US2408433A

US2408433A - Telephone system

Info

Publication number: US2408433A
Application number: US475745A
Authority: US
Inventors: William A Malthaner
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1943-02-13
Filing date: 1943-02-13
Publication date: 1946-10-01
Anticipated expiration: 1963-10-01

Description

Oct. 1, 1946. w. A. MAL'VI'HANER EELEPHONE SYSTEM Filed 'Feb. 13, 1943 14 Sheets-Sheet 1 R E M m NT EL VA mu A V our 2u\.# u 2: V t a 2st Set mum. k at at TEE u at vet $2. nut \Ei A Ti'ORNEY Oct. 1, 1946. w. A. MALTHAN'ER TELEPHONE SYSTEM I Filed Feb. 13, 194-5 14 Sheets-Sheet 5 INVEN W A. MALTHA/VER Oct. 1, 1946. w. A. MALTHANER. 2,403,433

' TELEPHONE SYSTEM Filed F915. 13, 1943 I 14 Sheets-Sheet 4 CALL VALLOTTER CCZ' 3 i I 5 Y Y INVENTOR I W14. MALTHANER I By D h A Tram/5K 0 1, 9 WQAIMA THANE'R 2,408,433

TELEPHONE SYSTEM Filed Feb; 15,1943 14 Sheets-Sheet 5 641.1. 41.1.0 rrm INVEN TOR I n14 MALTHANER {hi w A TTORWEY Oct. 1, 1946; W. A. MALTHANER 8,

- TELEPHONE SYSTEM I Filed Feb. 13, 1943 14 Sh'eets-Sheet 6 in! ,4. MAL THANER BY ATTORNEY Oct. 1, 1945.

w.- A. MAL'ri-IANER TELEPHONE sYsT Filed Feb. 13, 1943 14. Sheets- Sheet 8- Y [NI/ENTER MA. MALTHANER A r ms/EV Oct 1946. w. A; MALTHANER f 2,408,433.

- TELEPHONE SYSTEM Fil ed Feb. 13, 1943 14'Sheets-Sheet 10 J j I w 1 i 1 I 2" 2 s g r vol.

LIN

ENAQLER A c PULSE kEcE/vER.

INVENTOR It A. MALTHANER A from/Er Oct. 1,1946. w. A. MALTHANE R TELEPHoi fisYsTEM Filed Feb. 13, 1945 14 Sheets-Sheet 11 Lfl 4 mi Ta llll lllll 1 VVENILOR illll llllll llllH llll llllll-ll-ll llllllllllll n! A. MALTHANER BK rrow/5k 06b1, 1946-- w. A. MALTHANE IR 2,408,433

TELEPHONE SYSTEM 1 Filed Feb. 13, 1945 .14 SheetsSheet 12 MANUA L CENTRAL 0 Fr} a:

k Q b E TRUNK CIRCUIT TRUNK CIRCUIT TRUNK CIRCUIT II II l l Q j INVENTOR w ,4. MALTHANER H ATTORNEY Oct. 1, 1946. w. A. M LTHA ER v Q 3,

IELEPHONE SYSTEM Filed Feb; 13,1943 '14'Sheets-Sheet v15 A TI'ORNEV Oct. 1, 1946. w. A. MALTHANER 2,403,433

- 7 TELEPHONE SYSTEM Fi1ed Fe b. is, 1943 14 Sheds-Sheet 14 a U E z t.

k i E w g mvE/vmx? W A. MAL THA/VER 4 TTORNEV operated position. These three relays control the sequential allotment of an available link to a calling line, and the function of the relays CHI and CH2 in the positions shown is to route the start conductor I to the first available link in the office. As stated before, in the present embodiment of the invention it is assumed that the quantity of traffic from and between the various lines of the oflice, including trunk service to the central manual oflice, may be adequately handled by the use of only three links so that the relays CHI, CH2 and CH3 serve to route a calling line 7 to one of the three links depending upon which one of them is free in the order-of allotment.

