BE415524A - - Google Patents

Description

       

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  Improvements to telephone systems.

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   The present invention relates to telephone systems. It relates more particularly to telephone systems in which the interconnection between lines is effected by the operation of automatic switching devices. The object of the invention is to provide a new and improved telephone system in which the importance of the common elements blocked when a communication is established between two lines is reduced to a minimum.



   According to one of the characteristics of the invention, in a telephone system comprising several stages of main switches, through which a conversation takes place under the control of corresponding stages of bypass switches, the bypass switches are temporarily associated, at certain stages, with the main switches by the intermediary of a finder which controls the positioning of the main switches according to the positions taken by the bypass switches.



   According to another characteristic of the invention in a telephone system using digital switches for the establishment: temporary of a connection to a called line and non-digital switches for maintaining the connection established by the digital switches, digital and non-digital switches of one selector stage have access to different groups of digital and non-digital switches, respectively, of the next selector stage, with a digital one-stage switch associated with the non-digital switch correspondent by means of a finder who controls the establishment of the connection via the non-digital switch to resume a temporary connection established by the digital switches.



   According to another characteristic of the invention in a telephone system having a group of lines and

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 a group of connection circuits which allow communications to be established under the control of a bypass circuit, when a line is in call a connection is established by the bypass circuit which connects to a free connection circuit which combines the calling line and the connecting circuit.



   According to another characteristic of the invention in a telephone system the lines entering a selection stage are divided into groups, each group is associated with a group of outgoing lines and the seizure of an incoming line is used to activate a finder. having access to all outgoing lines to establish a connection with an outgoing line of the appropriate group, the operation of the searcher being controlled by a wire common to the group of incoming lines to which the calling line belongs.



   According to another feature of the invention in a telephone system a group of lines entering a digital selection stage has access to a certain number of groups of outgoing lines and an incoming line awaiting a call is connected to one of the lines of the group of outgoing lines. outgoing lines under the control of common devices temporarily associated with the calling line, the common devices being placed according to the designation assigned to a desired line in the group of outgoing lines, which selects a free line in the desired group and marked under Common Device Control, and establishes a connection between the incoming line and the marked outgoing line.



   According to another characteristic of the invention in a telephone system using researchers for the search for a calling line, the lines are divided into groups, each group being accessible to a certain number of searchers and having an associated starting relay. to these, the operation of the finder when a line of one of the groups is in

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 call instance being controlled jointly by the starter relay and the devices common to all the lines and having access to researchers accessible to all groups of lines.



   According to another characteristic of the invention two groups of switches each have access to a group of lines and a connection is established for any one of the groups of lines by first establishing a bypass connection by the interconnection of a line of any of these groups and of any of the switches which have access to it under the control of test units comprising a group wire, common to the group of lines in question.



   The attached plans which include figs. 1 to 12 show in the ordinary form of schematic circuits a sufficient part of the equipment of a telephone system to which the invention is applied to make it possible to understand it.



  More particularly fig. 1 to 6 show selection devices through which a connection can be established between the line of station A, fig. 1 and the line of station B, fig. 6; figs. 7 to 12 show common or bypass control equipment forming a train of bypass switches which are temporarily put into service during the establishment of a communication but which do not form part of the established conversation circuit; fig. 1S (on the sheet of fig. 3) shows an auxiliary circuit which can route a communication in the event that at a determined selection stage all the connection circuits are occupied and prevent connection to the requested line.



   In the conversation circuit two non-digital selection stages have been provided, they are represented respectively by the primary finder PF in FIG. 1 and the secondary researcher SF of FIG. 2; three digital selection stages are provided, they are represented by the selector

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 thousands TS, fig. 3, the hundreds PS selector, fig. 5 and the FS connector or final selector, fig. 6. When the normal decimal system is used, the three digital selection stages allow a final capacity of 10,000 lines.



   Each of the single movement rotary switches of FIGS. 1 to 6 is assumed to have 50 contacts in the bank of each brush as shown by the number 50 placed next to these switches. By the double brush artifice, as you can see, the capacity of each switch is increased to 100 pins.



