US2424577A - Long distance telephone signaling system - Google Patents

Description

Juli 29, 1947.

M. R. MAUGE LONG DISTANCE TELEPHONE SIGNALING SYSTEM Filed May 1, 1943 3 Sheets-Sheet l NV EN TO K fi/JECFL E M17066- July 29, 1947.

3 Sheets-Sheet 2 Filed May 1, 1943 Y $8 58" 3 n fin flan a an a as: a:

- INVENTQR All/Pal 1?. M17065 July 29, 1947.

M. R. MAUGE 2,424,577

LONG DISTANCE TELEPHONE SIGNALING SYSTEM 3 Sheets-Sheet 3 Filed May ,1, 194:5

ms warm :2 23% GENT iatented July 29, 1947 LONG DISTANCE TELEPHONE SIGNALING SYSTEM Marcel Roger Mange,

France, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 1, 1943, Serial No. 485,406 InFrance May 28, 1941 8 Claims. l

The present invention relates to electric signaling systems specially arranged for usein long distance electrical communication circuits [between central ofiices with which subscribers stations are connected. More particularly, the invention provides for the transmitting and interpreting of the various signals required for the establishment of longdistance telephone connections over different toll circuits.

The object of the invention is to provide long distance signaling circuits that require no modifications in the toll circuits and that can be used with incoming and outgoing circuits of any type, one-way or two-way, manual or automatic.

In long distance telephone systems, signaling is ordinarily effected by signals of frequencies comprised in the voice frequency band. In order to prevent wrong signals by speech currents,,it is customary to take the following precaution: Since the circuits may contain speech currents, the control signals are interpreted or countedonly if they are preceded by a preparatory signal composed of two given frequencies to the exclusion of any other. For best results, the receiving period of this preparatory signal should exceed 200 milli-seconds.

The interpretation or response to control signals may, however, be prevented by the presence of some other voice frequency current, orthe signals may be mutilated by echo suppressors. In order to avoid these drawbacks, the signals transmitted after the seizure of the speech circuit, which may cause interference, are repeated until an acknowledging signal is received at the point whence these control signals were transmitted.

The present invention has among its objects the provision of means for increasing the functional dependability of voice frequency signaling in such telephone systems, and the provision of arrangements for receiving the signaling pulses.

According to one feature of the invention, means are provided for readily increasing the duration of the preparatory signal so that the signal shall be effective only after a delay exceeding 200 milli-seconds, for example, after 300, 400 or 500 milli-seconds.

According to another feature of the invention, means are provided for increasing the duration of the dialing tone if the existing code suppressors are in danger of being locked. By dialing tone is meant the tone sent [by the called exchange when the equipment of that exchange is ready to receive the ringing pulses.

According to another feature of the invention,

Boulogne-Billancourt,

a stop dial (end of dialing) signal is sent by the originating exchange and then allows the switching in of the speech circuit. The disturbances due to improper switching in of the speech circuit during ringing is thus avoided.

According to another feature of the invention, the means provided for determining the duration of the transmitted signals are also arranged to analyze the duration of the received signals.

Means are provided for returning them to normal after each use.

According to another feature of the invention a, release signal of a particular form is transmitted in the system at the end of each connection, in order that the circuit shall be released at the end of the reception of the release signal of the calling party for a call to a subscriber and, for a call to the operator of the distant exchange, after reception of this signal and after the operator has pulled out the plug.

According to still another feature of the invention, the trunk seizure signal is repeated so that, in the event of simultaneous seizure of both sides of the trunk, the signals may not mutilate each other. The length of the interval between repeated or maintained signals is made different for signals of opposite transmission direction, in order to avoid perpetual collision between two simultaneously transmitted signals.

According to still another feature of the invention, diiferent seizure signals are used for the preselection of different exchanges connected in tandem.

According to another feature of the invention, the means provided for determining or verifying the duration of preparatory signals are also arranged to verify the intervals between repeated or maintained signals and to analyze the control signals. For that purpose, they are placed under the control of a relay that verifies the return to normal of these means after each use.

