United States Patent (19)
`Bloch et al.
`
`11)
`45)
`
`4,173,714
`Nov. 6, 1979
`
`54) COMMUNICATION CIRCUIT WITH
`COMBINED POWER FEED AND DATA
`TRANSMISSION OVER A PHANTOM
`CHANNEL
`75 Inventors: Alan Bloch, New York; Frank A.
`Coviello, Peekskill, both of N.Y.; Ira
`Guzik, Fairfield; Candido Puebla,
`Bridgeport, both of Conn.
`73) Assignee: TIE/Communications, Inc.,
`Stamford, Conn.
`21 Appl. No.: 803,243
`(22
`Filed:
`Jun. 3, 1977
`51) Int. Cl’............................................... H04Q5/18
`52 U.S. C. ..............
`... 179/99 R; 179/15 BB
`58 Field of Search ..................... 179/3, 4, 15 BB, 99
`(56)
`References Cited
`U.S. PATENT DOCUMENTS
`6/1940 Weissner ......................... 179/15 BB
`2,203,316
`3,701,854 10/1972 Anderson et al. ..................... 179/99
`3,787,631
`1/1974 Lewis ..............
`... 179/99
`3,843,845 10/1974 Ridley .....
`... 179/99
`3,973,085
`8/1976 Shiff ....................................... 179/99
`3,975,594
`8/1976 Guntersdorfer ...
`179/15 BB
`4,046,972 9/1977 Huizinga et al. ...................... 179/99
`FOREIGN PATENT DOCUMENTS
`1437032 5/1976 United Kingdom .
`Primary Examiner-Bernard Konick
`Assistant Examiner-Randall P. Myers
`Attorney, Agent, or Firm-Francis N. Carten
`
`ABSTRACT
`(57)
`The disclosure relates to a four conductor circuit ar
`rangement providing two communication channels be
`tween a control unit and a terminal device. Power feed
`and bi-directional signalling are accomplished simulta
`neously over the same four conductors used for the two
`commmunication channels without interference. The
`signalling from the control unit to the terminal is ac
`complished by means of a voltage modulator circuit in
`the control unit by which voltage pulses are transmitted
`via a phantom circuit arrangement over the four con
`ductors to the terminal unit. The voltage pulses are
`sensed from the phantom circuit in a data receiver
`which is located at the terminal unit. Connected to the
`phantom circuit in the terminal unit is a voltage regula
`tor circuit which receives power from the control unit
`via the four conductor phantom circuit interconnecting
`the control unit and the terminal. In series with the
`voltage regulator circuit is a constant current sink and
`an inductor. A current modulator circuit is provided in
`parallel for impressing current data pulses generated in
`the terminal onto the phantom circuit, which are, in
`turn, received by a receiver circuit in the control unit.
`A preferred data format is disclosed as a method for
`transmitting information between the control unit and
`the terminal.
`The preferred embodiment for the disclosed four con
`ductor communication circuit and the disclosed data
`format is in a key telephone system in which the control
`unit is a station card in a key service unit and the termi
`nal is a key telephone station set.
`
`12 Claims, 8 Drawing Figures
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`HPE 1025-0001
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`HPE Co. v. ChriMar Sys., Inc.
`IPR Pet. - U.S. Patent No. 8,902,760
`
`

`

`U.S. Patent Nov. 6, 1979
`U.S. Patent
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`Sheet 1 of 4
`Sheet 1 of 4
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`4,173,714
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`U.S. Patent Nov. 6, 1979
`
`Sheet 2 of 4
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`U.S. Patent Nov. 6, 1979
`U.S. Patent
`Nov. 6, 1979
`
`Sheet 3 of 4
`Sheet 3 of 4
`
`4,173,714
`4,173,714
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`HPE 1025-0004
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`HPE 1025-0004
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`

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`U.S. Patent Nov. 6
`U.S. Patent
`Nov. 6, 1979
`1979
`
`Sheet 4 of 4
`Sheet 4 of 4
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`4,173,714
`4,173,714
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`HPE 1025-0005
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`

