US2247055A - Telephone system - Google Patents

Description

. June 24-, 1941. HQVLAND 2,247,055 7 TELEPHONE SYSTEM Filed Nov. 28', 1939 3 Sheets-Sheet 1 I N 5 N TOR H. HOl LAND June 24, 1941. H, HQVLAND 2,247,055

' TELEPHONE SYSTEM Filed Nov. 28, 1939 s Sheets-Sheet 2 PBX FIG. 2

IHI-

' INVENTOR H HOVLAND Patented June 24, 1941 car o STATES PATENT orrlcs TELEPHONE SYSTEM Henry Hovland, Williston Park, N. Y., ass'ignor to Bell Telephone Laboratories, -Incorporated,'New York, N. Y., a corporation of New York K 7 Application November 28, 1939, Serial No. 306,470

14 Claims.

This invention relates to telephone systems and particularly to systems comprising subscriberof trunks therebetween, the assignment of-fewer subscribers numbers to the private branch exchange than there are trunks in the group, and a reduction in the number of level hunting connector switches needed for connecting with trunks to the private branch exchange.

The invention is a trunking arrangement in which each trunk of a group between a central office and a private branch exchange is connected to a set of terminals in the bank of a connector switch and to a set of terminals in the bank of a selector switch, the selector switch being directively responsive to the tens digit of the number assigned to the private branch exchange. Each connector switch having access to the trunks to the private branch exchange is multiplied to terminals in the banks of selector switches which are reached by the hundreds digit of the private branch exchange number and which are directively responsive to the tens digit of this number. In the level of these selectors which corresponds to the tens digit of a private branch exchange number, the first choice terminals, for instance, the first five sets of terminals, are connected directly to five trunks to the private branch exchange; and the last choice terminals, for instance, the last five sets of terminals, are connected to five of the connectors having access to the trunks to the private branch exchange.

A feature of the invention is the provision of selector switches corresponding. to the hundreds digit of numbers assigned to one or more private branch exchanges, each of these selectors having direct access in the level corresponding to, the

tens digit of a private branch exchange number to some of the trunks to the private branch exchange and also having access to connector switches each of which has access to all of the trunks to the private branch exchange. If there are a plurality of private branch exchanges each having a group of connector switches individual thereto, each level of the aforesaid selector switches corresponds to the tens digit of, and

gives access to, one of the private branch ex-' changes. The first choice terminals, for in,-

stance, the first five sets of terminals in each level, are connected directly to trunks to the private branch exchange; and each of the last choice terminals, for instance, the last five sets of terminals, are connected to'one of the connectors having access to all of the trunks to the private branchexchange. The selectors are divided into two or more groups, the selectors in each group-having access to some or all of the connectors associated with each of the private branch exchanges.

Another feature of the invention is a trunk circuit associated with the central office end of a two-way trunk to a private branch exchange, the trunk circuit being connected to terminals in the bflI'lkS'Of connectors, to terminals in the banks of selectors, and to terminals in the banks of line finders. A relay is provided for protecting the trunk against double connections, a low resistance winding being normally connected betweenv the test terminals of the connectors and the test terminals of the selectors and an operating winding being normally connected to the test terminals of the connectors. The connection of ground to the test terminal in the bank of any one of the selectors immediately guards the trunk againstseizure by a connector, and the connection of ground to thetest terminal in the bank of any one of the connectors immediately guards thetrunk again'stseizure by a selector. of seizure of the trunk of a selector, the windings of the-relay are differentially energized in series but the-relay does not operate; it seized by a connector, the energization of the operating winding effects immediate operation so that, if the trunk is simultaneously seized by a selector, the call from the connector is completed but a busy tone istransmitted to the other calling subscriber. A line circuit is associated with each trunk to the branch exchange, the cut-off relay being normally connected in parallel with the operating winding of the differential relay of the associated trunk circuit, the operation of the line relay on a call originating in the branch exchange being effective to connect the operating winding of the differential relay to the testterminal of the trunk in the banks of the line finders.

These and other features of the invention are further described by disclosure of a system in which the invention is embodied, one such system being shown schematically in the drawings which form-a part of this specification; The invention is, of course, not limited in its application to the particular system shown nor to switches of thetype shown but generally an- In case plicable to all automatic switching systems comprising an automatic central office and a private branch exchange.

Referring to the drawings:

Fig. 1 represents a central office CO comprising a subscribers line Li, line finder switches LF| and LF-Z, selector switches S|, S-2 and 8-3, connector switches C| and LHC, a line circuit LC and a trunk circuit T-J associated with a trunk outgoing to a private branch exchange;

Fig. 2 shows a private branch exchange PBX comprising a trunk circuit T--2 associated With a trunk to the central ofiice, a line-finder switch LF, a selector S, a connector C and acord ciri cuit CD at an attendants position; and

Fig. 3 shows the manner in which ten trunk circuits between a central ofiice and a private branch exchange are multipled to terminalsin the banks of both selectors and level hunting connectors.

