US2666810A - Switching system - Google Patents

Description

Jan. 19, 1954 E. G. ALIZON SWITCHING SYSTEM 4 Sheets-Sheet 2 Filed Jan. 31, 1951 m w Qk v m. .Il. 8v m5 Nm qv N u m f H. m 7. I

BY ATTORNKSZV Patented Jan. 19, 1954 UNITED STATES PATENT OFFICE SWITCHING SYSTEM Etienne G. Alizon, La Celle-Saint-Cloud, France Application January 31, 1951, Serial No. 208,794 Claims priority, application France April 6, 1949 11 Claims. (01. 179-18) The present invention relates to switching systems operating on the capacitive test principle, and more particularly to means employed with such systems permitting the identification of a preselected frequency with one of a plurality of frequencies as a step in the establishment of a desired configuration of the switching system. The present application is a continuation in part of my copending application Serial No. 153,594 filed April 3, 1950.

The invention has especial application in automatic telephone exchanges, and will be described in connection with such use, although it is not restricted thereto.

In my said copending application there are described rotary selector switches in which the selection of a chosen outgoing line is efiected without galvanic contact either between the scanning brushes of the selector rotor and the fixed contact pins of the stator or frame which are connected to the selector outgoing lines, or between the movable contacts connected to these brushes and the fixed contacts of the selector incoming line.

In such selectors the contact pins of at least one row in the frame are held at electric potentials of different frequencies according to their azimuth. In the course of a switching operation the rotor scanning brush which sweeps past these pins without contact therewith, forms successively with each such pin a variable condenser and thus experiences successive pulses of voltage having frequencies characteristic of the different outgoing lines. Although for simplicity they will be so described, these pins are preferably not pins at all, but are shaped like condenser plates. Neither are they contacts in the usual sense, since no galvanic contact is made therewith. They function rather as plates or electrodes of condensers.

The pulses are transmitted, via the condenser formed by a rotor contact connected to the rotor scanning brush and by a stator contact connected to a wire of the selector incoming line, to an identifying device charged with interrupting the motion of the rotor upon the receipt of an impulse of a predetermined frequency. At the moment of stopping, the rotor of the selector undergoes a translation in the direction of its axis and applies certain other brushes against the actual stator contact pins of the selected outgoing line. Likewise the rotor contacts connected to the scanning brushes are applied against the stator contacts connected to the incoming line. The capacitive connection furnished by the selector is thus made galvanic.

In view of cabling considerations which dictate a disposition of the pins of a given outgoing line along a single direction parallel to the axis of the selector rotor and in view of the time interval necessary for identification of the frequencies and stoppage of the selector, it is advantageous to advance thebrusheswhich make the capacitive tests by one or more azimuth intervals by reference to the other rotor brushes which are to make galvanic contact with the pins of the stator. In the traverse of the angular spacing separating these two azimuths there is made available the time necessary for the rotor to stop.

The present invention has for an object the provision of identification means operating on the successive pulses of voltage of different frequencies picked up by the scanning brushes of an associated selector and on a voltage of the characteristic frequency of the desired outgoing line. This characteristic frequency is chosen and made available to the identification means by a signal storage or recording device referred to in the telephone art as a sender. The identification means of the invention then stops the selector when the two frequencies which it receives are the same.

Such an identification means is composed essentially of two electron tubes operating as clippers to clip into square wave form the alternating voltages picked up by the scanning brushes of the selector and the alternating reference voltage provided by the sender, and of an electron tube for mixing these two voltages. The mixing stage includes in its plate circuit a linear resistance in parallel with a series combination of a condenser and a non-linear resistance. This parallel combination has two time constants according to the direction of the current flowing through it. As will be more fully explained below such a circuit produces in the mixing stage a plate voltage drop which is substantially zero when the two voltages applied to the mixer are of different frequencies and which on the contrary is substantial when these two voltages are of the same frequency and of opposite phase. The voltage pulse produced at the instant of coincidence of the two frequencies is used to stop the rotation of the selector rotor.

By reason or the dimensions customarily given to the contact pins of the fixed banks of such selectors, the capacity of a condenser formed by a scanning brush and a pin is small by comparison with the capacity of the condenser formed by the rotor contact connected with the scanning brush and the fixed contact connected to the incoming line, and also by comparison with the capacity between the conductors of the incoming line and ground. The result is a substantial attenuation in the voltage pulse picked up by the scanning brush. Consequently the invention provides for the identifying means amplifiers suitable to amplify the voltage pulses successively picked up by the scanning brushes.

In order to prevent stopping the selector on a chosen outgoing line if that line is busy, a separate scanning brush of the rotor tests capacitively for each output line a special contact pin which is held at an alternating voltage of special frequency when that line is busy. The voltage impulse picked up by this brush, when the desired line is busy,, is amplified and detected, and. the detected voltage serves to neutralize the stopping impulse generated by the selector and identifying means for the outgoing line in question.

For reasons of economy and compactness it-i's desirable to be able to associateasingleidentifica-- tion means with the selectors of anumber oisuc.-- cessive selector stages cooperating in the establishment of a communication" circuit. These se lectors are then held engaged by energization. of.

their stopping coils in parallel through a busy circuit extending throughout the complete chain of selectors.

In: the event that two selectors operatingto ward the establishment of difierent' communication circuits come into conflict on a single line; there occursa double or duplicate test or selection (1. e; with affirmative response) which; if allowed to be completed; must-then'becanceled by a complete disestablishment" of the switching chains up to and including the selectors 'sodoublyconnected to, a single line; This iol-ows necessarily because the identification of the line on" which conflict occurs is effected by each selector'without galvanic contact betweenits brushes'and'the stator pins, of that line, in consequenceof which" rec ognition ofthe double'selection is impossible'before completion of the circuit and further by virtue'of'the factthat' the holding coils of ail selectors in the cascade establishing a single communication circuit are connected in parallel on the same busy conditionwire.

The invention consequently provides means which systematically prevent the occurrence of duplicate selections. Upon the operation of" an identification means coupled with a selector of a given stage, such selector being at a given azimuth, these means for the prevention of duplicate selection disable other identification means coupled tothose selectors ofthe same stage which are indexed to azimuths characterized by the samefrequency as the azimuth of 'the'first selector. In'the event thattwo identification means operate to disable each other mutually, i. e. in the event of incipient duplicate selection; a baiancing effect'occurs which ultimately produces a completefunctioning of'one of these identification means and the disablement of the other.

The invention will be better understood by reference tothe following detaileddescription taken in conjunction with the accompanying drawings in which:

Fig. i is a schematic diagram of a selector switch of the capacitive test type, showing the rotor: contacts and brushes thereof and the contacts'of one azimuth interval of the stator.

Fig. 2 is a partial view of a selector switch of the'type'schematically shown in Fig. 1, Fig. 2 being taken in the direction of the axis of the switch rotor.