These and other features will be explained in detail in the following description given with ref erence to a specific embodiment shown in the accompanying drawings, in which:

Figs. 1 and 2, placed above one another, show a voice frequency signaling circuit incorporating features of the invention, and

Fig. 3 shows a two-way trunk circuit of a usual arrangement, with which is associated the signaling set circuit of Fig-s. 1 and 2. This arrangement is provided at the incoming and outgoing ends.

A typical code for the use of the signals is given below:

r indicates that the signal is repeated untiljreception of the dialing tone.

r indicates that the signal is repeated until acknowledgment of the receipt.

p indicates that the signal persists, with intervals,

while the cause. of operation lasts.

a: indicates a short pulse of frequency 2:.

y indicates. a short pulse of frequency y.

X indicates two successive short pulses of frequency ac.

Y. indicates two successive short pulses of frequ y 1 P indicates one long pulse of two frequencies r and y.

Terminals l to I (Fig. 1), which are grounded successively by the means determining the duration, are connected to terminals EP, RP, DS, DE, in accordance with the timing desired:

EP, duration of transmission ofthe preparatory signal.

RP, minimum duration for a; preparatory signal to be eificacious.

DS, length of interval between signals for the circuit used in outgoing call.

DE, same period for the circuit used in incoming call.

In order to facilitate. an understanding of the operation, the main function of the various signaling setrelays isgivenin the following table:

X and Rm respond to frequency. as.

Y and Ry respond to frequency 11.

G responds to all signals different from :r and y.

P, reception of a preparatory signal.

Rp, end of reception of the preparatory signal;

Pr, reception of acontrol signal.-

Py, reception of y control signal.

RI, reception of releasing signal.

Ac, reception. of the acknowledging signal.

D disconnects the line if. the. preparatory signal lasts more than 100 milli-seconds.

Cr, transmission of frequency w.

Cy, transmission of frequency y.

Cp, transmission of frequencies a; and y (preparatory signal) Se neutralizes the receiving members during the transmission.

Sac, control signal it instead of y.

Ls, control signal composed of two pulses instead of one.

Ia-Ib, pulse generators.

Ta-Tb, Tar-Ty, signal-duration counters,

Fp, end of transmission of the preparatory signal.

Fc, end of transmission of the control-signal.

Fi, end of interval between signals. and increase of the duration ofthe acknowledging. signal.

tion-counting relay.

M, holding relay. En, incoming call.

The method of operation of the circuit shown in Figs. 1, 2 and 3, will now be described for given types of calls.-

1 .When the circuit is free, signaling set holding relay M is in normal position, allowing the seizure of the circuit. It is assumed that the signaling set is connected to a two-way trunk circuit which in turn is connected to the operators positions which handles calls to the distant exchange. For calls coming from the distant exchange, the set is connectedto the usual subscribers line circuit. This;signaling set could also be connected to some other circuit, for example, to a selective trunk. circuit, without altering the operation.

Relay M being in normal position, the lamps LL signaling the free condition of the circuit are supplied with current over conductors l and 8 and one of the back contacts of relay M. Relay Cis connected to the third wire of the jacks (Fig. 3).

On plugging in, relay C operates in series with the 0 wire of the cord circuit: terminal E (Fig. 3) connected to ground or battery depending upon the cord circuit used at the exchange, winding of relay C, wire ll, back contact of relay M (Fig. 2), wire Ill'and Wire C of the cord circuit. Relay C causes the operation of relay B: battery, Winding of relayB, terminals L and M, front contact of relay C, ground onback contact of relay AP. Relay B short-circuits wires [0 and l I so that relay C can hold after energization of relay M; it causes the operation of rela M: battery, winding of relay M (Fig. 2), back contact of relay Rl, wire [2, front contact of relay B (Fig. 3), Wire l3, back contact of relay Py, ground (Fig.2). Relay M locks under the control of relay RI in normal position. It opens the wires whichnormally allow seizure of the circuit.

