information Centre on 008 033 821 or Melbourne 272 2555. [hp] ~i~K~~6 A Better Way. Just released: the HP 54602A scope with bandwidth up to 250MHz JWTHTM 125/A LARGE LCD DISPLAY MODULE A ve ry small telescopic STEPPER MOTORS These are brand new units, main body has a diameter of 58mm and a height of 25mm. Will operate from 5V, has 7.5 deg. steps, coil resistance 6.6 monocular IA viewe r. The assembled scope has high quality military grade optics, and employs a prefocussed IR image ohm and it is a two phase type. Six wi res. converte r tube. Thi s tube has a useful response ONLY from 600-1300 nM, thus also making it useful when working with IR LEDs, and IR laser systems. The scope has provision for a coaxial E.H.T. connection , and is supplied with a power supply kit, that features a ready assembled inverter on a small PCB. The power supply $12 ea. (Item No. 0140) easi ly fits into a small plastic case, and can be fitted with $269 r,~c~:~~I~ i~~~~uffi~:t At a ve ry small (Item No. 0121) We can also supply the in some laptop computers. Yellow•green same monocular viewer wi th its original power supply. Powered by a single 1.5V C type battery. In this case all that is needed is connecting the scope to the power supply unit via a coaxial cable, which is also supplied. (Item No. 0122) VISIBLE LASER DIODE POINTER-PEN r \ \ Brand new large Hitachi LM215XB display, as used Save by making your own LCD reflective type, 480 X 128 dot display, effective display area of 242 X 69 mm , has 14 LSI ICs 12V laser inverter kit. Th is combi nation incl udes a He-Ne visible red laser mounted on the rear of universal laser power the PCB. Includes four pages of data sheets. Limited stock at a sma ll ~~~~t~a~u~. their $39 ea. SINGLE CHIP MICRO COMPUTER (Item No. 0132) tube and one 12V supply kit - Mkll. This inverte r features easy construction since the transformer is already assembled and will drive all the 0.2-15mW tubes efficiently. Universal 12V supply plus a used 2-3mW tube or head sgg Small PCB assembly containing two ICs, one of which is a M usic generator (7910 CF). Th e other IC is probably a power amplifier (NECCTM701D). App ly 3-12V across two of connector pin s to obtain music from one of the IC pins. This information and a circuit diagram is provided. 5 $30 extra. Only with initial purchase. (Item No. 0105X) ..........-.............. 'C:"•.··-. t>:;/·:·-:•,.""• ~ and en hance all your presentations. Not a kit, but a comp lete commercial pen sized In the form of a telephone display. Based on an NEC uPD7503G single chip micro computer. Apply power to the un it and the lase r pointer at an display will show date and time (Xtal controlled osci llator on board) but incredible price! ON SPECIAL AT s1ao Small pen sized body. Runs on two small AAA battery. Battery life: 2-10 hours. Vis ible 5mW red (670nM) laser. Projects a visible red spot at more than 50 metres. As used for medical treatment by doctors and acupu ncturists. this is on ly a sma ll part of the mi cro comp uter's features: 4 bit single ch ip micro computer, 4 bit parallel ALU, 8 bit serial interface, 4096 X 8 bit words of ROM , 224 X 4 bit words of RAM , 23 general purpose 1/0 lines, 24 line segment outpu ts 15 switches: 5 off modern 3A panel mounted mains rated switches, 5 off modern 16A panel Powerful ball bearing identical small mains powered fans. transformers and four identical larger transformers . The transformers can be rewound for use in switches worth at least five tim_es our total price of s 16 FOR TH E COLLECTION Item No. 0150 LARGE ROT ARY SWITCHES 1mA whilst operating ,tfess than 10uA in standby. Connection to the .phone display can be made by with each order for one or more of the phone These brand units a re supplied in their original pack ing . Th ey are an option for backlighting Citizen LCD colour TVs. The sc reen glows a bri lli ant whi te co lour when the un it is powered from a 6V battery.' Draws approximately 50 mA. The displays. $ 15 They have at least th ree positions and are have currents rati ngs from 10-35A. Great for high power main s, and antenna switching. (Item No. 0133) pressure switch. horizontal adjust• 15 ONLY ea. Item No. 0151 Vertical and basis of a solar powered sign , e.g . house number. Effective sc reen size is 38 X 50 mm. The inve rte r PCB can be also used for powering backlighting ea . ments. Battery life more than 10 hours continuous. 5mW 670nM class III A laser. Range is about 150m, some would claim three times more. Shock tested to 2000g. One year warranty. ON SPECIAL AT (I tem No. 0144) 5 Small black anodised metal body. Removable PCD can be separated. Could be used as the 12 These large rotary switches have "snap in " action and all are rated at mains voltages . or higher. ONLY OR 5 for $60 sc reen and the inve rter S ea. new eq uipment. sg • 90 !