Fig; .3'shows schematically two multiple-d selectors.

Fig. 4 is a schematic diagram of an identifica tion means according to the invention.

Fig. 5 is a representation of certain voltage wave forms appearing in the circuit of Fig. 4 on the assumption that the two voltages applied to that circuit are of different frequencies, and

Fig. 6 is a representation similar to that of Fig. 5.but illustrating the corresponding waveforms inth'eevent. that the voltages applied to the identification means are of the'same frequency.

Referring now to Fig. 1, the stator of -theselector comprises a panel of pins providing two..out-. going lines per'azimuth. With each outgoing circuit' there: are: associated seven. pins thus making fourteen outgoing line pins per azimuth. These include three pins or electrodes of flat plate form 31, I37 and 231 for the first outgoing line and 31', I31" and 23'! for the second. The pins of fiat plate form constitute variable condensers with the scanning brushes of the rotor. In addition there are required for the galvanic contacts four round'pins, 33, I38; 238 and 338 for the first outgoingline and four similar round pins 38, I38, 235 and 338 for the second. These round pins come into galvanic contact with the corresponding brushes of the rotor upon the axial translation of the rotor when it is brought to rest,

The stator further includes four rings 3!, I3I-, 23! and 33! (which may be of semicircular shape as described in-my copending application above referred to) and four contact blades 32, I32; 232 and 332. Each ring and its corresponding contact blade are connected to one of the wires a, b, t and 0 of the'incoming line. The stator rings form condensers with corresponding rings of the rotor, all being concentric in the axis of rotation of the rotor, and'the contact blades come into contact with these rotorrings upon the translation of the rotor. In addition, upon such translation, the b.ades 232 and 332 respectively break and make contact with the fixed blades 233 and 333- upon this translation,

Of the four wiresa, b, t and o of the incoming line, a and b arethe talking conductors, t is the test wire and o is-the busy wire.

The selector rotor includes rings 4!, MI; 24! and 34!, mounted concentrically in its axis of rotation. The first three of these are connected each to the members of two diametrically-opposite and suitably-axiallydisp-aced pairs of brushes, viz. 5e, 5! and 50", 5! for the ring 4!; I50, I5! and M3, I5! for the ring !4!; and 250, 25'! and 250', 25! for the ring 24!; Those having reference characters which are multiples of I0 are scanning brushes while the others are-for the establishment of'galvanic contact.- The'ring 34'! is connected only to two diametrically opposite brushes 35! and 35!. The brushes 50, I53, 25-3 and 50', I50, 25$! are of fiat shape, whereas the brushes 5!, ISI, 25'l-'-and 5!, !5I', 25! are provided with 'spadesorlips as shown at The unprimed rotor members scan and make contact with onehalf of the outgoing lines-led to the selector, viz. the upper line for each azimuth, and the unprimed members,- which are axiallydisplaced fromtheir unprimed counterparts, cooperate-similarly with the remaining outgoing lines, of which one occupies'the lowerposition in the stator for each azimuth.

Whether the selector isidle or operating to establish a circuit, its rotor turns continuously with its incoming line rings 4!,- I4I, 24'! and 34! rotating respectively adjacent the rings 3I,-I3I, 2'3! and 33! of the stator. In the a'zimuthof the stator shown, the scanning'brushes- 5D, I50-and 250 rotate past the fiat stator pins 31, I3-I-and 231 of one outgoingline and the scanningbrushes 5fl, I50 and 250" rotate past the flat stator pins or electrodes 31, I31 and 23-1 of the other outgoinglin'e. The'lip'ped or curled rotor brushes 5!, I5!, 25! and 35! rotate'past the round statorpins 38, I38, 238 "and 338 of one outgoing line and the lipped rotor brushes 5!, I5I', 25! and 35! rotate past the round pins 33', I38, 238' and 338' of the other outgoingv line, all withoutgalvanic contact 'between'the rotating and the stationary elements. I

m the course of aselection operation-the scanning brushes 50, I50 and 250 (or 50', I50 and 250) of the rotor are moving past the flat pins 31, I31 and 231 (or 31', I31 and 231') of all the azimuth positions of the stator. Of each set of three stationary contacts at least one contact is held at an alternating voltage of frequency characteristic of the outgoing line to which it belongs. Consequently one or more of the flat brushes 50. I50, 250, or 50', I50, 250' pick up successive voltage pulses which are transmitted to the identification means which form the object of the present invention.

When the identification means receives and responds to that one of these successive pulses whose frequency is equal to a preselected frequency, a pulse is passed from the identification means through the wire t and via contacts 232 and 233 to energize the coil 35. The time of operation of the identification means and of the selector itself are so adjusted that the rotor of the selector executes a combined rotation and translation (in the sense of the arrow 8 I) imposed upon it by the pulse last referred to and stops with the lips 80 of its brushes 5|, I5I, 25I and 35I in contact with the round pins 38, I38, 238 and 338 (or similarly as to primed elements in the event the pulse of frequency equal to the preselected frequency is picked up from one of the contacts 31', I31 or 231).

Simultaneously the rotor rings AI, I4I, 24I and MI come into contact with the stator blades 32, I32, 232 and 332. The contact previously existing between 232 and 233 is opened by the pressure of 2 against 232 and contact is similarly made between 332 and 333, 233 and 333 being in the stator. Galvanic ocntact is thereby established between the wires a, b, t and o of the incoming line, and the wires A, B, T and O of the outgoing line. The wires of the second outgoing line for the azimuth shown coresponding to A, B, T and connecting respectively with 38', I38 238' and 33B-23l' have been omitted for simplicity, as have been the wires thereof corresponding to S and S.

Fig. 2 shows a view in plan of a selector, assumed to be a final selector. Fig. 2 shows a two interval difference in azimuth between the brushes I50 and II and a one interval difference in azimuth between the brushes 50 and 5I. The selector illustrated identifies the desired outgoing line first as to a frequency characteristic of the tens digit and then as to a frequency characteristic of the units digit of that line. The tens digit identification is first and therefore the scanning brush I50 which scans the stator contacts I3'I to which the tens digit frequencies are applied is two azimuth intervals in advance of the rotor galvanic contacts 5 I, I5I, 25I and 35 I. The brush 50 which scans the units digit contacts 31 is only one azimuth interval in advance of the rotor galvanic contacts.

Of course Fig. 2 is illustrative only; these azimuthal separations depend upon the time necessary to stop the rotor, having due regard to its speed of rotation.

Fig. 3 shows in simplified form and again for a single azimuth, two final selectors of the type of Fig. l, multipled together. The condensers respectively formed by the rings 3| and 4|, I3! and MI, 23I and 2M and by 33I and 3 (Fig. 1) are of large value compared to the variable condensers respectively formed by the brushes and pins 50 and 31, I50 and I31 and 250 and 231. They may therefore be neglected in a series connection. Consequently in Fig. 3 the incoming line of each selector has been shown with its wires 0, t, a and 1) directly connected to the scanning brushes 50, I50, 250 and to contact 35I.