   The commissioning of the switches used for conversation and that of the bypass circuit switches will be examined below.



   From the point of view of the operation of the bypass selection train, the calls are distributed by DS distributors, one group of which is assigned to groups of 100 subscriber lines in the present disclosure. Each distributor has a switch, such as the TSI switch, having access to 50 primary researchers, such as the PF primary researcher, fig. l. It is assumed that 10 primary researchers are sufficient to serve a sub-group of 100 subscriber lines and that 50 researchers therefore serve 5 groups of 100 lines, so they are all accessible by a 50-pin switch such as the switch. TSI, fig. 7.



   The group of 500 lines listed above are assigned as many distributors as necessary, as well as an additional distributor DS 'comprising a switch TS1'.



  The banks of the switches of the same group are suitably multiple among themselves as shown on the line. 740 connecting the two braces.



   The DS distributor has access, in common with all the other distributors associated with the same group of 500 lines and with distributors of other groups of 500 lines, to 25

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 thousands bypass selectors, one of which, BTS, is shown in fig. 9. Accordingly, each of the semicircles of bank contacts associated with the DS distributor brushes contains 25 contacts as indicated by the number (25) placed above the switch. This number is somewhat arbitrary because in practice it may be necessary to use a larger or smaller number of pins.



   In common with other bypass thousands selector, the BTS bypass thousands selector in fig. 9 (which has 10 levels: of bank contacts) can have access to the group of bypass eent selectors, one of which, BPS, is shown in fig. 11.



   To carry out the test of the called lines and to associate the final selectors (such as the FS selector in fig. 6) with the requested lines, final by-pass selectors are provided (such as the BFS final bypass selector in fig. 12) each of them having access to the terminals of five groups of 100 lines.



  Therefore the same final bypass selector can be reached from five levels of the selectors of hundreds bypass. The terminals shown opposite brushes 1114 to 1118 in the BPS bypass hundreds selector, fig. There, are supposed to be the first 5 pins of levels 1 to 5. It should be noted that with regard to the pins accessible to brushes 1115, 1116 and 1118 the terminals shown are multiplied together and are connected to wires 1137, 1136 and 1130 ending in the BFS bypass final selector. The second pins of the same levels can be multiplied in an identical way and connected to the second final bypass selector etc.

   With this arrangement the same final bypass selector can be entered following the emission of a number which directs the selector of hundreds bypass to any of the five levels. Therefore, so that the final bypass selector can make a

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 distinction between the five groups of 100 lines to which it has access, the contacts accessible by the line test brush 1117 of the BPS bypass hundreds selector are not multiplied between them but each of them is connected to a brush separate from the BFS final bypass selector, it follows that the group of 100 lines to which one is actually connected is determined by the level through which the final BFS bypass selector is entered.

   The levels from five to ten of the BPS bypass hundreds selector are likewise associated with another group of 500 lines through another group of final bypass selectors.



   The function of the non-digital switch TS1 in fig. 7 having been described in a general manner, attention will be paid to the first and second stage switches TS2 and TS3 of figs 8 and 10. The switch TS2 of fig. 8, has access to a maximum of 50 connection circuits such as the circuit interconnecting the secondary searcher in fig. 2 to the selector of ¯ thousand TS in fig. 3.

   When the BTS bypass thousands selector, fig. 9, is seized (via wires 741 to 745 by a distributor such as distributor DS in fig. 7) the selector TS2 operates to select a free connection circuit, including the secondary finder, such as the SF researcher, has access to the primary researcher (such as the PF primary researcher) activated by the switch TS1.

   Then, the circuits of the rod 7 and of FIG. 8 cooperate to cause the secondary searcher to search for the primary searcher used to search the calling line, as a result the conversation circuit extends from the calling line through a primary searcher and a secondary searcher to a thousands selector, such as the thousands selector TS in fig. 3.



   When the BPS bypass hundreds selector in fig.



