a binary coded decimal converter-a6a

8/8/2019 A Binary Coded Decimal Converter-A6a

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A BINARY CODED DECIMAL CONVERTERbyllartin ZisermanThe Norden LaboratoriesA Division of Norden-Ketay CorporationWhite Plains, New YorkSu mmrv This disadvantage has been eliminated inthe design of this analog-to-digital converter.

Of the many different coding schemes The output of this device is a totallypossible, the 8-4-2-1 binary decimal code was unambiguous 8-4-2-1 binary decimal code.felt to be the most useful for data handlingsystems. A transducer wes developed for Physical Designconverting shaft positions to digital numbersin this code. The converter is small, light The physical configuration of thisweight, and produces 10,000 binary coded converter is based on the commutator discsdecimal numbers in 100 revolutions of the input within the device. In order to get the greatestshaft. No ambiguity of number presentations is resolution within the smallest space, apossible due to the unique configuration of the compromise mas arrived at which gives this unitcommutating discs and the brush contacts. an O.D. of appraximately 2 inches. Similarly,the length of the unit is a function of the totalIntroduction count of the converter. Again, a compromisearrangement has produced a unit approximatelyIn the fields of data handling and data 4 inches long, exclusive of input shaftprocessing, there often arises a need for an extension and electrical connector. Figure 1,electrical signal indicative of a shaft a photograph of a similar type unit, illustratesposition. In analog systems, this is that its shape is very much like that of aaccomplished adequately by a synchro or synchro, even to a synchro-type mountingpotentiometer. For digital systems, this arrangement.analog information must be transformed. Thereare two mays in which this can be done. Theelectrical analog output of the transducer may ibe digitized or the transducer, itself, mayproduce an electrical output in digital form.Since the latter is the more efficient method,there have been marketed a number of thesedevices.

The outputs of these analog-to-digitalconverters have one common characteristic;viz., they produce voltages which representnumbers. Usually these voltages are of thebinary type; that is, they consist of twostates: one state representing "o", the otherrepresenting 1". The voltages representingthe digits ma y be produced simultaneously orsequentially. The number system in use may be Fig. 1binary, coded binary, decimal or coded decimal.Many data handling systems operate with Since most of the metallic members are ofcoded decimal numbers. In particular, the aluminum, it is anticipated that the weight of8-4-2-1 code is advantageous since it permits the unit will be approximately 10 ounces. Thethe relatively simple arithmetic operations of internal construction of the unit will bethe binary system, yet requires only very similar to that shown in Figure 2, a linesimple translation to the decimal system or drawing of a similar type converter. The threeother decimal codes such as teletypewriter, discs shown, each having a unique pattern on it,

etc. Ordinarily, the major disadvantage in the are joined through gear trains each having ause of an y weighted code fo r use in 10:1 ratio.transducers, lies in the fact that ambiguousnumbers ma y be generated in changing from one Adjacent to each disc is a stationarynumber to an adjacent number. There have been brush holder which holds and accuratelycodes devised that eliminate this ambiguity positions the small bifurcated brushes that areproblem. However, they are not weighted used to conduct the currents to and from thecodes and therefore require extensive patterns on the discs. At the rear of the unitequipment to translate them to a code that is a terminal board on which are mounted 30can be manipulated arithmetically. isolation crystal diodes. An electrical

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connector is provided at tlle rear of the unit The representation of a "O" in the codedwith a sufficient number of pins to carry all output is by means of a voltage at or very nearof the binary coded decinal bits a nd their ground potential. The "12) is represented by aco iplements. voltage that is negative with respect to ground.Converters can also be supplied for positivevoltages. This voltage is arbitrary but it isdesirable to keep the maximum voltage in theorder of 35 to 50 volts.The slip ring brushes are used fo r bringingout the information from the digit rings. Thecommutating brushes are used to bring theinformation into them. The commutating brushesreceive their information from some lesssignificant bit. One of them will be carryingcurrent while the other is not. When informationchanges, both of them change simultaneously.When a brush is carrying current, it will alwaysbe in contact with a tooth of one of thecommutating rings. In fact, a brush will beFig. 2 carrying current only when it is at leastone-half of a count of the least significantbit from the edge of a tooth. There will be nocurrent switching at the edges of a tooth due toRotating the input shaft of this converter any of the brushes except at the input ring.through 100 revolutions will produce a complete This improves the wearing qualities of the discs