~~:,r\~~:r~~~rc~· s5 Item No. 0152 $249 Price includes one rifle or pistol mount (Item No. $89 (Item No. 0111) We can also supply a similar kit which is based on an unmou nted co lli matin g lens which wi ll need to be supported in front of the laser diode. This kit includes a brand new 5mW/780nM IR laser diode, a suitable heat sink, a collimating lens, a driver kit (PCB and components) and the instructions. ea . Item No. 0147 Brand New high quality US made, amorphous glass Solar Panels. Dimensions: 150 X 150 X 2.5mm . In normal sunlight they deliver a full 1 Watt whilst charging 6V-8V consumption: less than instructions on the 220V/0.13A, 173mm diameter by 60mm deep. Thermally protected, finger guard included, removed from ad hesives are used between them or in coi l winding. There are four SOLAR PANEL GIVEAWAY addressing, can be battery operated (3.5-6V), extremely low battery uPD7503G and a circuit diagram of the phone display will be provided dissamble and rewind. Ferrite halves are clipped together and no That's a total of t5 New instru ction set enables direct memory simpl y pushing wires into ONLY Contains mainly 8 modern ferrite transformers which are very easy to LARGE COMPUTER FANS and 4 lines common the socket provided . One complete 15 page set of bar code reading , medical, etc. Limited stocks. mounted mains rated switches, and a mixture no less than 5 rotary switches: small and large. outputs for driving LCDs, (Item No. 0101) Unive rsal Supply Kil plus a new 10mW tube $299 (Item 0107) nM/5mW. We also supply a PCB components and instructions kit, for a suitable digital dri ver circuit that can be used to complete the laser transmitter. Suitable for Universal Supply Kit plus a new 5mW tube $189 (Item No. 0106) A mixture of no less than printers. but is suppl ied with an extra Brand New laser diode to su it . It produces a beam the.I can be foc ussed to a fin e dot or line. Barely visible, 780 comm unications . data links, perimeter protection , Universal Supply Kil plus a new 1mW·tube $129 (Item No. 0105) Suit teachers, doctors, businessmen, and other professionals. Improve r~igsd~s~lays 4 ea. Item No. 0146 One extra tube or head This precision collimator assembly was removed from working laser ONLY $45 Note that if either of the above lasers are coupled with a sui tab le receiver (E.A. Ap ril 91) a perimeter batteries! Can be connected in se ries, in parallel. or series / parallel. to increase the voltage, and/or current capac ity. For example, in ave rage sunlight , two of these panels connected in series will deliver approximately 150mA of charging current, into a floating 12V (14V) battery: 2 Watts! Four panels can be protection or data link wit h a range of a few ki lometres should be poss ible. We tried it ove r 300 metres. with no problems! We may stock a kit for the rece iver which would include the instructions. in July 92 . Approx imate pr ice S20 for the PCB. all on-board components . and a suitable relay. connected in series parallel to deliver 300 mA into a floating battery: 4 Watts' Compare the prices! All the panels have a weather protection film on rear, and the terminating clips are provided. $10 INCREDIBLE PRICING: or 10 for $85 (Item No. 0130) ea. DANGER! LASERS ARE NOT FOR KIDS then average the data for a single reading. This is effective for filtering out higher frequency noise, such as 50Hz interference. Software filtering of noise is not practical for all applications though. It works best for lowbandwidth signals that can be oversampled many times. Digital input & output signals Many data acquisition boards include several channels of digital input and output. These channels are usually TTL (0-5V) lines and are typically groupThe automotive fluids research program at EG&G Automotive Research Inc (San Antonio, Texas) is the type of industrial environment that tests the performance capabilities of SCXI, a ed into 4-way or 8-way low-noise, front-end signal conditioning system. ports. Each port may be dedicated as an input ment can add noise to the signals you potential difference adds unwanted or output port or may be software are acquiring. Switching high-current noise to the measurement. configurable. loads in nearby wiring can generate Some applications require that the magnetic fields that are coupled into Minimising noise reading or writing of digital patterns your signals through loops in your via software be synchronised with an external cabling. Signal wires close to AC Noise can be further minimised us- clock. Therefore, some boards include power lines can capacitively pick up ing software techniques. While soft- handshaking lines to time the reading noise. ware often assumes that the noise dif- and writing to the digital port. To minimise these problems, all sig- fers in some