The outgoing lines are distributed over fifty separate geometrical azimuths of the stator with two lines per azimuth. The fifty azimuths may for example occupy approximately a semicircle as set forth in my copending application Serial No. 153,594. To each stator azimuth there correspond two diametrically opposite sets of brushes in the rotor, byone of which the selector incoming line can be connected to one of the outgoing lines of that stator azimuth and by the other of which the incoming line can be connected to the other outgoing line of that stator azimuth. There are'accordingly one hundred relative azimuthal positions of the rotor and stator, each belonging to one of the selector outgoing lines. The selector will therefore be said to have one hundred azimuths.

Let it be assumed for example that each final stage selector gives access to one hundred outgoing lines numbered from 00 to 99 and that ten alternating voltages of frequencies FO-FQ are available. Let it be further assumed that for a given azimuth of the selector, azimuth 36 for selectors but in general comprise only one selection wire S for example, inasmuch as in general they have ten groups of outgoing lines or less. It is however to be understood that it is possible to effect with such group selectors selections at two or more characteristic frequencies.

The multipling wires A, B, T and O of an outgoing line of a group selector are respectively connected to the wires a, b, t and o of the incoming line of a selector of the following stage.

When the selector is stopped at the desired outgoing line by operation of the short pulse pro duced in the wire 15 as explained above, a battery permanently connected to the busy wire 0 keeps the selector stationary and engaged by feeding the winding 35 through the normally open (now closed) contact between 332 and 333 and through an economizer resistance 34. To the voltage of this battery there is added an alternating voltage of a special frequency which is therefore applied through the brush 35I to the pin 338 and consequently to the busy wire 0 of the chosen outgoing line. This voltage therefore indicates the busy condition of the outgoing line chosen by the selector.

The operation of the frequency identification means will be described with reference t Fig. 4. It will be described in connection with the selection of a chosen outgoing line by means of a final selector.

In Fig. 4, till represents a calling subscribers station. 402 represents the puls contacts of his calling dial, and 403 is his subscribers line. The dashed-line box indicated generally at I is the recording means or sender which, from the impulses received from the calling subscribers dial,

generates or selects the frequencies characteristic of the line called. Let it be assumed thatthe desired characteristic frequencies are fa for the dec ades digit and in for the units digit of the called line. 50 i, 405, and 406 are respectively the counting relay, the busy relay and the relay associated withthe end of the impulse train, the latter two 7 being delayed in release?- According to well unown'methods of operation relaytas'is energized as soon as 464 is energitd andremains-energized during the dialing; 4il6=remains energized during the transmission of one digitahdreopens at theend of the train 'ofimpiilses corrsponding to that digit.

Q91, 4fi8 a-nd'4fl9 are stepoy step rotary'switches having eleven azimuths: The wite'h 4illFop'erates to select the test frequenciesappropriate tic the successive digits-dialed.- It'advances through one step at each deenergizatioir of the relay 4 96 (opening of normally; opencontact 4661) at-the end of each digit dialed. Consequently-it directs the ground impulses; which reach its; brush through the normally open Contact 155i and through the normally closedcontactaiiliL-first toward the tens frequency selector aiisayandthen ae unitsfrequency selecton iil'iiu. I g A The stator pins of the selectoifi ifiiiai-andof the first row of-contactsin selector 409u'are connected to ten sources of alternating voltage;(not shown) of different frequencies of fo-fa. At the end of dialing the brush of 493a will-bepositioned' in-a particular azimuth corresponding to the tens digit and the brush of 409 will be positioned in a particular azimuth corresponding to the units dig-it. There then appears therefore-on wire 410 the frequency is proper to the tens thin the figure) and; on wire 4| L the frequency in proper to the units (f6 in the figure) The selector 4ll9u is provided with a. second row ofpins marked-from De, to Dt which are multipled to all units selectors such as 499.

The'characteristic test-frequency fa of the decadesdigit istransmitted through the normally closed side of contact 416i onrelay the to the input of a resistance-coupled thermionic amplifier 2 including a pentode 4l4 whose output is applied to the grid ofa clipper stage i includinga triode M8.

In the course of the rotationof the final selector rotor the potentials of different frequencies pickedup by its brush I 50 (Fig. '1) are transmitted by the wireb (see now Fig; 4) via the normally closed side of contact-4i82 (as-shown) on relay slow the input of a 'resistance-coupled thermionic amplifier 3 includin'gtwo pentodes ilfi and an. The output of tube 41-! is applied to the-grid of-a'clipper-stage Sincluding a triode 419. g

The clipper stages 4 and 5 preferably take the form of the two halves of a double triode, tubes 418 and H9 being thus combinedin a single envelope.

The variable condenser formed by the brush 50 in passing in front of a flat plate contact such as i3! (Fig. 1) possesses a small capacity by comparison with the capacity between ground and the wire b connecting this brush l58 to the input resistance 413 of the amplifier 3,- The result is that the potential across this input'resistance is substantially cophasal with the alternating voltage source which feeds'the contact-pin When the voltage applied to the input of the amplifier 2 is of the same frequency as that applied to the input of the amplifier 3, these two voltages are in phase; e. g. in view oftheir origin in a common-source permanently coupled-to thetest wire'S or S of the proper outgoing line or lines; the same-sourcebeingtapped by the applicable comparison frequency selector 408d or 469. The voltages,- at the outgoing terminals 420- and HI of the clipper stages-4 and'5 are 1 consequently ii phase opposition since one has- 8 undergone a single amplification-in 2 and the other two amplifications-in 3. Of course other meansmay'be-availed-of to bring the voltages at the output of the amplifiers 2 and-3 into phase opposition, when' of the same frequency, if desired;

The-output voltages of the clipper stages 4 and 5 are applied to a mixing stage 6 including a pentode 428; In tube 428 the voltage at the point 4214s applied to the control grid 423 and thatat 42s to the suppressor grid 422. Both of these grids are operated at zero bias, and the tube can-be cut 01f on either grid by the negative swing of the voltage applied thereto from the clipper 4 or 5 which is coupled thereto. The plate circuit of tube 428 includes a linear resistor 429 in parallel with a series combination ofthe condenser 4-30 and a non-linear resistor 43!. These three elements together constitute a circuit generally indicated at 1 which has two time constants. Ina preferred embodiment the condenser 339 is adjacent the source of B+ voltage HT; and the non-linear resistor 43l is adja centthe mixer tube plate. Moreover the larger of the two resistance values of 43! is preferably large compared to the resistance of 429. The non-linear resistance is poled to offer a low resistance to current flowing through it toward the mixer tube plate and to ofiera large resistance to current flowing through it toward the condenser. When the charging current for the condenser 430 flows from the junction 433 toward the junction 4'34 the'time constant of the circuit 1 is of low value. When the charging current flows from 434' toward 433, this time c onstant is large. More specifically, the circuit 1 is preferably so established that its short time constant is" short compared to the period of all test frequencies which may be applied to the amplifiers 2 and 3,and its long time constant is long compared tothe period of these frequencies.

