3~ou)ß

PRECISION CLOCK FOR PCsBuild this small add-on unit and put an end to the (often gross)inaccuracy of the real-time clock that ticks in your PC.Complete with a software driver, the precision clocksynchronizes your PC to DCF77, a time standard transmitteroperating in the long-wave band.

Design by B. Zschocke

~ many rc owners, tt is amazing.1 and frustrating ta note that even

the cheapest quartz-controlled Jittlealarm clock bought in a high-streetshop has a far better accuracy thanthe real-time clock in, say. a atate-of-the art 486-based PC. Although manype users will notice the clock inaccu-racy only twice a year (on changingfrom summer time to winter time andvtce ver-sa}. there are also cases whenlarge time errors on a pe have disas-trous consequences. For instance, ona small network, where one programsuch as Turbo Pascal ts shared. andthe MAKE option is used from time totime to produce code (MAKE automatt-cally compiles the latest version of the

program). In thts sttuation, it can hap-pen that the latest versfort of the soft-ware is overwrttten by an olderverston. because the associatcd timeinformation ts tncorrcct. It mayaIsohappen that a back-up program savesdata whtch is outdated. while the lat-est version is 'furgotten' - all becauseof an inaccurate system clock.

The inaccuracy of a PC clock mayhave several causes: supply voltagedips and surges when the computer isswnched on and off; large temperaturevanattons in the case: or agetng effectsof the quartz crystal. Furthermore.most PC clocks lack an adjustrnentpotnt. so that one ls totally dependenton the accuracy of a cheap crystal.

Atoms for the right time

The radio-controlled clock describedhere enables a PC to always 'know' thertght time. The time information re-ceived from the DCF77 transmitter inMainflingen. Ccnnany. is decoded andsubsequently applied to the PC via theRS232 sertal port. Thts ls done to savespace and energy required for an addi-tional PC insertion card. Further. thecurrent drain of the DCF77 recetvermodule and the associated interface isso small that an additional power sup-ply is not needed. Before dtscussingthe hardware. however, let us have alook at the source and structure of thetime information picked up by the re-cerver.

DCF7? IS a 50-kW transmitter oper-ating at 77.5 kHz from MainOingen,near Frankfurt in Germany. The sta-tion ts operated by the Physikalisch-Technische Bundesanstalt. and has arange between 1.500 and 2,500 km.The time Information transmitted byDCF?? is derived from a caesiumatomic clock which is claimed to beamong the most accurate in the world.

As wtth nearly every clock. an oscil-latory movement is used as the refer-ence. However, in contrast with , say. apendulum cJock where the length ofthe pendulum ts a measure of 'time', ora quartz clock. in which the self-reso-nance of a crystal is used as the refer-ence. an atomic clock is based on thefact that atomic energy levels are asso-ciated wtth certain frequencres. To un-derstand this relatton. a littleknowledge of atomtc physrcs ia re-quired.

Depending on their energy. atomscan take on different states. This phe-nomenon may be compared to a fuelgauge in a car: a high reading indi-cates a high energy, a low reading, alow energy. To change the mctcr read-ing from low to high, you have to fuelup, i.e .. energy has to be added .Likewise, the state of atoms can bechanged by adding energy. However. Itis only possible to go from a low stateto a high state by adding an exaclamount of energy. Adding Jess or moreenergy than the crtttcal amount willnot change the atom's state from lowto high. Because of this phenomenon.it ts possible to generate an ultra-exactfrequency by using radiation to add adefined amount of energy to the atom.

In the atomic clock. an amount ofcaesium is vaponzed at a temperatureof 100°C. Cacsturn is a meta I with alow melting temperature of 30 "C. Inthe Irrst stage of the c1ock. atoms with

ELEKTOR ELECTRONICS NOVEMBER 1993

a 'Iow' and 'high' energy state are sepa-ra ted , The 'Iow' level atoms arrive in aresonator in whtch they are exposed toradiation from a stgnal source operat-tng at a nominal frequency of9.192631770 GHz. The radtattön en-ergy ts transferred to the atoms, whichchange to the 'high' energy level. Afterbeing moved through the resonator,the atoms are collected and separatedagatn.