Onthe other hand, relay B causes the operation of. relay Ao: battery, winding of relay Ao (Fig. 3), terminals y and a, back contact of relay Sn, front contact of relay B to ground.

If the distant exchange is an automatic exchange, terminal Y will not be connected to terminal Z but to terminal 0, causing over conductor 2 the operation. of relay Fp (Fig. 2). Relay Fp then causes the sending of the seizure signal as follows: the energization of relay Si (Fig. 1) isefiected over the circuit: battery, resistance, winding of relay Si, front contact of relay Fp, back contact of relay Fa, ground. Relay S2, in operating. causes the operationof relay Ia: battery, windings of relay Ia, back contact of relay Th3, front contact of relay Si and ground over the; energizing circuit of this relay. Relay Ia over its front. contact causes -on the onehand the, operation of relay Ib and on the other the operation of relayTal: battery, winding of relay Tal, back contact of relays Tbl and Th2, back contact'of. relay Rp,-. ground on front contact of relay-Ia.

Relay II), in operating, short-circuits the energizing windingof relay Ia, which releases, Upon its release, the short circuit around relay Tbl is removed and it operates in series with relay Tal. On thev other hand, relay Ia causes the operation of.-relay Cy: battery, winding of relay Cy, back contactof relay Sr, front contact of relay Fp, back contact of relay F0 and front contact of relay Ia.

Relay Tbl -in operating causes the operation of relay Fc; battery, winding of relay Fc, front contact of relay, Si, front contact of relay F11,

back contact of relay Ls, front contact of relay Tbi to ground. Relay Fc locks up: .battery, Winding and front contact of relay Fc,uback contact of relay F2, front contact of relay Fp and ground over the energizing circuit of this relay. Relay Fp opens the energizingcircuit of relay Si and hence the locking circuit of relays Ta and Tbl and the energizing circuit. of relay Ia. Upon the release of relay Ia, the original energizing circuit of relay Ib is opened and this relay releases, allowing. re-energization .of; relay Ia. These relays operate as beating relays creating pulses at each closing of the front contact of relay Ia. These pulses are identical with dial pulses, relay Ia being about 66 milli-seconds on its front contact and 34 milli-seconds on its back contact. This beat frequency maintained because relay Ia releases upon the closing of the short circuit around its winding by the relay Ib, and the latter is retarded in its release by a rectifier cell placed in parallel with its energizing winding.

At each operation of relay Ia, a pair of relays Ta and Th operate as counting relays in known manner. After the energization of relay Fc, relay Cy cannot operate. Relay Cy thus operates once at the beginning of the operation, connecting a source of frequency 11 to the toll circuit across the back contacts of relay D and the windings of differential transformer T. Relay Cy causes the operation of slow-releasing re.- lay Se, which relay neutralizes the detector so that the latter may not be influenced lby an echo.

After the operation of relay Fp it has been seen that the energizing circuit of relay Si was opened, but this relay was held through its looking winding over the front contact of relay Tal. In this way, relay Si cannot release until after the release of relay Tal.

After its release, relay la is pulled up by'relay F0 in operating position: battery, winding of Ia, back contact of relay Th3, back contact o relay Si, back contact of relay Ft, ground to front contact of relay Fc. Relays Ia and. lb operate as before, causing at each energization the operation of one of the pairs of relays Ta and Tb.

At the end of this third pulse, relay Tb3 operates as has been explained, causing the release of relays Ta and Th2. A circuit is then closed for relay Tc; battery, winding of relay Tc, front contact of relay Tb3, back contact of relay Th2 and ground to the front contact of relay Fc ove the back contacts of Si and Ft.