cycle of 10,000 binary coded decimal numbers and brushes an d also permits larger tolerancesfrom 0 to 9999. The number is brought out on on the location of the brushes,16 wires simultaneously. At the same time, thecomplement of each bit is available on another The tw o co-mmutating brushes are spaced sogroup of 16 wires. Ambiguity of number that the angular distance between them is equalrepresentation is prevented from occurring by to the shortest tooth width of the digit theyhaving two commutating rings per bit of code on are carrying. iue to this spacing, there arethe discs a nd four contacting brushes per bit instances when both comimutating brushes will beof code. in contact with the same tooth segment of a ring.Since one of these brushes will be carrying a "1 "Each digit ring on a disc except the first, and the other a "Off, it is necessary to haveis composed of two toothed rings having their isolation diodes in the lines to these brushes.teeth interleaved and sepa:rated by a small The information output is then taken off theseinsulation region. The two commutating brushes lines ahead of the diodes,. The two brushes ofassociated with. each digit are positioned so the input ring, in conjunction with the inputthat they alternately contact both of the circuit, provide the pair of voltages to becommutating rings. The other two brushes are ai)plied to the second-order digit. The circuitryalways in contact with the slip ring portions of the converter then consists of a pair ofof the digit rings. The input ring is single push-pull voltages applied through isolationtoothed and has only two brushes associated diodes to a pair of commutating brushes. Thesewith it. Only one of these brushes is brushes are in contact with a digit ringpositioned to contact the toothed portion of consisting of two interleaved toothed rings.the ring. The other brush is in contact with The information is transferred to the slip ringsthe slip ring portion. and then to a pair of brushes in contact withthe slip rings. These are then the outputs ofthe digit rings. They also supply the inputsfo r a more significant digit, again, through aThe input to this unit may be either pair of isolation diodes.pulses or 1) C which is applied as a pair ofbistable voltages. However, the states of the Opetonpair are controlled by the converter. This isdone by the input commutating ring which The operation of this device can be seen bypresents either an open circuit or a short following the circuit diagram, Figure 3. Thecircuit to control the input voltages. These input circtLit is made up of the black box shountwo voltages constitute the bit and complement between the first two switching rings. Thisof the least significant bit of the least box and the switching ring of the leastsignificant decimal digit. Each bit coming significant digit, provide the push-pullfrom the converter is controlled in turn by a voltages for the converter. The crosshatchedless significant bit, and the orientation of the sections of the rings are the conductivepattern of the disc with respect to the brushes. portions, and the spaces between them indicate