The Iewer 'Iow level' atoms remainin the resonator, the more accuratethe resonator works. This enables acontrol system to adjust the resonatorfrequency for a minimum number ofatoms that rernain at the 'low energylevel. The resonator frequency so ob-ta.ined forms the reference [01' the timeinformat.ion transmitted by DCF77,The estimated accuracy of the caesiumreference is one second in 300,000years.

DCF77time informationIn order to transmit the complete timeinformatton, a protocol has beendrawn up that enables the exact timeand date to be contained in a pertodictransmission which lasts 59 seconds.A complete time code is transmittedevery minute, as illustrated in Fig. L.The transmitter frequency is 77.5 kHz,which is derived Irorn the caesium res-onator. This has the advantage of al-lowing the transmitter frequency to beused as a reference, since tts accuracyequals that of the umcbasc of the clock(phase errors may. of course. stilloccur in the path from the transmitterto the receiver]. The transmit fre-quency used lies in the long-waveband. which means that only theground wave is usable for transrrus-sion. Reflections of radio signals atthts wavelength are insigruficant. sopropagation is by one 'path' only. theground wave. Consequently, receptionof long-wave stgnals is virtually con-stant around the clock. In practice,DCF77 reliably covers most ofContinental Europe. the South ofScandtnavla, and the larger part of theUK and Ireland.

The coding systemThe coding system used on DCr77 wasdeveloped at a time when microproces-sors were virtually unknown. Hence. asimple coding system wtth BCD (btnary coded decimal) was chosen, al-lowing 'ordtnary' TTL components tobe used for decoding the time informa-tion. Each code word consists of59 brts (bit 0 through 58). One btt tstransmitted every second. A logtc oneis transmitted as a pulse with a lengthof 200 ms, and a logic zero, as a pulseof 100 ms. During the pulse time. the

ELEKTOR ELECTRONICS NOVEMBER 1993

PRECISION CLCOK FOR PCs

year

/.cl \0

month 8,H

day 01 2

wee~k ~ ,0108 ,

day 01 2 1

month~22010

hour

M~iscellaneousapplications

2)530 8'

104 i i \Pl,020 minute

~ 80524·1

Fig. 1, Schematic organization of the time/date information transmitted by the DCF77 timestandard tranamltter.

transnutter carrier ts redueed to 25%(amplitude modulation). The arnpli-tude reduction takes place at the startof a second. Since there are 60 sec-onds to the minute. and only 59 bitsare transmitted, the missing 60th bltIS used to mark the start of the timecode. The Iunction of each of the59 bits contained in the time code isgtven in Table 1.

Three times an hour-, during the19th, 39th and 59th minute, the sta-tion ident, DCF77. is transmitted inmorse. Each letter or number is trans-mitted between two second etgns byswitching the transmrttcr modulationdepth between 100% and 85% at a rateof 250 Hz. This tdenttftcauon is auto-mancally transmitted withou t inter-rupting the time code.

TheideaToday. a wide range of ready-madeDCF7? receivers/clocks is available atreasonable prices. In most cases, suchuntts are identified as 'radto-con-trolled clocks. The receiver section insuch a clock usually supphes the dig-ital time codc as 100-ms and 200-mspulses. The best known receiver IC tsthe U2775B. developed by TelefunkenAG for use in Junghans clocks. Apartfrom a ferrite rod antenna, this IC re-quires only a couple of external partsto make a sensitive and reliable DeF7?

receiver. The U27758 is capable of op-erattng at a low supply voltage of be-tween 1.2 V and 3.4 V, and has acurrent drain of only 0.5 mA

There are also ready-made recetvermodules based on the U2775B.Unfortunately, these usually supplyoutput pulses with a length of 160 msand 60 Ins because about 40 ms is re-quired for reliable decoding of the timernformanon.

