Volume Ii Part 11 (2/2)

Called-Line Busy:--It will first be a.s.sumed that the called line is busy. The testing operation at the connectors occurs in the second position of the side switch. If the called line is busy, it will be either because it is connected to by some other connector or because it has itself made a call. In the former case the private contacts of that line in the banks of all the connectors serving that hundreds group of lines will be grounded through the private wiper of some other connector. That this is so, may be seen by tracing the circuit from the private wiper on the shaft to the third side switch wiper which is grounded in the third position; the other connector that has already engaged the line will, of course, have its side switch in its final, or third position. Again, if the line called is busy, because a call has already been made from this line to some other line, the private contacts on the connectors corresponding to the line will be grounded, as will be seen by tracing from the private bank contacts, which are shown in Fig. 396, through the private wire to the line switch, which is shown in Fig. 389, and from thence to ground through the springs _19_ and _20_, which are brought together when the line switch is operated.

In any event, therefore, the determining condition of a busy line is that its private bank contacts on all connectors of its group shall be grounded. Under the present a.s.sumed condition, therefore, the connector wipers, which have been brought to the bank contacts of the desired line, will find a ground at the private bank contact. The connector shaft stops for an instant on the contacts of this busy line and immediately there follows over the rotary side of the line the inevitable single rotary impulse. This energizes the rotary relay and this, as usual, energizes the private magnet. Remembering now that the connector side switch is in its second position and that the private wiper of the connector has found a ground, we may trace back from the private wiper through the third side switch wiper to its second contact; thence through the contact springs _14_ and _15_, closed by the private magnet; thence through the release magnet; thence through the contact springs _16_ and _17_ of the calling battery supply relay to the live side of the battery. This calling battery supply relay will, at this time, have its core energized because the coil _18_ is in series with the rotary relay coil which, as just stated, was energized by the last rotary impulse. This series of operations has led to the energizing of the release magnet, and, as a result, the double dog of the connector is pulled out of the connector shaft ratchets and the shaft and the side switch are restored to their normal position.

Busy-Back Signal:--The connector has dropped back to normal in all respects. The calling subscriber, not knowing this, presses his ringing b.u.t.ton. This grounds the vertical side of the line at his station and operates the vertical relay at the connector. This steps the shaft of the connector up one step and causes the closure of the contacts _19_ and _20_ at the top of the connector shaft. This establishes a connection to a circuit carrying periodically interrupted battery current on which an inductive hum is placed. This circuit may be traced from this source through the springs _20_ and _19_ to the first wiper of the side switch, thence through the normally closed contacts of the ringing relay to the rotary side of the line, and the varying potential to which this path is subjected produces an inductive flow back to the calling subscriber's telephone, and gives him the necessary signal which consists of a hum or buzzing noise with which all users of automatic systems soon become familiar.

Release on Busy Connection:--The connector, since its last release, has been stepped up one notch and must again be released. When the subscriber hangs up his receiver after receiving the busy signal, he grounds both sides of his line momentarily by the action of the springs _21_, _22_, and _23_ of Fig. 384. This operates the rotary and the vertical relays on the connector simultaneously and brings together for the first time the springs _21_ and _22_ of Fig. 396. This establishes a connection from the battery through the springs _16_ and _17_ on the calling battery supply relay, thence through the release magnet of the connector, thence through the springs _22_ and _21_ of the vertical and the rotary relay, thence through the release trunk back to the second selector. From here the circuit pa.s.ses through the private wiper of that selector and the back release relay to ground through the third side switch wiper which is in the third position. Considering this circuit in respect to its action on the connector it is obvious that it energizes the release magnet on the connector which restores the connector to normal as before. At the second selector this circuit pa.s.sed through the back release relay, which closed a circuit through the release magnet and through the back release relay contacts, thence back over the second selector release trunk to the back release relay of the first selector, and through the third wiper of the side switch on that selector to ground, since that side switch also is in its third position. The current through this circuit energizes the release magnet of the second selector and restores it to its normal position and also energizes the back release relay of the first selector. This, in turn, closes the circuit from the battery through the release magnet of the first selector and contacts of the back release relay to ground. This works the release magnet of the first selector and restores that selector to normal. The contacts on the first selector release magnet, shown in Fig.

390, are closed by the action of the release magnet and this closes the path from ground back through the first selector release wire, and through the contacts _13_ and _14_ of the line switch, through the line switch release magnet to battery, and this restores the line switch to normal.