At the fourth pulse, relays Ta and Tbl operate again because the energizing circuit of relay Ia is not broken by the operation of relay Tb3, the back contact of this relay being shortcircuited by a back contact of relay Ap (Fig. 3). At the end of the fifth pulse, relay Td is operatd in a manner similar to that explained for relay Tc. 1

Through relays Ta, Tb, To and Td, a ground is placed after each pulse on terminals l to 1 (Fig. 1). Over terminal I relay F0 is energized and in the same manner, over terminal I, the interval-end relay Fi will be energized. This relay causes the release of relay F0 and, if the cause of operation of relay Fp has not ceased as explained later, the cycle of operations recommences: sending of a 1! signal, then determination of the length of the interval, etc.

At the distant exchange, the y signal applied to the toll circuit is sent through differential, transformer T to the detector circuit, which Will.

cause, after amplification and detection in a voice frequency tube circuit, the operation of relay Y, shown in the upper right-hand corner of Fig. 1. Relay Ry operates for the duration of the signal, causing, overthe back contact of relay P, the energization of relays M and F2. Relay Ap (Fig. 3) is energized in parallel with relay Ry: battery, winding of relay Ap, back contact of relay B, back contact of relay Sn, wire 42, ground to the front contact of relay Y.

Relay M locks as explained for the outgoing circuit, preventing the seizure of the circuit through the opening of the circuits of the availability lamp AL and of relay C. At the end of the signal, relay Si releases, allowing the operation of relays Ia and lb pulled up over wire 4 and the ground to relay Ap (Fig. 3). The operation of these relays i intended to lengthen if need be the signal that will be sent in return should the connection to the automatic exchange occur before the echo suppressor is unlocked.

Relay Ap locks over wire 38 and a ground furnished across the back contacts of relays Td and P. It causes the energization of relay At: battery, winding of At, front contact of relay Ap, back contact of relay Op, wire 33, back contact of relay P11, front contact of relay M, wire 9, back contact of relay B, wire l3, back contact of relay Py, ground. Relay En (Fig. 2) operates in parallel with the relay At over wire 21. It looks under the control of relay M, performing the switching required for the handling of an incoming call. Relay At closes a loop to the associated line circuit; terminals Bl, B2, wires 4| and 40 looped in the normal position of relay Rrr, resistances, self-inductance, winding of relay Fi, terminals R and Q, front contact of relay At, terminals A2 and Al. Relay Tm is connected to the grounded wire of the line circult. The automatic exchange is seized in a manner suited to the exchange and, when the automatic switches are in position to receive pulses, the line wire previously connected t the free ground is grounded across a winding of the supervisory relay. Relay Tm, which was short-circuited up to this moment, operates and closes over wire 6 the energizing circuit of relay Cw, which connects a source of frequency at to the toll circuit.

As has been explained before, relays Ta and Tb have been placed in operation independently of the engagement of the automatic chain and, after the operation of relay Th3, the circuit of relay Ia is broken, wires 20 and 2| being no longer looped owing to the energization of relay Ap. When relay Tm operates, these wires are connected and the pulses of Ia. can progress beyond the third pulse. When the fifth pulse is finished, relay Td operates as has been seen before, opening the locking circuit of relay Ap which releases. Relay Tm releases in turn, opening the circuit of relay Cw. From the preceding description it can be seen that, regardless of the time required for the engagement of the automatic chain, the duration of the :c signal will be lengthened if necessary to take into account the echo suppressor or relay Se of the outgoing circuit.

At the originating exchange, as has been explained, a y signal is sent every 700 milli-seconds; normally the a: dialing tone is received before the end of an interval, causing over wire 39 the energization of relay Sn, which opens the original energizing circuit of relayFp. This relay in releasing stops the operation of relays Ia, Ib, Ta and Tb. At the end of the a: signal, relay F2 energized: by this signal, releases,agiving..-oyer terminals X and P a signal :to indicatertmthe pulse sender'or to, the operator. the possibilityof dialing over the toll circuit. Thepulses act .upon relay Ca: through terminal H.