The line finder, selector and connector switches are of the well-knowntwo-motion step-by-step type and reference'may' be had to pages 53. to 65 of' the second edition of Automatic Telephony by Smith and Campbell for a description of the structure ofsuch switches and their operation when used as selectors or connectors. Reference may be had to the R. L.- Stokely Patent No. 1,799,654, April '7, 1931, for-disclosure of a twomotion step-by-step switch arranged for use as a line finder. Each of the switches LF -i, LF2, LF, SI,' CI and LHC are'represented in the drawings bya set of brushes and a singleset of terminals. The selector S2'is represented ing scheme and the total'number of subscribers.-

The selector S-3 representsthird selectors in an office in' which" onlyfirst selectors, second selectors and connectors-are needed-for complet-- ing connections between callingand called lines,

the'thirdselectors being-provided for giving directaccess to trunks" leading to'private branch exchanges which are large enough to require the use of level hunting connectors in which the entire bank serves a single private branch exchange;- An idle third selector S3 is seized in response to the dialing, ofthe hundreds digit of the number of "any one of as many as ten private branch exchanges. This selector is directively operated'infresponse to the tens digit of the called number to select a level in which the first choi'ce'sets of terminals, such as H, are connected directly to trunks leading to the private branch exchang and thelast choice sets of terminals such as l2 are'connected to level hunting connectors L'HC which have access to all of the trunks leading to the called private branch exchange. Each of the level hunting connectors, which may be similar to that disclosed in the patent to H. Hovland No. 1,747,169 of February 18, 1930, are accessible to some or all of the third selectors 8-3. These selectors are divided into groups, each group having access to certain of thelevel hunting connectors as is more particularly illustrated in Fig. '3. In' this figure there are ten trunkcircuits, five of which connect to the first five sets'of terminals in a level of the banks of a first subgroup of third selectors, the other five trunks being connected to th first five sets of terminals in the same level of the banks or a second subgroup of third selectors. Each of the ten trunks is also connected to a set of terminals in the banks of all of the level hunting connectors, five of which are shown in this figure. Each of the selectors in the first subgroup have first choice access to the trunk circuits T-! l to Tl5 and second choice access to the five level hunting connectors LHC-l to LHC-5, and each of the selectors in the second subgroup have first choice access to the five trunk circuits Tl6 to T-I0 and second choice access to the same five level hunting connectors LHC| to LHC'5.

v If more than fiv connectors are required, they may be divided into groups, each group accessible to some of the third selectors.

The trunks between the central ofiice and the private-branch exchange may be two-way trunks and the trunk circuits'T'l and T2 shown in the drawings are arranged for two-way service. On a call from a central office subscriber, the trunk T| may be seized eith r by a selector S3 o'r by a level hunting-connector LHC; and, in either case, seizure effects-the lighting'of an answering lamp AL associated with'a jack J at the attendants position in the private branch exchange. On acall from the private branch exchange to the central oflice' the trunk circuit T2 may be seized-eitherby the insertion of the plug of an operators'cord- CD in jack J or by the selection of this trunk circuit by a selector S in response to the dialing of the first digit at a calling station in the private branch exchange. Theseizure 'of'the trunk circuit T-2 effects the operation of the line circuit LC in the central office thereby connecting the trunk through a line finder LF2 to an associated first selector, not shown in the drawings; and the connection is'completed by selector and connector switches inresponse to the dialing of the number of the central ofiice subscriber.

Assume first that the subscriber A in the central office C0 calls a private branch exchange accessible to the'selector S-'3.' In'usual and wellknownmanner, the callingline L--I' is connected through a line finder switch LF-I to a first selector S-l which isdirectively operated by the dialing of the first digit of the number of the private branch exchange to select a corresponding" level and automatically operated to seize a set of terminals therein connected to an idle second selectors-4. The-second selector S-Z responds to-the'second digit dialed, which is the hundreds digit, toselect a corresponding level and a set of terminals therein leading to an idle third selector S-3. The seized selector S3 is directively operated in response'to the tens digit of the called subscribersnumber to select a correspondinglevel and t'oa'utomatically hunt ther in, Assume that one of the first'choice sets of terminals, such as H, is idle and is seized by. the selectors-land that this set of terminals is connected to the trunk circuit T- I. Upon seizure ofterminals H, a circuit is closed for operating the line relay 2| of trunk circuit T!. This circuit is traced'from battery through the upper winding of relay 2|, aback contact of relay 24 conductor'M, through the middle brush of each of switches S 3, S2, S! and LF'I, through the calling subscribers line and station, back through the upper brushes of each of switches LF--I, S-'-l, S-'2 and 3-3, conductor 3, a back contact of relay, 24,,and throughthe lower winding of relay 2! to ground: :Relay 2| closes a circuit for operating relay '22.. Relay-i22 also connects ground through its inner upper front contact, the upper back contact of relay 3|, the outer lower front contact of relay 22 in parallel with the lower back contact ofrelay 23,'upper.

winding of relay 29, inner lower backcontact of relay 38, thence through the lower winding of relay 29 to battery. Relay 29 does not operate because its upper and lower windings areoppo-i sitely energized. The inner lower back .contact of relay 35 is also connected through the inner upper back contact of relay 35, and through conductor 49 to the test terminals in the banks-of level hunting connectors having access to trunk circuit T--l.

connected to these test terminals is effective :to guard against seizure oftrunk circuit T l ,by any of these connectors.