The reason for the term _back release_ will now be apparent. The release operation at the connector is relayed back to the second selector; that of the second selector back to the first selector; and that of the first selector back to the line switch. Until this plan was adopted, the release magnet of each selector and connector involved in a connection was left bridged across the talking circuit so as to be available for release; and it sometimes occurred that a first selector would be released before a second selector or connector, which latter switches would thus be left off-normal until rescued by an attendant. The back release plan makes it impossible for the connection necessary for the release of a switch to be torn down until the release is actually accomplished.

Called Line Found Idle:--It will be remembered that, before the digression necessary to trace through the operations occurring upon the finding of a busy line, the connector wipers had been brought, by the influence of the calling subscriber's impulses, into engagement with the contacts of the desired line; that the connector side switch was in its second position; and that the final rotary impulse following the last series of selecting impulses had not been sent. The condition now to be a.s.sumed is that the called subscriber's line is free and the private wiper, therefore, has found and rests on an ungrounded private bank contact. The final rotary impulse which immediately follows will operate the rotary relay and this, in turn, will operate the private magnet.

This happened under the a.s.sumed condition that the line was busy, but in that case the release magnet was also operated at the same time and restored all conditions to normal. Under the present condition the operation of the private magnet will perform its usual function and move the side switch of the connector into its third position.

_Third Position of Side Switch._ When the side switch of the connector moves to its third position, it, as usual, cuts the talking circuit straight through from the vertical and the rotary sides of the trunk leading from the previous selector to the outgoing terminal of the subscriber's line, which may be traced upon Fig. 396 back through the line switch, shown in Fig. 389. Several things are to be noted about the talking circuit so established: First, the inclusion of the condensers in the vertical and the rotary sides of the connector circuit. The purpose of this will be referred to later. Second, the inclusion in this circuit at the connector of a pair of normally closed contacts in the ringing relay. It may be said in pa.s.sing that the ringing relay corresponds exactly in function to a ringing key in a manual switchboard. Third, the talking circuit leading from the connector to the called subscriber's line pa.s.ses on one side through the springs _24_ and _25_ of the bridge cut-off relay of the line switch, which is shown in Fig. 389. These springs are normally open and would prevent the completion of the talking circuit but for the fact that the bridge cut-off relay of the line switch is energized over the private wire leading to the connector bank and then through the connector wiper to the third side switch wiper which, at this time, is in its third position. The talking circuit is thus complete. The operation of this bridge cut-off relay on the line switch has not only completed the talking circuit but it has also opened the circuit of the trip magnet of the line switch so as to prevent the operation of the trip magnet by the subscriber on that line in case he should attempt to make a call during the interval between the time when his line was connected with by the connector and the time when he answers the call.

The third wiper of the connector side switch when moved into its third position, puts the ground on all of the private bank contacts of the line chosen and thus guards that line against connection by others, as already described. It also operates the bridge cut-off relay of the line switch as just mentioned.

The fourth wiper of the side switch, when moved into its third position, establishes such a connection as will place the ringing relay under the control of the vertical relay. This may be seen by tracing from ground to the vertical relay springs _23_ and _24_, thence through the normally closed upper pair of contacts on the private magnet, thence through the fourth wiper on the side switch to its third contact, thence through the ringing relay magnet, and through the springs _16_ and _17_ of the calling battery supply relay and to battery. The calling battery supply relay winding being in series with the vertical relay winding, the two operate together and close the two normally open points in the ringing relay circuit. This ringing relay acts as an ordinary ringing key and connects the generator to the called subscriber's line in an obvious manner, at the same time opening the talking circuit back of the ringing relay in order to prevent the ringing current chattering the relays in the circuit back of it. All that remains now is for the called subscriber to respond. When he does he closes the metallic circuit of the line through his talking apparatus.

_Battery Supply to Connected Subscriber._ Throughout the whole process of building up a connection, it will be remembered that both sides of the calling line are connected through the respective vertical and rotary relays involved in building up the connection with the live side of the battery. At the time when the connection is finally established and the called subscriber rung, both sides of the calling line are connected through various relay windings to the live side of the battery. Such a condition leaves both sides of the line at the same potential and, therefore, there is no tendency for current to flow through the calling subscriber's talking apparatus, even though it is connected across the circuit of the line. It remains, therefore, to be seen how these conditions are so changed after the building up of a connection as to supply the calling subscriber with talking current.