When the identification circuit is quiescent, i. e." when no voltagesare applied to the grids 422 and 423 of tube 428, this tube conducts and the plate current flowing through the resistance 429 produces a drop between points 433 and 434, bringing the point 434 to a given voltage V0.

When the identification circuit is in operation the voltage o'f the point 425 remains close to the value V0 if the two frequencies applied to tube 428 are diifer'ent. On the other hand the voltage of the point425 shifts to a higher value V1 corresponding to'cut-ofi of the tube 428 if the two frequencies applied to it are the same, as

\ willnow be described.

The terminal 425" at the output of the twotim'e" constant circuit! is connected to the input of atriggerineans shown as a monostable multivibratorfl including a normally non-conducting pentode 435 and a'normally'conducting triode 438. The terminal 425is connected to the screen grid 43'! of tube 435. This tube is cut off when the voltage of its screen grid is V0, and it conducts when this voltage is V1. The onset of plate current in tube 435 at'the instant when the voltage of the terminal425-reaches the value V1 produces reversal of the conduction phase in the multivibrator. Tube 438' then remains cut off, and-tube 435 conducts, during a specified time intervaldepending upon-the time constants of the various circuits connected to these tubes. The result-is a-positive pulse at the output terminal 44E of the multivibrator. This pulse is transmitted tln'oughthe normally closed side of contact-M63 tcthe plate of cold cathode tube 439 which thereupon comes into conduction, energizing the relay M6.

The circuit generally indicated at 9 including the cold cathode tube 439 and the relay 6 constitutes a switch which causes the identification means to pass from the decades circuit to the units circuit. By operation of the normally open sides of contacts M! and M52, now closed, the grids of tubes 4H4 and M5 in amplifiers 2 and 3 are now respectively connected to the wire 4!! and to the wire a. Wire 4 is energized at the frequencies fu proper to the units digit of the desired line and the wire a is connected to the brush 58 (Fig. 1) of the final selector whose operation is being considered.

In the manner described above the motion of the brush 59 past the pin 31 energized at the frequency ,fu causes a positive voltage pulse to appear at the outgoing terminals 44| of the multivibrator 3. This pulse, passing through the normally open side of contact 63 (now closed) brings into conduction the tube 442 inamplifier it, which functions as a stopping-pulse generator for the selector rotor. As shown, tube 442 is biased ofi in the absence of signal. At the junction 443 in the cathode circuit of amplifier Ill there then appears a positive pulse which is applied to the wire 13 of the incoming line via the wire 424. This impulse then passes via elements 232 and 2 3 (Fig. 1) to the stopping coil 35 of the finalselector under consideration. As previously explained, the selector rotor then undergoes translation in the direct on of the arrow 8| (Fi 1). Contact is interrupted between 232 and. 233 and established between 332 and 333. The coil 35 is maintained energized by the battery applied to the wire 0.

If the line reque ted is busy, the brush 253 of the selector picks up in moving past the flat electrode pin 23'! of this line a volta e of a frequency characteristic of the busy condition. This voltage is applied to the pin 23! of the selector which is hunting, i. e. that under consideration. from a source 42? (Fig. 4) via the transformer 426, wire 0 and contacts 332 and 333 of the selector which is stopped on the line desired (i. e. another selector in use in the circuit previously established) and the multipling wire 0, It is transmitted via the wire it of the selector which is hunting, the line 424 (Fig. 4) and normally open contact 4164 (now closed) to the input of a busy condition signal amplifier-detector l i (Fig. 4).

The amplifier-detector I l includes pentodes 445 and 445 coupled to a rectifier 449 by transformers 44'! and 448 which are tuned to the busy signal frequency. The outgoing signal from the tube 446 is rectified by the bridge rectifier 449 in conjunction with the resistance 450 and condenser 45I and the positive rectified voltage is applied to the control grid 453 of a pentode 452 which is normally cut off through 450 in the absence of such rectified voltage. Consequently if the desired line is busy, tube 452 is brought into conduction and a negative pulse appears in its plate circuit at 454 and is transmitted by the wire 460 to the suppressor grid 435 of tube 435 in the multivibrator circuit 3. Tube 435 is accordingly blocked and the arrival of a positive pulse from the terminal 425 no lon er causes a phase reversal in the multivibrator. Consequently no pulse appears at the output terminal 44l of the multivibrator nor at 443 and the selector does not stop at the occupied line.

It has been explained how the unbiasing of tube 442 produced a positive pulse on its cathode 10 at 443 with resultant stopping of the selector. Simultaneously a negative pulse appears at junction 444 in the plate of tube 442. This pulse is transmitted via line 451 to the suppressor grid 455 of pentode 452 and to the control grid 459 of triode 458. This triode constitutes an amplifier l2 for the suppression of duplicate selections.

The negative pulse originating at the plate of amplifier It cuts ofi tube 452. With 452 so cut ofi? the positive pulse formed at the cathode 443 of amplifier l9 and transmitted to the input of amplifier l I is unable to bring tube 452 into conduction even though applied to its grid 453. Consequently neutralization of the operation of the multivibrator is prevented.

The same negative pulse from the plate of the tube 442 similarly cuts off tube 458. 458 consequently sends a positive pulse through the normally open contact M65 (now closed) to the multiplying line Dt multiplied to the pins of the second bank of all selectors 40911 of all senders (in the figure to the wire Dt Therefore, during the occurrence of a positive stopping pulse sent by the tube 442 to line 424 and to wire t, (i. e. as the result of a prima facie successful identification or selection) tube 452 receives a negative pulse on its suppressor grid 455 via line 451, a positive pulse on its control grid 453 via line 462, and a positive pulse on'its screen grid 455 via line 46! (from the plate of 458). The magnitudes of these pulses are adjusted so that the tube 452 remain non-conducting during their occurrence. 7

The tubes 452 of other identification means operating at the same moment to effect final selection of a line having the same units digit are unblocked by the pulse arriving over the corresponding line Dr. There results a blocking of the tubes 435 of their associated multivibrators, preventing the identification circuits from bringing about a stopping of their associated selectors during the occurrence of the said pulse. The final selector under consideration is therefore able to efiect selection of the desired outgoing line.

If two identifying circuits simultaneously cause their selectors to stop on lines having the same units digit, there results either a mutual blocking and prevention of either identification circuit from effecting its stoppage if the circuits above described are of exactly the same characteristics, or else, as will usually be the case, these characteristics will not be identical and there will be a balancing of unequal circuits, one of the identification circuits causing a stoppage and disabling the other identification circuit.

The operation of the identification circuit will now be explained.