The operated condition of relay CHI indicates that the link to which it provides access is in the idle condition, the relay remaining locked until said link is taken into use, the locking circuit of said relay extending from battery, through the winding thereof, contacts of key KYI, conductor 2, No. lback'contacts of relay B of link No. 1 (Fig. 6), conductor 3, No. 3 front contacts of relay CH I to ground. The, locking circuit of relay CH2, which controls access to link No. 2, is traced over a path which extends from battery through the winding of said relay, contacts of key KY2, conductor 2' in the bracket line 4, the No. 1 back contacts of relay B in link No. 2 (said link being conventionally indicated in Fig. 11), conductor 3' in the bracket line 4, No. 6 contacts of relay CH2 to ground.

On the other hand, the operating circuit of relay CH3, which provides access to link No. 3, while traceable over the contacts of its associated key KY3, conductor 2" within the bracket line 5, through the No. 1 back contact of relay B of link No. 3 (said link being conventionally indicated in Fig. 11), and conductor 3" within the bracket line 5, terminates in the No. 3 normal contacts of relay. CH3 and also in the No. 3 open contacts of relay CHI.

In other words, for a calling line to have accessibility to link No. 1, relays CHI and CH2 must be operated and relay CH3 must be normal. It will be shown that as each link is allotted in rotation, the circuits of relay CHI, CH2 and CH3 are changed to provide for the advancement or Bearing in mind that relays CHI, CH2 are assumed to be operated and that relay CH3 is assumed to be normal, it may now be further assumed that a call is originated at station A (line for a trunk connection to the manual central ofiice. As before stated, the present invention is in the nature of an improvement over the small capacity oflice disclosed in my Patent 2,300,951, equipped either with a'dial device or an alternating current generating device, and in which'links are provided whereby local connections between the several lines may be established over said links by the dialing or keying the number of the wanted station. In the present invention, each of the stations is assumed to be provided with an alternating current device LG and that local connections between the several stations in the of; fice may be established over the links in the man ner described in said patent. Although the link circuit herein shown and used has been modified in somerespects over the link in said patent to provide for the performance of functions essential to the operation of the features of the Pres-' ent invention, its structure, is so far' as it perin which the stations thereof are a station line number,'that is, 20.

tains to the establishment of local connections, remains substantially unaltered. The operation ofthe link, therefore, with respect to the establisl'i'inent of local connections is not described herein, reference being made to the above patent for a description of said operation.

We may now assume that station A desires to establish a connection to the manual central oflice. To do so the station attendant removes the telephone instrument from its cradle and then waits for key tone upon receiving which he keys an arbitrarily assigned digit indicative of the trunk connected and, thereafter, keys his own The removal of the telephone instrument from the cradle completes a circuit for line relay L20 which extends from battery, over the No. 1 normal contacts of the hold magnet H020 associated with ver-- tical 20, winding of relay L20, ring conductor R, over the station loop, tip conductor T, No. 3 nor-- mally made contacts of relay L20 to ground on the No. 2 contacts of cut-oil relay C020. Relay L20 operates. Over itsNo. 3 front contacts, the ground terminal of its circuit is switched from: solid ground on the No. 2 contacts of relay C020 to ground through the secondary winding of transformer LBT to the primary winding of which is connected a source of distinctive tone which, as will be shortly described, is transmitted to the calling line in the event that no link is available for completing the connection.

Over its No. 1 and No. 2 contacts, relay L20 causes a circuit to be completed for'relay L020 of the call allotter circuit (there being one relay LO for each line) and relay ST of link No. l which, since relays CHI and CH2 of the link allotter are assumed to be operated and relay CH3 isassumed to be normal, is thepreferred link in the orderof selection. The circuit for the relays ST and L020 is traced as follows: ground through the No. 1 contacts of relay EI, winding of relay L020, conductor 6, No. 1 and No. 2 contacts of relay L020, start conductor I, No. 3 contacts of 'relay CH2, No. 1 contacts of relay CHI,

' conductor I. winding .of relay .ST to battery.