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insulating areas. The small arrowheads reference position, the brushes for the bits ofrepresent the brushes that are in contact with the second order decimal digit are in the samethe conductive areas. The rings are arranged position with respect to the pattern as the firstfrom top to bottom with the least significant order digit and similarly for any higher orderdigit at the top. The successively lower rings decimal digits. Therefore, all of the bitare the 2's bit, 4's bit and 8's bit of the outputs will be "I0" and all of the complementleast significant decimal digit. The switching outputs will be "'1".action may be seen by assuming a movrement of allthe brushes with respect to the pattern. For a positive increase in the digitalnumber output of the converter, all the brusheswill move to the right with respect to thepattern. As the brushes move to the right an dbefore the input commutating brush has reachedthe next switching point, there will be othercommutating brushes that make or break contactwith the teeth of a digit ring. However, nochange in number will occur since the brush thatis switching will always be carrying a "0". Asthe commutating brush of the first ring justmakes contact with the edge of the tooth, theshort circuit existing between the commutatingbrush an d the slip ring brush will cause the"black box" toggle to flip. The left outputwill now have the "l"-voltage and the righto.1o "~~"output will have the "O"-voltage.Fig. 3 The Bl-gutput is now a "1"t and itscomplement B1 is now a 'IO". Even though brushesE3 and Et are still engaging the same tooth, theback resistanci of CR2 prevents the "10"The brushes are illustrated in the zero appearing at B1 from being disturbed The B2position. The commutating brush of the first continues to have a "11"-output since it is nowring is just out of contact with the slip ring drawing current through the E3 commutatingbrush. This causes the "black box" toggle to brush. B2 remains at "0l" since the slip ringproduce a "O"-voltage at its left output and a brush, E5, is still not in contact with any"l"-voltage at its right output. The left blush carrying informtion, Similarly, B4 andoutput is also the B1-output, and the right B4 . remain as they were since their commutatingoiqtput of the bo x is its complement, the brushes are still engaging the same tooth. EvenB1-output. This large negativre "l"-voltage though commutating brush Ell has crossed over anappearing at the cathode of the isolation insulat4ng region and now engages a differentcrystal CR2 permits current to flow through to ring, B8 still remains with a "tl"-outputthe input brush of t4e second ring, to the slip because now Ell is carrying the "l"-informationring brush and the B2-output. It can easily be from B1. The output from the converter, asseen then that a "1" will appear at all the shown, has thus changed from 0 0000 to 0 0001,complement outputs, Even though the commutating If the brush assembly is moved another divisionbrushes, E3 and E4, are in contact with the same to the right, it will produce an output oftooth, the back resistance of CR1 prevents the 0 0010. As the brush assembly moves, thedisturbance of the "O" at the Bl-output. Since converter output will progress through all thethe slip ring brush, E5, is not carrying any binary coded decimal numbers up tocurrent, the output at B2 is a "on. The 1001 1001 1001 1001 or the equivalent of theB4output from slip ring brush E9 is also a "Ot" decimal 9999. One more change in the samesice there is no commutating brush carrying a direction will produce all zeroes again.voltage in contact with that ring. Similarly,the B8-output at slip ring brush E13 is also a Characteristics"0" even though there is a commutating brush,

Eill, in contact with the ring. It is a "0 " Figure 4 shows the commutator disc with itshowever, because the voltage it is carrying is patterns fo r the various digits. Thisthe B1-output which is a "10". particular disc will produce an output of twodecimal digits in one revolution. For eachThe next most significant bi t is the l's additional decimal digit required, another discbit of the second order decimal digit. The geared to the first by a factor of 10:1 wouldpatterns fo r the bits of this digit have the be necessary. The patterns fo r the successivesame configuration as those shown, except that discs will look exactly like the innermostthey ar e increased in scale by a factor of 10. four rings showm in Figure 4.Th e B8- an d B outputs are also the inputs forthe second order decimal digit through theirrespective isolation diodes. In the zero or217



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in the 8-4-2-1 code. This representation may beeasily converted to decimal output by means ofsimple matrices. AlU of the bits are availablesimultaneously, allowing a very high readingrate. In addition, the complement of each bitis also available at the same time. The torquerequired to rotate the input shaft is uniforman d low, approximately 0.3 inch-ounce. Theinput shaft can be turned at a rate of 150 rpm.Higher rates may be used with a correspcndingdecrease in life.Larger decimal counts may be had by addingadditional discs and 10:1 gear trains. Othercounts, such as 3600 or 6200 may be constructedin the same code, by using a slightly different

Fig. 4 pattern.In systems where a self-balancingpotentiometer is used to record a processThe "black box" toggle indicated previously variable, this converter ca n be used inmay be any sort of bistable device whose states conjunction with the recorder to produce acan be controlled by an open and short circuit. digital readout of the variable. Wh ere theIt may take the form of a biased Eccles-Jordan transducer output has a shaft position as thecircuit or it may be as simple as a network of analog of the variable, the co nv ert er can befour resistors. For pulse operation of the used directly to effect a digital readout.converter, a pulse source can be gated into the Since the unit furnishes a parallel output andconverter by means of the bistable circuit. can be read "on the fly", it provides a meansfor supplying data to high speed systems, suchThe converter produces an unambiguous as computers. It can also be used as the16-bit number representing four decimal digits sensing element in a digital servo system.

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a binary coded decimal converter-a6a

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