•Focus on: measunngwith a pe multimeter

More options fordigital multimeterswith anRS232 interface

Since a couple ofyears, multimeters

with an RS232 inter-face are available at

prices which areaffordable to the hob-byist. Thanks to their

flexibility and wideapplication range,

these multimeters arenow starting to

become popular. Fewusers of these instru-

ments, however,exploit the added

value and capacity ofthe connection with apc. This article aimsat showing what canbe done with the PClink, and how userscan write their own

programs.

By S WOjahn

36

Digital multirneters (DMMs) with aserial Interface are marked by theirability to connect the Instrument to anIBM compatible pe via a simple inter-face cable. The measurement results(values) are then sent to the pe for fur-ther processing. This approach createsa number of advantages and even newapplication ranges.

THE ADVANTAGESA useful application of the pe COI1-

nection couJd be a large display formeasurement values using the pemonitor, with the obvious aim ofimproving the legibility of test results,or for presentations. Alternatively,series of rneasurement values could bevisualised in diagrams, or stored infiles for processing accordmg to a cer-tain protocol. Having two or more ofthese DMMs at hand allows you tocreate a mathematical junction formeasurement vaJues supplied by theindividual instruments, one acting asa voltmeter and the other, an arnmeter.This arrangement already creates newmeasurement ranges.

A bit more extensive is the applica-tion of sensors for just about any phys-ical quantity, induding temperature,light intensity, pressure, humidity, andothers. In that case, the output of thesensor; or the sensor interface, suppliesa voltage er resistance which can bemeasured, while unit-conversion isrequired to the correct value er dimen-sion, ustng a look-up table or an estab-lished calibration functian. This systemaJlows users to simply read the correctvalue trom the pe screen, without theconstant need to do conversion sums.This 'calibration feature' also enablesnearly a11 measurement equipmentwithout a pe interface, for example, aGeiger-Muller counter or a decibelmeter; to be hooked up to a pe, simplyby using the DM1v1 as a kind of ana-logue-to-digital converter (ADC).

TRE MULTIMETERTf you da not yet count yourself ruTIongthe owners of a digital multimeter witha seria! interface, yau will no doubtwelcome sorne advice as regards

Elektor Electronics 4/97

Figure 1. Readoutoptions ot DigiSeopetor Windows: traceroscilloscope, mea-surement value list,bar-graph and giantdisplay.

Figure 2. Strueture ota simple contral sys-tem: the resistancevalue ot a tempera-ture sensor (e.g.GTF150tromGreisinger) is eeu-brated in software. Aslong as the tempera-ture remains be/owthe nominal level(here, 80"C), the PCswitches on theheater via the re/aycard. System controland measurementresults indication arearranged by the pro-gram Windows Labo-ratory Software incombination with theDMM type HC506 tromHung Chang or VC506(Volteraft) and an 8-channel Centroniesrelay card.

Elektor Electronics

-0"" DCV 036.6 mVDCV 058.6 mVDCV 065.3 mVDCV 058.0 mVDCV 058.0 mVDCV 055.2 mVDCV 043.4 mVDCV 061.9 mVDCV 059.0 mVDCV 052.7 mVDCV 043.4 mVDCV 057.6 mVDCV 088.2 mVDCV 085.7 mVDCV 091.9 mVnrv nan Q

Page

....:J -86.3222 mV

choosing such an instrument. The gen-eral advice in selecting a suitable (andaffordable) instrument is to lock forone that has the desired measurementranges, The standard ranges such asvoltage. current, resistance and fre-quency are, of course, offered by vir-tually a11of today's more advancedDMNls. [f you want additional rangessuch as capacitance, inductance, tran-sistor/diode tesüng, logic testing. tem-

perature {internal or using aseparateprobe), dB sound pressure level (Pro-tek 506 or Voltcraft/Hung ChangVC 506), or power (M-3860M), thenYOllshould choose a matehing instru-ment.

External measurement range exten-sions are, however, also possible, aswill be seen further on in this article.Regarding the connection to the pe, itshould be noted that the software you

4/97 37

Figure 3. Here, a dBmeter with vo/tageoutput is applied asan A-D converter incon/unction w;th anRS232-DMM as part 01a PC-controlled mea-surement system. TheDMM ;s an M-3850 setto the mV range, theprogram, MeasurePC.

have avaiJable does not necessarilysupport aU instruments, because someDMMs use a slightly modified dataformat (for example. the Protek 506 orVC 506, and the M-3860M).