The designers airn was to find asimple way of ustng a DCF77 recervermodule with 'shortened' output pulsesto decode the time information withthe aid of a pe. This has been achievedby turning the pulses into charactersconveyed to the pe via a serial link op-erattng at a low bit rate. In thts sys-tem, the binary code 000011111 (OrH)represents a 60-ms pulse. and0000000001 (OOH),a 160-ms pulse.The leading 0 in the code is the startbtt, and the trailing I is the stop btt.The advantage of tlus approach ts thatthe software is relieved from pulse de-coding. which is taken over by the ser-ial Interface. Furthermore. theInterface ensures that an interrupt isgcncratcd only once a second. At sucha slow interrupt rate, the PC is hardlyslowed down. The functions that re-main for the software are limiled tofinding the start of the time code. de-coding the actual Information. andconveying the decoded time to the op-

-m-=UTERS AND MICROPROCESSORS

00 ers

ce lN4148

"IC, 100~ lN414B K',,,Q '"++

" ,'G s

0; lN4148

0;

oe rxo

lN4148

BF245B BC547B ". '"

GI, ,i\. 11

2,1 N4148

9311058·119J2001 - 11 . 11

Fig. 2. Circuit diagram of the interface between the pe and the DCF77 receiver module.

erating system installed on the P'C. RTS line, wluch is between +7 V and+12 V. At the same time. capacitor C5ts charged to the TxD Une level, whichis between -7 V and -12 V. Diodes D4and D5 protect the circuit againstdarnage caused by an incorrectly iru-tialized RS232 port. The voltages butltup across the capacitors form the sym-metrical supply voltages for opampICI. The opamp gives the digital time

The serial interfaceThe circuit diagram of the sertal inter-face between the DCF77 receiver mod-ule and the PC's RS232 port is gtven inFig. 2. The interface Is powered via theRTS and TxD lines. Capaetter C4 ischarged via diode D3 to the level of the

COMPONENTS LIST

Resistors:1 4kQ71 lkn51 22kn1 100n

RlR2R3R4

OCF77 receiver module: Conrad',2order code 190969-66.OCF77 ferrite antenna: Conradl-'order code 535630-66.Plastic enciosure 60x50x30 mm;e.g., type E406 (Boplaj''Printed circuit board 930058 plussoftware 1871 (see page 70)

The control software for this project isalso available separately: order code1871 (see page 70).

Capacitors:2 tOOnF1 10!,F/16V radial2 100!'F/25V radial

Cl,C3C2C4;C5

Semiconductors:1 LEO 3mm green8 lN41481 LEO 3mm red1 BF245B1 BC547B1 TL071

0102-09010TlT2ICl

1 Conrad Electronic Nederland BV, P.O.Box 12, 7500 AA Enschede, Holland.Tel (+31) 53 282000. Fax: (+31) 53283075.

2 C-I Electronics, P.O. Box 22089,6360 AB Nuth, Holland. Fax: (+31) 45241877.Miscellaneous:

1 9-way sub-D socket Kl

code applied to its etnput a level thatenables it to be recogntzed as a sertalsignal by the PC·s RS232 port.

Generating a euttable supply volt-age for the DCF77 recerver module is alittle more complex. Here. a greenLED. Dt. 15used to stabilize the mod-ules supply voltage. A stable voltage ofabout 1.8 V is obtained at a small CUf-

rent already.Extensive decoupling is necessary

in the interface, because cross-inter-ference occurs easily between ana-lague and digital sub-circuits. Inparticular, every care has been takento prevent the digital secttons upset-ting the operatton of the sensitive ana-logue receiver. A very effectivedecoupling device is the high internalresistance formed by T! -R2, a currentsource inserted in the positive supplyto the receiver module. Together wilhcapacitors Cl and C2. thrs currentsource forms a filter that affords a veryhigh degree of decoupling of the re-cctvcr's power supply lines. The combl-nation D2-RI is also connected to themodule power supply. and provides areference voltage of 0.6 V at the -tnputof the opamp. This is necessary to en-able the opamp to recognize the highand low levels su pplied by the recetvermodule.

The last part of the Interface is the

Fig. 3. Track layout and component mount-ing plan of the printed circuit board designedtor the receiver-to-PC interface.

ELEKTOR ELECTRONICS NOVEMBER1993

Bit Meaning Function

0-14 Miscellaneous applications / as required.15 R Antenna bit, 0 = normal antenna; 1 = spare antenna.