`

`15
`
`25
`
`1.
`
`COMMUNICATION CIRCUIT WITH COMBINED
`POWERFEED AND DATA TRANSMISSION OVER
`A PHANTOM CHANNEL
`
`4,173,714
`2
`The power feed and bi-directional signalling arrange
`ment is described in British Pat. No. 1,437,032, also
`assigned to the Plessey Company. Power is transmitted
`from the control unit to the terminal by way of two
`conductors; simultaneously, data signals, generated in
`the control unit, are transmitted over the same two
`conductors to the terminal where they are sensed by a
`data receiver and applied to the station set. At the sta
`tion set itself, a series regulator senses the voltage im
`pressed on the two conductors and regulates that volt
`age in order to supply the terminal station set with d.c.
`voltage. In series with the series regulator is a constant
`current source, which is modulated by a current modu
`lator in response to data generated by the terminal. The
`data generated by the terminal gives information about
`the characteristics of the terminal itself, and these data
`are transmitted over the two conductors back to the
`control unit where they are sensed by a receiver respon
`sive to the current pulses and are used in the terminal to
`control the terminal itself.
`In modern communication systems, such as key tele
`phone systems, there is often a requirement that there be
`two communication channels connecting the control
`unit and the terminal. For example, in a key telephone
`system it is often desirable that a user of a key telephone
`station set, who is communicating over a first channel,
`(for example on an outside call over a central office line)
`be able to simultaneously talk over a second communi
`cation channel to someone connected to the key tele
`phone system over an intercom line. Thus, there is a
`requirement that there be provided not one, but two
`audio communication channels over the four conduc
`tors connecting the control unit and the terminal, with
`the simultaneous provision of data signalling in both
`directions (from the control unit to the terminal and
`from the terminal to the control unit) with simultaneous
`provision of power being supplied from the control unit
`to the terminal over the four conductors.
`Thus this invention provides a circuit arrangement in
`which a control unit and a terminal are interconnected
`by means of only four conductors and in which two
`communication channels, bi-directional signalling be
`tween the control unit and the terminal and power
`transmission from the control unit to the terminal is
`provided.
`This invention also provides a method for transmit
`ting control data over the four conductor arrangement
`by which various circuits in a key telephone station set
`may be controlled and/or monitored. The invention
`further provides a method of dial pulse signalling simul
`taneously with control data transmission.
`SUMMARY OF THE INVENTION
`This invention relates to a circuit arrangement in
`which first and second communication channels are
`provided over two conductor pairs which are simulta
`neously used for power feed and bi-directional signal
`ling between first and second equipment units. These
`first and second equipment units, in a preferred embodi
`ment, are a control unit and key telephone station sets in
`a key telephone system.
`A first pair of conductors is used as one information
`channel. The other pair of conductors provides the
`second information channel. Each conductor pair is
`terminated at the control unit and at the terminal unit
`with a transformer. Each transformer has one center tap
`winding, the ends of the center tapped winding being
`connected to the conductor pair.
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to a circuit arrangement for
`providing communication channels and power and sig
`10
`nalling circuits between a control unit and a terminal.
`Specifically, the invention relates to such a circuit ar
`rangement in a key telephone system.
`2. Description of the Prior Art
`There has been a continuing effort on behalf of the
`telephone industry to reduce the cabling and cross-con
`necting equipment requirements for key telephone sys
`tems. These requirements are based in the desire to
`allow a key telephone set to be interchangeable, and
`also for the desire to reduce service assignments at the 20
`telephone sets. So called electronic key telephone sys
`tems have been proposed in recent years in an effort to
`reduce the cabling and cross-connecting requirements
`of the older key telephone systems, and in addition
`provide features not possible in the prior systems.
`For example, in a series of papers published at the
`1970 International Conference on Communications, an
`electronic key telephone system is described in which
`the control unit and each telephone station set is inter
`connected by means of six conductors. Two of these six 30
`conductors are used for a voice circuit communication
`channel. The other four conductors are used for data
`transmission: two conductors for transmission in one
`direction; the other two conductors for data transmis
`sion in the other direction. One paper presented at the 35
`1970 International Conference on Communications
`entitled "Exploratory Common Control Key Tele
`phone System: System and Functional Description', by
`L. P. Fabiano, Jr. of Bell Telephone Laboratories, Inc.,
`describes how the six conductor circuit arrangement 40
`among four of the six conductors is used to transmit
`data to and from each station set and how four of the
`conductors are connected in a phantom pair arrange
`ment for power transmission. Each conductor pair has a
`center tapped transformer connected both at the con- 45
`trol unit and at the station set. Power is supplied from
`the control unit to the station set over the phantom pair
`arrangement, while data in one direction is being sup
`plied on one pair of the four conductors and data from
`the station set to the control unit is being returned over 50
`the other pair of the four conductors. The other two
`leads of the total of six leads, as mentioned above, are
`used only as a communication channel.
`Key telephone systems have been known in which
`station sets are controlled and powered from a control 55
`unit via only four conductors connecting the control
`unit and each station set. For example, British Pat. No.
`1,437,031, assigned to the Plessey Company Limited of
`Great Britain, describes a four conductor circuit ar
`rangement which connects the control unit for a tele- 60
`phone system to each station set. This British patent
`describes a "house telephone system' in which the four
`conductors connecting the control unit to the terminal
`are divided into two pairs of conductors, one pair being
`used for power feed and bi-directional signalling, the 65
`other pair being used solely as a communication channel
`or voice circuit path between the station set and th
`control unit.
`
`HPE 1025-0006
`
`