An additional selection aspcctshould be the measurement accurncy,If you are after relanvely high accu-racy, you should value low tolerances.and have your test instrument dulycalibrated. A special position in thisrespect ls held by the digital multime-ter type M-4650-CR. While all otherDMMs use a 3)2 or 3)4 digit display, theM-4650-CR boasts a 4)2-digit LCDreadout, which is also fully copied tothe PC Although the accuracy is sohigh in some measurement rangesthat the last digit has no mearung, thisproblem does not occur with othermeasurement ranges (for example,resistance, where the tolerance is0.15% or 3 digits). Furthermore. cali-bration is helpful with this meter,

Other buying criteria are featuressuch as automatie maximum/minimumhold, measurernent value storage, dualdisplays, auto-ranging (which should,incidentally, be switched off with com-puter use), and others.

38

THE P'C CONNECTIONTo be able to use the DMM with acomputer, you will need a suitableinterface cable and, of course, a PC Inmany cases, the cable is inc1uded inthe DM1v1paekage. Tf not, you have tobuy it separately at about flO. Be sureto come horne with the cable that'sright for your DMJv1,however; becausethree are three different typcs around.

AB regards the PC, there are hardlyrestrietions: a11you will need in mostcases is an IBM eompatible PC whichhas at least one free serial port (alsocalled RS232 or COM). This require-ment may still cause problems. how-ever; if a mouse and a modern alreedyoccupy COMI and COM2, and a ser-ial communication extension card hasto be instalIed. Unfortunately, that maynot be possible in some cases - worse,it may not work at all because of inter-rupt conflicts.

Basically, it is immaterial whetheryou use an old 8088 XT or a PentiumPC sporring RS232 FIFa. The mainpoint Is that the software to be used

runs on the avaiJable PC, and then,Windows software simply requires atleast an 80386 processor.

THE SOFTWARElf you are unable or unwilling to writeyour own, special. program for theprocessing of measurement va lues,there is no option but to rely on avail-able software. CommerciaUy availablesoftware may be divided in twoclasses. Some programs are restrietedto simple displaying of data (with themonitor aeting as a large readout or along-period osciUoscope), or the abil-tty to store rneasured values in theform of strings. Other prograllls inaddition offer the possibility of logiecombinitation or conversion of mea-surement values, thereby adding tothe useful range extensions alreadymentioned.

As a representative of the simplerdass we should mention the programOigiScope, which is available in DOS01' Windows versions. OigiScope forDOS offers large readouts, a long-period oscilloscope (y(t) diagram), bar-graph, measurement value listing anddata storage in text formet, whiehenables programmers to load mea-surement data into a table calculationprogram or a data bank, not forgettingthe option of manual edinng of allmeasured data. A negative aspect ofthis software is that diagrams can notbe printed. The operanon. unfortu-nately, requires some getting used to,and will certainly fail to appeal thoseof you who are used to the 'Windowslook and feel'. A definite advantage is,however; that DigiScope runs withoutproblems on an old XT or AT com-puter, under DOS, provided, of course,an EGA or VGA graplucs card isinstalled. All this allows an old. avail-able, PC to be given a useful applica-tion as part of say- an measurement

Figure 4. Data format01 the measurementdata strlng. The HungChangNo/tcraft 506and the M-3860M usesllght/y different for-mats.

example 1:

example 2:

I ':neasuremen~14

unit ·I~ndmeasured valuerecognition

970035 ·15range

Elektor Electronics 4/97

Figure 5. Program list-Ing tor a simple mea-surement va/ue dis-play in BASIC.

station, while the new Pentium com-puter remains available for other appli-cations. This divicling of tasks betweenthe new and the old computer hasobvious advantages fOT Iong-pertodmeasurements.

Under Wmdows, DigiScope bringsyou an appealmg, Windows-compliantuser Interface. The internal operationof the program is, however, not muchdifferent fram the DOS version (seeFigure 1). The only advantage is theavailable printing option for measure-ment diagrams. Unfortunately, thisfunction is poorly implemented, andfails to deliver satisfactory hard copy.An additional shortcoming is that onlyolle measurement instrument may beconnected, although that problem canbe eliminated simply by starting theprogram several times over.

The Protek 506 multimeter (or theVoltcraft 506) is already supplied withWindows software. As regards perfor-mance, this is at the same level asDigiScope for Windows.

In the better league are programssuch as MeasurePC (original Germanname: MessPC) and Windows Labo-ratory Software (WLS). Both enablemeasurement values to be converted,so that the data processing and thereadout have the right unit of mea-sure when a certain sensor type isconnected.

MeasurePC affers two possibilities formeasurement value conversion: if thefunctional relation between the mea-sured quantity and the desired unit isknown, then a function may beentered for the value calculation. Thisconversion function should either beknown, say, frorn the sensordatasheets, or be established by regres-sion of measurement value patrs usingmathernatics software. As an alterna-tive, the program offers direct entry ofvalue pairs, which, by interpolation(linear or with splines), are used tocompute the conversion function. Thevalue pairs required to do this areeither taken from the sensorsdatasheet, or from reference measure-ments obtained from a calibratedinstrument specifically designed forthe relevant quantity. The softwateoffers readouts on the pe monitor aswell as a long-penod (tracer) oscillo-scope with data storage. Regrettably, abar-graph readout and measu rementvalue Hsängs are not provided.