Both relays L020 and ST operate in this circuit. Relay L02 0, upon operating, completes acircuit for relay N, the path of which may be traced from ground on the N0. 1 contacts of relay L020, conductor I 0, No. 1 back contacts of relay M, No. 4 contacts of relay MS, winding of relay N to battery. Relay N operates and completes a circuit for relay M which extends from ground on the No. 2 contacts of relay N, No. 1. contacts of relay MS, winding of relay M to battery. Relay M operates, opens the circuit of relay N at its No. 1 back contacts and causes thelatter relay to release. The release of relay N opens the operating circuit of relay M but this latter relay, however, remainslocked to ground on conductor I0 under the control of the No. 1 contacts of relay MS and its own No. 2 contacts. When relay MS operates, as described below, relay M releases. This cycle for operating relays M and'N ordinarily performs no useful function, but will perform an emergency function which will be ex plained in due course. At the same time, relay L020 completes a circuit for relay E which may be traced from ground on the N0. 1 contact of relay EI, No. 2 contacts of relay L020, conductor 8, serially through the No. 1 back contacts of the individual link relays LK3, LKZ and LKI in the call allotter, conductor 9, winding of relay E to battery. Upon operating, relay E locks to ground on the No. 1 contacts of relay L020 and,

No.2 normally made contacts of relay MS, the I windings of relay El .and E2 in parallel to battery. Relays El" andEZ operate but relay L020, which operated from ground'on the No. 1 contacts of relay El, now holds locked to ground on its own No. 1 contacts from battery through relay ST'of the selected link, as previously traced to'conduc-tor 6, winding of relay L020 and its No. 2contacts, conductor-8 as traced to conductor 9, contacts of relay E to ground on the No. 1 contacts of relay L020. Hencef-with the operation and locking of relay L020 individual to the callingstat-ionA and with theoperation of relay El and E2, all otherflinesmay not receive service if'cal-ls are initiated'therea-t during the time when stati'onA is havinga link assigned to its since, with relays El and "El operateino ground is available for completing the circuit of any other relay LO- individual to a calling line.

When relay El "operates, it completes a circuit for relay MS, said circuit being traced from ground on conductor 10, No; 14 contacts of relay El, winding of relay MS to battery. Since relay MS has slow-operating characteristics, it operates after an interval, at which time it opens the locking path-of relay N, supplies holding ground over its -No. 2 contacts for-relays E I: and E2 and,"over its No. 3 contacts,-applies ground On conductor ill to conductor If-l; Ground on conductor H is used to complete the circuit of the link select magnet-9 in virtue of operations which will be shortly described and which have been taking place in the link as the result of the operation of relayST therein.

In'the event that, due to sometrouble cc'ndition,'relay E -ofthe call allotter' fails to operate, relay 'MS cannot "operate and ground will not be applied-to 'conductorll *over its No. 3 contacts. If possible under these circumstances, it is desirable to "complete the connection-and so emergency means are provided for applying ground'to' conductor -I| when relay -MS fails to operate. lt will be remembered that, upon'the operation of relay LO20, a circuit "was 'completed for relay N, and'thatprior to the operation of' relay MS, relay "N caused' the' operation or relay M. Now; when relay M operates,*it opens 'the circuit': of relay N, but relay M remainslocked "overits 'Nol 2 contacts to"groundon "conductor "Hi, and it will be observed that, with relay"-N normal and relay M-operated; --ground-on con ductor 1015 applied to conductor ll "via the No. lfrontcontacts 'of relay M and the No. 1 contacts of-relay'N.

Returning, now, to "the operation-ofrelay'ST of the allotted link No. 1,'a circuit is'completed for select'magnet' fi, the path-'of which maybe trace-d from battery, through thewinding of said magnet, conductor FL'No. 2 contacts *of relayB, No. i contactsof relay-ST, conductor H, to-the aioretraced ground thereon. Select "magnet '9 operates and conditions thegroup of cross-points in the ninth horizontal 'leve'l'for *-selection,,including "cross-point "9 on "vertical 21!] through which the calling loop willbe extended into-the as shown hereinafter. Select magnet 9,

