US2249429A - Teletypewriter switching system - Google Patents

Description

July 15, 1941. F. s. KINKl-:AD

TELETYPEWRITER SWITCHING lSYSTEMV Filed March 1s',4 19:59

2 Sheets-Sheet l ATTORNEY July 15, 1941. F. s. KINKEAD 2,249,429 TELETYPEWRITER SWITCHING' SYSTEM Filed Marc-2h 18, 1939 l 2 Sheets-Sheet 2 POLAR/250 /N VEA/TOR By E s f/NKEAD A TTOR/VE V Patented July 15, 1941 TLETYPEWRITER SWITCHING SYSTEM Fullerton S. Kinkead, New` York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a-corporation of New York Application VMarch 18, 1939,`Serial N o. 262,681

25 Claims. (Cl. 17'8'-.71)

This invention relates to telegraph and teletypewriter switching systems and particularly to new transmission and supervisory features for a switching system'including a calling subscribers station and subscribers line circuit interconnectedthrough a subscribers cord circuit to a called subscribers line and station circuit.

In certain direct current systems of telegraph communication., signaling is performed Vby 'opening and closing a telegraph line, so 'that current of full strength vand Yzero current are transmitted for 'marking and spacing signals. In other direct current systems, currents of opposite polarity and 'of the same magnitude are impressed on theline with short intervals of' Zero current between the signal elements while the armature ofthe'telegraph relay is'traveling between contacts. There are also other :types of direct 'current telegraph communication signaling, inicludingan arrangement wherein current of one polarity 'and of-a certain magnitude is transmitted for'the marking 'condition 'and 4current vof opposite'polarity and reduced magnitude istran'smitted for the spacing condition.

When such' systems of communication signaling are usedin teletypewriter switching .systems they introduce a major difficulty inherent vin'each inthat relays in the transmission path controlling supervisory signals have a tendency to release.

In addition to communication signals in manual teletypewriter switching systems, it is necessary to transmit calling, disconnect, recalLand break signals'to control'the 'connection of subscribers through a manual switchboard as well as to regulate communication between subscribers. The character of the communication signals used in the switching system generally dominates the arrangement of the line, supervisory and break signal apparatus. The Yreception Vof false supervisory signals and the non-registering of intended supervisory signals are inherent in the operation of such systems.

Subscribers teletypewriter switching stations are vgenerally interconnected through associated central-switching stations by means of loop circuits. Two general types of loops, namely, closed and open loops, have been used for this purpose. A closed loop is one which is normally closed during'the idle period and during communication signaling. It is open for the transmission of calling and supervisory signals. It has a number of 'serious disadvantages and is being generally replaced by open loop circuits. These latter loops are normally open during the idle period and are closed for calling andwcommunication signaling. One loop conductor is ordinarily grounded for the transmission of supervisory signals. Although open loop circuits are generally moresatisfactory than the 'closed loop circuits, as presently used, they too have a number of disadvantages. Perhaps the most serious'of these disadvantages is that the supervisoryrelay, which is ordinarily a differential relay has a tendency to release during communication signaling, requiring additional apparatusI to prevent the registering of yfalse supervisory signals. Further, in the present open loops, in the transmission of calling and supervisory signals, one of the loop conductors is connected to ground. On long-loops, the Variation in ground potentiallbetween the station and the oiiice is at timesconsiderable, seriously interfering with the transmission of thesesignals. Another `objection is that directcurrent flows'rom the central station .to ground atthe subscriber-s station while lcalling and supervisory signals are being transmitted causing electrolysis. To overcome these objections, the open loop subscribers circuits .and'cooperating cord circuits `disclosedherein are arranged to maintain unidirectional current in the loop and through the winding ofthe supervisory relays at all times during communication signaling, including the interval vwl'iilethe-armature ofthe relay which transmits into the loop is traveling between its marking and spacing contacts. Further,- the circuits are arranged -so that it is not necessary to ground one loop conductor to transmit supervisory signals, thus eliminating'the objectionable electrolysis effect and .the effect of variation in ground potentials between the subscribers station and the central station.

In the copending application of W. W. Cramer and applicant,-Serial No. 225,898, iiled August 20, 1938, there are disclosed means for maintaining unidirectional current through the transmission path in certain telegraph circuits at all times, while the circuits are conditioned for communication, that is, during the transmission of marking and'spacing signal elements as Well as during the intervals between the transmission of said elements while the armature of the sending relay is traveling. 'The invention therein is shown as applied to two specific circuits, namely, (l) a full metallic trunk or tie line circuit which Would -be Vused ordinarily in interconnecting two teletypewriterfprivate branch exchange switchboards, and (2) a subscribers extension line circuit' used in connecting subscribers stations'remote from a switchboard to the'switchboard.

The invention herein now makes it possible to extend the principles disclosed in the abovenamed application so that they may be applied to an operating system instead' of limited to-the individual circuits des-cribed'in said application,

Inthe invention herein. circuits have been devised making possible the positive control of supervisory signals in a connection involving a calling subscribers Aline and station circuit intercuit is shown at the left of the dotted line.

connected through a cord circuit to a called subscribers line and station circuit.

An object of this invention is to provide a cord circuit wherein relays controlling supervisory signals are positively controlled.