`

`10
`
`15
`
`4,173,714
`4.
`3.
`current pulse from the key telephone station set to the
`The communication channels, typically audio chan
`key service unit.
`nels, are created by using each pair of conductors via
`transformer coupling to the terminal and to the control
`BRIEF DESCRIPTION OF THE DRAWINGS
`unit, there being an information receiver (typically a
`small speaker) and generator (typically a microphone)
`The invention will be described by referring to the
`accompanying drawings, in which:
`for each channel in the terminal and there being means
`to receive and transmit information for each channel at
`FIG. 1 shows in block diagram form the circuit ar
`the control unit.
`rangement by which two communication channels and
`Data information flow between the control unit and
`means for supplying power from the control unit to the
`the terminal and power feed from the control unit to the
`terminal, transmitting data from the control unit to the
`terminal is accomplished via a phantom pair circuit
`terminal, and transmitting data from the terminal to the
`arrangement in which the control unit circuitry is con
`control unit are provided over four conductors con
`nected to the two center taps of the transformers termi
`necting the control unit and the terminal;
`nating the two pair of conductors at the control unit.
`FIG. 2 shows the d.c. supply circuit and modulating
`Connections to the center tap connections of the trans
`circuit within the control unit for applying d.c. power
`formers terminating the two conductor pairs at the
`and data signals to the four conductors connecting the
`terminal complete the phantom pair circuit. A d.c. volt
`control unit and terminal,
`age source is connected at the control unit to the phan
`FIG. 3 shows details of the voltage regulator circuit
`tom pair circuit arrangement. The d.c. voltage is ap
`located in the terminal for receiving the d.c. power
`plied to the phantom circuit arrangement, and is sensed
`20
`from the four conductors and for providing different
`and regulated by a voltage regulator in the terminal
`voltage levels with which to power the terminal logic
`connected in series at the terminal end of the phantom
`circuits and components,
`pair.
`FIG. 4 shows the detailed circuit of the receiver
`Data signals are applied to the phantom pair at the
`located in the terminal which is responsive to the data
`control unit by means of a voltage modulator. These
`25
`signals generated in the control unit and applied to the
`data signals, transmitted from the control unit to the
`four conductors connecting the control unit and the
`terminal, are sensed at the terminal end of the phantom
`terminal,
`pair by means of a data receiver. Information is trans
`FIG. 5 shows the details of the current modulator
`mitted to the control unit from the terminal by modulat
`circuit located in the terminal, responsive to data from
`ing a resistor in parallel with a constant current sink at
`30
`the logic circuitry in the terminal, for applying current
`the terminal end of the circuit arrangement. This pro
`modulated pulses onto the four conductors connecting
`vides current modulation of the constant current in the
`the control unit and the terminal,
`terminal circuit connected to the phantom pair. Current
`pulses therefore are applied via the phantom pair, the
`FIG. 6 shows the circuit in the control unit which
`senses the current modulation from the terminal which
`pulses being sensed in the control unit by means of a
`35
`is applied to the four conductors connecting the control
`data receiver. An additional feature of the invention
`provides at the terminal end of the circuit arrangement,
`unit and the terminal, and
`an inductor selected so that the terminal circuit presents
`FIGS. 7A and 7B show the data format with which
`a very high impedance to the phantom pair line as
`the modulators in the control unit and the terminal
`viewed from the control unit end of the arrangement,
`provide information to and from the control unit and
`thereby preventing signal reflections returning to the
`the terminal.
`control unit from the terminal end. A very high impe
`DESCRIPTION OF THE INVENTION
`dance also aids in the generation of current pulses
`General Description of The Circuit Arrangement
`which result from modulation of the constant current
`sink in the terminal circuit.
`45
`FIG. 1 shows in block diagram form the general
`According to another feature of the invention, the
`aspects of this invention. Four conductors, 1,2,3,4 are
`data to each terminal is provided by means of dividing
`shown connecting a control unit 10 and a terminal 20.
`time into N segments, where N is preferably selected to
`Although the preferred application for the invention is
`be the integer 34. Each of these N segments, called a
`in a key telephone system, the inventive circuit arrange
`word, is further divided into five sub-time elements. A
`50
`ment can find application in many different control
`pulse during the first of these time elements of a word is
`unit/terminal applications.
`used in each word to indicate the start of a new word.
`Each conductor pair is terminated by a transformer.
`A second element is used to transmit pulses from the
`Transformers T1 and T2 terminate the conductor pair
`control unit to the terminal indicating control informa
`1, 2 and conductor pair 3, 4 in terminal unit 20 while
`tion about a particular terminal circuit. A third time
`transformers T3 and T4 terminate the conductor pair 1
`element during each word is used to transmit status
`and 2 and conductor pair 3 and 4 in control unit 10.
`information about a particular terminal circuit from the
`Each transformer has a center tap connection on the
`terminal to the control unit. The other time elements are
`transformer winding connected to the conductor pair.
`left idle in each word. Typically the first word, labeled
`Center tap connections are located at the control unit 10
`the 0th word is used as a synchronizing word, and the
`(C3 and C4) and at the terminal unit 20 (C1 and C2). A
`following N-1 words are used to convey information
`"phantom circuit' is created via the center tap connec
`between the control unit and the terminal about individ
`tions C1 and C2, through the conductors 1,2,3,4, to the
`ual terminal circuits.
`center taps C3 and C4 of transformers T3 and T4. This
`When the inventive data format is used in the pre
`arrangement is known as a phantom pair because in a
`ferred embodiment of a key telephone system, informa
`65
`closed circuit connected to taps C3 and C4 of T3 and T4
`tion concerning the condition of the hookswitch and the
`presence of a dial pulse during signalling is transmitted
`and to taps C1 and C2 of T1 and T2, the current sup
`plied at a center tap point will divide at the center tap
`during the first time interval of each word by sending a
`
`55
`
`HPE 1025-0007
`
`