, uponoperating, completes a circuitfrom-ground over its contact and paralleled "conductor I 8, to the commonwindings of relays S-and'-S!, caus- "in-g sai'drelays' to" operate and complete a circuit "for "cut-01f relay I CQZ-lk th 'circuit of said-relay being "traced "from *ground" on the i No.- 3 -contacts oirelays S and Si, No. 3 contacts of relay L020, conductor l9,"wind-ing of r'elayrCO20 to battery. Relay C020, upon operating, closes an obvious circuitfor .hold magnet H021! of the Vertical '29 to'which line-zlllstati'on A) is connected and. because selectmagnet 9 is operatedcnoss-point 9 on verticalyZD is also operated. This crosspoint 9 remains operated after the circuit of select magnet Shae been opened, and since this is standard cross bar switch operation, it will not be amplified anyfurther; When hold image net H020 operates, it releases line. relay L28 which, in turn, releases :relayLOZfl and the latter, upon releasing, opens the circuit of 0020 and. further disconnects ground from start conductor l which caused the operation of relay ST. Relay C020 (as well as every other relay CO)- is slow release and slow operate and will holdup for the nonce until a holding circuit therefor will have been completed in the link as described hereinafter. Also, while conductor l is now opened, the fact that cross-point 9 on vertical 28 is closed, relay'ST will not release, it havinga locking circuitt'hrough across-point contact, which circuit may be traced over the No. 2 contacts of said relay, No. 1 contacts of relay T2, conductor-1m, No. 5 normally made contacts of relay LCI, conductor Sl,"No."3 contacts of cross-point 9 on vertical 26, resistors RC and RClto ground.

Since cross-point 9 on vertical '2!) is closed, the line tip and ring conductors T and Rare joined to the'linktip and ring conductors LTzand LR, respectively, over the No. 6 and No. 5 contacts of the operated cross-point 9, respectively, and this joining of conductors completes an operating path for relay A of the link, which circuit may be traced from battery, through'the lower winding of s'aid'relay, No. 2 back contacts cfrelay HS, conductor LT, No, 6 contacts of crosspoint 9 on'vrtioal-Zll, line conductor '1 over the calling line loop, ring conductor R, No. 5 contacts -of cross-point 9 on vertical 20, conductor LR of the link, No. 3 back contacts of relay HS, upper winding of relay A to ground. Relay A operates and completes a circuit for relay B which extends from ground on conductor S'I,'No. 5 and No. 4 normally made continuity contacts on relay LCI, conductor I02, front contacts of relayAfNo. 4 normally made continuity contacts of relayB, :winding of relay B to battery. Relay l3 operates and looks over its No. 4 front contacts, conductor mi; No. 5 normally made continuity contacts on relay L'Cl, to ground on conductor SI. At its No. 7 contacts relay B now completes a holding circuit for C022 which traces from battery through the winding of "said relay C020, No. 4 and No. 2 contacts in series of crosspoint '9 in vertical 20,;conductor 51, No. '7 contacts of relay B, front contactsd'f relay A conductor I02 and thence as traced to ground on conductor SI. Relay 0020 thus holds'operated, in turn holding magnet H028 which keeps crosspoint 9 closed. At its No. '2-ccntacts, relay B opensconductor l-1,-thereby causin the release of select magnet 9. However, since the hold magnet H026 is being held, cross-point 9 on vertical 20 will likewisebe held. 'Relay'B further completes a circuit from ground over its No-'8 contacts to a relay (not-shown) in the power charge circuit I00 by means of which, when the "relay is operated, circuits are closed thereby that willincrease the-chargingrate to the link-power supply duringfthe time that-the I lin-k is' being used and, over its "No. '5 an'd No. "6

contacts relay B further applies ground. to hold conductors 20 and 2I, respectively, for-holding and operating certain other relays as noted ,below. Over its No. 1 contacts, relay B opens the locking circuit of relay CHI in the allotter circuit, which, upon releasing, advances the common start conductor I over the No. 3 contacts of relay CH2, No. 2 contacts of relay CHI, No. 1 contacts .of relay CH2 to conductor I which further extends to relay ST of link No. 2 (conventionally indicated in Fi 11) It is now evident that if, at the time a call is initiated at station A (or any other station) link I is busy or is otherwise made unavailable, its relay CHI in the link allotter circuit would be normal and the start conductor I, with ground applied thereto, would be connected to conductor 1' of link 2 to cause the operation therein of relay ST in the manner described for relay ST of link I. On the other hand, if both links I and 2 are busy or otherwise made unavailable, relay CH2 as well as relay CHI will be normal since their respective locking circuits will be opened at the No. 1 contacts of relay B of their respective links.