A further object of this invention is to provide a cord circuit cooperating with a subscribers line and station circuit in a manual teletypewriter switching system in which there is either full current of a specific polarityor reduced current of the same polarity owing at all times while a connection is established and conditioned for communication, as well as during Comunication and during the intervals between the transmission of Y signal elements, so that relays controlling supervisory signals may be controlled positively.

A further object of this invention is to provide an open loop circuit in which it is not necessary to ground one conductor in order to transmit a supervisory signal, thus overcoming the difculty of electrolysis and the difficulty due to difference in ground potential between the subscribers station and the central station.

A feature of this invention is a cord circuit having a two-relay repeater therein, wherein each relay has a line winding and a balancing winding and wherein, connected to said balancing winding, there is a potentiometer device to maintain current through said line winding and the transmission circuit connected thereto during the g transmission of both elements of the two-element communication signaling code and during the interval while the armature of the signal transmitting relay is traveling.

A further feature of this invention is a relay having a winding in the transmission path of the cord circuit and means in said cord circuit to maintain unidirectional current through said winding at all times while the circuit is conditioned for communication and while it is transmitting marking and spacing signals and during the intervals between said signals.

A further feature of this invention is an arrangement for changing the biasing current through a polar relay which relay controls the Supervisory signals in the cord circuit to compensate for different line leakance conditions.

These and other features will be made apparent in the description which follows.

The invention may be more fully understood from reference to the associated drawings.

Fig. 1 shows a teletypewriter subscribers station and line circuit which serve in the following description as a calling circuit. The station ciris interconnected by means of two conductors to the line circuit apparatus, shown at the right of the dotted line, which is located at a switchboard in a central oice. The jack ||0 ordinarily has multiple appearances located at a plurality of positions.

Fig. 2 shows a cord circuit and an operators position circuit which serve to interconnect calling subscribers to other circuits and to communicate with subscribers. The position circuit is shown at the bottom of the gure below teletypewriter key 2|3. The cord and position circuits are located in a switchboard at a central office.

Fig. 3 shows a second teletypewriter subscribc ers station and line circuit which serve in the following description as a called circuit. It is the inverse of Fig. l. The line circuit apparatus shown at the left of the dotted line is located at the switchboard and is connected by means of Cil two conductors 4to the apparatus at a subscribers -station shown to the right of the dotted line.

The jack 3| 0 is also multipled before a plurality of operators at various positions.

The circuits will now be described in detail.

Refer now to Fig. 2. This circuit represents a single cord circuit, which is one of a group of cords at a particular operators position in a switchboard, and a single position circuit which is flexibly interconnected as required to any cord at a particular position. The circuits as shown are normal for the idle condition. It will be observed that .the position circuit shown at the bottom of the figure, below key 2|3, is not operatively connected to the transmission circuit of the cord while key 2|3 is in the position shown.

The circuits through the bottom or biasing windings of relays 2I| and 2|2, which constitute the cord circuit repeater, terminate in a potentiometer in each case. In the case of relay 2 I 2 the potentiometer circuit consists of grounded battery connected through resistances 250 and 249 to ground. For relay 2II the circuit consists of grounded battery connected through resistances 244 and 243 to ground. Similarly the two relays in the repeater in the operators position circuit, namely relays 228 and 229, have potentiometers connected to their bottom or biasing windings. For relay 228 the circuit extends from grounded battery through resistances 230 and 23| to ground and for relay 229, from grounded battery through resistances 231 and 236 to ground.

In the case of each of these relays the circuit through their bottom windings connects to a twobranch parallel circuit. One branch of the parallel circuit extends in each instance through the top or line winding of the same relay. The second branch extends through the armature of a second relay, which forms with the rst the second of a pair of repeating relays. Thus a circuit may be traced from a point between resistances 249 and 259, through the bottom winding of relay 2|2 to the junction of two parallel branches. The first branch extends through the top or line windingof relay 2|2 and is continued through the loop circuit connectedto the calling end of the cord, to be traced hereinafter. The second branch extends through resistance 294 and the armature of relay 2| which may be connected to either battery or ground for the marking or spacing condition. Similarly, for relay 2||, the circuit through its bottom or biasing winding extends from a point between resistances 243 and 244 through said winding to a parallel circuit. The first branch of said circuit extends through the top or line winding of relay 2|| and then `through the tipconductor of the loop connected to the answering end of the cord to be traced hereinafter. The second branch extends through resistance 295 to the armature of relay 2|2 where it connects to either battery or ground for the marking or spacing condition.

In the case of relay 229 the biasing circuit extends from a point between resistances 236 and 231 through the bottom or biasing winding to a parallel circuit. The first branch extends through the top or line winding of relay 229 to a transmission circuit to be traced hereinafter. The second branch connects to the armature of relay 223 where it is connected to ground through resistance 405 for marking and through resistance 491 to negative battery for spacing. For relay 228 the biasing circuit extends from a point between resistances 230 and 23| through the bottom or b-iasing winding of relay 228 to a parallel circuit. The rst branch extends through the top `or line winding of relay 228 to a transmission circuit to be traced hereinafter. The second branch extends through resistance 432 to the armature of relay 220 where it may be connected through resistance 408 to negative battery for marking or to ground for spacing.