In Fig. '5 there are shown as functions of time:

At A, the sinusoidal voltage applied to the input resistance 4 I 2 of the amplifier 2;

At B, a sinusoidal voltage, of frequency assumed difierent from that of A, applied to the input resistance 4 l 3 of the amplifier 3;

At C, the output voltage of the amplifier 2;

At D, the output voltage of the amplifier 3;

At E, the voltage at the output terminal 420 of the clipping stage'4;

At F, the voltage at the output terminal 42! of the clipping stage 5;

At G, the voltage of the junction 434; and

At the voltage at the terminal 425 at the output of the two-time constant circuit 1,

The voltage G is equal to V1 when the tube 428 is cut off, i. e. when at least one of the two grids 422 and 423 is sufficiently negative.-- 'This.will

be true at any timeduring the existence of either the pulse 51H of voltage E or the pulse 502- or voltage F. The voltage G at junction 43.4 is equal to V when the tube 428 is conducting, i. e. when the potentials of the two grids 422 and 523 are simultaneously at or close to zero. fhis will be true-at any time during the co-existence of the pulse 503 of voltage E. and of the pulse 554 of voltage F.

When the voltage G changes from the value V0 to V1, the charging current of the condenser 330 flows from 434 towards 433, and the voltage I-I increases relatively slowly since the time constant of the circuit 7 is large for this direction of charging current. This corresponds to the portion 525 of voltage wave form H. When the voltageG passes from the value V1 to V0, the charging current flows from 433 to 434, and the voltage H declines relatively quickly since the time constant of the circuit '1 is then small. Thisis represented by the portion 506 in wave form H. In consequence of the shape of wave form H, as long as the brush I50 for example is passing in front of. the pins I 31 for azimuths energized at frequencies diiTerent from the frequency fa, tube 328 will be successively. cut off and allowed to conduct in a variable time sequence depending upon the relative values of the said frequencies. In view of thedissymmetry of the time constants of the circuit 1, the voltage at the terminal 425 which isv represented by th -peaks of the wave form H will remain close to V0.

In Fig. 6 there are shown as functions of time:

At A, a sinusoidal voltage applied to the input resistor M2 of amplifier 2;

At B", a sinusoidal voltage. assumed of the same frequencies as that of A, applied to the input resistor M 3 of amplifier 3;

At C, the output voltage of amplifier 2;

At D, the out ut voltage of amplifier 3;

At E. the voltage at the output terminal 428 of the clipper stage 4;

At F. the voltage at the output terminal 42'; of the clipper stage 5 (this voltage is now permanentlv in phase opposition with the olt-age E) At G. the voltage at the junction 434' in circuit 1; and

At H. the voltage at the output terminal 425 of the two-time constant circuit 1.

Since under these conditions at. least one of the two grids 422 or 423is always strongl negative; the tube 428 is permanently cut oif. The volta e G is fixedly eoual to V1. In Fig. 6 the slope 6M represents the passage of. the voltage G from thevalue- V0 to-the value V1 at the initial moment of identity in the frequencies of. the voltages a plied to the grids 4.22 and423. The slope at 602 represents the passage of. thevol'tage G from V1 to V0 when this identity ceases.

When wave form G changes. at the time of slope- Bill from the'value V0 to V1, the plate current flows from junction 43A to junction 433, and the voltage H increases exponentially according to the longer of the two time constants of they 12 It is to be understood that: the: embodiment of theinventionabove described is exemplary only and, that modifications. in: the particular circuits describedcan beeffected' by those skilled in the art without departingifrom the scope of the present invention.

I; claim:

1. In automatic switching equipment employing capacitive testing in line finding, means to detect identity in frequency between an A. C. voltage of aparticular frequency characteristic of a desiredlineand'one of asuccession of'A. C. voltages ofjdifierent frequencies scanned from a plurality of lines including. said. desired line, said means comprising. a. multigrid. electron tube havingtwo of its grids operatedsubstantially at zero bias, meansto apply to one of saidgrids in succession square waves havingrespectively as repetition rates theirequencies of the A. C. voltages scanned from the said plurality of lines, means to applyto the other. of. said grids a square wave voltage havingasrepetition rate theirequency of the voltagecharacteristic.of the desired line, the voltage applied to said. other grid being 180 out of phase with that-of the same repetition rate applied2to thefirstof-said grids,and a parallel circuit. connected between: the plate of said multigrid tube-and'the source of platevoltage therefor, said circuitincluding a nonlinear resistor and a capacitor in series-in one leg thereof and a resistor intheotherleg thereofisaid nonlinear resistor being disposed between said; plate and capacitor and being poled to permita rapid charging of the; capacitorduring conduction in said tube but to retard, discharge of the capacitor when said tube is cut oil.

2. In automatic telephone switching equipment. employing; capacitive testing for the identification ofa desired outgoing line in a selector, means to identify'an-A, C. voltage of a given frequency picked up by a scanning brush of a selector witha: cophasal A. C. voltage of the same frequency whichisv characteristic of a desired outgoing line of the selector, said means comprising means to invert the phase of one of the said voltages, meansto clip each of said voltages intosqua-re wave shape,,a multielectrode vacuuum tubeserving as.a,-mixer-,- means to apply said voltages after clipping thereof and inversion of one thereof to; separate grids of the'mixer tube, a capacitor; and; a. non linear-resistor in series connected in. parallel; with a resistor in the plate circuit of the mixer tube with: the non-linear resistor between, the: tube, plate and the capacitor; the nonlinear resistorbeing poled to permit a-rapid; charging of thecapacitor during conduction in the-'mixer tube but to, retard discharge of the capacitor when; the mixer tube is cutoff, whereby, upon coexistence at the grids of the mixer tube of identical frequencies for a. time long by comparison with the period of either of said frequencies, the junctionbetween the capacitor and the non-linear resistor. rises to the plate supply voltage of the mixer tube.

3. In automatic. telephony, in combination, a selector includingan incoming; line ,having, a tall;- ing. conductor and a busy. test conductor; said selector further including for each of a plurality of outgoing lines connected, thereto two fixed electrodes of, which a. first one, is energized at an A. C. voltage; of a frequency characteristic of its line and of which the second is energized when itsline isbusyby. an. A. Qyoltage o f a frequency characteristic oi the. husyicondition; said. selector further including a movable, member, a first scanning brush in the movable member forming with said first electrodes, upon motion of the member, capacitors for the transmission of the said line-characteristic voltages to the movable member, a second scanning brush in the movable member forming with the said second electrodes. upon motion of the member, capacitors for the transmission of the busy condition voltage to the movable member, means to transmit the voltages induced upon said first and second scanning brushes to the talking conductor and to the busy test conductor of the selector incoming line respectively; and line-finding apparatus comprising two thermionic amplifiers of which one provides to the signal passed therethrough substantially 180 more phase change than the other provides to the signal passed through said other, means to apply to one of said amplifiers the voltages appearing on said talking conductor, means to apply to the other of said amplifiers from a source cophasal with that employed to energize the first fixed electrode of a desired outgoingline a voltage of the frequency characteristic of the desired outgoing line, a clipping tube coupled to the output terminals of each of said amplifiers, a mixer tube having its control and suppressor grids operated at zero bias and capable of being cut olf on either of its said grids, means to couple separately the clipping tubes to the control and suppressor grids of the mixer tube, a capacitor and a non-linear resistor in series connected in parallel with resistor in the plate circuit of the mixer tube, a monostable multivibrator, means to couple the junction between the capacitor and non-linear resistor to a grid of the normally nonconducting tube in the multivibrator, means actuated by reversal in the conduction phase of the multivibrator to initiate a stopping of the selector movable member, a tuned amplifier tuned to the busy condition frequency having its input coupled to the busy test conductor of the selector incoming line, and means coupled to the output of the tuned amplifier responsive to the appearance of a signal thereon and adapted to disable the multivibrator.

