US 3576531 A Description (OCR text may contain errors) United States Patent [72] inventor Larkin B. Scott Fort Worth, Tex. [2]] Appl. No. 553,568 [22] Filed May 27, 1966 [45] Patented Apr. 27, 1971 [73] Assignee The Perkin-Elmer Corporation Norwalk, Conn. [54] COMPARATOR CIRCUIT ARRANGEMENT 6 Claims, 5 Drawing Figs. [52] US. Cl ..1 340/1462, 307/235 [51] Int. Cl G06f 7/02 [50] Field of Search 340/ 146.2; 307/231; 340/172; 307/235; 328/146 [56] References Cited UNITED STATES PATENTS 2,885,613 5/1959 Myracle et a1. 340/l46.2(X) NUMBER SOURCE 3,155,840 11/1964 Hoeschele, Jr. 307/231 3,218,609 11/1965 Shaw 340/1462 3,237,025 2/1966 Clapper.. 340/1462 3,328,599 6/1967 Stupar 328/146(X) Primary Examiner- Eugene G. Botz AttorneyEdward R. Hyde, Jr. ABSTRACT: Circuit means are provided for converting a first number A which occurs in digital form to a first plurality of analog representations and a second number B which occurs in digital form to a second plurality of analog representations; for comparing the amplitude of the numbers in analog form; for providing a plurality of digital representations indicative of these relative analog magnitudes; and for comparing the digital representations thereby providing an indication as to the relative magnitudes of the numbers A and B. NUMBfR SOURCE PATENTED APR27 12m SHEET 1 BF 3 INVENTOR. ii B. SCOZZ izm s w 558m mwmvs HTTOR Nli' Y. COMPARATOR CIRCUIT ARRANGEMENT This invention relates to circuit arrangements for comparing two numbers and for providing an output voltage indicative 'of the relative magnitude of the numbers. Theinventionrelates more particularly to a comparator for comparing numbers occurring in digital form There are many applications in which it is desirable to compare the magnitude of two numbers occurring in digital form and to provide an indication as to which is the larger and the smaller of the two. The-need for such a comparison exists, for example, in many forms of digital computers and in other digital equipment. The digits of a number are generally represented electrically by discrete, predetermined voltage levels. For example, in a digital numbering system having a radix-2, each digit of the number can alternatively assume two discrete electrical values and thus can be represented by a bistable electrical device such as two switching contacts or a fIip fIop circuit. The digits of the number can also be represented electronically by the presence or absence of pulses. The digital form is distinguished from an analog representation wherein the complete number is represented by a single voltage level, for example. The analog voltage level can assume any of many levels corresponding to the magnitude of the particular number being represented. Since in an analog arrangement, two numbers are represented by voltage levels derived from separate sources, a relatively uncomplex arrangement can be provided to effect comparison of these numbers. However, because digital numbers are represented by a plurality of voltages for example, it is necessary to compare corresponding digits of the same order of the two numbers in order to provide an indication as to the relative magnitude of the numbers. This arrangement therefore requires a relatively larger number of circuit components and is thus substantially more expensive than a comparator for comparing numbers occurring in analog form. It is an object of the present invention to provide an improved circuit arrangement for comparing two numbers occurring in digital form. It is another object of the present invention to provide a comparator for digital numbers requiring relatively less components and which is relatively less expensive than prior comparators. In accordance with a feature of the present invention, cir cuit means are provided for converting a-first number A which occurs in digital form to a first plurality of analog representations and a second number B which occurs in digital form to a second plurality of analog representations; for comparing the amplitude of the numbers in analog form; for providing a plurality of digital representations indicative of these relative analog magnitudes; and for comparing the digital representations thereby providing an indication as to the relative magnitudes of the numbers A and B. These and other features and objects of the invention will be apparent with reference to the following specification and drawings wherein: FIG. I is a block diagram illustrating an embodiment of the invention; 7 FIG. 2 is a block diagram illustrating in more detail a converter and comparator stage of FIG. 1; FIG. 3 is a schematic circuit diagram of the converter and comparator of FIG. 2; FIG. 4 is a schematic circuit diagram of a logic circuit 7 .shown in block form in FIG. I; and FIG. is a logic truth table for the logic circuit of FIG. I. In FIG. 1, a source 10 ofa first number A in digital form and a source 12 of a second number B in digital form are shown coupled to converter and comparator stages 14, I6 and I8. These sources, which may comprise registers in digital equipment, each provide output voltages representing n digits of the particular number. Although numbering systems of other radix may be provided, the sources I0 and I2 provide output voltages representing numbers