  11 is seized (through wires 931 to 935 by a

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 thousands bypass selector such as the BTS selector in fig. 9) switch TS3 selects a function - resulting in a hundreds selector (such as the PS selector fig. 5) which is accessible to a particular thousands selector, such as TS, activated by the previous switch, such as TS2.



  Then, the circuits of fig. 8 and cooperate to put the thousands selector in position on the junction selected by the switch TS3, thereby the conversation circuit extends from the calling line to a hundreds selector, such as the selector PS of FIG. 5. -
Then a common level wire, such as the common fifth level wire 1138 is marked by the position taken by the BPS bypass hundreds selector and the entered hundreds selector PS is controlled by the circuits of FIG. 10 to select a final selector from the group determined by the level, from which the BPS bypass hundreds selector has been operated.



   In principle, the final bypass selector entered (such as the BFS final bypass selector, fig. 12) operates to establish the connection with the called line and to carry out the test to ensure that it is busy or free, then the final selector, such as the FS final selector, fig. 6, completes the conversation circuit to the line, if the latter is free, under the control of the circuits of fig. 10 in conjunction with the circuits of fig. 4, and as determined by the position taken by the hundreds bypass selector and the final bypass selector (such as the BPS selector, fig. 11, and the BFS selector, fig. 12).



   As will be explained in more detail below: (1) the connection established to a bypass thousands selector, such as the BT3 selector, fig. 9, through a distributor such as that shown in FIG. 7 is released as soon as the cir-

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 Conversation output has been established to a thousands selector, such as TS, under the control of the non-digital stage and first stage switches, after which the connection is maintained between the calling line and the thousands selector by pass through a switch such as switch TS2;

   (2) as soon as the connection has been extended by a thousands selector (such as TS fig. 3) to a hundreds selector (such as PS fig. 5) the thousands selector bypass (such as BTS, fig. 9) and the associated switch are released, the connection to a bypass hundreds selector (such as BPS) of the calling line is maintained through a switch, such as TS3;

   finally (3) as soon as the connection has been established to the requested line (through a hundreds selector such as PS fig. 5), and a final selector (such as FS fig. 6) the connection of the calling line to the bypass hundreds selector (such as BPS fig. 7 via a switch such as TS3) is released and the bypass hundreds selector, as well as the final bypass selector are released and ready to serve a new call.



   From the general description given above of the operation of the system it can be seen that a first stage switch, fig. 8, by performing the test of the connection circuits to which it has access must not only find a free circuit but must find a free circuit of which the secondary researcher such as SF has access to the primary connection circuit to which the connection has been made by the non-digital switch of fig. 7 because it would be useless to select a connection circuit in a group which does not have access to the primary researchers of the calling group. In order to enable this group test, a common group thread 200 is provided for the group of secondary researchers in which the secondary researcher SF is located.

   All these secondary researchers have their benches connected in multiples in the usual way, and the pins

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 of these multiple contacts are normally connected through the distributor to the primary searchers of the groups of 100 lines. The common group wire 200 is multiple to the contacts on the right side of the distributor DF1, such that an individual wire, such as 124, from each of the primary junctions accessible to the secondary researchers of the group is connected to the common group wire 200.



   At the other end the common group wire 200 is multiple at various sets of contacts at the top of the. distributor DF5, so that each of the access points dtun. A switch to a secondary finder such as SF includes a wire such as 235 connected to common group wire 200. Following the normal practice of keeping junctions accessible by various switches, a switch such as TS2 may have access to terminals. secondary researchers in a number of groups, as has been said above, and by performing a test in its bench the switch can encounter functions of several groups, and can successively test several wires of common groups such as 200 .



  All of these common group wires are, however, kept separate from each other and no bond exists between one common group wire and another, so that the group test can be done accurately in a way that will be described later.



   Likewise, a separate common group wire (such as 300) is provided for each group of thousands selectors (such as TS) to direct the search for the second stage switches.



   Each of the different distributors, DF1 to DF10, shown on the lans can be made up of connection blocks suitably mounted with rope rings. Banes wires are attached to the terminals of some of these terminal blocks, while wires terminating at switches

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 of the next floor are attached to the pins of other blocks, flying wires being pulled between the blocks as needed to make the various interconnections of the bench wires with the jumper wires.