However, as soon as these relays are both normal, a circuit is completed for relay CH3 which extends from battery through its winding, contacts of key KY3, conductor 2" within bracket line 5, No. 1 contacts of relay B of link 3 (conventionally indicated in Fig. 11), conductor 3" to ground over the No. 1 normally made contacts of relay CHI. Relay CH3 operates and, with relays CHI and. CH2 normal, represents the condition in which links I and 2 are unavailable but link 3 is available. If, now, a call is initiated at a line station, as for example, station A, ground on start conductor I is extended to conductor 1" which is connected to the winding of relay ST of link 3, the path being from ground on conductor I as applied thereto from conductor 6 through the No. 2 and No. 3 contacts of relay L20, No. 4 contacts of relay CH2, No. 2 contacts of relay CH3, conductor 1", to the winding of relay ST of said link, causing the operation of said relay and the connection of the associated link 3 to the calling line.

If all three links are unavailable, relay CH3 will be normal along with relays CHI and CH2. The initiation of a call under these circumstances will cause ground on the start conductor I to be applied to conductor 22 via the No. 4 contacts of relay CH2, No. 1 contacts of relay CH3, No. 2 contacts of relay CHI, No. 2 contacts of relay CH2, to conductor 22. This conductor extends (within bracket line I3) to the power tone and alarm circuit III! and completes a circuit therein which causes a distinct Link busy tone to be applied to

conductors

23 and 24 which are connected to primary winding terminals of busy tone transformer LBT. Since the secondary winding of this transformer is connected to ground and iscompleted into a circuit over the winding of the line relay L- to battery on the upper normally made contacts of the hold magnet HO, the subscriber at the calling station will hear the tone as an indication that, due to unavailable links, the connection cannot be established.

As soon as a link becomes available, say link I, a circuit is completed for relay CHI which extends from battery through its winding, thence as traced to conductor 3, No. 5 contacts of relay CH2 to ground. Relay CHI operates and lock to ground over its No. 3 front contacts to reestablish the path of start conductor I to the windin of relay ST of link I in readiness for'seizure on 8 the next call. If, link 2 becomes available, relay CH2 operates over a circuit traceable from battery, through its winding, thence as traced to conductor 3' to ground on the No. 3 normally made contacts of relay CH3.

Returning, now, to the operation of relay ST of link I, assumed to have been taken into use as a result of a call having been initiated at station A, ground on the No. 3 contacts of said relay completes a circuit for relay LSI of its private link connector and this circuit is tracedfrom ground on said contacts, conductor 58, winding of relay LSI, serially through the No. 2 normally made continuity contacts of relay LS2 and LS3 (the former being individual to link 2 and the latter individual to link 3), to battery. Relay LSI operates, connects operating battery to its winding through its own No. 2 front contacts and, over its No. 1 front contacts, closes two circuits, the first extending from ground, Winding of relay LCI to battery, and the second extending from the same ground, conductor I03, No. 3 contacts of relay DCT, winding of relay LCI to battery. Both relays LCI and LCI operate, the former to connect the translator and alternating current receiver shown in Figs. 9 and 10, respectively, to the link and the latter to connect the trunk marker of Fig. 14 to the link.

In connection with the circuit for operating relay LCI (or LC2' or L03), it should be observed that the link is designed to control the setting up of two types of connections, namely, a local connection between two local stations as, for instance, between stations A and B, and a trunk connection between a local station as, for instance, station A and a trunk extending to the desired central oflice. In the case of a local connection no trunk is required and, therefore, the services of the trunk marker (Fig. 14) may be dispensed with whereas, on a trunk connection, its services are required and, therefore, connection of the trunk marker to the link is necessary. Discrimination between the two types of connections is achieved by the first digit that will be keyed by the calling subscriber. Since, in the present embodiment of the invention, the private branch exchange is limited to twenty lines with each line having a numerical designation from 20 to 39 depending upon the cross bar vertical in the cross bar switch to which the designated line is connected, and since for the establishment of a local connection the calling subscribermerely keys the digits of the wanted number as set forth in my above-mentioned patent, whereas, for a trunk connection, he keys the trunk digit ahead of the digits of his own station designation, it is obvious that if the first digit keyed by a subscriber is a 2 or 3 and such digits are not used for trunk digit designations, said digits may be used to apprise the link that the connection will be local and, therefore, that the use of the trunk marker can be dispensed with. As described in myabove-mentioned patent, the keying of a