4. In automatic telephony, in combination, a selector including an incoming line having a talking conductor and a busy test conductor; said selector further including for each of a plurality of outgoing lines connected thereto two fixed electrodes of which a first one is energized at an A. C. voltage of a frequency characteristic of its line and of which the second is energized when its line is busy by an A. C. voltage of a frequency characteristic of the busy condition; said selector further including a movable member, a first scanning brush in the movable member forming with said first electrodes successively, upon motion of the member, capacitors for the transmission of the said line-characteristic voltages to the movable member, a second scanningbrush in the movable member forming with the said second electrodes successively, upon motion of the member, capacitors for the transmission of the busy condition voltage to the movable member, means to transmit the voltages induced upon said first and second scanning brushes to a talking conductor and to the busy test conductor of the selector incoming line respectively, and linefinding apparatus comprising two thermionic amplifiers, means to apply to one of said amplifiers the voltages appearing on said talking conductor, means to apply to the other of said amplifiers a voltage of a frequency characteristic of a. desired outgoing line, a multigrid mixer tube capable of being cut oil on either of two of its grids, means to couple separately the amplifier outputs to the said two grids of the mixer tube, a capacitor and a non-linear resistor in series connected in parallel with a resistor in the plate circuit of the mixer tube, a monostable multivibrator, means to couple the junction between the capacity and non-linear resistance to a grid of the normally non-conducting tube in the multivibrator, and means actuated by reversal in the conduction phase of the multivibrator to initiate a stopping of the selector movable member, a tuned amplifier tuned to the busy condition frequency having its input coupled to the busy test conductor of the selector incoming line, and means coupled to the output of the tuned amplifier responsive to the appearance of a signal thereon to disable the multivibrator.

5. In a system of automatic telephony wherein each of a plurality of selectors includes an incoming line having a talking conductor and a busy test conductor; said selectors each further including for each of a plurality of outgoing lines connected thereto two fixed electrodes of which a first one is energized at an A. C. voltage of a frequency characteristic of its line and of which the second is energized when its line is busy by an A. C. voltage of a frequency characteristic of the busy condition, said selectors each further including movable means to scan capacitively the said electrodes of its lines and to transmit to a talking conductor and to the busy test conductor of the selector incoming line respectively the line characteristic frequencies and the busy condition frequency, the improvement which comprises identifying means associable with said selectors and adapted to efiect selection of a desired outgoing line of an associated selector when free and to prevent selection thereof when busy, said identifying means comprising two thermionic amplifiers, means to apply to one of said amplifiers the voltages appearing on said talking conductor of the associated selector, means to apply to the other of said amplifiers a comparison voltage of a frequency characteristic of a desired outgoing line, said comparison voltage being supplied at such phase that it appears at the output to the other of said amplifiers substantially out of phase with the voltage of the same frequency appearing at the output of the first of said amplifiers, a clipping tube coupled to the output of each of said amplifiers, a multigrid mixer tube capable of being cut off on either of two of its grids, means to couple separately the clipping tubes to the said grids of the mixer tube, a capacitor and a non-linear resistor in series connected in parallel with a resistor in the plate circuit of the mixer tube, a thermionic relay, means to couple the junction between the capacitor and non-linear resistor to a grid of the thermionic relay, means actuated by operation of the relay initiating a stopping of the selector movable'scanning means, a tuned amplifier tuned to the busy condition frequency coupled to the busy test conductor of the selector incoming line, and means'coupled to the output of the tuned amplifier responsive to the'appearance of a signal thereon to disable the relay.

6. In a system of automatic telephony wherein each of a plurality of selectors includes for each of a plurality of outgoing lines connected thereto a fixed electrode energized at an A. C. voltage of a frequency characteristic of its line and movable means to scan capacitively the said electrodes and to transmit the linecharacteris'tic 15 voltages thereof to a talking-conductor of the selector incoming line, and wherein each of a plurality of senders associable with the selectors of said plurality of selectors supplies upon call of a calling station for comparison a voltage of the frequency characteristic of a desired outgoing line cophasally with the voltage on the electrodes of that line in said selectors, each such sender including for the selection of such characteristic comparison frequencies a step-by-step rotary switch having a second row of contacts whose rotor blade is linked to the rotor blade of the characteristic comparison frequency selection contact row, the contacts of said second row being multipled tocorresponding contacts of other such senders available for association with selectors of the same stage, the improvement which comprises a plurality of identifying means separately associable with the said selectors, each identifying means of said plurality of identifying means serving to effect selection of a desired outgoing line of an associated selector when free and to prevent duplicate selection of the same outgoing line by other selectors of the same stage, each identifying means of said plurality of identifying means comprising two thermionic amplifiers of which one provides to the signal passed therethrough substantially 180 more phase change than the other provides to the signal passed through said other, means to apply to one of said amplifiers the voltages appearing on the said talking conductor of an associated selector, means to apply to the other of said amplifiers from an associated sender a comparison voltage of a frequency characteristic of a desired outgoing line of said associated selector, a clipping tube coupled to the output terminals of each of said amplifiers, a pentode mixer tube having its control and suppressor grids operated at zero bias and capable of being cut off on either of its two said grids, means to couple separately the clipping tubes to the control and suppressor grids of the mixer tube, a capacitor and a nonlinear resistor in series connected in parallel with a resistor in the plate circuit of the mixer tube with the non-linear resistance adjacent the mixer tube plate and poled to permit a rapid charging of the capacitor during conduction in the mixer tube but to retard discharge of the capacitor when the mixer tube is cut off,- a monostable multivibrator, means to couple the junction between the capacitor and non-linear resistor to a grid of the normally non-conducting tube in the multivibrator, means actuated by reversal in the conduction phase of the multivibrator to initiate a stopping of the movable scanning means in said associated selector, a multivibrator disabling-pulse generating multigrid tube biased to cut off on one of its grids and having its plate coupled to a grid of the normally non-conducting tube of the multivibrator, a duplicate-selection blocking tube operated at zero bias having its grid coupled to a second grid of the multivibrator disabling tube and having its plate coupled to a third grid of the multivibrator disabling tube and, via the multipling contacts of the second row in the comparison frequency selecting switch of said associated sender, to the plates of the duplicate-selection blocking tubes of other identifying means of said plurality of identifying means instantaneously associated with senders effecting through selectors of the same stage selection of lines having the same characteristic frequency, and means operable 16 multivibrator to apply a negative pulse to the duplicate-selection blocking tube grid, whereby upon reversal of conduction phase in the multivibrator the multivibrator disabling tubes of said other identifying means are brought into conduction.