   At this point it may be mentioned that the switches are usually mounted in groups of 10 or 20 and that the banks of 10 switches are usually permanently multiple by what is called a multiple bank and that a group of bank wires connects one of the banks. 10 switches to one connection block. The group of 10 can then be multiplied with one or more other groups of 10 if desired, the entire group thus formed is connected to jumper wires as the initiated know.

   This brief explanation is given with the aim of making clear the use of the various distributors which are those which are normally encountered in automatic offices to allow the correct connection of the connection blocks and the pins of the junctions and also to understand that these dispatchers are not unique to the new system discussed here.



   After the general description which has just been given, a more detailed description of the various devices will be undertaken. For this purpose it will first be admitted that the subscriber connected to station A in FIG. 1 wishes to converse with the subscriber connected to set B in fig 6.



   When picking up at station A the relay 104 of the individual LC line air- bake draws wires 101 and 102 into the circuit and through the contacts of relay 105. Pulling relay 104 disconnects test wire 103 from the multiple. on the called side of the calling side test lead and connects it to ground to mark the calling line Unoccupied, it connects resistor 107 to its armature / bottom in parallel with the coil of relay 105 to indicate that the line is on. appeal body in the benches of researchers, such as PF,

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 having access to the calling line; at its middle armature it places a positive, coming from the contacts of relay 105, on wire 120 belonging to the group of 100 lines which contains the calling line.



   The relay 701 of the group of relays 701 to 705 now attracts under the action of the positive placed on the wire 120.



  Pulling relay 701 puts a positive on wire 745 at its inner frame while at its outer frame it connects wire 746 to wire 751 belonging to the associated 100 line group. Wire 751 is interconnected to distributor DF4 to wire 131 associated with primary finder PF, fig. 1, and to each of the corresponding threads (not shown) associated with the other researchers in the same group of 100 lines.



   Relay 708 of the DS distributor attracts through the contacts of relay 709 due to the presence of a positive on wire 747. In addition, the starting relay of each of the free distributors associated with the group of 500 lines attracts at the same time. time. Pulling relay 708 to its upper armature closes the control circuit of electro 720 of non-digital switch TS1, to. its lower armature closes the control circuit of the electronic 730 of the DS distributor through the contacts of the relays 709 and 711.

   As a result, the electro 720 of the switch TS1 working like a buzzer advances the brushes 721 to 729 step by step in the contact bank in search of a free connection circuit in the group of 100 lines, and that the DS distributor electro 730 operates to advance brushes 731 to 735 in search of a free bypass thousands selector. The search for a selector of thousands by-pass by the DS distributor is carried out under the control of relay 711 while the search for a junction to a free primary searcher is carried out under simultaneous control. relays 707 and 710.



   At its lower armature, relay 708 closes the black

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 fires polarizing windings of relays 709, 710 and 711.



  None of these relays attract when closing the circuit of their polarizing winding because their construction also requires for them to attract that the test winding be closed, this one is represented in each case as the top winding. of the relay.



   It will be assumed that the DTS bypass thousands selector, fig. 9, is the first that was found free by the DS distributor. If this is the case, when the brushes 731 to 735 arrive on the contacts where the wires 741 to 745 end, the test brush 733 encounters a negative on the wire 743 brought through the contacts of the key 812, of the Arming 843 of relay 804 and its idle contact, wire 853, BPS selector engagement contacts 907, and relay coil 907.

   When the brush 733 of the DS distributor encounters this negative it closes the circuit borrowing the contacts of relay 712, those of relay 709, the test winding 711, the lower inner armature of the relay 708, the positive, The relay 711 immediately draws into this circuit and opens the control circuit of the electro 730 to its lower armature, in this way it stops the operation of the distributor brushes which are engaged on a free BTS bypass thousands selector; it connects the brush 735 to the lower bearing of the relay 713, it immediately marks the thousands selector BTS bypass engaged by bypassing its test coil, which places a direct positive on the brush '733;

   to its upper armature it connects the negative brush to the positive brush 732 through the contacts of the relay 709, which immediately attracts the relay 901 of the BTS bypass selector.