It is by means of these potentiometer arrangements that it is possible to maintain unidirectional current flowing in the transmission circuits while the armature of the opposite relay of a pair of repeating relays 'is traveling 'and while it is connected to ground. This will become more apparent in the following description.

When the cord circuit per Fig. 2 is in the idle condition and the position circuit is also in the idle condition and disconnected from the cords,

the armatures of the repeater relays 2|| and I* 2|2 in the cord and 228 and 229 in the 'position circuit are all held in engagement with their left-hand or marking contacts as indicated. While the cord is idle relays 202 and 265 are released as indicated and teletypewriter key 253 is in the position shown. Relays 252 and 265 while released connect ground to the top or line windings of relays 2|| and 2|2, respectively, to hold these relays in the marking position. These circuits may be traced as follows: For relay 2|| the circuit may be traced from ground, through 'contact 219 of relay 232, resistance 403, contacts 2|9 and 225 of key 2|3 in parallel, through the top winding of relay 2| l, resistance 295, armature and contacts 241 of relay 2|2 and resistance Vlill to negative battery. For relay 2|2 the circuit may be traced from ground, through 'contacts 28|] of relay 255, resistance 25|, top winding of relay 2|2, resistance 2323, armatureand 'contacts 245 'of'relay 2|| and resistance 495 to ne'ga'tiin l battery. The circuit for relay 223 maybe traced from ground, through resistance 296, contacts 221 of key 2|3, top Winding of relay 228, resistance 402, armature and 'contacts 23 of relay 229 and resistance 428 to negative fba'tte'ry. The i f armatures of said relays to their left-hand or f marking contacts, preponderates over the effect of the current flowing in 'the bottom or biasing windings, which tends to operate the armatures to engage their right-hand or spacing contacts and the armatures 'are 'all held on their marking contacts while the 'circuits are idle.

When the ycircuit per Fig. 2 is idle, the spacing contact 245 of relay 2`|| is connected'through resistance 299 and `the winding of 'relay 251 to ground. When key 213 is in the position shown, the spacing contact 248 ci relay 2|2 is connected through resistance M2 and contacts 216 to key 2 I3 through the winding of relay 228 to ground. When key 213 is operated, the termination of the spacing contact is transferred from ground through the Winding of relay 238 todirect ground connected to contact 2|5 of key 2 |3.

When the cord per Fig. 2 is idle, relays 268 and 251 a're released. For Ythis condition a biasing current ows through the bottom windings of relays 201 and 262. For relay 201 the circuit extends from ground, through the bottom winding of relay 231, through the armature of 'relay 268, shunting resistance 4l l, through resistance MB to battery. For relay 222 the circuit may be traced from ground, through 'the bottom winding of relay 262, the armature of relay 251, shuntin-g resistance 258, through resistance 259 'to |battery. The effect of the biasing current is to tend to voperate the armature of 'each of these relays to engage'the contacts shown. When current ows in the top or line windings of relays 201 and 262, its eiiect is to tend to disenga'ge the armatures of these relays Vfrom these contacts.

1. The subscriber originates a call Il? the subscriber at station A wishes to call the vsubscriber at station B, she operates power switch |ll| to close contacts |26). This closes a circuit from negative battery, through 'resistance H3, contacts IM of relay |29, contacts |22 of key li, printer magnet |22, sender contacts |23 which short-circuit resistance |95, break key 'Ifl which short-circuits resistance |06, contacts ||1 of relay E39 and the winding of relay' 'to ground. Relay is operated, closing contacts H3 to supply battery through the vfilament of lamp |2, lighting the lamp. Y

` 2. The operator answersthe call Invrespon's'e to the lighted' lamp signal ||2, the operator at the central oice 'inserts plug 201 associated with the answering `end of 'the `cord per Fig. 2 into jack Hi). Relays' 202 and `H39 operate vover a circuit from'battery through the `winding of relay 292, sleeve of plug 20|, sleeve of jack lill, and the winding of relay '|09 to ground. The operation of these two-relays eX- tends the tip and ring conductors of the subscribers loop into the cord circuit. A circuit may now be traced from ground, ythrough the top winding of relay 201, contacts 21| of relay 222, th'e ring of plug 29|, ring of jack l0, contacts' l |'B of relay 'm9, break key |24 which shunts resistance |255, sender contacts |23 which shunt resistance |05, prin-ter magnet |22, contacts '|20 of key ||l|, contacts H5 of relay |139, "tip of jack l 1B, ltip of plug 22|, vcontacts '298er relay 29.2, contacts 222 of vringing key 25|, winding' of rellay 250, contacts 2'l9 and 225 'of lthe tele'typewriter key 2|'3 'in parallel, top winding of relay 2H, resistance 295, armature o'f relay 2|2, contacts 2131 of relay 25.2, through 4resistance 4M to negative battery. Relays 231 and 2 |'0 operate and relay 2'|| is held on -its marking "contact 245. The operation of relay 2|@ in turn operates relay 299 over 'an obvious circuit. The operation o'f relay 229 supplies negative battery 'through 'its 'upper contacts, resistance 42|, winding of relay 221 to'ground. The currentthus furnished augments 'that supplied around 'the loop from negal tive battery connected to contacts 241 to hold relay 221 operated. The resistance of the 'to'p winding of relay 201 isI low vrelative to the Value cf resistance Mil. The operation of relayv21 prevents lamp 2M 'from lighting by ydisconnecting ground from the armature of relay 221, p'reventing the operation of relay 293. The armature of relay 2|2 continues to be held against its'marking Contact '241 over the circuit heretoforeY traced to ground 'through contacts 280.