16 Al 1: change to summer or winter time in next hour.17 Zl Time zone 1: 1 = summer time; 0 = winter time.

18 Z2 Time zone 2.19 A2 1: switching second folIows.

20 S Always 1; start marker for time/date transmission.

21 1 Minutes bit; value 1.22 2 Minutes bit; value 2.23 4 Minutes bit; value 4.24 8 Minutes bit; value 8.

25 10 Minutes bit; value 10.26 20 Minutes bit; value 20.27 40 Minutes bit; value 40.28 Pl Parity bit for all bits transmitted so far (complement

to give an even number).

29 1 Hours bit; value 1.30 2 Hours blt: velue 2.31 4 Hours bit; value 4.32 8 Hours bit: value 8.33 10 Hours bit; value 10.34 20 Hours bit; value 20.35 P2 Parity bit for all bits transmitted so far.

36 1 OaV of month bit, value 1.37 2 Oay of month bit, value 2.38 4 Dav of month bit, value 4,

39 8 Dav of month bit, value 8,40 10 Dav of month bit, value 10,41 20 Dav of month bit, value 20.

42 1 Oay of week bit; value 1.43 2 Day of week bit; value 2.44 4 Oay of week bit; value 4.

45 1 Month of vear bit; value 1.46 2 Month of vear bit; value 2,47 4 Month of vear bit; value 4,

48 8 Month of year bit; value 8.49 10 Month of year bit; value 10,

50 1 Vear bit; value 1,51 2 Vear bit; value 2.52 4 Year bit; value 4.53 8 Vear bit; value 8.54 10 Vear bit; value 10,55 20 Vear bit; value 20,

56 40 Vear bit; value 40,57 80 Vear bit; value 80,58 P3 Parity bit for all bits transmitted so tar.

Table 1. Meaning of all bits contained in the time information signal.

ctrcutt around transistor T2. whichdrives an LED. DlO. to stgnal receptionof time pulses recetved from DCF77. A6-mA current source, T2-R4-DB-D9,isused to cope with the wide range of

voltages that can occur on an RS-232interface. In this way. the LED alwayslights at a constant intenstty. whilebeing supplied via the DTR line.Consequently, the opamp and receiver

ELEKTORELECTRONICSNOVEMBER1993

PRECISIUNCLCOKFORPCs

supplies are nolloaded by the LED,

ConstructionThe compact printed circutt board de-stgned for the Interface is shown inFig. 3. Construction is simple. First fitthe passive components, then the ac-ttve components. You may want to fitcapacitors C4 and Cs last because theyare fairly tall, and comp1icate the fit-ting of the parts around thern.

The DCF77 receiver module men-tioned in the parts lrst is connected tothe interface via four wires, for whichsolder pins are provided. The positivesupply voltage is connected to the sol-der point marked '+' on the receivermcdule. while the grau nd wirc goes tothe point marked '-'. The module hastwo further connections. The pointmarked with an outgoing arrow is thesignal output, which is connected tothe stgnal input on the interface board.The last connection is marked with an'tnput' arrow. Thts input is used to ac-tuate the module, and must be con-nected to tts positive supply. At theinput side of the receiver module, con-ncct the ready-made ferrite rod an-tenna to the indicated points on theprinted circuit board. If the receivermodule Is fitted into an encloaure, twopoints should be observed. First, theenclosu re rnay not be a metal typesince that would make reception of theDCF77 stgnals on the ferrite rod an-tenna impossible. Second, to reduceinterference to an absolute minimum,the recerver module must be fitted at areasonable distance from the interface.

An alternative to the receiver mod-ule mentioned in the parts Iist is aready-rnade OCF77 clock which con-tains an U2775B. lf you happen tohave such a clock, open it to see tf itcontains this IC. If so, proceed as fol-lows: IC ptn 14 is the stgnal output,pin 1 is ground. Apower supply con-nection ts not required, stnce the clockis usually battery-operated. Pin 13must be conneeted permanently to theclock's supply voltage, and any otherconnection to it must be broken.Usually, the processor in the clockswitches the receiver Ie off after a cer-tain pertod. This is done to save power.However, even if the U2775B ts perma-nently on, the clock will run for morethan a year on a fresh penlight battery.A suitable clock is, for tnstance, ELVskit number 4434 (also avaüable ready-made: order code 4435). For more in-formation, contact ELV, Postfach1000. D-2950 Leer, Gerrnany.Telephone: (+49) 491 600888. Fax:(+49) 491 7016.