in binary form. Included in each source is a plurality of stages having bistable elements. The bistable elements can comprise for example, electronic flip-flop circuits or an electromechanical arrangement of switch contacts such as is provided by relaysFor simplifica tion of the drawings, a plurality of source voltages E to E and E, to E are shown derived from associated stages and applied to the converter 14 via a single cable. Similarly, a plum of voltages mr a-n ton-a) m m and ema) m n) representing higher order digits of the numbers are also shown applied via associated cables to other comparator stages in FIG. 1. Each of the comparator stages 14, 16 and 18 are adapted to convert that portion of the numbers A and B represented by the digital input voltage to analog form, to compare the magnitude of the analog voltages, and to provide a digital output voltage indicative as to the relative magnitude of the analog voltages. FIG. 2 is a block diagram illustrating the stage 14 in greater detail. The converter and comparator 14 includes a circuit means for converting the plurality of digital input voltages E,," to E5 to a single analog representation and the plurality of digital input voltages E to E} similarly to an analog representation. In FIG. 2 this conversion means is shown to be the summing networks 20 and 22. The output of these networks occurring at terminals 24 and 26 respectively are analog voltage levels indicative of the magnitudes of the numbers represented by the four, lowest order, and least significant digits of the numbers A and B. These analog voltages are applied simultaneously to a circuit means, shown to be a difference amplifier 27, for indicating their relative difference in amplitude and providing an output voltage indication. The amplifier output voltage indication is in digital form and occurs at terminals 28 and 29. At times, the particular circuit arrangement employed may render it impractical to provide a difference amplifier output voltage indication at the logic voltage levels employed throughout the comparator apparatus. Thus, an electronic switch 30 is included for providing the desired digital output representation at logic levels cor-- responding to those employed in the comparator apparatus. The output voltage at terminals 32 and 33 is therefore in digital form, is indicative of the relative magnitudes of the lower order digits of the numbers A and B, and occurs at voltage levels compatible with logic voltage levels employed in other portions of the comparator apparatus. In a similar manner, output voltages occurring at terminals 34 and 35 are indicative of the relative magnitudes of higher order, more significant digits of the numbers A and B, while output voltages occurring at terminals 37 and 39 are indicative of the relative magnitudes of the highest order, most significant digits. In accordance with a feature of the invention, the comparison and digital indication of the relative magnitudes of the numbers A and B is provided at terminals 32 and 33. In accordance with another feature of the invention, when the numbers A and B include a number of digits n which renders the comparison in a single stage 14 impractical, then pluralities of digits are compared independently and the output indication of the converter-comparator stages are then compared. - An arrangement of this type is illustrated in FIG. 1. The output of the converter-comparator stages 14, 16 and 18 are applied to a digital comparator comprising associated logic circuits 36, 38 and 40 respectively. These logical circuits include an arrangement of NOR gates, the circuit of which is described in more detail hereinafter. Each of the logical circuits function to 'provide an output voltage indicative of the relative maggenerates a voltage which controls all lower order logical circuits. Under these conditions, the outputs from lower orders are independent of the magnitudes of lower order digits of A and B. For example, the logical circuit 36 is adapted, when E 43 and E each represent a logical 0. to compare the voltages E and E and to indicate at terminals 41 and 42 which of the two is the greater. However, when the digits E is greater than E then outputs of logic circuits 36 and 38 will be governed by E and is independent of the relative magnitudes of E and 15 A logical expression relating input and output voltages for the logic circuit is given by: A0 A A 1 Similarly Ba=EB'+ Er-E (2) where E E are the output voltages from a logic circuit, E and E are input voltages from an associated converter-comparator stage, and E,,' and E input voltages coupled from the next higher order logic circuit. For example, with respect to the logic circuit 36 of FIG. 1, The expression of equation I may be rearranged with the aid of DeMorganss Theorem and other well-known Boolean algebra relations to a formula readily recognizable with respect to the NOR circuit arrangement of FIG. 1. This expression is: A truth table is shown in FIG. illustrating the various logical outputs for the various logic input combinations to a logic circuit. The output of the logical circuit 36 is thus an indication as to the relative magnitudes of the digital numbers A and B. Thus, the arrangement of FIG. 1 avoids a digit by digit comparison as