3. The operator connects her position circuit to the cord scriber, the operator at the position where the cord circuit per Fig. 2 is located must connect her position circuit, which is common to all the cords at her position, to the particular cord circuit which has been used in establishing the connection to the calling subscriber. The operator therefore actuates teletypewriter key 2|3 to its alternate position. This connects the tip of the subscribers loop heretofore traced through contacts 226 of key 2|3, the top winding of relay 226, resistance 462, through the armature and marking contact 234 of relay 229 and resistance 468 to negative battery. The connection through the line winding of relay 2| I, which is disconnected from the subscribers loop, is extended through contacts 224 of key`2I3, printer magnet 242, sending contacts 246 and resistance 24| in parallel, the top winding of relay 229, armature and contacts 232 of relay 226, and resistance 466 to ground. The operation of key 2|3 changes the termination of spacing contact 248 as heretofore described. It also disconnects ground which was connected through contacts 2|8 from the spacing contact 235 of relay 229. This contact remains connected to ground, however, through resistance 469.

The operator may now communicate with the subscriber. The signals transmitted from the sending contacts |63 in subscribers station A, Fig. l, will actuate relay 228. This in turn will actuate the operators printer magnet 242 as well as relay 2||. The operation of relay 2|I, of course, is ineffectual as the calling end of the cord is not connected at this time, the transmitting conductor being terminated, over a circuit heretofore traced, in ground connected to contacts 286 at relay 265.

4. The operator calls the called party- The calling subscriber may request a connection to another subscribers line connected to the same exchange. The operator will thereupon connect plug 268, associated with the calling end of the same cord circuit per Fig. 2 used in answering the calling subscriber, into jack 3|6 per Fig. 3, corresponding to the called partys line. Thereupon relays 369 and 265 are operated simultaneously over a circuit from battery, through the winding of relay 265, sleeve of plug 268, sleeve of jack 3|6, and the winding of relay 369 to ground. The operation of relay 265 connects the tip and ring of the cord circuit to the tip and ring of the called subscribers loop. The called subscribers loop is open at power switch 36|. The tip circuit extends from negative battery, through resistance 465, contacts 245 and armature of relay 2| I, resistance 294, top winding of relay 2|2, winding of relay 255, contacts 292 of key 25|, contacts 276 of relay 265, tip of plug 268, tip of jack 3I6, contacts 3I5 of relay 369, contacts 3|9 of power switch 36|, through ringer 361 and condenser 363 to ground. The ring circuit extends from open contacts 326 of key 36|, through receiving magnet 362, sending contacts 363, which short-circuit resistance 366, break key 364 which short-circuits resistance 365, contacts 356 of relay 369, ring of jack 3|6, ring of plug 258. contact 26| of relay 265 through the top or line winding of relay 262 to ground. The armature of relay 262 when the relay is unoperated is connected to ground. A circuit may therefore be traced from said ground, through the armature of relay 262, contact 282 of relay 265 and the winding of relay 264 to battery, operating relay 264. This lights the calling lamp 261 as a signal that the called subscriber has not yet answered. The lamp circuit may be traced from ground, through contacts 236 and the filament of lamp 261 to battery. Relay 263 is also operated in turn by relay 264 over a circuit from ground, through contacts 285 of relay 264 and the top winding of relay 263 to battery. The operation of relay 263 at this time is ineffectual, however, as the circuit from interrupter 266 is open at contact 28T due to the operation of relay 254.

Since the tip of the subscribers loop in Fig. 3 is connected through condenser 368 to ground, relay 255 is released at this time, as is also relay 256. through contacts 266 used. to terminate the line winding of relay 2|2 is disconnected. However, its function is performed by ground through contacts 234 of relay 264, which relay is opel'- ated. The operator now rings on the calling cord. Ringing is furnished by operating key 25|. The ringing supply is alternating current superimposed on direct current. The supply circuit is in the form of a potentiometer which may be traced from ground, through the source of alternating current 252, condenser 253 and inductance 254 to battery. When key 25| is operated to ring the called party, the ringing circuit is extended through contacts 29| of key 25|, contacts 219 of relay 265, tip of plug 263, tip of jack 3|6, contacts 3|5 of relay 369, contacts 3|9 of power switch 36|, ringer 361 and condenser 368 to ground. Ringer 361 operates.

5. Called party answers In response to the ringing signal the called party operates power switch 36| to t'he alternate position to close the loop circuit by closing contacts 326. Relay 262 operates when power switch 36| is closed. This relay will operate Y whether the switch is closed during the ringing or silent interval. If the power switch 36| is operated while ringing key 25| is operated, relay 262 will be operated over the loop from the direct current supply in the ringing circuit. If the power switch 36| is closed during the silent interval while key 25| is in its normal position, relay 262 isoperated from the negative battery supplied around the loop from marking contacts 245 of relay 2| I. 'I'he operation of relay 262 removes ground from the circuit through contact 282 of relay 265 and the winding of relay 264, releasing relay 264. This in turn extinguishes lamp 261 as an indication that the called party has answered.