The aforementioned operation of relay 2| also: closes a circuit for operatingrelay 35; andrelay 25 looks to ground at the back contact of relay private branch exchange, the winding of relay,

53 being normally bridged across the conductors of trunk T. Relay 53 closes a circuit foroperating relay 56. Relay 56 closes a circuit for operating relay 51. Relay 5'! connects ground to conductor T3, to mark the trunk circuit T- -2 busy; in

the banks of all selectorsS having access to this; trunk circuit, and connects battery through re-u sistor 50 to the sleeve of jacks J. to markthe trunk circuit T2 busy at each of these jacks.v

When the calling subscriber dials the next digit of the private branch exchange number, relay 2| is alternately released and reoperated in response to each of the impulses in' the train. Relay 22 is,- slow to release and remains operated during the response of relay 2i to -dial impulsesr The release of relay 2!, in response to thefirst impulse,

closes a circuit for operating relay-23. Relay 23 is a slow-to-release relay which remains operated until all of the impulses in the trainhave been received, Relay 23 closes a circuit, including, its outer lower front contact and an upper back contact of relay 3%, for operatively energizing the winding of relay 28. Relay23 closes a locking circuit through its inner lowerfront contact toground at a front contact of relay 22. At its middle upper front contact, relay 28 connects ground to conductor 46, operatively energizing the winding of cut-off relay 42 of line circuit LC. The connection of ground to conductor 45 shortcircuits the upper winding of relay 29 and maintains the energization of the lower winding of relay 29. Relay 2% is thereupon operated by its lower winding, closing a circuit for operating relay 25. Relay closes a circuit for operating relay and closes a short circuit around the winding of relay 21. Relay 30 opens the operating circuit of relay 28, closes a connection from ground through the middle upper front contact of relay 2%, conductor 46, the inner lower front contact of relay 25, upper front contact of relay 2!], to conductor 49 to continue the marking of the trunk circuit T-l as busy in thebanksof the level hunting connectors. Relay 3flalso opens Since the upper winding of relay 29 is a low resistance winding, the potential thus the connection between the'two windings of relay 29;and connect ground through its innerlower front contact, the lower back contact of relay 23 in parallel with the outer lower front contact of relay 22, and through the upper back contact of relay 3!, to conductor l5. The Ope on of relay 30 also causes the release of relay 35; and the release of relay 35 disconnects battery and ground from the conductors of trunk T, releasing relay 53. Relay 53 opens the operating circuit of relay 52 but relay 56 is slow in releasing and, before it can release, another operating circuit is closed by the operation of relay 54 as hereinafter described, With relay 28 operated and relay 23 released, a circuit is closed from the grounded, ring-- ing current generator 32, through the inner upper front contact of relay 2%, lower winding of relay 26, upperback contact of relay 23, lower back contact of relay 26, lowermost front contact of relay 28, lower conductor of trunk T, conductor 52 of trunk circuit T.-2,'windin g of relay 53in parallel with winding of relay 54 and condenser 55 in series, through upper back contacts of relay 61, an upper back contact of relay 52, conductor 5|, the upper conductor of'trunk T, uppermost front contact of relay 2%, to ground at a back contact of relay 25. While this ringing circuit is closed, ringing tone is transmitted through condensers land is and-conductor M to the calling subscriber. The ringing current operatively energizes the winding of relay 5d and may cause relay 53 to vibrate. Relay 53 will be operated by direct current from the tripping battery during the silent part of the ringing cycle. Relay 54 closes a circuit through the winding of relay 56 and so also does relay 53. Relay 56 maintains the circuit for operating relay 5'5. With relays 5t and 51 both operated, a circuit is closed for operating relay 58; and relay 58 closes a circuit for lighting the answering lamp AL. Relay 58 also closes a circuit for operating relay 52; and relay 59 closes a circuit through the lower back contact of relay 68 for locking relay 58.

When an attendant at the private branch exchange answers the call by inserting the plug of a cord CD in jack J, a circuit is closed at the jack for holding relay 51 operated; and a circuit is closed through the tip and ring springs of the jack, middle lower front contact of relay 51 and outer back contacts of relay 6'? for operating relay 6|. Relay 6| closes a holding circuit for relay 56, closes a circuit for operating relay 62, and connects ground toconductor 1%. Relay 52, which is slow in operating, opens the shunt throughresistor 64 across the winding of relay 65, and completes a circuit path from conductor 5| through the winding of relay 55, inner upper and outer lower front contacts of relay 62, to conductor 52. The winding of relay 55 is thus bridged across the conductors of trunk T and ground is connected through resistor. 69 and a lower front contact of relay 52 to conductor 52 to cause an increase in'the current through the lower, tripping winding of relay 26 of trunk cir cuit Tl. Relay 62 also closes a circuit for operating relay 53. Relay 63 locks independent of relay 58 and connects the winding of relay 65 across conductors 5i and 52 independent of relay 62; Relay 25 is operated suficiently to close its inner upper front contact; and the energization of its upper, locking winding completes the operation of relay 25. Relay 26 opens the ringing circuit and connects relay 24 to'the conductors of trunk T; and relays 24 and 65,

operate in series through'resistor 69 to ground at relay 56.. Relay 65 closes a circuit for operating relay 56; and relay 66 disconnects the tripping ground from conductors 5| and 52 and closes circuits for operating relays 61 and 68.