`

`5
`
`4,173,714
`6
`5
`indicator, the status of a hold key or privacy release
`connection flow over two conductors and be received
`key, etc.
`in recombined form at the other end.
`Power is supplied from the control unit 10 to the
`Connected to both conductor pairs 1,2 and 3,4 at each
`terminal 20 by applying d.c. current from the d.c.
`end is circuitry necessary to create a complete commu
`sources 10, 12 via the phantom circuit to terminal 20
`nication channel. For example, at the terminal 20 end of
`where it flows through voltage regulator 500. Voltage
`transformer T1, connected to pair 1,2, is a circuit 30,
`regulator 500 divides and regulates the d.c. voltage
`which can be a transmitter and receiver set commonly
`supplied via the phantom circuit and applies the divided
`found in a telephone handset. Circuit 31, which is con
`and regulated voltage levels to different elements
`nected to transformer T2, is an information receiver and
`within the terminal 20.
`generator which might be a built-in microphone and
`10
`The invention allows simultaneous provision via only
`speaker of a telephone station. Connected to the side
`four conductors 1,2,3,4 the following: two complete
`opposite pair 1.2 of transformer T3 in control unit 10, is
`communication channels between the control unit and
`a conductor pair 5, 6. Connected to the other side of
`the terminal, means to provide control information
`transformer T4 is a conductor pair 7,8 which may be
`from the control unit 10 to the terminal 20, means to
`15
`connected by other circuits in the control unit to other
`provide status and request information in the terminal
`information receivers and generators. Thus, two com
`20 back to the control unit 10 and means to supply
`plete information channels over the four conductors
`power from the control unit 10 to the terminal 20. As
`1,2,3,4 are provided in this circuit arrangement.
`will be discussed below, means are also simultaneously
`Superimposed upon the communication channels as
`provided to allow dial pulse signalling over the four
`20
`described above, are means for supplying power from
`conductors.
`the control unit 10 to the terminal unit 20 via the phan
`tom pair comprising the center tap connections C3, C4
`Detailed Description of the Voltage Modulator 100 in
`through the balanced conductors 1,2,3,4, to the center
`Control Unit 10
`tap connections C1 and C2 of transformers T1 and T2.
`FIG. 2 shows the modulator 100 which applies data
`25
`D.C. voltage sources 10, 12 each of voltage E1 are con
`from the control unit 10 to the terminal 20 on the phan
`nected in series with resistors R1 and R2 to center tap
`tom circuit via transformer center tap connections C3
`connections C3 and C4. Data from the terminal is super
`and C4. Voltage sources 10 and 12, being of the same
`imposed on the d.c. level supplied by voltage sources
`voltage value (25 volts), are connected in series so as to
`10, 12 by modulator 100.
`force current in the same direction; that is from terminal
`30
`In terminal 20, receiver circuit 150 responds to the
`center tap connection C3 via resistor R5 and the emit
`d.c. voltage pulses appearing between center-tap con
`ter-collector path of transistor Q2 and then through
`nections C1 and C2 of transformers T1 and T2 and
`voltage sources 12 and 10 to the collector-emitter path
`produces a data signal proportional to the data modul
`of transistor Q1 and then to resistor R4 to the center tap
`lated by modulator circuit 100 in control unit 10. The
`connection C4. Transistors Q1 and Q2 are normally
`35