   As the brushes 721 to 729 of the switch TS1 run through the contacts of their bank under the action of the electro 720, the group brush 721 successively connects the terminal

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 higher of relay 707 than wires such as wire 131, but this has no effect until the brush engages a contact associated with a connection circuit assigned to the group of 100 calling lines, the connection circuit terminating at the primary finder PF, for example.

   When brush 721 reaches the pin to which wire 131 is connected, the attraction circuit of relay 707 closes through the right terminal of the secondary winding of transformer 706 (individual to distributor DS ) through relay 707, brush 721 and wire 131, wire 751 and from there through the contacts of relay 701 and wire 746 to the left terminal of the secondary winding of transformer 706.



  Relay 707, as well as other similar test relays which will be discussed later, is an AC relay of the protected type, the protection on the core preventing vibration of the relay in the well known manner. Pulling relay 707 extends the positive of the lower middle contacts of the attracted relay 708 up to the winding, testing relay 710 and thence to brush 726. Assuming that the connection circuit leading to the primary finder PS is free. The test winding of relay 710 is fed through wire 136, the contacts of relay 119 of the primary finder PF, key 134 and resistor 118.

   By pulling relay 710 immediately shunt its test winding to apply positive directly to wire 136 and thereby mark the connection circuit as Unoccupied, it opens the circuit of electro 720 of switch 721 to stop brush movement 721. to 729 which are engaged on the pins of the connection circuit leading to the primary seeker PF.

   As soon as the relay 710 and the relay 711 are attracted which indicates that the switches TS1 and DS are both in position a test circuit is prepared for the winding; upper relay

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   709 and the control circuit of the PF seized primary seeker closes through the curetais contacts 709,710 and 711 via the control brush 722. As the PF primary searcher is the one selected, the brush 722 rests on the pin of wire 132 and the circuit extends to the own control electro 117 of the PF seeker. Therefore the electro It? starts dragging brushes 111 through 116 looking for the calling line.

   At the same time a positive is placed on the right terminal of the test winding of the line test relay 709 through the contacts of the relays 710 and 711, which allows the relay 709 to test the lines, to determine if they are. or not pending appeal, through the brush 723, the test lead 133, and the test brushes 113 and 116 of the PF researcher.



   When the PF primary seeker brushes reach the calling line pins, the test brush 113 closes the relay 709 test circuit, fig. 7, through the contacts of the relay 105, the contacts of the attracted relay 104 and the resistor 107. It should be noted that the relay 709 will not attract through a cut-off relay which is not bypassed by a resistor such as 107.



   By pulling the relay 709 opens the control circuit of the primary seeker PF and bypasses its test winding to its inner upper armature to mark the calling line entered, in the multiple of the seeker. The relay 105 which is attracted at the moment removes the positive from the starting wire 120, it disconnects the resistor 107 and substitutes the counter 106 for it, it disconnects the positive and the relay of the wires 102 and 101.



   In the distributor the wire 709 also connects the positive and negative brushes 724 and 725 through the corresponding brushes 732 and 731 of the distributor to the wires 742 and 741 of the BTS bypass selector at the same time as it

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 removes the preliminary bridge of attraction closed through the upper contacts of the relay? The relay 901 of the BTS bypass thousands selector therefore remains drawn into the circuit of the calling line through the brushes 112 and 111 of the primary seeker PF.



   Distributor relay 709 also opens at one point the special circuit for blocking the upper winding of relay 713 at its lower armature, at its inner armature it disconnects relay 708 from the associated wire 747 and extends it through the relay contacts. 712 Up to brush 733, at the same time it disconnects bqlai 733 from the earth previously placed on the test brush through the contacts of relay 711 and the lower middle armature of relay 708.