To ring a second exchange connected in. transit with the first, theseizureof the trunk-.would'be effected in a different way, for example, through a second jack causing, in addition tothe energization of relay-Fp, the energization of relay Ls. Two. pulses of frequencyy will be sent, the circuit'of relay Fv being connected no longer to Tblibut to Tb2:through relay Ls. Discrimination ,would be effected thenat the distantexchange to put the call through in the second direction.

The dialingpulses cause the energization of relay-Rizal; th'e distant/exchange. Thepulses are transmittedtothe automatic chain through the opening of wireslllz'and 4|. At the firsta: pulse, relay Sn is operated over wire and locks under the control of. relay At the end of the transmission of'the digits, the outgoing circuit causes the. sending of a. y signal causing at the distant exchange the operation of relay Fn over wire 42. At the.originat ing exchange, relay Fn is operated: across terminal I over a circuitnot shown in the drawing.

RelayFn, at the outgoing as well as at the incomingend, closes the. circuit of signaling-set relay D; battery, winding of relay D, back' contacts. of relays Th2, Th3, Tc, Par, wire 34, front contact of relay Fn, wire 38, back contacts. of relays .Id and P.

Relay D closes. the talking. circuit and the operatorv can hear the tones transmitted by the automatic exchange.

When the called. party answers, the direction: of the current supplyis reversed and relay Su, which was short-circuited by a rectifier cell, now: operates, closing the circuit of relay Sa: (Fig. 2) battery, winding of relay Sm, wire 23, back contact of relay C122, front contact of relay Suand ground .over the locking circuit of relay At. Re-- lay Sa: locks: battery, windingand frontcontact. of relay Sat, backcontact of relay.Ac, wire 28, front contact of relay Cuz, ground. Relay C12 isenergized: battery, winding of relay Cvz, wire 22; front contact of relay S33, groundonwire 21; re-= lay CUz locksandopens-the energizing circuit of relay Sm. On the other hand, relaySrcauses the energization of relay Pm: battery, winding of relay.P:c, back contact of relay Ac, frontcontact of relay Sx, front contactof .re1ay-M to-- ground. Relay-.D releases.

ThePa: signalis now transmitted 'as' follows.- Relay Pw'causes the energization of relay Cpabattery, winding of relay Cp, back contacts of .relays Fp and E0, wire 5, backcontactof relay. Op, wire.36, front.contact of relay Pagbackcontacts of relays Ry andRm, ground.

RelayCp connects the signal ofrcomposite frequencies a: and y to.the. tollicircuit and acts upon the pulse-generating relays under the control of relay Si in normal position. As already. explained, a ground is connected successively to terminals l to 1. When terminalEP is connected to ground, relay Fp is operated: battery, winding of :relay F12, front contact of relay-0p, terminal. Ep, ground. Relay Fp locks and opens theem ergizing circuit of the pulse-generating relays, relays Ta and To releasing. The compositesignal P is no longer sent- Relay Fp acts upon thepulse relays as. already described through relay-St in: operating position. It should be notedthatrelay Sti'cannotmperate until after the-release of relays -Ta. and Tb; thus ensuring the return to normal. ofall'j.the: relays used for determining the duration of signals.-

Relays Ia,.Ib, Ta, Tb operate again. Upon the first: energizationof relay Ia, relay Ca: is operated and-at the-end of thefirst pulse, relay Fc operates asia'lready. described. Relay Cm has operated Once connecting'an. a: signal to the toll circuit. Asalreadydescribed, relays Ta and Tb are used thento .insertan interval between the sending of.:the Pm signals, relay F2 operating at the end of. 'the:interval when terminal DE, this being an incoming. call, is connected to ground. The operation: continues until the arrival of the Py acknowledging signal.

Upon arrival of this signal, the composite freuencyP 'causesthe operation of relays Rat: and Ry. throughthe detector. These two relays, in operating, close.the energizing circuit of relay S2; whichewill .not' operate until the release of relays Ta and Tb. On the other hand, relayFz' operates through relays Ra: and Ry. Relay Fi causes the release of relay Fc. Relays Ta. and TI) releaseandrelayst operates.