After the called party answers and ringing key 25| has been restored to normal, relay 255 operates as its winding is included in the loop circuit heretofore traced through the calling end of the cord and the loop is now closed. The operation of relay 255 in turn operates relay 255 over an obvious circuit. The operation of relay 256 establishes a circuit from negative battery through its top contacts and resistance 266 to the ring conductor of the called subscribers loop, to supplement the operating circuit around said loop for relay 252. This feature is the equivalent of that performed when relay 26S operates, furnishing negative battery through its top conta-cts to supplement the operating circuit around the calling subscribers loop for relay 261. Relay 263 is released when relay 264 releases, asground is removed from the top winding when contacts 285 are open. Relay 263 is provided with a locking path which may be When relay 265 operates, the ground traced from battery through the bottom winding and bottom contacts of relay 263, contacts 283 of relay.265, through contacts 226 of key 2|3 to ground but relay 263 is not locked as key 2| 3 is operated at this time and the key remains operated until the subscribers are in communication. When lamp 261 is extinguished the operator may restore her teletypewriter key 2|3 to normal, disconnecting her position circuit from -the cord and reconnecting the tip of the calling subscribers loop to the cord circuit repeater, or she may leave her position circuit in the communication path momentarily to insure that the calling and called parties have been interconnected satisfactorily.

6. Subscribers communicate with each other The manner in which the circuits function when the operators position circuit remains in the communication channel will rst be described. The circuit connecting subscribers station A with the repeater in the position circuit may be traced from negative battery, through resistance 4218 to marking contact 234 of relay 229, armature of relay 228, resistance 482, top winding of relay 228, contacts 220 of key 2|3, winding of relay 2| 0, contacts 289 of key 25|, around the calling subscribers loop and back to its termination through the top winding of relay 281 to ground as heretofore traced. For this condition the current flowing in the loop is fixed by a proper choice of battery and resistance values at 25 mils. When the calling subscriber operates the contacts of her sender |03 to send a spacing signal, the shunt around resistance |25 is opened and resistance |05 is inserted in series in the loop. For this condition the current is reduced [tol 5 mils. When current of 25 mils flows through the top winding of relay 223, its effect preponderates over the effect of the current in the lower or biasing winding of relay 228, holding the armature of relay 228 in engagement with its marking contact 232. When the line currentl is reduced for the spacing condition to 5 mils, the effect of the biasing current in the lower winding of relay 228 preponderates over the effect of the line current in the top Winding of said relay and the armature of relay 223 is operated to engage its spacing vcontact 233. These signals in turn are transmitted over the transmitting path connecting the armature of relay 228 Ithrough the top or line winding of relay 2| and the armature and marking contact of relay 2|2 to negative battery. Relay 2| follows these signals. In thisn latter transmitting circuit also the current is 25 milliamperes for the marking condition and milliamperes in the same direction for the spacing condition. When the armature of relay 228 is on its spacing contact 233 and the' negative battery connected thereto opposes the negative battery connected to contact 241 of relay 2|2, which remains on marking, the 5 milliampere current in the transmitting conductor is supplied from the mid-point of the potentiometer connected to the bottom winding of relay 229. This potentiometer is also effective to maintain current in the transmitting conductor while the armature of relay 228 is traveling between contacts 232 and 233; As relay 2|| follows the signals repeated from relay 228, the signals are in turn repeated into the called subscribers loop. For the marking condition negative battery is connected to contact 246 of relay 2| I. For the spacing condition ground is connected through contacts 248 of relay 2| For the marking condition current of 25 milliamperesflows around the called subscribers loop. For'thespacingcondition current of 5 milliamperes is supplied from the potentiometer device around the called subscribers loop. As the armature travels between the contacts current of 5 milliamperes is maintained in the loop. Receiving magnet 352 at the called subscribers station follows these signals.

When the called subscriber at station B operates sender contacts 303, the shunt around resistance' 3505 is alternately closed and opened. The current in the 4loop is 25 mils for the marking condition when the resistance is shunted and 5 mils for the spacing condition when the resistance is eliectively in circuit. Relay 2|2 is operated betweenits marking and spacing contacts in response to these signals. The signals are repeated into the circuit extending through the top winding of relay 22B to ground connected through contacts 233 and the armature of relay 212,8. `Relay 229 in turn follows these signals, repeating them into the calling subscribers loop. Receiving magnet |02 at subscribers station A responds to these signals. For these two transmitting circuits alsov the current is maintained at 25 mils for the marking condition and 5 mils 4for the spacing' condition and during the travel time of the transmitting relay.