referred to previously and compares two digital numbers in a manner which reduces the number of components previously required. Circuit arrangements for effecting the conversion and comparison of stages l4, l6 and 18 and the logical comparison of stages 36, 38 and 40 are illustrated in FIGS. 3 and 4. FIG. 3 illustrates a circuit arrangement for the analog converter, difference amplifier and switch arrangement of FIG. 2. A digital to analog converter is provided for each group of input digital voltages and comprises two resistive summing networks, indicated generally as 50 and 52. Each of these networks includes a resistor associated with an applied digital voltage and having a value of resistance weighted in accordance with the order of the digit in the number. Input voltages are derived from stages of the sources and 12 and are applied to these resistances. A typical stage of these sources is indicated generally as 53 in FIG. 3. Weighting of the resistors 54 to 60 and 62 to 68 is accomplished in a wellknown manner for providing the desired conversion. For example, for a number of radix (2) and having n digits, the value of resistance corresponding to a digit of order k can be determined by selecting a value of resistance R for the nth order and then calculating the value of resistance for the lower order k. This calculation is made according to the equation where k=0 for the least significant digit. For the converter of FIG. 3, the resistances would thus have the values: R60=R68=R,,, where the value of R, is preselected. According to equation (5), R58=R66=2 R56=R64=4 R, and R54=R62=8 R,,. In the particular arrangement of this circuit for which component values are indicated hereinafter, the weighted resistors of the networks 50 and 52 provide approximately 1 volt steps from volts, when the input digit voltages represent a number having a logical value of 0, to 0 volts when each. input digit has a logical value of 1. Generally, the degree of circuit reliability will decrease as the number of resistors in the networks 50 and 52 increases and corresponding voltage differentials decrease. As indicated, and in accordance with a feature of the invention, a plurality of converter-comparator stages are provided. By the use of a plurality of these stages, the number of resistors in the networks 50 and 52 can be appropriately preselected to provide the desired degree of accuracy and reliability. Analog output voltages which appear between terminal 24 and ground and between terminal 26 and ground are applied to associated emitter follower amplifier stages having transistor amplifying devices 70 and 72. Collector electrodes of these transistors are connected to ground potential and the emitter electrodes are returned through resistors 74 and 76 to a current regulator stage 77. The current regulator stage 77 includes a transistor 78 having a collector electrode coupled in series with the resistors 74 and 76 and having an emitter electrode maintained at a positive potential by a voltage divider formed by the resistors 80 and 82. Base electrode biasing current for this transistor is provided by a second voltage divider formed by the resistors 84 and 86. The operation of this current regulator stage is described in more detail hereinafter. The circuit of FIG. 3 and other circuits described hereinafter are arranged to function at positive logical levels; i.e., Truth or a logic 1 is represented by a relatively positive voltage level and Falsity or logic 0 is represented by a relatively more negative voltage level, generally near ground potential. Digital voltages applied to converter resistors 54 through 60 and representing a relatively larger number cause a more positive voltage to appear at terminal 33 than a relatively smaller number applied to these same resistors. A similar result occurs with respect to the voltages applied to the resistors of network 52 and to the voltage occurring at terminal 32. Voltages at the emitter electrodes of transistors 70 and 72 are applied to base and emitter electrodes of transistors and 92 of a nonlinear differential amplifier. The emitter of the transistor 90 is connected to the output of the emitter follower transistor 72 while its base electrode is connected to the output of the emitter follower transistor 70. Similarly, the emitter of the transistor 92 is connected to the output of the emitter follower transistor 70 while its base electrode is connected to the output of the emitter follower transistor 72. An operating potential is derived from a source of potential -l-E and is applied to collector electrodes of these transistors through load resistors 93 and 95. In operation, when the voltage at terminal 24 is more negative than the voltage at terminal 26, then the bias on the transistor 90 is increased while the bias on the transistor 92 is simultaneously decreased. The collector current of the transistor 90 decreases correspondingly and the transistor 90 is driven to collector current cutoff while the transistor 92 is driven to collector current saturation. The differential amplifier is thus adapted to distinguish between the analog voltages at the terminals 24 and 26 and to cause an accompanying saturation and cutoff in the associated transistors. The differential amplifier