The pulse-generating relays operate under the control .of relays R50 and Ry which are operated.

If a. frequency different from frequencies a: and*y were received, relay G would be operated by -thedetector, opening thecircuit of relay Ry. RelaysJIa and Tb could not progress further.

If-only frequencies a: and yare received,- and if thesignallastslong enough for terminal RP to be connected to ground, relay P operates to indicatethereception of-a. preparatory signal. At theend of the reception of this signal, relays R1: and Ry release and relay Rp operates: battery, winding-of relay-Rp, front contact of relay P,

back contacts of relays Rp, Ry and Rat, ground.

RelayP' is slow to release to hold until the reception of 'the control signal, which will cause, since-this is a'y signal, the energization of relay Ac: battery, winding of relay Ac, front'contact of-relay Sat, back-contact of relay-Ac, front contactsof relays Rp'and Ry, back contact-0f relay Rm, ground.- This relay causes the release of relays Sa: and Pa, which stops the sending of Pr signals.

If the signal received does not cause the energization of relay Ry, for the reasons already explained; relay-Acdoes not operate and, at the endof the reception of the signal, relay Cp 0peratesagain after the release of relays Rx and Ry, sending the preparatory signal in the same manner as when the calling party removes the receiver; However, the duration of this signal is increased by-thereleasing time of relays P and F2 to-take into-account the echo suppressor whichof relaytFc, wire 15, back contact of relay Ap,

wire [4, front contactof relay M, ground. At the beginning o-f'the reception of the preparatory signal,- when relayTbZ operates, relay D releases.

RelayPrc causes the energization of relay Col which locks and changes theresistance of relay 0 totransmit the supervision to the operator.

Dependingupon'the-type of cord-circuit used at theexchange, point SU is connected to point HR (high resistance during conversation) for BB. (low resistance during conversation).

On the other hand, relay Pa: causes the energization of relay Cp, the P11 signal is sent as described for the Pa! signal, the control signal being 1/ instead of :17, relay Sm being in normal position. At the end of the control signal, relay Fe in op erating causes the release of relay Pr, which stops the operation of relays Ta and Tb. Relay D operates again, closing the talking circuit.

When the called party releases, the direction of the current supply is reversed again. Relay Su releases, causing over wire IS the energize.- tion of relay Pr. The Py signal is sent to the originating exchange where relay Py is operated, its operation being ensured in the same manner as that of relay Px, which has been described before. Relay Py locks: battery, winding and front contact of relay Py, back contacts in parallel of relays Th2 and Td, back contact of relay En,

wire 28, front contact of relay C, back contact ofrelay Ap, ground, This provides supervision over wire 3| or 32 and brings into-operation relays Ia, 11), Ta, Tb for the purpose of causing the opening of its locking circuit if a new Py signal is not received.

Relay F2 operates at each new signal, ensuring the return to normal of relays Ta and Tb.

When the operator pulls out the plug, relay C releases, opening the locking circuit of relay Py. Relay B releases, closing the circuit of relay Pr: battery, winding of relay Paz, wire I6, back contacts of relays At and Op, wire 33, front contact of relay M, wire 9, back contact of relay B, wire I 3, back contact of relay Py, ground.

Relay Pa: causes the sending of the Py signal. At the distant exchange, relay Py operates, opening the locking circuit of relay At, which releases, releasing the automatic chain.

The releasing signal is then sent from the dis-. tant exchange to the originating exchange. For this, relay Ls is operated upon the release of relay At; battery, winding of relay Ls, front contacts of relays En, Pu, and M, wire 9, back contact of relay B, wire I3, front contact of relay Py, back contact of relay Fc, ground. Relay Pat is operated, the sending of the preparatory signal is efiected, the 1; control signal is sent but relay Ls being operated, relay Fc does not operate until after the energization of relay Th2. Two y pulses are therefore sent constituting the Y control signal characteristic of the release. At the end of this control signal, relay Fe, in operating,- opens the locking circuit of relay M which, in releasing, releases all the locked and closed relays, allowing new seizure of the circuit.