digit

2 or 3 as the first digit will cause the operation of relay T2 of the link or relay T3 (if digit 3 is keyed) followed by relay T2 (if digit 3 is keyed), for registering therein the

digit

2 or 3, respectively, whichever digit it may be. Relay T2 completes a circuit for relay CI (as set forth in said patent) and the latter, upon operation, completes a circuit for relay DCT of the trunk marker connector. This circuit may be traced as follows: Battery through the winding of relay DCT, No. 4 normally'made continuity contacts of said relay, conductor I04, serially through the zieosea N0. 8 contacts of the individual trunk relays TKT, 'IK6, TK5, and TI (assuming that four trunks are provided between the private branch exchange and the central office), No. Qbackcon- .tacts of relay RTT, conductor I05, No. 3 front contacts of relay LCI, conductor I06, No.5 contacts of relay CI, to ground on the No. 5 contacts of relay B. Relay DCT operates over the above circuit, locks over its No. 4 front contacts, conductor I08, to ground on the No. 6 contacts of relay ON which is operated at this time aswill be shown, and opens the circuit of relayLCl which releases and, in so doing, disconnects the trunk marker from the link.

In the event of a trunk connection, however,

the subscriber first keys the trunk digit designation 4 and. thereafter, follows this digit by keying in succession the two digits of his own I number which, in the case of station A, would be a 2 and 0, Under these circumstances, relay T2 will not be operated until the digit 2 is keyed, and inthe interval between thekeying of the trunk digit and the keying of the tens digit of the calling line, the trunk marker remains connected to the link. I

- {The operation of relay LCI, transfers the holding circuitof relay Bqfrom' groundonconductor ,Sl. andconductor ll; as traced thereto over the normally made continuity contactsof relay LC] to the same conductor SI as traced thereto from conductor l0l, No. front contacts of relay LCI, -conductor,84-,'No. 8 back-contacts of relay NC, conductor 81, No. 6 frontcontacts of relay LCI, to conductor SI. It also transfers over its No. 4 front contacts the holding ground for out off relay CO which now holds over conductor IN to ground on the No. 5 contacts of relay COK. Relay LCI (or relay L02 or relay L03 depending upon which of the relays LSI, LS2 or LS3 is operated) also performs certain other func tions in they progressiveestablishment of the connection which are best described when these functions comeinto play. 7 Relay LCI closes a circuit fromground on its No. 5 contactsover conductor to the winding K of relay ON which operates to complete a circuit over its No. 1 contacts to a relay (not shown) in the Time out circuit I20, and applies-ground over its Nos. 3 to 6 contacts, inclusive, to the con- I nected conductors for controlling certain relays '10 The object of operating relay LSI and, locally therefrom, relays LC! and LCI is, as before stated, to connect to the link when taken into use to the common alternating current pulsereceiver and translator shown in Figs. 9 and 10 and-tothe trunk marker shown in Fig. 14. When these circuits are connected to the link (the signal for which is the operation of relay LS! the receiver and the trunk marker must be made unavailable to the other two links. This is done by removing the operating ground for relays L02, L03, M32 and L03 at the No, 1 back'contacts ofrelay LSI so that, if either relay LS2 or LS3 should have its operating circuit established by relay ST of

links

2 and ,3, respectively, the circuit'of relays LC2 and LC2 or L03 and L03 will be cut off at the No. 1 back contacts of relay LSl. Onv the other hand, if the link taken into use happens to be

link

2 or 3, then the corresponding operation of relay LS2 or LS3, respectively, will out offthe operating circuit of relay LSI at their respective No. 2 normally made continuity contacts so that if link I is seized thereafter, the failure of relay LSI to operatewill prevent the operation of relays LCI and LC! and thereby prevent the connection of the common alternating current receiver, translator and trunk marker simultaneously to two calling links.