After the operator has been assured that the parties are properly interconnected, key 2|3 will be restored to normal. This disconnects the position circuit from the cord. Thereafter, the calling subscribers transmitter will control relay 2| directly instead of through relay 228. Relay` will repeat'the callingsig'nals to Ythe calledV 2|| subscribers station.l The called subscribers sender will control relay 2| 2.` Relay 2|2 will control printer magnet |02 directly instead of through relay 228. v

Relays 2`|0 and 25,5 follow all signals. The small spacing current of 5 mils is not sufficient to hold them operated. The release of relays 2m and 25,5V during tends'to release relays 209 and 256, but these latter are slow-to-release relays. Relays 201 and 202. also tend to release on4 the spacing condition. 2.01 and 22vrel'ease the loops. When the It isrequired that relays when the `subscribers open loops are opened the leakance path between conductors may be as low as 50,000 ohinsl 1 This permits 2.6 milliamperes residual current tofiow in theloop. As the relays have to release on this condition, there is a tendencyfor them to release on the 5 milliampere condition, which represents but 2.4 milliamperes, additional current.

The circuits of this disclosure are adapted to meet practical limitations at present encountered in the operation of a practical system. Sincel the-'total'loop leakance including conductors and terminal equipment in afioopl f 3s miles of 1,9 gauge wire might be as low as 50,000 ohms, these circuitsV are 'designed' tol cooperate with a loop of this order of magnitude. As this leakance is reduced due tochanged battery terminal arrangements or by other means, or as relays 201 and 282'are made more critical, it will be possible to make the relays discriminate more closely between V`the spacing and leakance current without aitendency to vrelease for the spacing condition. As the discrimination is improved it will be possible to dispense with relays 289 and 255 communication signalingv 5 milliampere whose function it is to aid relays 201 and 262 respectively during the spacing interval.,

7. Transmitting a break signal between subscribers stations The manner in Which a break signal is transmitted between subscribers will now be illustrated.

Let us asume that the subscriber at station B is transmitting to station A andthe subscriber at station A Wishes to break. To do this break key |04 at station A will be operated. For this condition resistance IOS is inserted in series in the loop. This resistance is of such a value as to permit l mils to flow in the loop. Relay 2li) which follows signals, operating on 25 and releasing on 5 mils, also releases on 10 mils. The break will be long enough to release relay 209. The release of relay 299 will remove the negative battery connected through its contact from theend of the loop and relay 261 will of course remain connected in the loop without the aid of relay 299. Key 2I3 is in its normal position for this condition. The tip of the calling subscriber's loop is extended through the line Winding of relay 2H and resistance 295 to the armature of relay 2l2. When the armature of relay 2I2 is on its spacing contact mils only will flow through the top winding of relay 2| I. For this condition the effect of current through the bottom winding of relay 2H preponderates and the armature of relay 2H is operated to its spacing contact. When the armature of relay 2H is on spacing, it momentarily transmits only 5 mils around the loop through station B. Relay 255 releases and after a suiiicient interval relay 256 also releases. This removes direct ground connected through the bottom contacts of relay 256 to spacing contact 24B of relay 2H and connects said contact instead through the winding of relay 251 to ground.

For the normal spacing condition there is what in eiect is a potentiometer comprising two arms connected to the top winding of relay 2I2. The rst arm comprises battery, resistance 250, resistance 249 and ground. The second arm comprises resistance 290, resistance 299 and normally ground through the bottom contact of relay. 256. For this condition the direct ground through the bottom contact of relay 256 is removed and the resistance of the winding of relay -251 is inserted in series to ground in the second arm. These resistances and the battery are so proportioned that for this condition the current is changed from 5 mils to 10 mils, so 10 mils flows around the loop through subscribers station B. The reason for this is to care for the condition when both subscribers try to break at the same time and introduce resistance in each instance into the loop by inserting in the circuits the resistances which normally are shunted by their respective break keys. If there were but 5 mils ilowing this current would be further reduced and it might be diicult for relays 291 and 262 to discriminate between the reduced current and the normal leakage current. When relay 256 releases and 251 is effectively connected, relay 251 operates removing the shunt around resistance 258. This lowers the current in the biasing winding of relay 262, the circuit for which normally extends from battery through resistance 259 and the contacts of relay 251 through the bottom of biasing winding and relay 262 to ground. When relay 251 operates, resistance 258 is inserted in series in the circuit. The reason for this is to holdrelay 262 operated on the reduced direct current for the double break condition thus preventing a false disconnect.

The voltage impressed on the loop ends for the spacing condition is about 40 per cent of normal, so that the leakage current for this condition is 40 per cent of normal or approximately 1 mil. If the subscriber disconnects during this interval Ythe loop is broken but 1 mil leakance current flows through relay 252. The relay is arranged to release for this condition to give a disconnect signal. The effect of the biasing current in other words when relay 251 is operated, overcomes the l l mil leakance and relay 262 releases, but if the subscriber has not disconnected and 4 mils are flowing in the loop, the eiiect of the loop current overpowers the biasing current and relay 262 holds.

8. The called party recalls the operator The manner in which a subscriber recalls an operator while his station is still connected to a cord will now be described. Let us assume that the subscriber at station B wishes to recall the operator.

to close the loop, relay 262 reoperates in turn releasing relay 251%. This establishes a circuit from ground through the interrupter 265, the top contacts of relay 2&3, contacts 281 of relay 264 and the filament of lamp 251 to battery. This will provide a flashing lamp as a recall signal. The operator responds by operating her key 2|3 to its alternate position. This breaks ground 'from the holding winding of relay 253 at contact 225 and relay 253 releases, disconnecting inter rupter 256 and extinguishing the call lamp.