At the originating exchange, the preparatory signal causes, as is known, the energization. of relay R10, the control signal being composed of two pulses. Relays TaI and Tbl and then TaZ'an'd Th2 operate. Since the signal was not of frequency at, relay Pa: is in normal position. Relay RI is operated: battery, winding of relay RI, back contact of relay Pr, front contact of relay Rp, back contact of relay Si, front contact of relay Th2, ground.

Relay RI transfers the locking circuit of relay M to the energization circuit of relay RI. This relay looks over the front contact of relay M. At the end of the signal, when relay Rp releases, relays M and RI release, releasing the circuit.

Thus, it will be seen that the release is not effected until the two exchanges have sent their releasing signal precluding any possibility of locking.

Furthermore, the release signal being differ- 10 out from the other signals, it is easy to provide such an arrangement that the circuits usedat exchanges seized in transit shall respond only to the release signal.

The calling of the operator of the distant exchange is possible up to the time the called party answers. The operator of the originating exchange momentarily depresses the ringing key of the cord circuit, causing the momentary energization of relay Ao over wires 23 and 30. Relays Sr and Ls are operated and locked, relay Pat is operated and, aswill be readily understood from the foregoing description, the preparatory signal followed by two pulses of frequency a: is sent. This combination constitutes the signal called PX. As already explained, relay CvI operates over wire 35. Relay Ls connects the ringing return to the cord circuit.

At the distant exchange, relay Pa: operates and, since the control signal is made up of two pulses, relay Op is operated; battery, winding of relay Op, back contact of relay Cvz, wire 31, front contacts of relays Pa: and R10, back contact of relay Si, front contact of relay Th2, ground.

Relay Op is locked: battery, winding and front contact of relay Op, wire 35, back contact of relay Py, front contact of relay M, wire 9, back contact of relay B, wire I3, back-contact of relay Py, ground.

Relay Op closes the circuit of the ringing lamps. One of the operators listens in by plugging in a cord circuit.

RelayC operates, its circuit being closed by one of the contacts of relay Op, which shortcircuits wires III and II opened by relay N of the signaling set. Relay B operates, opening the locking circuit ofrelay Op. The ringing lamps are extinguished, signaling-set relay D is operated upon the energization of relay C and the conversation cantake place.

To ring back the operator, the same operations are performed and, this time, relay Op is not 01 erated but the clearing relay of the cord circuit is momentarily operated.

Upon the arrival of the PX signal, relay Pa: of the signaling set is operated and locked. When relay Op releases acknowledging signal P11 is sent,

causing at the originating exchange the release of relays Ls, Pa: as already described.

It has been seen that when the operator pulled out the plug signal Py was sent continuously. In the case of a connectiontowards a subscriber, this signal causes the releaseof the automatic chain. In the case of a call between two operators,"this signal provides the supervision through variation of the resistance of relay C. Furthermore, at the distant exchange, the cord circuits do not generally have supervisory relays'on the answering side, and therelay Op is operated momentarily at the same time as relay Py, affording supervision by the operator. The release signal is not sent until after the two operators have pulled out the plugs.

In the example shown in Fig. 3, the operator does not dial, the distant exchange being a manual exchange. In that case, relay A0 is operated as described before, causing over wires 23 and 30 the energization of relays Fa: and Ls. Relay Pa: is operated then: battery, winding of relay Pr, back contact of relay Ac, front contact of relay S0: or Ls, ground to the front contact of relay M. Signal PX is sent then in known manner, and the circuit operates as already described for the ringing of the operators.

The incoming circuit could be connected also to the toll automatic chaini or :to an: incomin selector'circuit without theioperationof the signalingset'being therebyimodified.'