The alternating currentreceiver of Fig. 10 is adapted to receive alternating current impulses vised, by an appropriate warning tone, to transrelay ONA, conductor 16, No. 4 contacts of relay LCI, conductor .15, No. 1 back contacts of relay- C,.conductor 2| to ground on the No.- 6 contacts of relay B. Relay ONA operates, locks over'its No. 3 front contacts under the control of the No. 1

contacts of relay HR to ground on conductor 60 and, at the appropriate time, performs certain functions which arenoted below. The operation .of the relay in the Time out circuit I20 initiates the operation ofa group of relays orsimilar in strumentalitiesby means of which a predeter trunk or called number as the case may be," i

When that interval is terminated,

mit the called line or trunk designation. Accordingly, when relay ONA operates, it closes theprimarycircuit of a low frequency tone derived fr'om the tone oscillator 10 through adjustable resistance pad 62' and transformer 6|. The path is traced as follows: from the lower terminal of the secondary winding of transformer 6i, conductor [4, serially through the No. 1 contacts of relays TKl-TK J, inclusive in the order named, conductor 15, No. 3 contacts of relay RR,.No. 2 contacts of relay RA, No. 2 contacts of relay' ON, No. 2 contacts of relay ONA, conductor, through the low-pass filter 6 3 to the upper terminal of the secondary winding of transformer 6|. The tone frequencies produced by the oscillator 10 are thus induced over the circuit above traced.

Through a transformer arrangement (not shown) in the low-pass filter 63, the low frequencies' are induced in the closed circuit com prising the transformer 68, a transformer in said low-pass filter 6 3 andfa transformer in high-pass filterfi l. Beyond the secondary winding of the transformer (not shown) in the filter 64 are connected suitable'filter networks (not shown) which are designed to pass only frequencies within the signal frequencyband of the frequencies that are produced by the station generator LG when, operated, Hence the tone frequencies proe v'duced byoscillator 10 will not pass into the volume limiter, 65 or into the enabler 80. They will, however, be induced into the secondary winding oftransformer 68 if said secondary winding forms part of a closed electrical system, which it is, because conductors H and l'2v are but extensions oftheline conductors T and B into the link and alternating currentreceiver connected thereto. The path of. this circuit is traced as follows: From the left side of the secondary winding of 11 transformer 68, conductor H, No. 2 back contacts of relay HR, conductor 78, No. 6 front contacts of relay LCI, conductor 54, condenser C2, No. 2 back contacts of relay HS, conductor LT, N0. 6 contacts of cross-point 9 on vertical 20, conductor T, over the loop of station A, conductor R, No. contacts of cross point 1-) on vertical 20, conductor LR, No. 3 back contacts of relay HS, condenser 03, conductor 55, No. '7 front contacts of relay LCI, conductor 14, No. 3 back contacts of relay HR, conductor 12 to the right terminal of the secondary winding of transformer 68. Inasmuch as the receiver of the telephone instrument of station A is included in the aboveclosed circuit, the tone induced thereover is heard by the subscriber, which he understands as a signal to proceed with keying the number of the wanted subscriber or trunk.

If the subscriber fails to key the wanted number or trunk (including the digits of his own number) in the time taken for relay HR to operate as above set forth, it is presumed that the connection so far set up was due .to the telephone instrument at the station being accidentally removed from its cradle and not restored. In any event, the fact that but three links are provided to serve twenty lines makes it imperative that no link be unduly kept out of service after key tone is transmitted to the calling line without performing the necessary connectioncompleting functions that follow the transmission of the wanted number designation. Hence if the subscriber fails to transmit the number by the time relay I-IR, operates, relay ONA is unlocked, a circuit is completed from ground over the No.- 4 contacts of relay HR, No. 4 contacts of relay ONA to a relay (not shown) in the howler circuit I30 which operates to complete a sharp howler tone which is applied over the No. 2 and No. 3 front contacts of relay HR to conductors I3 and 74, both of said conductors extending over the calling loop (as previously traced) including the station telephone instrument. The tone is sufficiently loud to be heard at a distance from the receiver and is intended to apprise the subscriber that the telephone in strument should be restored, an occurrence that will cause the link to be disconnected and restored to normal as described hereinafter.