  The continued attraction of relay 708 therefore depends on the positive applied to. wire 743 via: key 812, contacts of relays 804 and 808, wire 853 and contacts of relay drawn from selector BTS. Note that the coil of relay 708 is shunted by a resistor. This shunt placed at the ends of the coil of relay 708 has a slight tendency to make this relay slow to release, to prevent its release when its circuit is transferred from wire 747 to brush 733.



   Relay 701 assigned to the 100-line group that contains the line for station A now attracts as a result of the pull of relay 105, even though there is another line waiting for a call in the same 100-line group. At this moment the wire 747, common to the five starter relays 70l to 705 is no longer connected to the earth even if there is a line pending call in one of the five groups of 100 lines associated with the DS distributor. If we remember that each free distributor in the group containing the distributor DS is put into service following the placement of a starting positive on wire 747 we will understand that all the others

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 will stop and be disengaged as a result of removing the positive on wire 747 when relay 701 releases.



   When the BTS bypass thousands selector is entered as stated above, and relay 901 has drawn through wires 851 and 852, relay 902 draws under the action of the line relay through the wires. contacts of relay 904. At its lower armature, relay 902 opens at one point the circuit of the release electro 912 and prepares the circuit of the ascension electro; at its upper inner frame it places a positive on the polarizing winding of relay 904; it applies a positive to the start wire 855 to activate the associated switch TS2; it puts a positive on wire 853 to provide a blocking circuit to the distributor DS; finally, it connects the test winding of relay 904 to test brush 923.



   Due to the presence of a positive on wire 853, the upper winding of relay 903 is energized through engagement oontacts 907. Relay 903 attracts and disconnects the rotation electro 911 to its lower armature. interior; it closes at another point the circuit of the ascension electro to its lower armature; at its upper armature it disconnects the positive from the lower winding of the relay 901 and substitutes for it a positive taken through the contacts 909 and the secondary winding of the invitation tone transformer 951.

   Consequently, the calling subscriber receives this tone which indicates to him that he can start sending the digits of the number of the called subscriber. when a positive is placed on wire 855 through relay 902, the circuit of relay 801 closes through frame 844 and the inner top frame of relay 803.



  The relay 801 therefore attracts in series with the control electro 820, it closes a direct circuit for the electro which therefore operates and opens the circuit of the relay 801. The relay 801 releases

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 and opens the circuit of the control electro 820 which allows the electro to drop out and again close the circuit of the relay 801. This interaction between the pulsation relay and the electro continues until that the relay 803 attracts through the contacts of the relay 807 and the brush 826 when it encounters a free connection circuit associated with a secondary searcher having access to the primary searcher used to establish the connection to the calling line.



   As the brushes 821 to 830 turn on the contacts by the action of the control electro 820, the brush 825 connects the relay 827 successively to the wires such as the wire 235 but this has no effect until this point. that the switch T21 has come to be put in position, as we said above, and that the relay 710 has drawn. After the attraction of the relay 710, each time the brush 825 encounters a connection circuit in the group (containing the searcher SF) predetermined by the selection, of the primary searcher PF, the attraction circuit of the relay 817 closes via a wire such as 235, relay 807 attracts and connects relay 803 to brush 326.

   The current source which attracts relay 805 is the secondary winding of transformer 811 (individual to the circuit shown) to which the relay is connected through the lower inner armatures of relays 805 and 804. The other terminal of the secondary winding of the individual transformer 811 is connected through the lower middle frame and the idle contact of the relay 805, the wire 744, the brush 734 of the distributor DS, the contacts of the drawn relays 710 and 711, the brush 729, the wire 124 to 200 thread assigned to the group of secondary researchers containing the secondary researcher SF. This group of secondary researchers was definitely marked by the position of the brush 729 of the switch TS1, because the wires 139 and 124 only end at the wire 200.

   It will be understood, in fact, that other researchers

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 Primary can have their wires, such as 144, interconnected with other groups of secondary seekers and therefore with other common group wires instead of wire 200. Relay 807 being drawn through wire 200 as explained above, the relay 803 is connected to the brush 826 and can therefore test the connection circuits over which the latter passes and which belong to the group of secondary researchers to which the common group wire 200 is assigned.