Furthermore. MeasurePC offersuseful functions such as smoothing ofthe measurement curve, and post-conversion of measurement strings

Elektor Electronics

PRINT "Press space to quit", initia1ise interface :OPEN "COM2:1200,N,7,2,RS,CS,DS,CO" FOR RANDOMAS #100

PRINT #1, "0"; , request meas. data00

IN$ = INPUT$(l, 11)PRINT IN$;

LOOP UNTIL IN$ = CBR$(13)FOR Z = 1 TO 3000 'short delay loopNEXT Z

LOOP UNTIL INKEY$ = CBR$(32)CLOSE #1END

(post-calibration). The program runsunder DOS and works with anygraphies ea.rd on any pc. This abilitymakes it very weH suited to use on anold pc. The graphlcs display of astring of measured values is adequate,while printer output as well as dia-gram exporting (using the bitmap for-mat) are possible. A zoom function forbetter detailed displaying of certarnscreen areas is, however; not pro-vided. Unfortunately, this program,too, allows on.ly one multimeter to beconnected.

Not so with Windows LaboratorySoftware (WLS): totally in line withWindows thinking, four D:M1v1smaybe connected simultaneously, whilethis program spoils you with a weU-designed, message-controHed userinterface ('shell'). WLS requires onlyone equation to allow mathernaticalcombination of measurement va luescaprured by different digitalmultime-ters. If necessary, integration er differ-entiation may be included in yourequation. On the down side, calibra-tion or value conversion by means ofa look-up table Is not provided.

The measurement value indicators(whieh rnay be freely placed anywhereon the screen) inc1ude a digital read-out, an analogue (moving-coil) scale, abar-graph. a y(t) recorder (long-periodoscilloscope) a measurement value Iist,a rnax/min. readout, and triggering onthese, in any combination. An excellentpoint is the control of the variousinstruments by means of the messagesexchanged between them. A messageis a number between 1 and 9999 sentwhenever an instrument registers astatus change. If for instance, the trig-ger level is reached on a certain triggerinstrument, this is automaticallyprompted to transmit a messagewhich initiates, say, the writing of a y(t)grapf

The message instruments indude aswitching clock, a stopwatch, a docksource, a message counter; a sound sig-nal, or the possibility to drive an 8-channel relay card. The relay card(pnce approx. E35) is connected to aparallel printer port (LPT), allowingthe relays to be actuated by individualmessages. By clever coupling of the

4/97

Instruments via the various messages,it becomes possfble to create extremelyuseful switching systems, or accom-plish synchronised measuring andautomation tasks (Figure 2). Despttesome minor flaws, the weIl-designedconcept and matehing user interface,all at a price of less than E25, give Win-dows Laboratory Software (WLS) theedge over the other programs men-tioned in this article.

THE USE OF SENSORSThe use of sensors results in a tremen-dous increase in the possibilities of adigital multimeter. Cost-effective sen-sors are available these days for manyphysical quantities inc1uding temper-ature, pressure, light intensity , mag-netic field strength and relativehumidity. Such sensors usually supplya voltage or a resistance value. Next.these output values have to be con-verted (calibrated) before they appearwith the proper value and unit. Sucha functicn is already provided by soft-ware products such as Wi.ndows Lab-oratory Software (WLS) and Mea-surePC. lf the requtred conversion isnot dear from the sensor s datasheets,you can't avoid acquiring them thehard way, i.e., by means of a referencemeasurement on a calibrated instru-mcnt. Although that may hardly be aproblem with a temperature sensor,things may become considerably morecomplex if you want to measure, say,ozone levels. That is, if you do nothave the datasheet.

Quite simple, by contrast, is theconnection of an analogue meter; forexample, a deeibel (dB) meter (Fig-ure 3). All you need to do is tap thevoltage at the meter indicator, andmeasure it using a DMM switched tothe rniIlivolt (01V) range. A teehnieallyclean method of doing so is to fit thedB meter with a small jack socket andmaking a suitable cable for connectingthe output voltage to the multimeterinputs.

The conversion from measuredvoltage to the required unit is easilyaccomplished with the aid of a coupleof measurement pairs which may beread from the analogue instrument orthe DMM. In this way, the sensor (the

39

Figure 6. üsting of ameasurement valuedisplay written in De/-phi-Pascal under Win-dows.

analogue measuring device) itself isemployed for the calibration.