known way from the As mentioned earlier, you may need desired signal, the simplest technique signal conditioning circuitry for your nal wires should be as short as possible. You may also want to use shielded is just to average readings over a pe- digital channels, usually to provide or twisted-pair cabling for better noise riod. isolation. The digital 1/0 channels of performance. You should be careful Consider an application that rea data acquisition board may also be to avoid ground loops in your signal quires a temperature reading once per used to control relays to switch loads connections. A ground loop occurs second. Instead of performing a sin- such as solenoids, lights, fans, mowhen the signal source is referenced gle reading every second, the soft- tors, pumps and so on. to a ground that is at a different poten- ware can sample the temperature senRelays may be either electrometial to the measurement system. _This sor several times at a higher rate and chanical or solid state. Solid state input relays can also be used to sense large voltages (AC or DC) from proximity switches, thermostats or limit computers which don't provide the switches and convert them to digital Budget 16-channel standard ±12V power supply rails. TTL signals. Both electromechanical A/0 card The PC-126 is also compatible with and solid-state relay boards are availThe PC-126 is a 12-bit 16-chan- the PC-26 and PC-30 range of data able from vendors for direct connecnel AID card capable of sampling acquisition cards. tion to digital 1/0 ports of data acquirates of up to 50kHz with two 12-bit Supplied with the PC-126 is driver sition boards. D/A channels and 16 digital 1/0 lines. software (compatible with most comData acquisition software The inputs and outputs have DIP pilers), driver source code and the switch selectable ranges. Advanced Status 30 data acquisition software. Software is often the most critical triggering and clocking modes are . Drivers are also available for the component of a data acquisition sysintegral to the card as well as a DIP PC-126 to run under Labtech Notetem. A whole spectrum of software switch selectable bus address to book and Labwindows. options exist, each with important avoid bus contention. For more information, contact trade-offs and advantages. The card has a low power conBoston Technology, PO Box 415, The first option is to program the sumption and can be used in laptop Milsons Point, NSW 2061. data acquisition board yourself at the hardware level. Data acquisition SEPTEMBER1992 23 Relay output board from Procon Procon Technology has released an externally mounted high cu rrent relay board , the PC-10-NR. Designed and manufactured in Australia, this board provides eight relay outputs capable of switching 1A at 11 0VAC or 30V DC. Also included are eight optocoupler inputs with 1000V RMS isolation. Up to seven of these boards may be used in conjunction with Procon 's bidirectional parallel port cards. A new feature of the board is the ability to turn off all outputs if new data is not received within 10 seconds (user selectable) . This function may also be disabled, allowing outputs to remain in their last state indefinitely. For further information , contact Procon Technology, PO Box 655, Mt. Waverley, Vic 3149. Phone (03) 807 5660. repeatedly fills the buffer as the program accesses and processes the data. This method allows large amounts of data to be processed with limited amounts of memory. Another important feature of driver software is the ability to maximise hardware performance. To do this, full support ofDMA-driven and interruptdriven data acquisition is required. However, there are certain DMA techniques and features that can optimise hardware performance on different hardware platforms. For example, EISA bus computers include a faster, more advanced DMA controller. While this DMA controller is compatible with AT and XT computers, the DMA controller implements certain features , such as buffer chaining, that are valuable for high speed data acquisition. Macintosh II computers, on the other hand, do not include a DMA controller. Therefore, driver software and the accompanying hardware must provide the services to attain full hardware throughput rates. Acquisition~analysispresentation software boards are typically register-based, ie, they include a number of digital registers that control the operation of the board. The developer may use a standard programming language, such as C or BASIC, to write series of binary codes to the data acquisition board to control its operation. While this method affords the highest level of flexibility, it is also the most difficult and time consuming, especially for the inexperienced programmer who must know 1 the details of programming all hardware , including the board, the PC