Relay 6? closes a circuit for holding relay 56 operated and opens the circuits through the windings of relays 6| and 53. Relay 6| releases relay 62, reconnecting resistor 64 in shunt with denser 55 remain bridged across the trunk. Withrelay 26 of trunk circuit T-| and relay 61 of trunk circuit T2 both operated, a talking con-' nection is completed from the calling line through condensers I! and I8 of trunk circuit TI, the outer front contacts of relays 26 and 28, the conductors of trunk T, conductors 5| and 52 of trunk circuit T2, winding'of relay 65, outer front contacts of relay 6?, a front contact of relay 51, through the tip and ring co'n-' ductors of jack J and plug of cord CD, to the operators telephone set.

When the calling subscriber releases the connection, relay 2| releases, closing the circuit for' operating relay 23. The continued release of relay 2| causes the release of relay 22. Relay 22 disconnects ground from conductor I5 thereby effecting the return to normal of switches LF-l, S|, S2 and S3 in usual and wellknown manner. The release of relay 22 also effects the release of relays 23, 26 and 28. The release of relay 26 disconnects relay 24- from the trunk thus releasing relay 65 of trunk circuit T2 and causing a disconnect signal in the operators cord circuit CD. The release of relay 65 causes the release of relay 66. The release of relay 66 causes the release of relays 61, 68 and 56. the operator disconnects the cord from jack J.

When the plug is removed from jack J, relay 51 releases and the busy ground is disconnected from conductor 13. The release of relay 28 of trunk circuit T-| causes the release of relays 29 and 25 in succession. The release of relay 25 opens the energizing circuit of the slow-torelease relay 30. branch exchange has released the trunk in response to the disconnect signal, relay 3!] releases and the trunk cincllit T2 is in normal condi tion ready for use on another call. 7

If the operator at the privatebranch exchange fails to remove the plug of cord CD from jack J in response to the disconnect signal, relay 6| reoperates when relay 6! releases; and relay 6| holds relay 56 operated and reoperates relay 62.

Relay 6| connects ground potential to conductor 13 to prevent seizure of the trunk circuit T2, bridges the winding of relay 65 across conductors 5| and 52, and also connects ground at the back contact of relay 66 through resistor 69 and conductor 52 to the lower conductor of trunk T;

thereby operating the line relay 4| of the line circuit LC associated with trunk T in the central c-fiice. Relay 65' does not reo-perate since the upper conductor of trunk T is not connected to ground in the central ofiice. I trunk circuit Tl operates in series withthe line relay 4|, the short circuit across the'w'ind ing of relay 21 having been opened bythe release of relay 25.

release of relay 25 does not cause' the release Relay 51 is held to ground at jack J until" If the operator at the private Relay 21 of The operation of relay 27- closes a holding circuit for relay 30-'so' that the of relay 30. Withrelay 30 operated, ground potential is connected through its inner lower front contact, lower back contact of relay 23, and upper back contact of relay 3| to conductor |5 to prevent seizure of trunk circuit T2 by any of the selectors S3, and ground potential is connected through the lower back contact of relay 25 and upper front contact of relay 36 to conductor 49 to prevent seizure of trunk circuit T2 by any .ofthe. level hunting connectors LHC. The line relay 4| connects the Winding of the cut-off relay 42, .throughconductor 43 to test terminals in the banks of line-finder switches LF-2 having access to the trunk T. Relay 4| also closes a circuit .for operating a group relay 46 and relay ,4'0 initiates the operation of an idle one of the line. finders to connect the trunk T to a first selector in usual and Well-known manner. When a line finder finds the terminals of the calling trunk, ground is connected to the test terminal in usual manner to efiect the operation of the cut-off relay 42;.and the operation of relay 42 causes the release of relays 4| and 46. The connection of ground to the test terminal also causes the energization of the operating winding oflrelay 29a Relay 29 closes the circuit for operating relay 25; and relay 25 closes the circuit for operating'relay 30 and closes the short circuit around the winding of relay 21. Relay 30 continues to connect busy ground to conductorl5- and connects the test terminal of the line finder to the test terminals in the banks of the connectors. The trunk T and associated trunk circuits are thus held until the attendant at the private branch-exchange removes the plug and a permanent signal alarm may be operated in the central oiiice; When the plug is removed, thereleas'e of relays 6| and 62 causes the release of relays 56 and 5'! in succession. The release of relay 51 disconnects guarding potential irom conductor 13' and from jacks J. Relay 62- disconnects 'resistor 69 from conductors 5| and 52 thereby causing the release of the first selector and line-finder switches in the central oifice. 'When ground is disconnected from the test terminal of the line finder, relays 42 and 29 release; 'The release of relay 29 causes the successive release of relays 25 and 30 thereby disconnecting the busy ground potential from conductors l5 and 49. The trunk T is now free for seizure at either end. I