thereby provides digital voltages at tenninals 28 and 29 which are indicative of the relative magnitudes of the digital input voltages E and B Since the differential amplifier is required to distinguish between voltage levels over a range of voltages and provide a digital output, it is operated in a nonlinear manner. In doing so, the amplifier is arranged to respond only to a minimum preselected voltage deviation between the input analog voltages. Deviations between these analog voltages which are less than the preselected minimum will not cause an output change. A dead band or tolerance band is thereby provided. This band is established in the arrangement of FIG. 3 by virtue of the base-emitter diode voltage of the transistors 92 and 90, which as indicated hereinafter are silicon transistors. Alternatively, voltage dropping means such as diodes can be coupled in the base leads of other types of transistors when the desired voltage drop cannot be obtained from the base-emitter diode I of the transistor. In a particularjarrangement of this circuit for which component values are enumerated hereinafter, the transistor is adapted to respond to a voltage differential of 0.5 volts or greater. I The emitters of 94 and 96 are kept at a constant voltage by the regulating action of 78. Although the voltages at the collector of the transistors 90 and 92 are in digital form and are representative of the magnitude of the input numbers E,, and E it is not always feasible to provide voltages at these collectors which are compatible with the logical voltage levels used throughout the apparatus. Accordingly, transistors 94 and 96 are provided for matching the converter and comparator to the voltage levels of associated logic circuits. The collectors of the transistors 90 and 92 are coupled to base electrodes of switching transistors 94 and 96 respectively and drive the latter transistors to cutoff or saturation. The switching transistor collector electrodes are coupled via resistors 98 and 100 and terminals 33 and 32 respectively to NOR gates in the logical circuit 36 (FIG. 4). The circuit arrangement for the logic stages 36, 38 and 40 of FIG. 1 for comparing the digital outputs from an associated converter and comparator stage along with a digital output of a more significant logic stage is illustrated in FIG. 4. The logic circuit of FIG. 4 includes NOR gates formed by transistors 110 of A are greater than 8 causes relatively positive and negative voltages at terminals 114 and 116 respectively. Accordingly, a positive voltage at terminal 114 causes transistors 110 and 134 to conduct and a relatively negative appears at their collector electrodes. A corresponding negative voltage at terminal 116 causes transistor 130 to cut off, Thus the inputs to transistor 130 are both negative, 130 cuts off, and its collector electrode voltage level. The logic circuit 36 includes input resistors 118 and 120 and resistors 122 and 124 coupled to a source of bias potential (E Operating potential is applied to the collector electrodes of transistors 110 and 112 via load resistors 126 and 128 respectively. The output of NOR gate transistor 110 is applied viaa resistor 132 to a transistor 130 of a successive NOR gate. The output of NOR gate transistor 112 is similarly applied via a resistor' 136 to a transistor 134 of successive NOR gate. Input voltages from the preceeding logical stage, as for example stage 38, are also cross coupled, as indicated, via a resistor 138 to the transistor 134 and via a resistor 140 to the transistor 130. Bias is established at the base of the transistors 130 and 134 by resistors 142 and 146 respectively. Operating potential is derived from a source +E and is applied to the collector electrodes of these transistors through resistors 148 and 150. The output from this logical circuit is provided at terminals 41 and 42. Similarly, output terminals 43 and 44 are provided for logic stage 38 (FIG. 1) and output terminals 45 and 46 are provided for logic stage 40. Since a higher order digital representative voltage is not applied to the logical circuit 40, and since it is at times desirable to employ similar circuit modules in the circuit arrangement, the more significant digit input terminals are grounded. Alternatively, the output voltages of terminals 37 and 39 can be applied to more significant input terminals of the logical stage 38. The circuit arrangement of FIG. 4 operates in accordance with the logical expressions 1 through 4. Positive input voltage levels or negative going pulses to NOR .gate transistors 110, 112, 130 and 134 represent a logic 1 while more negative levels represent a logic 0. A 1 output at terminal 41, . represented by a relatively positive voltage level, indicates that the number E,,'" is greater than or equal to the number Eg and conversely a 1 or relatively positive voltage level at terminal 42 indicates that the opposite is true. When inputs to the logical circuit are equal the output at terminals 41 and 42 will be relatively negative and equal. A truth table for various logical input combinations at terminals 32, 33, 114, and 116 is shown in FIG. 5 along with the corresponding outputs at terminals 41 and 42. As indicated previously, an input positive level from a higher order