The connection between the inter-face board and the RS232 port on thePC Is best made in 4-core screenedcable. The screening of the cable is

COMPUTERS AND MICROPROCESSORS

Please mentton ELEKTOR ELECTRONICS when contacüng advertisers

CABLES CAPACITORS

usecl for the ground (SG) connectton.Für best results It is recommended

to install the receiver moduJe or theclock at a distance of at leasl 4 m fromthe pe or lhe rnorutor. This helps tokeep interference caused by stray radl-ation to aminimum. LED 010 ts conve-niently used to find a suitable locationfor the recerver module. Reception tsall rtght when the LED Ilasbes at a reg-ular rate.

SoftwareThe control software for the precisionclock 18a so-called device drtver, ca lIedDCFCLOCK.SYS. which ts supplied ondisk through Dur Readers Servicestorder code 1871). Jt must be Installedafter the mouse driver in the CON-FIG.SYS file. Consequently, it is not al-lowed to call the mouse driver in theAUTOEXEC.BAT file. Any other orderof initialization is only allowed tf it tscertain that the mouse driver. whtlelooking for the mouse. does not affectthe port to which the clock interface tsconnected. 11should be noted that thesoftware does not run on pe-XI com-puters.

The drtver initialization command hasthe following options:

P=nThis sets the RS232 port the receiveris connected to. Valid numbers Ior nare 1 to 4: the default Is 2. By default,the drtver assumes lhe following inter-rupt lines for the ports:

COM1: IRQ4

VIDEO HEADS SPEAKERS

COM2: IRQ3COM3: IRQ5COM4: IRQ7

1= nThis is used to change the interruplline asstgnment Irorn the default val ue.Valid numbers for n are 0 to 15. If youwant to set the port and tntcrrupt line.this parameter must be given after P =n. Owing to missing hardware. IRQ 8-15 could not be tested.

S=nIf n ;::;:1. the driver will adjust the real-ttme clock in your PC-AT after everycorrectly received time Informationpacket. Setting n to 0 will disable thisfunction. The default is 11 ;::;: 1.

B=nThis sets the 510'5 divisor rate andthus the baudrate. You only needthts option If the pulse widths suppliedby your receiver deviate considerablyfrom the standard (60/l60ms). Thedefault ts 2500, which equals 46 baud.

D=nGenerally. the driver checks whetheranother program or driver alreadyuses the COM port in mterrupt-drivenmode. If so, an error report is pro-duced. Thts test may be dtsabled bysetttng n to 1. The default is O. i.e .. runthe test.

W=nIf n = 1. the driver will erase the COMport's l/O-address from the BIOS datasegment. This is necessary u you arerunning MS-Windows, whtch resets all

CONNECTORS SEMICONOUCTORS

ports at slartup and would otherwiseturn the recetver's power off. Clearingthe address makes the port invisiblefor Windows. and the clock conttnucsto work. The default is n ;::;:O.

T=nThis allows the driver to be used in atime zone different from that of theDCF?? transmitter, for instance. inthe UK. where GMT ;::;:CET minus onehour during most of the year.Whenever a correct packet is received,this value will be added to. or sub-tracted from. the 'hours' information.Parameter n may be negative or posi-live.

Practical useStart your PC wtthout the precisionclock connected. Call up the time anddate. lf the clock is fast, you may havea sm all problern to fix - see beiow.Next, with the driver included in theCONFIG.SYS file , and the recetvermod ule fitted in a suttable posttton.the pe may be started again. Fromthen on, the clock in the PC ticks atatomic preciston. 00 take care. how-ever, if the original PC clock used to be'fast', t.e., running well ahead of thecorrect time. Once the preoiston clockhas taken over from a 'fast' clock. newfiles suddenly havc older date/time In-formation than the original ones. It ts.therefore, better to wait with savingnew files until the precision clock hasgained on the 'old' time. From then on,old and new files can not be swappedany more by accident. •

ELEKTOR ELECTRONICS NOVEMBER 1993

COMPUTERS AND MICROPROCESSORS

Fig. 2. Circuit diagram of the interface between the PC and the DCF77 receiver module.

erating system installed on the PC.