`output from receiver 150 is applied to logic circuits in
`biased in their linear range so as to be almost fully con
`the terminal for controlling various circuits and equip
`ducting; that is, only a small voltage drop occurs across
`ments within the terminal.
`Q1 and Q2, and thus nearly the entire voltage source
`An a.c. high impedance element 600 is connected in
`magnitude of voltage sources 10 and 12 appear across
`series with voltage regulator 500 and constant current
`center taps C3 and C4.
`sink 300 to provide a circuit terminating phantom line
`The data to be transmitted from the control unit to
`1,2 and 3,4 so as to substantially eliminate reflections on
`the unit 10 to the terminal 20 is modulated within the
`the line 1,2 and 3,4.
`circuit outlined by dashed lines and labelled 100. The
`Data is transmitted from the terminal 20 to the con
`circuit operates to vary the base biases of transistors Q1
`trol unit 10 over conductors 1,2,3,4 by applying termi
`and Q2 synchronously with the data input on lead 103.
`45
`nal 20 data pulses to modulator 200, which switches a
`Thus, the voltage appearing from C3 to C4, v, is a
`resistor 201 in and out of parallel connection with the
`modulated voltage from the average voltage, which is
`phantom circuit connected to center taps C1 and C2,
`approximately the sum of source voltages 10, 12 (about
`the current of which is being supplied by the control
`50 volts).
`unit 10. Because constant current sink 300 draws con
`When a data pulse appears at the connection point
`50
`stant current from the closed circuit, and because resis
`between resistors R6 and R7, the voltage output of the
`tor R201 is switched in and out of parallel connection
`differential amplifier 151 varies the base-emitter bias of
`with the constant current sink, current modulation in
`transistor Q1 so as to create a voltage drop from the
`synchronism with the terminal 20 data signals is pro
`collector to the emitter of transistor Q1 in synchronism
`with the data pulse input of lead 103. Differential ampli
`duced on conductors 1,2,3,4.
`55
`The current pulses are sensed at the control unit 10 by
`fier 152 is slaved to the output of transistor Q1 via resis
`receiver 400 by comparing the voltages across resistors
`tors R10 and R11. Thus, the signal appearing at the
`R1 and R2 which are proportional to current changes
`emitter of transistor Q1 is also applied to the base of
`through them. The data produced by receiver 400 is in
`transistor Q2. The change in base-emitter bias of transis
`response to the current modulation imposed by modula
`tor Q2 causes a voltage drop from the emitter to the
`tor 200 in terminal 20. Data signals received by receiver
`collector of transistor Q2 similar to that across the col
`400 are then applied to other logic circuits within the
`lector-emitter of transistor Q1. The voltage pulse ap
`control unit 10 to provide information with respect to
`pearing on lead 103 therefore also appears across the
`terminals C3 and C4 and is superimposed upon the d.c.
`the status of different elements of terminal 20. For ex
`ample, where terminal 20 is a key telephone station set,
`voltage of sources 10, 12. Thus there is applied to termi
`65
`the data being supplied by logic circuitry within the
`nals C3 and C4 a composite signal comprising the sum
`of voltages of sources 10 and 12 less the normal voltage
`terminal 20 to modulator 200 might be the status of
`drops across the emitter collector paths of Q1 and Q2,
`different line keys in the station set or the status of an
`
`HPE 1025-0008
`
`