interrupt controller, PC memory and so on . Driver software If you don 't have the time or interest to learn the details of your computer and plug-in board hardware , you can use data acquisition driver software. Driver software typically consists of a library of function calls usable from a standard programming language. These function calls provide a high-l evel interface to control the standard functions of the plug-in board. For example, a function called SCAN_OP may configure, initiate and complete a multiple channel scan24 SILICO N CHIP ning data acquisition operation of a predetermined number of points. The function call would include parameters to indicate the channels to be scanned, the amplifier gains to be used, the sampling rate and the total number of data points to collect. The driver responds to this one function call by programming the plug-in board, the DMA controller, the interrupt controller and CPU to scan the channels as requested. Some driver software consists of little more than a collection of routines to exercise the very basic operations of the data acquisition hardware. At the other end of the spectrum, some drivers act as a complete operating system. These provide transparent DMA (direct memory access) and interrupts for multiple boards and multiple functions (analog in, analog out, digital) on single boards. They may also provide different buffering techniques, such as doublebuffering, to maximise performance of the hardware. With double buffering, data is streamed into a memory buffer using DMA or interrupts concurrently while the application program pulls out data from the buffer. 1,'he DMA or interrupt-driven process While driver software simplifies the operation of collecting data using plug-in boards, a typical acquisition involves more than the simple acquisition of voltage readings. For example, most applications also involve some type of processing, manipulation, display and storage of the collected data. A user of driver software will code these operations with the programming language they happen to be using. However, several software packages are available that address the development of entire applications. These packages simplify not only the acquisition of data but also the development of a complete data acquisition application. Anyone using one of these software packages configures and develops an application ranging from menus to standard programming languages to graphical programming. With graphical programming, the user places graphical icons representing different acquisition and processing operations on a workspace. Lines are drawn , or wired, between icons to represent the flow of data in the program. For example, a user might wire output data from an icon that collects temperature measurements to an icon This is an example of a temperature measurement application configured with the LabVIEW graphical (icon-based) programming system. Lab VIEW Vis (virtual instruments) have graphical front panels as the user interface and a block diagram as the program. You can customise your graphical user interface with graphs, strip charts, knobs, slide controls and imported graphical images. that averages a series of measurements. The icon may also be wired to a terminal to display the measurements on a strip chart. One of the first decisions to be made when configuring a data acquisition and control application is to decide which computer system will be used. -...... = . . === -----·. ·:-..T"..a..1 00000 00000 aioai..:ai -.:;t N ,- NT""" YHFHFHF>YI- ~ ;, r:i;l3 =ii~•£= Pfii{{jfeeson s PC MARKETPLACE P/L TEL(02) 4186711 FAX (02) 4186713 Mail Order Address: P.O.Box1100 Lane Cove NSW 2066 ,Major Credit Cards accepted. Prices apply for mooth of issue Dealer enquiries welcome. ATTENTION: FAX / MODEM OWNERS If you are using QL2 Software, then upgrade to the latest Version 2 for only $40.00 including P&P. This new version has a number of new features. These cover the range of 80x86-based PC/XT/AT/EISA computers, IBM PS/ 2 computers and Macintosh computers. You will want to consider the bus architecture, the processing power and operating system of a platform before making a decision. The I/O bus architecture of the different computer platforms can be a factor in the performance of your application. The 16-bit AT bus, used in most of today's 80286, 80386 and 80486 computers, has a maximum data transfer rate of about 1.6 megabytes/ second. Boards built with 8-bit XT bus in- . terfaces can be used in AT computers and both XT and AT boards can be used with the 32-bit EISA bus. The EISA bus, used in 80386 and 80486 computers, has a maximum rate of 33 Mb/sec and expanded DMA capabilities. EISA boards also can be configured automatically with software without having to use jumpers or switches. IBM PS/2 computers use the Micro-Channel Architecture bus, a 16 or 32-bit bus with capabilities similar to the EISA bus. Apple's Macintosh II and Quadra computers use the NuBus, ~ ~~~ ~ - - - - ~-11"";.;.....