If a selector S'3 is operated to select the level including trunks T to a private branch exchange and all 'of the first choice sets of terminals are busy, the brushes continue to hunt over the second choice sets of terminals until an idle level hunting connector LHC is found. The selected level hunting connector receives the last digit ialed' by the calling subscriber and is operated in well-known manner to select an idle trunk T to the private branch exchange. Upon seizure oftru'nk T by a level hunting connector LHC, groundpotential is connected, in usual manner, through its test brush to conductor 49 to guard against seizure by any other level hunting connectorjand is further connected through the upper low're'sistance winding of relay 29 to conductor |5 to guard against seizure of the trunk by any of the selectors. This ground connection also causesthe operation of cut-off relay 42 and causes the successive operation of relays 29, 25 and 3D. Relay 42 prevents the operation of line relay 4| Relay 30 connects ground potential directlyto conductor l5 as hereinbefore described toprevent' seizure of trunk circuit Tl by any of the selectors S-3. The connector LHC transmits ringing current over trunk T; and the trunk circuit T2 operates as hereinbefore described to light the answering lamp AL and mark the trunk circuit T2 busy. When the call is answered by an attendant at the private branch exchange, the trunk circuit T-2 functions as hereinbefore described and the talking connection is completed by the level hunting connector in usual manner. When the calling subscriber releases the connection, the connector and preceding switches in the central office are returned to normal and the trunk circuits T-l and T'-2 are restored to normal.

If, on a call from the private branch exchange to the central ofilce, the trunk circuit T-2 is seized by an attendant, the insertion of a plug in jack J causes the operation of relays 51' and 6|. If the trunk circuit T-2 is seized by a selector S in the private branch exchange, relay El operatesin serieswith the calling extension. In; either case. relay 6| closes .circuits for operating relays 55 and 62 and connects ground potential to conductor 13 to prevent seizure of trunk circuit T2 by a selectorIS. 'Relay'56 closes a circuit for operating relay 51. and relay 51 connects busy. potential'tothe sleeves of jacks J. With yrelay E2 operated, ground is connected through resistor E9'to conductor. 52 to effect the operat on of the line relay 4| in the central'office and extension of the trunk through a line finder to a first selector. Relay 2'! of trunk circuit T.--! is operated in series with relay H, closing the circuit for operating relayBll. Relay 39 connectsbusy-marking, ground potential to conductors l5 and 49 as above described. When a l ne finder finds the terminals of trunk T, ground is connected through the test brush, operating the cut-off relay 427to release relays 40 and 4!. Relay 42 also extends this ground potential through its left front contact toconductor 46 and the lower winding of relay 29. Relay 28 operates closing the circuitfor operating relay 25 and relay 25-short-circuits relay 21. With relays 25 and Sllyboth operated, the

ground connected through the test brush of the line finder LF-fZ to conductor 46 is extended through conductor 49-to the test terminals-in the banks of the level hunting connectors. Upon connection of trunk T through the brushes of a l ne finder LF--2 to the line-relay. (nct'shown) of an associated first selector, relay B5 of trunk circuit T2 is operated Relay, 65 closes the circuit for o erating relay 66.: Relay 65 disconnects' ground from conductors 5i and 52 and loses the circuits for operating relays 1i! and 6%. Relay 6'! connects the conductors of the trunk to the calling operator or calling extension, causes the release of relays BI and 62. The release of relay 62 reconnects resistor 64 in parallel with the winding of relay fi fi and opens the short circuit across conductors 5I and152 thereby clearing the connectionrfor transmission of dial impulses to theswitches' in the-central office. Relay 65 may follow dial impulses; "but, being slow in releasing, relay BG-hclds relays 61 and 68. The impulses, transmitted from the calling operators or subscribers dial, eifect the operation of selector and -connectorswitches in the central office to complete theconnection. If the call is to a subscriber's line. the connection is under the control of the calling subscriber or operator in the private branch exchange. If the call is to an operator. the connection is under the joint control of the called operator and the calling subscriber or operator, in which case release only by the calling subscriber or operator effects the successive release of relays 65, 66, 61 and 68. The release of relay 8'! bridges the winding of relay 53 across the trunk to hold the switches in the central oifice. Relay 53 operates, closing a circuit for holding relay 56; and relay 56 holds relay 5'! to maintain the busy-marking of trunk circuit T2 and hold the switches in the private branch exchange. When the calling subscriber again removes the receiver or the attendant again inserts the plug of a cord, relays 6i and 62 are reoperated, relay 53 releases, relays 65, 65, 61 and 68 reoperate, and relays BI and 62 again release. When the connection is released by the calling station or attendant in the private branch exchange and also by the called operator in the central office, the line finder, selector and connector switches in the central office are returned to normal. and ground is disconnected from conductor 43 to release the cutofi relay #2. Relays 29, 25 and 30 of trunk circuit T! are thereupon released. In the private branch exchange the release of relays 65, 65, 6! and (-38 effects the release of relays 56 and 5'! and the disconnection of ground potential from conductor l3 and the disconnection of guarding potential from jacks J.

If the trunk circuit T'2 is seized at substantially the same timeby both a selector 8-3 and a level hunting connector LHC, relay 29 is op erated by the ground connected to conductor 49 by the level hunting connector. The operation of relay 29 causes the operation of relays 25 and 3B. Relay 2! is operated in series with they line 'of the subscriber connected to trunk circuit T2 through a selector S-3.