The calling subscriber can get no current until the called subscriber responds. When the connection is first made with the called subscriber's line, battery connection to his line is made from the live side of battery through the normally closed contacts of the calling battery supply relay, thence through the winding _25_ of the called battery supply relay to the vertical side of the called line. The grounded side of the battery is connected to the rotary side of his line through the third wiper of the connector and the coil _26_ of the called battery supply relay. As a result, this subscriber receives proper talking current through the coils _25_ and _26_, and this relay is operated by the flow of this current. The operation of this called battery supply relay merely s.h.i.+fts the connection of the rotary side of the calling subscriber's line from its normal battery connection, to ground, and thus the battery is placed straight across the calling subscriber's line so as to supply talking current. This supply circuit to the calling subscriber may be traced from the live side of the battery through the winding _13_ of the calling battery supply relay and the winding of the vertical relay to the vertical side of the line, and from the grounded side of battery through the third side switch wiper in its third position to the now closed pair of contacts in the called battery supply relay through the coil _18_ of the calling battery supply relay and the coil of the rotary relay to the rotary side of the line.

It will be noted that the system of battery supply is that of the standard condenser and r.e.t.a.r.dation coil scheme largely employed in manual practice; and that aside from the coils through which the battery current is supplied to the connected subscribers, there are no taps from, or bridges across, the two sides of the talking circuit.

=Release after Conversation.= It remains now only to secure the disconnection of the subscribers after they are through talking. When the calling subscriber hangs up, the whole disconnection is brought about, all of the apparatus, including connector, selectors, and line switch, returning to normal. This is done by the back release system and is accomplished in almost the same way as has already been described in connection with the disconnect after an unsuccessful call. There is this difference, however: after an unsuccessful call when the line called for was found busy, the release was made while the connector side switch was in its normal position. In the present case, the release must be made with the connector side switch in its third position and with the talking battery bridged across the metallic circuit rather than connected between each limb of the line and ground. It must be remembered that the calling battery supply relay, while traversed by current during the conversation, is not magnetically energized because, with the current flowing through the metallic circuit of the line, the two windings exert a differential effect. As soon, however, as the calling subscriber hangs up his receiver, this differential action ceases, due to the grounding of both sides of the line at the subscriber's station. This relay, therefore, operates and cuts off battery from the called battery supply relay and this, in turn, releases its armature and thus changes the connection of the rotary side of the calling line from ground to live side of the battery. The normal condition of the battery connection now being restored, both the vertical and the rotary relays at the connector become operated, due to the ground on both sides of the line at the subscriber's station, and this, as we have seen, is the condition which brings about the operation of the connector release magnet, and the relaying back of the disconnect impulse successively through the selectors to the line switch.

=Multi-Office System.= In exchanges involving more than one office, the same general principles and mode of operation already outlined apply. If the total number of subscribers in the multi-office exchange is to be less than ten thousand, then four digit numbers suffice, and the first movement of the dial may be made to select the office into which the connection is to go, the subscribers' lines being so numbered with respect to the offices that each office will contain only certain thousands. The choosing of the thousand by the calling subscriber, therefore, takes care in itself of the choice of offices. Where, however, a multi-office exchange is to provide for connections among a greater number of lines than ten thousand and less than one hundred thousand, then it will take five movements of the dial to make the selection--the five movements corresponding either to the five digits in a number or to the name of an office, as indicated on the dial, and the four digits of a smaller number. The lines may all carry five digit numbers or, what is considered better practice, may be designated by an office name followed by a four digit number. In this latter case the numbers of the subscribers' lines will in each case be contained in one or more of the tens of thousands groups, no number having more than four digits. And the first movement of the dial, whether the name or number plan be adopted, will select an office; or, looking at it another way, will select a group of ten thousand and this being done, the next four successive movements of the dial will select the numbers in that ten thousand in just the some way that has been already described.

Certain difficulties arise, however, in multi-office working due to the fact that the three-wire trunks between offices would in most cases be objectionable. As long as the trunks extend between the various groups of apparatus in the same office, it is cheaper to provide three wires for each of them than it is to make any additional complication in the apparatus. Where the trunking is done between offices, however, the system may be so modified as to work over two wire inter-office trunks.

_The Trunk Repeater._ The purpose of the trunk repeater is to enable the inter-office trunking to be done over two wires. It may be said that the trunk repeater is a device placed in the outgoing trunk circuit at the office in which a call originates, which will do over the two wires of the trunk leading from it to the distant office just the same thing that the subscriber's signal transmitter does over the two wires of the subscriber's lines. It has certain other functions in regard to feeding the battery for talking purposes back to the calling subscriber's line, taking the place in this respect of the calling battery feed relay in the connector in a single office exchange.