The flashing feature on both ends of the cord operate similarly and this condition will not be described for a recall sent from subscribers station A.

9. The subscribers disconnect When the subscriber at station B disconnects relay 252 releases. This connects ground through the armatureof relay 292 and contact 282 of relay 265 through the Winding of relay 254 to battery operating relay 254. The operation of relay 26:3 lights lamp 261 as a disconnect signal. Relay 263 also operates over its locking path to ground connectedY through contacts 225 of teletypewriter key 2l3 which is now normal. The operation of relay 253 performs no useful function at the moment as the flashing circuit is broken at contact 281. For disconnect during the marking condition relay 25 remains released. With relay 251 released there is full biasing current through relay 252. There is full leakance current through the line winding of relay 252 and for this condition the full biasing current predominates, releasing relay 262 to ring in lamp 261. Similarly,

when the subscriber at station A disconnects,

lamp 204 is lighted. When the operator removes plugs 2m and 26.8 the circuit per Fig, 2 is restoredto Y normal.

A busy test feature is furnished in Fig. 2. Before inserting a plug into a jack, the operator touches the tip of her plug to .the sleeve of the jack. If a cord is already connected to a multiple appearance of the same jack, battery connected to the winding of relay 282 or relay 255 will be connected to the sleeve of the jack and a circuit will be extended, in the case of the calling cord, through the tip of plug 2%, contacts 218 of Yrelay 265, winding of relay 239 to ground, operating relay 239. This will light lamp 238 as an indication that the line is busy. Similarly in the case of the answering plug, a circuit will be extended through the tip of plug 255i, contacts 259 lof relay 262 and the winding of relay 239 to kground toprovide the same test.

1l. General Although this invention has been shown herein as applied to a manual switching telegraph system, it is to be understood that the principles of operation disclosed herein may be used as well in mechanical switching telegraph systems, wherein it may be necessary to perform, not necessarily positive control of visual supervisory signals, but functions requiring positive control of a relay having a winding in a communication path which as presently arranged is subject to release due to current reversals or intervals of zero current. Such circuits as the link circuit in a mechanical system, which is generally the equivalent of a cord circuit in a manual system, may be very readily rearranged by one skilled in the art to operate according to the principles disclosed in this invention.

What is claimed is:

1. In a teletypewriter switching system, a cord circuit, an end in said cord circuit conditioned to connect to a subscribers loop circuit to form with said loop an operable telegraph path, means in said cord for establishing in said path a current of a first value and in a first direction as the rst element of a two-element communication signaling code, and means also in said cord for establishing in said path a current of a second value and in the same direction as said rst direction as the second element of said two-element cornmunication signaling code.

2. In a teletypewriter switching system, an operators position circuit, a cord circuit and a subscribers loop circuit, means in said circuits for interconnecting said circuits to form an operable telegraph path through said circuits, means in said position circuit for establishing in said path a current of a rst value and in a rst direction as a first element of a two-element communication signaling code, and means also in said position circuit for establishing in said path a current of a second value and in the same direction as said rst direction as a second element ofsaid two-element communication signaling code.

3. In a teletypewriter switching system, a cord circuit and an operators position circuit, means 6 inV said-` circuits for interconnecting said circuits to form an operable telegraph path, means also in said circuits for establishing in said path a current of a rst value and in a first direction as f 4. In a teletypewriter switching system, a cord circuit, a repeater therein, a flexible switching means ln said cord, a communication channelA connecting said repeaterand said means, and means also in said cord for establishinga flow of current of two different values in the same direction in said channel as two different elements of a communication signaling code as said repeater repeats communication signals toward said switching means.

5. In a teletypewriter switching system, a link circuit, an answering and a calling switch in saidv circuit, a repeater in said circuit,l a first telegraph channel connecting said answering switch and said repeater, a second telegraph channelA connecting vsaid calling switch and said repeater, and means connected tosaid circuit to impose current of two different valuesr in one and the same direction in one of said channels to comprise the elements of atwo-current communication signaling code.

6. InV a. teletypewriter switching Y system, a switching .circuit forflexibly interconnecting one of a plurality ofcalling circuits-withaspecified u called circuit, a repeater in said switching circuit, a telegraph channel in said switching cirengage a marking and a spacing contact on saidrelay for thetransmission of communication signals, and means in said circuit for imposing on:

said channel unidirectional current while` said armature is in engagement with each of said contacts and while it travels between said contacts.

8. In a telegraph switchingl system, a subscribers line and station circuit, a cordcircuit, means for interconnecting said circuits to form an operable telegraph path, means in said circuits for transmitting current of a iirst value and in a rst direction asone element of a two-element communication signaling code in signaling from said station to vsaid cord and control means in said station circuit forchanging the value of said current while its direction remains the same as said rfirst direction as a secondfelement of said two-element communication signaling code.

9. In a telegraph switching system, a rst subscribers station, a second subscribers station, a

central switching station intermediate said stations, an operable telegraph communication channel connecting said subscribers stations through saidl switching station, means in said channel for transmitting through said channel current of two different-values each of the same polarity to form the elements of .a two-element communication4 code as said irst station transmits toward said second-station.