   For convenience, it will be assumed that the connection circuit containing the secondary seeker SF is the first of the preselected group which has been found free, in this case the rotation of the brushes of the switch TS2 continues until the terminals of this connection circuit are reached, the brush 826 then meets the wire 236 to which is connected a negative through the resistor 210, the contacts 209 and those of the relay 208. The relay 803 attracts and stops the search operation of the switch.

   Relay 803 also prepares the circuit of relay 804 but this circuit does not become effective immediately because it remains open to the lower inner frame of relay 808 which remains attracted to the lower contacts of relay 801 during the entire operation. research tion.



   A moment after the end of this operation brought by the relay 803, the relay 808 releases and closes the attraction circuit of the relay 804. This relay attracts and closes its own blocking circuit to its preliminary contact 845; it opens at one point the search circuit to armature 844 as well as its original attraction circuit; at armature 843 it opens at one point the interconnection between wires 853 and 854, leaving these wires connected together through the connectors. - superior interior tacts of the relay released 808; on armature 842 it places a direct positive through brush 826 on wire 236 to keep the connection circuit

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 against any subsequent capture; at armature 841 it prepares the control and test circuit under the control of relay 802.

   At its lower inner frame, relay 804 disconnects relay 807 and substitutes relay 806 for it, while at its lower frame it connects relay 802 for a reason which will be explained later.



   At this point it will be noted that the relay 808 remains attracted during the search operation of the switch TS2 to keep the circuit of the relay 804 open momentarily after the attraction of the relay 803 to prevent a false operation which may otherwise occur as follows: if it is assumed that the test of a free connection circuit begins while the switch brushes are on one of the last connection circuits of the group; and that these are all occupied, relay 807 attracts but relay 803 cannot not attract under the action of one of the test leads of the occupied connection circuits of the group, but in the case where. the first connection circuit of the next group is free, there is the possibility for relay 803 to be operated before relay 807 has had time to release.



   With the circuit of relay 804 being kept open for a short space of time by relay 808, the momentary attraction of relay 803 as indicated above causes a temporary cessation of the forward movement; this resumes as soon as the relay 807 releases and opens the circuit of the relay 803 which releases in its turn. This occurs before relay 808 has had time to release due to the temporary release of relay 801.



   When relay 802 is connected to the lower contacts of relay 804 as shown above it attracts through wire 745, brush 735 and contacts of series relays 711 and 712 / with the lower winding of relay 713 of the DS distributor. Relay 713 attracts and disconnects broom 734

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 brush 729 and transfers it to brush 728, preparing to position the selected secondary searcher SF on the particular primary searcher (PF) through which the connection has been established to the calling line.



   To prepare for the release of the distributor, the relay 713 connects its lower winding directly to the brush 735 at its lower inner frame; it closes a new blocking circuit for relay 708 at its upper middle armature, while at its upper armature it closes the circuit of relay 712. Relay 712 thus attracts and at its upper contact ensures the release of the relay. relay 708 when relay 713 releases. Relay 713 also closes at one point a special blocking circuit for itself at its lower armature, to prevent releasing of relay 713 and loosening of the circuit before pulling relay 709 when the primary seeker PF found the calling line.



   In the switch TS2, when the relay 802 draws in series with the relay 713, it closes the circuit of the control electro 207 of the secondary seeker SF through the drawn armature 841, the armature 848, the contacts of relay 806, broom 821 and wire 231. The clean-controlled electro 207 now operates to drive the secondary finder brushes 201 to 206 until the relay 806 draws through the broom 822, wire 232 and the broom. 203 or 206, on which the primary finder is located.



   In the example proposed, the primary researcher in service is the researcher PF accessible to the brushes 201 to 203 of the secondary researcher SF. When brushes 201 through 203 arrive at the pins to which wires 121 through 123 are connected, relay 806 draws through brush 203 and wire 123, this circuit includes contacts of relay 119 of the

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 PF primary seeker, wire 138, brush 728, relay contacts 713, relay contacts 711 and 710, brush 734, wire 744, relay contacts 805, the secondary winding of the individual transformer 811, the contacts of relay 805 and the inner lower armature of relay 804.