•-! IN ANYONE'S LANGUAGE. PROCON TECHNOLOGY LETS YOU TAKE CONTROL! We manufacture a wide range oflow-cost digital 1/0 boards. Each board features: 8 opto-isolated inputs (12/24/48V AC or DC), 8 relay outputs (switching up to 10 Amps at 250VAC), LEDs indicate 1/0 status and IBM-PC software is included. An industrial version with plug-in relays is also available. The system features: External mounting (up to 30 metres from computer) operating through any IBM-PC bi-directional printer port and capable of expanding to 240 1/0. Applications: Home or business security systems, process monitoring and control, laboratory automation, quality control testing, robot control and energy management. SUPER SPECIAL 101 Enhanced Keyboard Was $99.00 NOW ONLY $59.00 GRAPHIC CARDS/ MONITORS VGA 512K VGA 1MB 14" Mono Monitor 14" SVGA Monitor $ 95.00 $ 149.00 $ 195.00 $ 499.00 FLOPPY DISK DRIVES 1.4MB3.5" 1.2MB 5.25" 3.5" TO 5.25" Bracket $ 105.00 $ 120.00 $ 9.00 PROCONtechnology Box 655, Mt. Waverley, VIC. 3149. TEL: (03) 807 566o FAX: (03) 807 8220 • Yes! High-speed drivers are available for GWBASIC, Quick.BASIC, TurboBASIC, QuickC, TurboC and TurboPascal. Our file 1/0 driver also allows many other programs and languages to be used. E.g. DBASE, Clipper, COBOi, FORTRAN, MODULA-2 etc. SEPTEMBER 1992 25 a 32-bit, 37.5Mb/s bus with automatic this 640Kb limit, however. configuration. One potential drawback . Windows 3 and 3.1 take better adof the Macintosh NuBus is that, un- vantage of the hardware capabilities like other popular platforms, the sysof 80386 and 80486 computers. With tem does not include a DMA controlWindows 3, users have access up to ler for plug-in boards. Therefore, high16Mb of RAM and even more virtual performance NuBus boards may inmemory (the hard disk is used as exclude a DMA controller for high tra RAM) . However, Windows 3 has throughput. been criticised for its lack ofreal-time performance, mainly due to the slugThe performance of these I/O buses, especially the 32-bit buses, will be gish response of Windows to hardadequate for all but the most demand- ware interrupts. ing applications. The operating sysDespite this, high throughputs can tem and application software more be realised if the data acquisition often determines the ultimate success driver software and hardware takes of a data acquisition and control apthe necessary steps to work around plication. The operating systems com- the problem. First, problems can be monly used on PC/XT/AT/EISA and substantially reduced by using a virPS/2 computers are DOS and Wintual interrupt service device (VISRD) dows. that has quicker access to hardware interrupts. Second, the data acquisiDOS vs. Windows tion board should have large FIFO DOS has been standard for PCs for buffers on board to buffer data during several years and a huge supply of interrupt delays. software tools exists. Its real-time caThe third method uses alternating pabilities are sufficient for most data DMA channels. Data acquisition acquisition applications but the main boards typically use hardware interdrawback is that it limits the amount rupts to signal the CPU that the DMA of memory that can be used by an controller must be reprogrammed with application to 640Kb. Memory extend- the next memory segment. The DMA ers are available that help work around transfers halt as the controller waits to be reprogrammed. The problem is compounded in Windows because memory buffers in Enhanced mode can be heavily segmented into different physical memory locations. The DMA controller must be reprogrammed for each individual memory segment. With dual-channel DMA, however, the board automatically switches DMA channels at the end of a memory segment. DMA immediately begins on the second DMA channel while the interrupt is asserted and the CPU reprograms the first DMA channel for the following memory segment. This technique can be useful in DOS as well as in Windows. SC Acknowledgement Our thanks to Tony O'Donnell , of National Instruments Australia for this article, photos and diagrams. The Authors, Ash Razdan and David Potter, are senior staff members of National Instruments, at Austin, Texas, USA. ,-----------------, :Fax/Modem: The single handed I : I ":~~,o~ . 11 $349.00 I Send/Receive faxes from your PC I Your faxes can now be sent directly from your PC without having to print them first. You can view incoming faxes on-screen and print them using a standard dot matrix printer. Justthinkofthetime/papersavings . I I I I I I I I I I I : I , Extensive communication and fax software included. 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