Relay 2| closes the circuit for operating relay 22. The call incoming to trunk circuit T--! from the level hunting connector may be completed but abusy tone signal is transmitted from tone source 20 through the inner lower front contact of relay. 2!, inner upper front contact of relay 25, lower back contact of relay 2%. condenser I3 and conductor Id to the subscriber calling through the selector S- 3. If this subscriber dials, relay 28 cannot be operated since its operating circuit is open at the outer upper back contact of relay 3!). When the subscriber receiving the busytone releases, relays 2! and 22 release and relay 23 is temporarily operated. When relay 22 releases. ground is disconnected from conductor I5. to release the selector S-3 and, preceding switches. when relay 23 releases, ground is reconnected to conductor l5 to prevent seizure by another selector.

}er. 32,.the brush of which is momentarily con- ,.nected to batteryonce every thirty seconds. As

soon. after the release of relay 24 as the interrupter 32 connects battery to relay 3!, relay 3| operates and locks independent of the interrupter.

Relay 31 connects ground to the conductor leading to the alarm. 34 and connects ,interrupter 33 to the Winding of relay 28. As soon afterrelay it operates asthe brush of interrupter 33 is connected to ground, which may be eighteen seconds after the brush of interrupter 32 is connected to battery, the winding of relay 28 is short-circuited and relay 28 releases. The release of relay 2% disconnects relay 24 and talking condensers l1 and I 8 from the trunk thus preventing the call from the selector from interfering with the seizure of trunk T by a level hunting connector or from interfering with calls from the private branch exchange to the central oflice. If and when the subscriber calling through the selector releases, relays 2| and 22 release and relay 23 operates temporarily. The release of relay 22 causes the release of relay 26 and relay 26 causes the release of relay 3|.

Each time the trunk circuit TZ is seized by a' selector S3, the momentary operation of relay 23, when the connection is released, closes a circuit for operating the register 39.

Although in the arrangement shown in the drawing the trunks to the private branch ex change are connected to terminals in the banks of selectors and level hunting connectors in the central office, the invention is applicable to an arrangement in which the trunks are connected to the banks of rotary connectors instead of level hunting connectors.

What is claimed is:

1. In a telephone system, subscribers lines, a central oflice, a private branch exchange, a group of trunks between said oflice and exchange, selectors and connectors in said central oflice, and means for connecting a calling line with a selector, the trunks in said group being multipled to first choice sets of terminals in a level of the bank of each of said selectors and to terminals in the banks of said connectors, each of the remaining sets of terminals in said level of the banks of said selectors being connected to one of said connectors.

3. In a telephone system, an automatic ofiice comprising lines, subscribers stations, a dial at each station, selectors, trunk hunting connectors, and means for connecting a calling line with a selector, a private branch exchange, a group of trunks between said ofiice and said branch exchange, each of said trunks being connected to first choiceterminals in a level of the banks of selectors and to terminals in the banks of trunk hunting connectors, each of said connectors being connected to other terminals in said level of the banks of selectors, means in each of said connectors for transmitting ringing current over any of said trunks, means in the branch exchange for each trunk for marking the trunk busy in the branch exchange in response to ringing current from the central'office, and means for each trunk for marking the trunk busy in the branch exchange in response to seizure of the trunk by one of said selectors.

4. In a telephone system according to claim 3, means for each trunk effective upon seizure of the trunk by one of said selectors for marking the trunk busy to the other selectorsand to said connectors, and means for each trunk eifective upon seizure of the trunk by one of said connectors for marking the trunk busy to the other connectors and to said selectors.

5. In a telephone system, an automatic office comprising lines, subscribers stations, a dial at each station, selectors, trunk hunting connectors, and switching means for connecting a calling line with a selector, a private branch exchange, a

group of two-way trunks between said oflice and said branch exchange, each of said trunks being connected to first choice terminals in a level of the banks of selectors in the central office and to terminals in the banks of said trunk hunting connectors, each of said connectors being connected to terminals in said level of the banks in the central office for closing a circuit through the-trunk to" operate said circuit means, means in each trunk circuit efiective upon seizure by a selector in the central ofiice for absorbing the dialimpulses' thereafter created by the dialing of any remaining digits of the branch exchange number by the calling subscriber, and means in each trunk circuit effective after the absorption of said dial impulses 'for transmitting ringing 'curr'ent over said trunk to operate the signal means associated with said trunk in the branch exchange and to maintain the marking of said tru'nk'as busy 'in' the branch exchange.

'6. Ina telephone system, an automatic office comprising lines, subscribers stations, a dial at each station, selectors, trunk hunting connectors, and switching means for connecting a calling line with a selector, a private branch exchange comprising selectors and a manual switchboard, a group of two-way trunks between said office and saidbranchexchange, each of said trunks being connected to a jack at said switchboard, to

terminalsinthe'banks of the selectors in the branch exchange, to first choice terminals in a level of 'thebanksof selectors in the central office and to terminals in the banks of said trunk hunting connectors, each of said connectors being connected to te'rminals'in said level of the banks 'of selectors, trunk circuits in said oifice one for each of said trunks, signal means for each of said trunks in said branch exchange, cords in the "branch' exchange for'connecting with said jacks, circuit means at said branch exchange for each trunk for marking the trunk busy in the branch exchange, means in each of said trunk circuits responsive to seizure by any one of said selectors in the central office for closing a circuit through the trunk to operate's'aid circuit means, means in 'ea'ch trunk circuit effective upon seizure by a selector-in the central 'ofiice for absorbing the dial impulses thereafter'cre'ated by the dialing of any remaining digits of the branch exchange number by the calling subscriber, and means in 'each trunk circuit effective after the absorption 'of said dialimpulses for transmitting ringing current oversaid trunk to operate the signal means'associateaj with said trunk in the branch exchange and to ma'intain'the marking of said trunk as busy in the branch exchange.