The serial interfaceThe clrcutt diagram of the serial inter-face between the DCF77 receiver mod-ule and the PC's RS232 port is grven inFig. 2. The interface is powered via theRTS and TxD lines. Capacitor C4 ischarged via diode D3 to the level of the

RTS line, which is between +7 Vand+12 V. At the same time, capacitor C5is charged to the TxD line Ievel, whichis between=-? V and -12 V. Diodes D4and D5 protect the circuit againstdamage caused by an incorrectly inl-tialized RS232 port. The voltages builtup across the capacitors form the syrn-metrical supply voltages for opampICl. The opamp gtves the digital time

COMPONENTS LIST

filesistors:1 4kQ71c 1kQf5

22kQ1 100Q-8: ->1F ·~F~

R1'R2'rRS;R4

-t:

Gapacitors:!~fw ~P0n,f '{!0~

1 10IlF/16V radial2' 1'00liF/25V radial

.!Ct!~3C2

!'C4;'C5

Semiconductors:l' t:E:D3mm green'8 1.N41481 LEO 3mm redf BF245B' '1 BC547B·1. lJLOZ1

"01,·02-D901QTl ,"T2ICl

Miscellaneöus: '1, 9-way sub-Desocket K1

DCF77 receiver module: €onrad1•2

otder code 190969-66.DCF77 fetrite antenn~: Conrad1,2

ofdercode 535630-6E>.1, Plastlcenclosure,60x50x30 mm;

e.g., type E406 (sopla)21'" Printed cifcuit'boa'rd 930058 plus

software1!8H(see page 70)Tpe cpntrgl s9ftw!lre f~r ttlls p.r0jegt is"also availableseparately: order code1871~seepage 70).

1 Conrad Electronic Nederland BV, P.O.Box 12, 7500AAEnschede, Holtänd.Tel (+31) 53 282000. Fax: (+31) 53283075.

2C-1 Electronics, P.O. Box 22Ö89,6360 ~B Nuth, Holland. Fax: (+31) 45,241877.

code applied to its +input a level thatenables it to be recogntzed as aserialsignal by the PC's RS232 port.

Generating a suitable supply volt-age for the DCF77 receiver module is alittle more cornplex. Here, a greenLED, DI, is used to stabilize the mod-ule's supply voltage. A stable voltage ofabout 1.8 V is obtained at a small cur-rent already.

Extensive decoupling is necessaryin the interface, because cross-inter-ference occurs easily between ana-logue and digital sub-circuits. Inparticular, every care has been takento prevent the digital sections upset-ting the operation of the sensitive ana-logue receiver. A very effectivedecoupling device is the high internalresistance formed by TI-R2, a currentsource inserted in the positive supplyto the recetver module. Together withcapacitors Cl and C2, this currentsource forms a filter that affords a veryhigh degree of decoupling of the re-ceiver's power supply lines. The combi-nation D2-RI is also connected to themodule power supply, and provides areference voltage of 0.6 V at the -inputof the opamp. This is necessary to en-able the opamp to recognize the highand low levels supplied by the receivermodule.

The last part of the interface is the

Fig. 3. Track layout and component mount-ing plan of the printed circuit board designedfor the receiver-to-PC interface.

ELEKTOR ELECTRONICS NOVEMBER 1993

amplifier?). Theoutput pulse is picked up bya microphone and fed back into thc IMPwhere it is digitized and then fed into anIBM (compatible) computer via the printerport. The IMP software then analyses theinput via Fourier transforms and outputs theresults on to the cornputer sereen in graph-ieal form in the shapeof arnplitude and phaseresponse curves. Full control is via the Pe.The amplifier output can be sampled via aprobe to correct for errors in the pu Ise spec-trum and amplificr response.