5. In a system of automatic telephony wherein each of a plurality of selectors includes for each of a plurality of outgoing lines connected thereto two fixed electrodes of which a first one is energized at an A. C. voltage of a frequency characteristic of its line and of which the second is adapted to be energized when its line is busy by an A. C. voltage of a frequency characteristic of the busy condition, each such selector including movable means to scan capacitively the said electrodes of its outgoing lines and to transmit to a talking conductor and to the busy test conductor of its incoming line respectively the line characteristic frequencies and the busy condition frequency so scanned, and wherein each of a plurality of senders associable with said selectors is adapted to supply for comparison a voltage of the frequency characteristic of a desired outgoing line, each such sender including for the selection of such characteristic comparison frequencies a step-by-step rotary switch having a second row of contacts whose rotor blade is linked to the rotor blade of the characteristic comparison frequency selection contact row, the contacts of said second row being multipled to corresponding contacts of other such senders available for association with selectors of the same stage, the improvement which comprises a plurality of identifying means separately associable with said selectors, each identifying means of said plurality of identifying means being adapted to effect selection of a desired outgoing line of an associated selector when free, to prevent selection of said desired outgoing line when busy, and to prevent duplicate selection of the same outgoing line by other selectors of the same stage, each identifying means of said plurality of identifying means comprising two thermionic amplifiers of which one provides to the signal passed there- .through substantially more phase change "than the other provides to the signal passed through said other, means to apply to one of said amplifiers the voltages appearing on the said talking conductor of an associated selector, means to apply to the other of said amplifiers from an associated sender a comparison voltage of a frequency characteristic of adesired outgoing line of said associated selector, said voltage being at the input to the other of said amplifiers cophasal with the voltage of the same frequency appearing at the input to the first of said amplifiers, a clipping tube coupled to the output terminals of each of said amplifiers, a multigrid mixed tube capable of being cut off on either of two of its grids, means to couple separately the clipping tubes to the said grids of the mixer tube, a capacitor and a non-linear resistor in series connected in parallel with a resistor in the plate circuit of the mixer tube, a monostable multivibrator, means to couple the junction between the capacitor and non-linear resistor to a grid of the normally non-conducting tube in the multivibrator, means actuated by reversal in the conduction phase of the multivibrator to initiate a stopping of the movable scanning means in -sai d associated selector, a tuned amplifier tuned to the busy condition frequency coupled to the busy ,test conductor of the incoming line of said asso- -c at ed selector, means coupled to the output of the tuned amplifier responsive to the appearance of a signal thereon to disable the multivibrator, a multivibrator disabling-pulse generating multigrid tube biased to cut off on one of its grids and having its plate coupled to a grid of the normally non-conducting tube of the multivibrator, a rectifier coupled across the output of the tuned amplifier and having its positive D. C. terminal connected to a grid of the multivibrator disabling tube, a duplicate-selection blocking tube operated at zero bias having its grid coup-led to a second grid of the multivibrator disabling tube and having its plate coupled to a third grid of the multivibrator disabling tube, and, via the inultipling contacts of the second row in the comparison frequency selecting switch of said associated senders, to the plates of the duplicate-selection blocking tubes of other identifying means of said plurality of identifying means instantaneously associated with senders effecting through selectors of the same stage selection of lines having the same characteristic frequency, and means operable upon reversal of the conduction phase in the multivibrator to apply a negative pulse to the duplicate-selection blocking tube grid, whereby upon reversal of conduction phase in the multivibrator the multivibrator disabling tubes of said other identifying means are brought into conduction.

8. In a system of automatic telephony employing capacitive testing in line finding by means of selector scanning brushes moving past fixed electrodes and wherein a selector includes for each outgoing line connected thereto three fixed test electrodes of which one is energized from a source of A. C. voltage of a frequency characteristic of the tens digit of that line, of which a second is energized from a source of A. C. voltage of a frequency characteristic of the units digit of that line and of which the third is energized when that line is busy from a source of A. C. voltage of a frequency characteristic of the busy condition, and wherein a sender provides upon call of a calling station for comparison with the first two of said voltages A. C. voltages of frequencies characteristic of the tens and units digits of a desired line, said sender including for the selection of the units comparison voltage a multiple contact switch having a second set of contacts multipled to the like contacts of all senders available for association with selectors of the same stage, the improvement which comprises identification means adapted to stop the selector on a desired outgoing line when free, to prevent its stopping on the desired outgoing line when busy, and to prevent other selectors of the same stage from making a confiicting selection of a desired line, said means comprising two thermionic amplifiers, a multigrid mixer tube having two of its grids operated at zero bias, means to apply the outputs of said thermionic amplifiers separately to saidmixer tube grids, a two-time constant impedance including a capacitor and a non-linear resistor in series connected in parallel with a resistor, said impedance being connected in the plate circuit of the mixer tube, a monostable multivibrator having a grid of its normally non-conducting tube coupled to the junction between the capacitor and non-linear resistor, a normally cut off selector rotor stopping-pulse generator tube which when conducting generates on its cathode a positive pulse for application to the selector rotor stopping means and which generates simultaneously a negative pulse on its plate, a busy r '18 signal amplifier tuned to the'busy condition frequency, a multigrid multivibrator-blocking tube biased to cut oif on one of its grids and having its plate coupled to a grid of the normally non-conducting tube of the multivibrator, a rectifier coupled across the output of the busy'signal amplifier and having its positive'l). C. terminal applied tothe cut off grid'of the multivibrator-blocking tube, a normally conducting duplicate-line selection blocking tube having a grid coupled to the plate of the selector rotor stopping-pulsgenerating tube and to a second grid of the multivibrator-blocking tube; a relay, 2. first contact on said relay connected when deenergized to couple one of said thermionic amplifiers to the tens digit A. C. voltage in the sender and when energized to couple the said one of the thermionic amplifiers to the units digit A. C. voltage in the sender, a second contact on said relay connected when de-energized to couple the other of said thermionic amplifiers to the selector rotor brush which scans the tens test electrodes of the selector outgoing lines and adapted when energized to couple the other of said theermionic amplifiers to the selector rotor brush which scans the units test electrodes of the second outgoing line, a third contact on said relay connected when de-energized to connect the plate of the normally conducting tube of the multivibrator to the actuating element of said relay and connected when energized to connect said plate to a grid of the selector rotor stopping-pulse generating tube, a fourth contact on said relay connected when energized to connect the busy test wire of the selector incoming line to the input of the busy signal amplifier, and a fifth contact on said relay connected when energized to connect the plate of the duplicate selection-blocking tube to the rotor blade of the second row of contacts in the units digit frequency selector of the sender.