10. In a telegraph switching system, a subscribers station, a central switching station', a telegraph loop connecting said stations having `a first and a second end terminating in said switching station, a cord circuit at said switching station having a rst conductor and a second conductor, means in saidcircuits for connecting said first conductor to said rst end and said second conductor to said second end, a telegraph repeater connected in said cord to said rst conductor, and a polar relay in said cord having a winding in series in said second conductor.

11. In a telegraph cord circuit, a polar relay controlling supervisory signals, said relay having a winding in series with a telegraph line subject to leakance, means for impressing a potential of a rst magnitude on said line at a first time, means for impressing a potential of a second magnitude, substantially different from said iirst magnitude on said line at a second time, both of said potentials being so conditioned as to change the magnitude and maintain the direction of the leakage current in said line, a biasing winding on said relay, variable resistance means connected to said biasing winding and means for controlling said resistance as said pctential is changed to compensate for said changed leakage current.

12. In a telegraph system, an operable telegraph loop, a -telegraph repeater connected to the first end and a polar supervisory relay connected to the second end oi said loop, a winding on said relay in series with said loop, means connected to said loop for imposing a potential of a first magnitude and a rst polarity between the ends of said loop for the marking signaling condition, means for changing said potential to a second magnitude and maintaining said polarity for the spacing signaling condition, and means for changing said potential to a third magnitude and maintaining said polarity for the breaking condition.

1-3. A telegraph cord circuit connected to a telegraph loop circuit, a first and a second telegraph signaling conductor in said circuits, a polar relay controlling a supervisory signal, said relay having a rst winding in series in said rst conductor, a first source of potential connected between s-aid conductors for a first open circuit condition, leakage current of a ilrst magnitude ilowing in said conductors for said rst condition, a second source of potential connected between said conduct-ors for a second open circuit condition, leakage current of a second magnitude flowing in said conductors for s-aid second condition, a biasing circuit including a second winding on said relay, and means connected to said biasing circuit for changing the current in said biasing circuit for said two conditions.

14. In a telegraph switching sys-tem, a first subscribers station, a second subscribers station, a central station, an operable telegraph path interconnecting said subscribers stations including a cor-d circuit at said central station, means connected to said path for imposing on saidpath currents of two different values, both of the same polarity, to form elements of a twoelement communication signal-ing code, and supervisory signal means in said cord responsive to control means at said rst subscribers station.

15. A system in accordance with claim 14 in which the supervisory signal means is of a plurality of characters.

16. A system in accordance with claim 14 in which the control means is a parallel circuit having a plurality of branches in series in said path.

17. A system in accordance with claim 14 including means connected to said path for stopping the transmission of communication sign-als from said second station to said first station and conditioning said path for the transmission of lil.

communication signals in the opposite direction.

18. A system in accordance with claim 14 in which said supervisory signal is a recall signal to attract the operators attention to said path.

19. In a cord circuit in a telegraph switching system, a method of preventing the false release of a relay controlling a supervisory signal in said cord which comprises (1) transmitting direct current of dilerent magnitudes and of the same polarity through a telegraph channel in said cord .tc form the elements ci a two-element communication signaling code, and (2) transmitting direct current of the same polarity through said channel during the intervals between the transmission of said signal elements, so that direct current uniform in direction flows through a winding of said supervisory relay at all times while said cord circuit is operably connected in la communication channel.

20. In a teletypewriter switching system, a cord circuit, a telegraph communication path in said circuit, a relay controlling a supervisory signal in said circuit, a winding of said relay in series in said path, and means in said circuit for maintaining a direct current uniform in direction through said winding while communication signals are being transmitted through said path.

21. In -a teletypewriter switching system, a subscribers line and station loop circuit, a cord circuit connected to said line circuit to form therewith an operable telegraph path, means in said circuits for maintaining current through said loop at all times during communication, and means in said circuits in response .to the opening of said loop so that no current flows in either loop conductor to operate a supervisory signal ln said cord.

22. In a direct current telegraph transmission system, a switching office, a manual cord circuit in said -oiiice connected to a subscribers line circuit from said ofIice to a subscribers station, means for telegraphing over said circuits by marking current of a certain value and spacing current of a substantially different value and of the same polarity as said marking current.

23. A telegraph system in accordance with claim 22, including means for transmitting a break signal current over said circuits of a value intermediate said marking and said spacing current and of lthe same polarity as said marking and said spacing current.

24In a direct current telegraph transmission system, a switching cnice, a manual cord circuit in said ofce connected through a rs't subscribers line circuit to a first subscribers station. circuit and through a second subscribers line circuit to a second subscribers station -circuit t-o form an operable telegraph circuit connecting a rst and a second telegraph station, and means in said circuits for telegraphing thereover by marking current of a first value and of a rst polarity and spacing current of a substantially different value from and of' the saine polarity as said marking current,

25. A telegraph system in accordance with yclaim 24 including means for transmitting a break signal current over said circuits of a value intermediate said marking and spacing current and of the same polarity as said marking and said spacing current.

FULLERTON S. KINKEAD.

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