`

`4,173,714
`7
`and the modulated voltage caused by the data pulses on
`lead 103.
`Transistors Q1 and Q2 are of a kind denominated
`MPS U05 and MPSU55, manufactured by the Motor
`ola Co. and are described at pages 1087 and 1107 in a 5
`publication by Motorola Semiconductor Products Inc.
`entitled "Semiconductor Data Library', Series A, Vol
`ume 3, 1974. Differential amplifiers 151 and 152 are of a
`type denominated LM 1458 manufactured by the Na
`tional Semiconductor Company and are described at
`10
`pages 2-238 in a publication by National Semiconduc
`tor Company entitled "Linear Integrated Circuits',
`published February, 1975. Biasing for the differential
`amplifiers is shown as in FIG. 2, and is provided by
`resistors R6 and R7 to provide the proper bias to the
`negative input of differential amplifier 151 and by resis
`tors R15 and R16 in conjunction with voltage source 14
`to the positive input terminal of differential amplifier
`151. In a similar fashion, the biasing for differential
`20
`amplifier 152 inputs is provided by resistor R18 to the
`positive input and by resistors R8 and R9 to the nega
`tive input terminal.
`Detailed Description of the Voltage Regulator 500 in
`25
`Terminal 20
`Connected to the center tap connections C1 and C2
`in terminal 20 is a transformer T3, voltage regulator
`500, and constant current sink 300, as shown in FIG. 3.
`The voltage regulator circuit 500 comprises a series 30
`connection of three Zener diodes Z1, Z2 and Z3 each of
`which has a capacitor connected in parallel with it. The
`Zener diodes sense the current flowing through the line
`from C2 to C1. With sufficient voltage applied across
`C1 to C2, each Zener diode breaks down having a con
`35
`stant voltage across it. The capacitors, C10, C20 and
`C30 connected across Zener diodes Z1, Z2 and Z3,
`provide filtering so as to provide a substantially con
`stant voltage at points P1, P2, P3 and P4. The Zener
`diodes preferred in this invention are 1N4737A and
`40
`1N4733A diodes manufactured by Motorola Semicon
`ductor Products, Inc. and are described at page 1-101 of
`a publication entitled "Semiconductor Data Library',
`Volume 1, Series A, 1974, by Motorola Semiconductor
`Products.
`45
`Point P4 between Zener diodes Z1 and Z2 is taken as
`an arbitrary reference point from which the voltage of
`other points P1, P2 and P3 may be measured. In the
`preferred embodiment as shown in FIG. 3, point P1 is
`7.5 volts higher than point P4; P2 is 7.5 volts negative 50
`with respect to P4; and P3 is 12.6 volts negative with
`respect to P4. Points P1, P2 and P3 provide a supply of
`d.c. voltages to operate circuitry within terminal 20;
`they also provide power for biasing and powering the
`modulator 200, receiver 150 and the constant current
`source 300.
`Transformer T3, acting as an indicator, is inserted in
`the series line between points C2 and C1 in order to
`provide a high impedance for the pulse transmission
`circuit in the control unit 10 in order to substantially
`prevent reflections from the terminal 20 as the pulses
`are transmitted from the control unit 10 to the terminal
`20. In this regard the constant current sink 300 has an
`internal high impedance and therefore the impedance of
`transformer T3, as inserted into the line between points 65
`C1 and C2 and the impedance of constant current sink
`300 create a balanced line, thereby substantially pre
`venting reflections when pulses are transmitted.
`
`8
`Data Receiver in Terminal
`FIG. 4 shows the details of receiver 150 in terminal
`20. Data pulses are sensed on the line at terminals C1
`and C2 which are the center tap connections of trans
`former T1 and T2. Voltage pulses modulated at the
`control unit by modulator 100 appear across the termi
`nals C1, C2. These voltage pulses are then applied to