9. In a system of automatic telephony employing capacitive testing in line finding by means of selector rotor scanning brushes moving past fixed stator electrodes and wherein each of a plurality of final selectors includes for each outgoing line connected thereto three fixed test electrodes of which one is energized from a source of A. C. voltage of a frequency characteristic of the tens digit of that line, of which a second is energized from a source of A. C. voltage of a frequency characteristic of the units digit of that line and of which the third is energized when that line is busy from a source of A. C. voltage of a frequency characteristic of the busy condition, and wherein each of a plurality of senders associable with such selector provides for comparison with the first two of said voltages A. C. voltages of frequencies characteristic of the tens and units digits of a desired line, all sources of voltage of the same frequency employed being cophasal, each such sender including for the selection of the units comparison voltage a stepby-step rotary switch having a second row of contacts multipled to the like contacts of all senders available for association with selectors of the same stage, the improvement which comprises identification means adapted to stopthe selector on a desired outgoing line when free, to prevent its stopping on the desired outgoing line when busy, and to prevent other selectors of the same stage from making a substantially simultaneous selection of a desired line, "said means comprising two thermionic amplifiers of which one provides to the signal passed therefiers, a pentode mixer tube having its control and suppressor grids operated at zero bias, means to apply the outputs of said clippers separately to said mixer tube grids, .a two-time constant .impedance including in series a capacitor anda non-linear resistor connected-in parallel with a 1 linear resistor, said impedance being connected in the plate circuit of the mixer tube with the non-linear resistor adjacent the mixer tube plate and'being poled topermit a rapid charging or the capacitor when plate current flows in the mixer tube but to retard discharge of the capacitor when the mixer tube is cut ofi,a.monostable multivibrator-having a grid of its normally non-conducting tube coupled to the junction between the capacitor and non-linear reslstor a L normally cut off selector rotor stopping-pulse generator 'tube which when conducting generates on its cathode apositive pulse for application tothe selector rotor stoppingmeans via the busy test wire of the selector incoming lineand generates simultaneously. a negative pulse on its plate, a busy signal amplifier tuned to thebusy condition frequency, a pentode .multivibratorblocking tube biased tocut off on its control grid and having its platecoupled to a secondrg'ridiof the normallynon-conducting tube or" the multivibrator, a .rectifier coupled across the output of the busy signal amplifier and having its positive D. C. terminal appliedto the control grid of the multivibrator-blocking tube, a normally conducting duplicate-line selection blocking tube havingits gridoperated at zerowbiasand coupled to the plate of the selectorrotor-stopping .pulse generating tube and tothesuppressorgrid of the multivibrator-blocking tube, a.relay, a I

first contact on said relayconnected-when deenergizedto couple one .of said thermionic am .plifiers to the tens digit A. C. voltage in-the sender and when energized to couple-the said one of the thermionic amplifiers to the .units digit A. C. voltage in the sender, asecond contact on said relay connected when de-energized to couple the other .of said thermionic amplifiers to the-selector. rotor brush which scans the tens test electrodes of-the selector outgoing lines and adapted when energized to couple theother of said thermionic amplifiers to .-.the selector rotor brush which scans the units' test electrodes of the second outgoinglinaa third contacton said relay connected whenrde-energized vto connect the plate of the normally conducting tube of the multivibrator to the-actuating element of saidrelay and connected when energizedto connect it to a grid of the selector rotor stoppingpulse generating tube, a fourth contact on said relay connected when energized to connectthe .busy test wire of the selector incomingline to the input of the busysignal amplifier, and. affifth contact on said relay connected when energized to connect the plate of the duplicate selectionblocking tube to the rotor blade ofthehsecond row of contacts in-the units digit frequency selector of the sender.

.101 In a system of automatic telephonyineluding a plurality ofselectors, a plurality of senders and a plurality of identifying. means each adapted to stop an associated selectoron an'outgoing line thereof indexed by an associated sender, the outgoing lines of: said selectors being multipled together and the line indexing means ofsaidsenders having contacts coupled-thereto which are multipled to'like contacts of all of "said senders, each of said identifying meansincluding a relay adapted to generate a selector stopping signal upon identification of an outgoing line of an associated selector with the line indexed by an associated sender and further including a relay disabling amplifier actuable upon receipt by said identifying means from the associated selector of a signal indicating a busy condition for the desired outgoing line, the improvement which comprises means adapted .to prevent duplicate selection of a single line by two of said selectors, said means including for each of said identifying means a duplicate selection blocking tube having itsgrid connected to a terminal of said relayexperiencing a negative signal uponactuation thereof, said blocking tube having its plate coupled to a control :grid of said relay disabling amplifiensaid blocking tube having its plate coupled via the multipling contacts of the sender associated with the identifying means of which said blocking tubeisia part to the plates of corresponding blocking tubes of other identifying means of saidlplurality of identifying .meansinstantaneously associated withsenders indexedto the same outgoing line.

11. In a system of automatic telephonyincludingaapluralityofi selectorsra pluralityof senders and-aplurality of identifyingmeansleach adapted to stop an associated selectoizon-Lan.outgoingline thereof indexed by an associated sender, .the.out- .goingilines of said selectorsbeing multipled to- .gether and the line indexing means of said senders having contacts coupled. .thereto'lwhich .are multipled to like contacts of .all .of .said senders, eachof said. identifyingmeans including a relay adapted to generate .a.,selector.stopping signal .uponidentification. o'fsan. outgoingsline .of an iassociatedqselector with ;.the .line .indexed by anassociated senderand further including-a relay disabling a amplifier actuable .upon receiptby said identifying meansufrom .theassociated selector of/asignal 'indicatinga busy conditionlfor the. desired outgoing line, the improvementwhich comprises means adapted to prevent ..duplicate selectionvof a single lineby two of said; selectors, saidmeans including foreach of .saididentifyingzme'ans a duplicate selectionlblocking relay adapted to be actuatedupon selectionof the line .by a selector associated witthe identifying-means of whichv said duplicate selection blocking relay is a part; said duplicate selection blockingrelay being coupled to a controLgrid of the relay'disabling amplifier of its own identifyingrmeans and, via the multipling contacts of. its associated sender, to thelplatesof corresponding duplicate selection blocking relays of .other identifying means of said plurality of identifyingmeanszintors of said pluralityofselectors instantaneously associated with senders indexed to thesame outgoing line .are actuated.

ETIENNE G. ALIZON.

Name Date Mirick Sept. 4, 41928 Number

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