IM P allows the collection and analysisof 12·bilanalogue data up 104,095 sampIesin length and sam pIe rates are selectable ateilher61.44I kHz or 1.92 kHz wh ich, alongwith the internal filtering, allows rneasure-ments from several hertz to 20 kHz.

Further information from Falcon AcousticsUd, TaborHouse, Norwich Road, Mulbarton,Norfolk NR14 8JT, England. Telephone+44 (0)508 78272; fax +44 (0)508 70986.

CORRECTIONSDIGITAL DIAL (January 1994)

An attentive reader has drawn our auennon10 the fact that the digital dial can not beused in conjunction with the receiver illus-trated (a Yaesu TypeFRG· 7) since the IF ofihat receiver is much too high for the dia!.Sorry for that oversight! [Editor]

VHF/UHF TUNER (Oct/Nov 1993)The tuner module used in this design is noIon ger in produciion with Philips and itsavailability will thus become a problem.Fortunately, the Type UV916H is an excel-lent alternative. The snag is, however, thatthis unit is slightly smaller than the UV8 16,

so that the antenna connectorno longer pro-trudes from the enclosure. This ean be over-come by terrninating the aruenna eable intoa coaxial plug and making the entrance holeslightly larger. Moreover, one of the twoearth tags of the UV9 I6H must be eonnectedat a different position.

LETTERSSCART SWITCHING BOX

(December 1993)I have a few problems with this project,which I believe have to do with the conncc-tions. Pin 1 of one connector is linked 10pin 2 of the other. The same is true ofpins 3and 19, which are linked to pins ö and Zü re-spectively. All other pins are interlinked asone would expcct, i.e., pin 5 to pin 5, pin 10to pin 10, and so on.L. Bastiaenssen

1naSCARTcable. the wiresfor the video andaudio connections are always crossed. Thatis why the video outpui (pin 19) at one endof the cable is linked to the video input(pin 20) at the other end. This arrangementensures that the input of one piece of equip-ment is always connected properly to theoutput of another. There is, therefore, noth-ing wrong witn your cable.

Note that two pie ces of equipment mustne ver be connected simultaneousty to K3and K5/K6. Use SCART connector K3 or thephono plugs K5/K6, but not both at the sametime! [Editor]

PRECISION CLOCK FOR PCs(November 1993)

1haveencounteredaproblem with the.Precision

Uni! 4, Gibbs Reed FarmPashley RoadTJCEHURSTTN57HETelephone: (0580) 200 657 (Na.tiollal)or +44 580 200 657 (International)Fax: (0580) 200 616 (National)01" -t44 580 200 616 (International)

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clock for PCs. I have an IBM (compatible)PC486 and have, as stated in the article,eomplemented the CONFIG.SYS file withthe following (last) line:DEVICE C:\MSDOS\DCFCLOCK.SYS.

I should be pleased ifyou would tell me:1. Whereto should the files of the softwareprovided (DCFCLOCK.ASM, DCF·CLOCK.DOC and DCFCLOCK.SYS becopied? To the root, the MSDOS or a sepa-rate directory?2. Onee the files have becn loaded, how isthe program ealled up to initialize thedriverand to fill thc options P, I, S, Band D?(M. Meersschaut)

Thefile DCFCLOC.ASM is the assembler list-ing of the program. which you no longerneed (it is of interest only to dyed-in-the-wool programmers). The file DCF·CLOCK.DOC contains the instructions forthe program, which you can readwith awordprocessing program. lt is not necessary 10store this file on a hard diksk.

The only program that you need to copyto the hard disk is DCFCLOCK.SYS. Placethisfile in the directory containing the DOScommandste.g., C:\DOS). Add a line that in·dicates where the computer can find thatprogram to CONFIG.SYS (in C:I), e.g. DE·V/CE; C:\DOSIDCFCLOCK.SYS. Other suf·fixes may be addedfor changing certain set-tings (see DOCfile), but even without thesethe system should work correctly.

Note, however, that the receiver circuit mustbe connected to the COM port 2 and that thecomputer must be restarted ofter the softwarehas been instal/ed. The program will then au·tomaticalty set the correct time in the inter-nal clock of the PC every minute.

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ELEKTOR ELECTRONICS MARCH 1994