logic stage such as at terminal 114 will govern the operation of the FIG. 4 circuit. For example, an indication from logical stage 38-that more significant digits appears relatively positive. The inputs to terminal 32 and 33 will control the output only when inputs to terminals 114 and 116 are both negative, i.e., the more significant digits of A and B are equal. The output voltages at terminals 41 and 42 can be employed with suitable buffers to operate relays, etc. in accordance with a desired control function which is to accompany the comparison. In addition, terminals 41 and 42 can be coupled to further logical circuits such as an AND gate to effect'some function which is to accompany equality of the numbers A and B. While other component values and arrangements may be selected in practicing the present invention, the following components, voltages and signal levels have been successfully employed in a particular arrangement of the invention. Transistors: N%II5%2, 94, 110, 112, 130, 134-Sprague type PNP-7 2, 78, 96, 94, 702N 3638 (Epoxy). Resistors: 1 60, 6830.1 1%, mil type Rn 60 Filmister. 58, 6615 1%, mil type Rn 60 Filmister. 56, 647.5 1%, mil type Rn 60 Filmister. 54, 62-374 1%, mil type Rn 60 Filmister. 74, 7668 V4 watt, 10%. 80, 84-2.2 watt, 10%. 82, 862.2 $4 watt, 10%. 93, 2.2 )4 watt, 10%. 98, IOU-5.6 )4 watt, 10%. 122, 124, 142, 14675*, 4 watt, 10%. 118, 120, 132, 138, 136, 14O7.5 watt, 10%. 126, 128, 148, 1504.7 watt, 10%. 1 8" '2= Input levels to networks Q and 2 Logic 1=0 Logic 0= 15" Output levels Terminal 41: A= B= Logic 1 0 A B=Logic 0= +28 Terminal 42: B=A=Logie 1=0 B A=Logic 0=+28" An improved circuit arrangement has thus been described for comparing numbers occuring in digital form, which requires relatively fewer components, and which is accompanied by greater circuit economy. While a particular embodiment of the invention has been described hereinbefore, it will be understood that various changes and modifications can be provided therein without departing from the spirit of the invention or the scope of the appended claims. Iclaim: l. A comparator circuit arrangement comprising: first and second resistive summing networks for summing a plurality of digital voltages applied to each network and for providing equivalent analog output voltages, means for applying a plurality of digital voltages representing a first number A to said first resistive network and a plurality of digital voltages representing-a second number B to said second resistive network, first and second emitter follower amplifier stages and a differential amplifier having first and second transistor amplifying devices, means coupling said first and second resistive networks to said first and second emitter follower amplifiers respecand second transistor amplifying devices respectively, means coupling an output voltage from said second emitter follower amplifier to emitter and base electrode of said first and second transistor amplifying devices respectively, and said resistive networks and amplifier stages arranged for providing a digital output voltage indicative of the relative magnitudes of the numbers AandB 2. A comparator circuit arrangement comprising: first circuit means providing a plurality of output voltages representative of a first number A having n digits, second circuit means providing a plurality of output voltages representative of a second number B having it digits, third and fourth circuit means each adapted to convert first and second pluralities of digital voltages to first and second analog voltages respectively and for providing first and second digital output voltages indicative of the relative amplitudes of the analog voltages, means coupling a plurality of voltages representing less significant digits of said numbers A and B from said first and from said second circuit means respectively to said third circuit means and coupling a plurality of voltages representing more significant digits of said numbers A and B from said'first and from said second circuit means to said fourth circuit means, a digital comparator circuit adapted to provide an output voltage indicative of the relative magnitude of a plurality of digital numbers applied thereto, and means coupling the digital voltages from said third and fourthcircuit means to said comparator circuit. 3. The comparator of claim 2 wherein said third and fourth ir t eshins ysfit i ensswd its! mev le voltage converters and a differential amplifier coupled to said converter and adapted to compare and provide a digital output voltage representative of the relative magnitude of said. represent a number larger than the more significant digits of B, and said digital comparator circuit includes a circuit arrangement for providing an output voltage E when the number A is larger than the number B and an output voltage E when the number B is larger than the number A. 5. The comparator of claim 4 wherein said digital comparator circuit is arranged to provide the output voltage E A0 in accordance with the logical relation: EA0=EA,+( i 6. The comparator of claim 4 wherein said digital comparator circuit is arranged to provide the output voltage B in accordance with the logical relation: Patent Citations
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