Provided you know what you aredoing, the same may be achieved witha digital instrument by "tapping' a volt-age at a suitable point just befare rheA-to-D converter.

YOUR OWN PROGRAMSIf you are not satlsfied with theoptions and features offered by theavailable software, you may want toconsider writing your own programsusing whatever programming skillsyou have. Alas, programming an oscil-loscope ts out of the question becausethe DMMs discussed here supply mea-surement data at a rate of two per sec-ond only.The software may be written in ahigher programming language such asBASIC, C, Pascal, Visual BASIC, er DeI·phi. First, the serial interface has to beinitialised to 1,200 baud (9,600 baud forthe M-3860), no parity, 7 data bits inASCIJ format, 2 stop bits and softwarehandshaking. That's all there is toprepenng for the transfer of rneasure-ment data - all the computer (or theprogram) has to da is request a mea-surement value. This is achieved bytransmitting the ASCII character 'D'(#68) to the DMM, which responds byreturning astring containing the cur-rent rneasurement value, the unit andthe measurement range. The receivedstring has a length 01 14 bytes (Fig-ure 4). An exception is formed by theProtek 506 (or VC 506) and the M-386OM. On being told to do so, the M·3860M uses a baudrare of 9,600 bits/s,and transmits the measurement valuesof the main display and its three sub-displays in a single data packet with alength of 56 bytes each. The Protek 506transmits at 1,200 baud and uses avariable data packet length of between6 and 15 bytes.

A simple demonstration program isshown by the listing in Figure 5. Theresults of a conversion to Delphi-Pas-cal running under Windows are seenin Figure 6. Both prograrns have beenkept as simple and spartan as possibleto emphasise their crucial function ofcontinuously requesting a measure-ment value string and displaying itwithout any kind 01 filtering. It shouldbe noted that these example programsare not geared to the Protek 506 andM-3860M, although it should not betoo difficult to adapt the relevant rou-tines. Obvtously, if the measurementvalues are to be subjected to further

Elektor Electronics 4/97

unit Main; { Protocol unit for indication of measurementdata from a DMM with interface; not suitablefor Protek 506, VC 506 or M-3860M }

interface

usessysUtils, WinTypes, WinProcs, Messages, Classes, Graphics,Controls, Forms, Dialogs, StdCtrls, ExtCtrls;

typeTFormMain = class(TForm)

LabelDMM: TLabel; { should be placed on FormMain }Timer: TTimer; { Timer also placed on FormMainprocedure FormCreate(Sender: TObject);procedure FOrmDestroy(Sender: TObject);procedure TimerTimer{Sender: TObject);private

ComPort : integer;end;

var FOrmMain: TFormMaini

implementation

{$R *.DFM}

procedure TFormMain.FormCreate(Sender: TObject);Var DCB : TDCB;begin

ComPort: =OpenCOMM(' COM2' ,100,100); { initialise Port }GetcoMMState(ComPort, DCB); { load standard settings }with DCB do beg in { implement own settings }

Baudrate:=1200;ByteSize:=7;Parity:=O;Stopbits:=2;Flags:=2;

end;SetCOMMState(DCB); { set parameter}Timer.Enabled:=true; { activate timer

end;

procedure TFOrmMain.FormDestroy(Sender: TObject)ibegin

Timer.Enabled:=false; { deactivate Timer}CloseCOMM (Comport); { close Port }

end;

{ TimerTimer is OnTimer refresh (interval 1000)procedure TFormMain.TimerTimer(Sender: TObject);var Buf array[O ..100] of char; { receive buffer

ComStat : TComStat;begin

WriteCOMM(ComPort,'D',I); { request meas. data }Repeat

GetCommError(ComPort, comStat);Application.ProcessMessagesi { don't block application ... }

until ComStat.cblnQue=14; { wait for daten}ReadCOMM(ComPort, Buf, 20); { read out meas. data }FlushComm(ComPort, 1) i { clear receiver queue }LabelDMM. Caption :=Copy(StrPas (Buf), 1,13); { display meas. data }

end;

end.

processing, the measurement stringwill have to be disassembled into itscomponents as shown in Figure 4. For-tunately, even less expertenred pro-grammers should not find it too diffi-cult to adapt and extend the examplelistings to their personal requirements,or convert them into another pro-gramnung language. (970035)

Windows Laboratory Software dis-tributors:

Conrad Electronic, 0·92240 Hirseheu.Germany, tel. (+49) 1805312111.

Reichelt Elektronik-Vertrieb, PO. Box1320, 0-26449 Sande, Germany.

Digiscope (Windows and DOS) isavailable trom Conrad Etectronic.

MeasurePC (MessPC) is supplied byUmschau Software, no. Box 110262,0-60037 Frankfurt, Germany. Tet.(+49) 230481854.

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