`operational amplifier 180 through resistor-capacitor
`paths created by resistors R160 and R161 and capacitors
`C162 and C163.
`Operational amplifier 180 is connected as a differen
`tial amplifier such that the difference in voltages ap
`pearing at its plus and minus terminals are measured.
`Differential amplifier 180 operates to sense only the
`differences in voltage appearing at its input terminals,
`yet it is not sensitive to any common mode voltages
`which appear on both of its input terminal. This feature
`insures that any common mode voltages which appear
`on the conductors 1,2,3 and 4 connecting the control
`unit and the terminal will be rejected.
`Operational amplifier 180 has a feedback resistor
`R166 connected from its output terminal to its negative
`input terminal thereby providing negative feedback and
`an overall gain across the amplifier from input to out
`put. The signal appearing at point 190 thus represents
`the amplified difference in voltages appearing from
`center-tap points C1 and C2.
`The signal at point P190 is applied to the operational
`amplifier 181 which is connected as a comparator am
`plifier. Amplifier 181 has a resistor R171 connected
`from its output terminal P191 back to its positive input
`terminal thereby insuring that voltage pulses applied to
`its negative terminal will not cause oscillation at its
`input. The output of the receiver 150 appears at point
`P191 from which the data pulses are applied to logic
`circuitry in the terminal for controlling various func
`tions and apparatus in terminal 20. Operational amplifi
`ers 180, 181 may be Motorola 1458 operational amplifi
`ers identified above.
`Description of the Current Modulator 200 in Terminal
`20
`The detailed circuitry of the modulator 200 shown in
`FIG. 1 is illustrated in FIG. 5. Terminal logic circuitry
`in terminal 20 applies data to the modulator 200 which
`is to be transmitted to the control unit for sensing infor
`mation about the terminal. The data is applied to the
`negative terminal through resistor R220 of amplifier 201
`which is connected as a comparator amplifier. A large
`resistor R222 is connected from the output of amplifier
`201 back to the positive input which is connected via
`resistor R221 to a common point. This large resistor
`connected around the positive feedback path of ampli
`fier 201 insures that the output of amplifier 201 at point
`P250 will not oscillate when the leading edges of pulses
`are applied to the negative input terminal of amplifier
`2011,
`The pulses appearing at point P250 are applied first to
`a transistor Q212 via resistor R223 for inverting the
`pulse, which is then applied via resistor 226 to the posi
`tive input of amplifier 202. The pulse at point P250 is
`also applied to amplifier 203 at its positive terminal via
`resistor R227. The amplifiers 202 and 203 are connected
`as level shifting negative feedback amplifiers whereby
`the outputs of amplifiers 202 and 203 at points P251 and
`P252 serve to bias the transistors Q210 and Q211 to their
`
`15
`
`55
`
`HPE 1025-0009
`
`

`

`4,173,714
`full on condition when a data pulse is applied to the
`circuit.
`Thus, the transistors Q210 and Q211 are turned on for
`the duration of the pulse, thereby connecting resistor
`R201 directly across the line defined by terminals C1
`and C2. Referring to FIG. 1 it can be seen that when
`resistor 201 is in parallel across the phantom circuit line
`defined by center-taps C1 and C2, it changes the current
`supplied from control unit 10 via the phantom circuit
`10
`defined by transformers T1, T2, T3, T4 and line con
`ductors 1,2,3,4 to equal current through R201 and con
`stant current sink 300. When resistor 201 is not con
`nected across center-taps C1 and C2, the current drawn
`from the line 1,2,3 and 4 is less than when it is in the
`circuit.