[Ill.u.s.tration: Fig. 397. Circuits of Trunk Repeater]

The circuits of a trunk repeater are shown in Fig. 397. In considering it, it must be understood that the three wires entering the figure at the left are the vertical, rotary, and release wires of a second selector trunk leading from the first selector banks in the same office.

The two wires leading from the right of the figure are those extending to the distant office, and terminate there in second selectors. The vertical and the rotary sides of this trunk as shown at the left will receive the impulses from the subscriber's station coming through the line switch and the first selector, as usual. The vertical impulses will pa.s.s through the winding of the vertical relay and through the winding _1_ of the calling battery supply relay and thence to battery, the same as on a connector. These impulses will work the armatures of both of these relays in unison. The movements of the vertical relay armature in response to these impulses will cause corresponding impulses to flow over a circuit which may be traced from ground, through the springs _3_ and _2_ of the vertical relay, the springs _4_ and _5_ of the bridged relay _6_ and thence to the vertical side of the trunk and to the distant office, where it pa.s.ses into a second selector and through its vertical relay to battery. Thus the vertical impulses are pa.s.sed on over the two-wire trunk to the second selector at the distant office. It becomes necessary, however, to prevent these impulses from pa.s.sing back through the winding of the bridge relay _6_ and this is done by means of the sluggish relay _7_. This relay receives local battery impulses in unison with those sent over the trunk by the vertical relay, these being supplied from the battery at the local office through the contacts _8_ and _9_ of the calling battery supply relay, which works in unison with the vertical relay. These rapidly recurring impulses are too fast for the sluggish relay _7_ to follow. And this relay merely pulls up its armature and cuts off both sides of the trunk leading back to the first selector. The rotary impulses are repeated to the rotary side of the two-wire trunk in a similar way.

Considering now the operation of the trunk repeater in the reverse direction, the action of the bridging relay _6_ is of vital importance.

Normally both sides of trunk line are connected to the live side of the battery and, therefore, there is no difference of potential between them and no tendency to operate the bridged relay. When the connection has been fully established to the subscriber at the distant office, and that subscriber has responded, the action of his battery supply relay will, as before stated, change the connection of the rotary side of the line from battery to ground, and thus bridge the battery at the distant exchange across the trunk. This action will pull up the bridged relay _6_ at the trunk repeater and will perform exactly the same function with respect to the connection of the battery with the calling subscriber's line. In other words, it will change the connection of the rotary side of the calling line from battery to ground, thus establis.h.i.+ng the necessary difference in potential to give the calling subscriber the necessary current for transmission purposes. The disconnect feature is about the same as already described. When the calling subscriber hangs up his receiver both the vertical and rotary relays of the trunk repeater operate, which places the ground on both sides of the two-wire trunk to the distant office, which is the condition for releasing all of the apparatus there.

For the purpose of convenience the simplified diagram of Fig. 398 has been prepared, which shows the complete connection from a calling subscriber to a called subscriber in a multi-office exchange, wherein the first movement of the dial is employed to establish the connection to the proper office and the four succeeding movements to make a selection among ten thousand lines in that office. This circuit, therefore, employs at the first office the line switch, the first selector, and the trunk repeater; and at the second office the second selector, third selector, connector, and line switch.

The third selector is omitted from Fig. 398, but this will cause no confusion, since it is exactly like the second selector. The circuits shown are exactly like those previously described but in drawing them the main idea has been to simplify the connections to the greatest possible extent at a sacrifice in the clearness with which the mechanical inter-relation of parts is shown. No correct understanding of the circuits of an automatic system is possible without a clear idea of the mechanical functions performed by the different parts, and, therefore, we have described what are apparently the more complex circuit drawings first. It is believed that the student, in attempting to gain an understanding of this marvel of mechanical and electrical intricacy, will find his task less burdensome if he will refer freely to both the simplified circuit drawing of Fig. 398 and the more complex ones preceding it. By doing so he will often be enabled to clear up a doubtful circuit point from the simpler diagram and a doubtful mechanical point from those diagrams which represent more clearly the mechanical relation of parts.

[Ill.u.s.tration: Fig. 398. Connection between a Calling and a Called Subscriber in an Automatic System]

=Automatic Sub-Offices.= Obviously, the system of trunking employed in automatic exchanges lends itself with great facility to the subdivision of an exchange into a large number of comparatively small office districts and the establishment of branch offices or sub-offices at the centers of these districts.

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