7.I'n a telephone system according to claim 6',

a source of talking current in each trunk cirf'cjuit, "me'ans in each trunk circuit responsive to the methane: a cord with the jack of the 'triinkf'tofanswer'the call for disconnecting the r'inging'current source and closinga talking circuit, the closure of said talking circuit being e'fie'ctive to maintain the marking of said trunk as busy in the banks of the 'selectors'in the branch exchange. 1

8. In a telephone system, an automatic central office comprising lines, subscribers stations, a dial at each station, selectors, and trunk hunting connectors, a private branch exchange, trunksbetween said office and said exchange, each of said trunks being connected to terminals in a level of the banks of selectors and to terminals in the banks of trunk hunting connectors; each of said connectors being connected to a-set of terminals in said level of some of said selectors, means for connecting a calling line wasuecror, means for each of said trunks responsive to seizure by any one of said selectors for closing a signaling circuit over said trunk, means in said branch exchange for each of'said trunks for marking the trunk busy, means in the branch exchange for each of said trunks for operating the busy-marking means responsive to closure of the signaling circuit upon'seizure of the trunk by a selector, means in said, automatic ofiice for each of said trunks for absorbing impulses'transmitted from the dial at a calling station and for thereupon connecting a source of ringing ,cur-. rent to the trunk, a calling signal in the branch exchange for each of said trunks, means in the branch exchange for each of said trunks responsive to ringing current from the centrali office for operating the calling signal and for operating the busy-marking means, means in the branch exchange for seizing any one of said trunks onv a call to the central ofiice, andmeans in the central ofiice for each of said trunks re sponsive to seizure of the trunkby a selector or a connector in the central office and responsive to seizure in the branch exchange for marking the trunk busy in the central oflice.

9. In a telephone system, an automatic central office comprising lines, subscribers stations,-a dial at each station, line finders, selectors, and trunk hunting connectors, a private branch exchange, trunks between said office and said exchange, each of said trunks being connected to terminals in a level of the banks of selectors, to terminals in the banks of trunk hunting'connectors and to terminals in the banks of line finders, each of said connectors being connected 'to a set of terminals in said level of some of said selectors, line circuits in said ofiice, one for each of said trunks, means for connecting a calling line to a selector, means for each of said trunks responsive to seizure by any one of said selectors for closing a signaling circuit over said trunk, means in said branch exchan e for each of said trimks for marking the trunk busy, means in the branch exchange for each of said trunks foreperating the busy-marking means responsive to closure of the signaling circuit upon seizure of the trunk by a selector, means in said automatic ofiioe for each of said trunks for absorbing impulses transmitted from the dial at a calling station and for thereupon connecting a source of ringing current to the trunk, a calling signal in the branch exchange for each of said trunks, means in the branch exchange for each of said trunks responsive to ringing current from the central office for operating the calling signal and for operating the busy-marking means, means in the branch exchange for seizing one of said trunks on a call to the central oflice, and means in the central office for each of said trunks responsive to seizure of the trunk by a selector or a connector in the central office and responsive to seizure in the branch exchange for marking theztrunk busy in the central office.

'10. In atelephone system, an automatic onice comprising lines, subscribers stations, a dial at each station, selectors, level hunting connectors,

and means for connecting a calling line to a,

selector, a private branch exchange, trunks between said oflice and said branch exchange, each of said trunks being connected to first choice terminals in a'level of the banks of some of said selectors and to terminals in the banks of all of said connectors, each of said connectors being connected to terminals in said level of some of said selectors, trunk circuits in said office, one for each of said trunks, trunk circuits in said exchange, one for each of said trunks, means in each of said trunk circuits at said office responsive to seizure by any one of said selectors for marking said trunk busy to the other selectors and to said connectors and for transmitting a seizure signal to the branch exchange, means in each of said trunk circuits at said ofiice responsive to seizure of the trunk by any one of said connectors for marking the trunk busy to the other connectors and to said selectors, means in each of said connectors for connecting a source of ringing current to a seized one of said trunks, busymarking means in each of said trunk circuits at the branch exchange responsive to a seizure signal transmitted from the trunk circuit at the central office end of the trunk, and means in each of said trunk circuits at the branch exchange responsive to ringing current from the central office.