Assuming, however, that relay HR has not operated, the subscriber, upon hearing key tone, operates the device LG to transmit the wanted designation which, since the connection desired is to a trunk extending to the manual central office, will consist of the arbitrary digit 4 indicating a trunk to said central oifice, followed by the subscribers own number 20. He does this by first operating a key in the device LG bearing the numeral 4 thereon and then, in succession, similar keys bearing the

numerals

2 and 0, respectively. The depression of each key causes two out of five reeds to be plucked according to a code combination individual to the numeral indicated, and the reeds are set into vibration to produce two alternating currents of specific frequencies which are now transmitted over a circuit which may be traced as previously to conductors l3 and '14 at the No. 2 and No. 3 back contacts, respectively, of relay HR and, thereafter, to conductors H and 72, respectively, connecting with the secondary winding of transformer 68. These frequencies are induced into the circuit comprising the primary winding of said transformer and the high-pass filter 64.

The alternating current pulse receiver shown in Fig. 10 is a conventional disclosure of the receiver shown in Patent 2,269,022 issued to G. Hecht and A. A. Lundstroin on January 6, 1942. It comprises the channel composed of transformer 68, the high-pass filter 64, the volume limiter'65,'the transformer 66, the adjustable pad 61, five detector amplifiers fA-JE with relays F'A-FE in the respective anode circuits thereof and a suitable filter BPF intermediate the pad 61 and each one of the detector amplifier tubes fA-fE, each of said filters being designed to pass only the frequency intended for the particular channel in which the filter is included. Between the high-pass filter 64 and the volume limiter 65 is bridged the primary winding of transformer H2 to the secondary winding of which is connected the enabler 80. This enabler is identical to that disclosed and described in the abovementioned patent to G. Hecht and A. A. Lundstro-in and is shown only in conventional form except for polarized relay 11. The enabler re- 'sponds to a portion of the energy in each of the signal pulses that pass through the highpass filter 64 and functions to operate polarized relay 1'! after a predetermined interval. Relay 1'! "grounds conductor 18 which is connected to the cathode of each of the detector tubes JA-fE thereby'activating them to respond to the portion of the signal energy that passes through the volume limiter 65. Since the operation of the enabler 80' is completely described in the above-mentioned patent to G. Hecht and A, A. Lundstrom, reference is made to said patent for a complete description thereof, only so much of said description being supplied herein as will be necessary to an understanding of the present invention. V

It will further be noted that the No. 1 contacts of relay ONA are bridged acrossthe highpass filter 64 (through conductors H6 and ill) to short-circuit said filter when the relay is normal. This is a safety measure to insure that if, at the time the subscriber initiates the call, the Nos. 5 and 6 contacts of cross-point 9 on vertical 20 close last, and should clos after the alternating current receiver is connected to the link, the impulse produced over the line loop will not find a passage into the high-pass filter. For if it does and this pulse contains frequency components within the signal frequency band, these frequencies would falsely operate one or more of the signal channels. By the time relay ONA has operated, however, the pulse will have been dissipated, the short circuit is removed and the receiver is connected to the line as above described.

As said before, each detector amplifier tube fA iE is preceded by a filter BPF which passes only the frequency intended for the channel in which the tube therein is to operate on the energy carried by said frequency. It is assumed that the device LG is capable of producing five different frequencies which, for convenience, may be designated A-E, inclusive, and that, in accordance with the operation of the receiver as described in the above-mentioned patent to G. Hecht and A. A. Lundstrom, frequency A will cause the operation of tube fA, frequency B will cause the operation of tube IE and so on up to and including frequency E which will cause the operation of tube IE.

As already mentioned, each detector amplifier tube has a relay F- connected in its anode circuit, and a front contactof this relay is further connected to the winding of "one of the five re-