While a specific embodiment of'ithe invention has been described, itisclear thatl it is'iiin'no'way limited but on the contrary-is capable oflnumercus modifications and adaptations 'withinrt'h'e scope-f the claims.

'Whatis'claimed is:

1. In a voice" frequency:signalingsystem,' two switching points, means including trunk lines and switching means tor-establishing a circuit between said points, means at the first'po'int for sending control signals preceded by preparatory signals over said circuit at a frequency comprise'd within the voicefrequency band, means at the second point for 'sending an a'cknowledgingsignal over said circuit, acknowledging-signal responsive means at the first point, means at the first point for repeating the preparatory signal until the last mentioned means responds; andmeans at the first point'for varying-the duration-of the preparatory signal and forren'dering it fiecti-ve only afterthe expirationof a' predeterminedperiod of "time. 7

2. The .systemaccording to "claim- 1,' and in which said means for varying the-durationof the preparatory signal "analyzes theduration 'o'f'the acknowledging signals.

3. Inavoice frequency-signaling systemi two switching-points, means'including trunk -linesan'd switching means for est-abli-shingua' circuit between *saicl tpoints, means at the' first "point' for sending control signals preceded 'by' preparatory signals over said'circuitat a frequency-comprised withinthe -voice frequency band,--means for sending dialing impulses from the' first to the second point, meansat the second-point --respjon'si-ve to said pulses,- .a dialing tone signal *sent from the second to the first point-indicatingreadiness for said-pulses,-and means for yaPying the -LduratiOn of the'dialing tonesignal. V

4. In a voice frequency sig rialing system; two switching points, means inclddingtrunk lines and switchingmeans' for partially establishing: a talk nectingzsaid :points, switching means-atizeach point for seizing a trunk line; means at each point for sending a signalitothe other 'point uponnthe seizure-"0f atrunkline; meansfor repeating the signalsentfrom a first toa'secondpoint'at a certain interval, and means for repeatingthe signal 'sent' fromthe second to the first pointat a different "interval.

i6. The'system'ac'corvding to claim 5, and'means for varying theduration of 'the'seizure signal depending ion the 'pointtowhich the seized trunk leads.

7; in a'voice frequency signaling system, two exchangesrmeans including trunks'and switching means for establishing a, talking circuit between saidi'exchanges, aplurality'of numerically designated'rlines associated with each exchange, means at ea-ch exchange for sending control signals to the -oth'er "overfsaid "circuit 1 at a :frequency comprised within the voice:frequency band,'"means at each- -:'exchan'gc' for sending to theother "-an acknowledging 'signal over: said icircuit, acknowledging'signal responsive means at each'exchange, means at eaich exchangeT-orsen'ding impulses'to the other representing 'the numerical designations of lines, andme'ans at 'each exchange for determining the duration of the signals sent therefrom and analyzing thesignalsreceived thereat.

8 In a voice frequency signaling system, a plurality of exchanges, trunks=forinterconnecting said I eiich-anges, numerically designated subscribers" lines terminating ateach exchange, operators positions'atthe exchanges, means at each exchange for sending control signals to the other, means 'at e'ach-exchange for sending acknowledging signals tothe other; means at I each exchange for sending'imp'ulsesto the other representing the numericalfdesignation of avline, switching means for-establishing a talking circuit between subscribers at two exchanges over a trunk"and"f0r establishinga talking circuit between a subscriber at one exchange and an operator 'at' the other "exchange, means at each exchange -for transmitting a release signal, means ateach exchange responsive to the release signal received for releasing the talking circuit established to a called subscriber; and operator co'ntr'olled means for-'releasingthe talk-ing circuit established-to her position.

MARCEL ROGERMAUGE.

' REFERENCES CITED "The following references are of record in the file of thisp'atent:

UNITED STATES PATENTS Number Name Date 12,119,239 "Meszar May 31, 1938 2306 236 QWalsh Dec. '22, I942

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