4.11. In an automatic telephone ofiice comprising dial subscribers lines, selectors, trunk hunting connectors, trunks and trunk circuits, one for each of said trunks, connected to terminals in the banks of selectors and connectors, each of said connectors being connected to terminals in the banks of some of said selectors, and means in each of said trunk circuits responsive to seizure of the trunk circuit by a selector or connector for marking the trunk busy in the banks of the selectors and connectors, said last-mentioned means comprising a relay having a winding normally connected between the test terminals of the trunk in the banks of said selectors and the test terminals of the trunk in the banks of said connectors, said winding having a low resistance so that the connection of a busy-marking potential to either side of said winding upon seizure of the trunk by a selector or a connector marks the trunk as busy in the banks of all of said selectors and connectors, said relay having another winding normally connected in series with the low resistance windingto the test terminals of the trunk in the banks of said selectors, said windings being difierentially and non-operatively energized upon seizure of the trunk by a selector, said other winding being operatively energized upon seizure of the trunk by a connector, and means controlled by the relay for continuing the busy-marking of the trunk in the banks of the selectors and connectors.

12. In an automatic telephone onice comprising dial subscribers lines, line finders, selectors, trunk hunting connectors, trunks and trunk circuits, one for each of said trunks, connected to terminals in the banks of line finders, selectors and connectors, each" of said connectors being connected to terminals in the banks of some of said selectors, and means in each of said trunk circuits responsive to seizure of the trunk circuit by a line finder, selector, or connector for marking the trunk busy in the banks of the selectors and connectors, said last-mentioned means'comprising a relay having a Winding normally connected between the test terminals of the trunk in the banks of said selectors and the test terminals of the trunk in the banks of said connectors, said winding having a low resistance so that the connection of a busy-marking potential to either side of said Winding upon seizure of the trunk bya line finder, selector or connector marks the trunk as busy to all of said selectors and connectors, said relay having another Winding normally connected in series With the low resistance Winding to the test terminals of the trunk in the banks of said selectors, said windings being differentially and non-operatively energized upon seizure of the trunk by a selector, said other Winding being operatively energized upon seizure of the trunk by a line finder or a connector, and means controlled by the relay for continuing the busymarking of the trunk in the banks of the selectors and connectors.

13. In a telephone system, an automatic oflice, a private branch exchange, subscribers lines, a group of two-Way trunks between said oflice and said exchange, selectors and trunk hunting connectors in said automatic office, said trunks being multipled to first choice terminals in a level of the banks of said selectors and to terminals in a level of said connectors, each of the other terminals in said level of the selectors being connected to one of the connectors, trunk circuits, one for each of said trunks, means for connecting a calling line in said automatic oifice with an idle one of said selectors, means comprising a dial at the calling subscribers station for directively operating the associated one of said selectors to select said level, means in said selector for seizing the first idle set of terminals in said level, means in each of said connectors responsive to a train of dial impulses from a connected calling station for operating the connector to seize an idle one of said trunks, means in each of said trunk circuits effective upon seizure by said selector for absorbing the impulses thereafter created by the dialing of the remaining digits of the number of said private branch exchange, means in each of said trunk circuits effective only upon seizure by one of said selectors for immediately sending a seizure signal to said private branch exchange, selectors in said branch exchange, said trunks being multipled to terminals in the bank of said selectors in the branch exchange, means in said branch exchange for each trunk responsive to said seizure signal from the trunk circuit in said oflice for marking said trunk busy in the banks of said r selectors in the branch exchange, means in each of said trunk circuits effective only when seized by a selector and after receiving the dial impulses of said remaining digits for connecting a source of ringing current to the trunk, means in each of saidconnectors for connecting a source of ringing current to a seized one of said trunks, an operators position in said branch exchange, signaling'means for each of said trunks at said operators position operatively responsive to ringing current from the automatic office, means comprising a cord for answering the call, alarm means common to said trunk circuits, and means ineach of said'trunk circuits for operating said alarm means in the event that a calling subscriber fails to release the connection over a trunk to the branch exchange Within a predetermined interval of time after the trunk is released by the answering operator.

14. In atelephone system, an automatic oilice, a private branch exchange, subscribers lines, a group of two-way trunks between said office and said exchange, selectors and trunk hunting connectors in saidautomatic office, said trunks being multipled to first choice terminals in a level of the banks of said selectors and to terminals in a level of said connectors, each of the other terminals in said level of the selectors being connected to one of the. connectors, trunk circuits, one for eachof said trunks, means for connecting a calling line in said automatic ofiice with an idle one of said selectors, means comprising a dial at the calling, subscriberfs station for directively operating the associated-one of said selectors to select said level, means in said selector for seizing the first idle set of terminals in said level, means in each of said connectors responsive to a train of dial impulses from a connected calling station for operating the connector to seize an idle one of said trunks, means in each of said trunk circuits effective ,upon seizure by said selector for absorbing the impulsesv thereafter created by the dialing of the remaining digits of the number of said private branch exchange, means in each of said trunk circuits effective only upon seizure by one of said selectors for immediately sending a seizure signal to said. private branch exchange, selectors in said branch exchange, said trunks being multipled to terminals in the bank of said selectors in the branch exchange, means in said branch exchange for each trunk responsive to said seizure signal from thetrunk circuit in said ofiicefor marking saidtrunk busy in the banks of said selectors in the branchexchange, means in each of said trunk circuits effective only when seized by a selector andafter receiving the dial impulses of said remaining digits for connecting a source of ringing current to the trunk, means in each of said connectors for connecting a source of ringing current to a seized one of said trunks, an operators position in said branch exchange, and signaling means for each of said trunks at said operators position operatively responsive to ringing current from the automatic office.

HENRY HOVLAND.

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