   By pulling the relay 806 opens the control circuit described above for the electro 207 of the secondary seeker SF at its upper contact, thereby terminating the search for the secondary seeker, while at its lower armature. interior it closes the attraction circuit of relay 805. When relay 805 attracts it locks at armature 849 on wire 855; it opens at another point the control circuit to the armature 848; it connects the negative and positive wires of the secondary finder to the corresponding wires of the thousands bypass selector at armatures 846 and 847; at its lower inner and middle armatures it transfers the secondary winding of the individual test transformer 811 to the first digital selection stage; at its lower armature it opens the circuit of relays 802 and 715.

   Relay 802 thereby releases and opens the control circuit at another point and at the same time applies armature ground 841 to brush 822, which grounds brushes 203 and 206 through wire 232. earth on brush 203 extends to wire 123 where it acts as a blocking earth as soon as relay 119 attracts.



   In the DS distributor, the opening of the circuit from the lower winding of relay 713 to the lower contacts of relay 805, fig. 8, has the effect of immediately releasing relay 713, although operation of switch TS2 and secondary seeker SF has occurred before primary seeker PF has found the calling line. when
In this case / relay 713 releases, it opens the circuit of relay 712 and closes the circuit of relay 119 of the primary researcher PF through the contacts of relay 712 and wire 137.

  <Desc / Clms Page number 23>

 



  When the relay 119 attracts it locks on itself to the test wire, grounded through wire 133, to its inner upper frame; at its three remaining reinforcements it connects the finder brushes to the junction wires 121, 122 and 123, at the same time disconnecting the line marking wire 138 from the wire 123.



   Relay 713 also opens the circuit of relay 708, an auxiliary circuit of which is kept open by relay 712. Relay 708 therefore releases and opens the circuit of the biasing and blocking windings of relays 709, 710 and 711 which releases these three relays. Relay 712 remains held pulled due to its delay in releasing until relays 709, 710 and 711 have released.



  The relay 709 disconnects the positive and negative brushes 724 and 725, which opens the connection initially established between the calling line and the BTS bypass thousands selector; relay 709 also connects relay 708 to wire 747 to allow distributor DS to start again when a subsequent call occurs.



   The calling line is now connected to the thousand bypass selector B.TS through brushes 111 and 112 of the primary searcher PF, the contacts of relay 119, brushes 201 and 202 of the secondary searcher SF, brushes 824 and 823 of the switch TS2, the armatures 846 e 847 of the relay 805 The relay 119 is now attracted by means of the wire 123 through the brushes 203 and 822 by the earth placed on the brush 822 through the contacts of the relay 802 and the armature attracted 842 of relay 804.



   From the above description it can be seen that the distributor DS and the switch TS1 cooperate to connect the calling line very quickly to a free bypass selector through a primary searcher and that the secondary searcher is then operated to resume. the connection established

  <Desc / Clms Page number 24>

 through these machines. This arrangement has two purposes, it firstly enables the calling subscriber to be given rapid access to the thousands bypass selector so that he can start sending the pulses without delay; second, it greatly simplifies the primary and secondary researchers at the expense of a somewhat more complicated construction of the distributor and the switch TS2 which, however, are less numerous than the primary and secondary researchers.



   When the calling subscriber hears the invitation tone to be transmitted sent over his line by transformer 951, as shown above, he transmits the first digit of the desired number; relay 901 of the BTS bypass selector momentarily releases a number of times corresponding to the value of the digit emitted. Each time he releases the relay 901 closes the circuit of the ascension electro 910 through the lower armatures of the relays 902 and 903. As a result of the operation of the electro 910 the brushes 921 to 925 are raised step by step. until they are placed in front of the desired level of the bank contacts.



   The lower winding of relay 903 is supplied in series with the ascension electro 910. The presence of a ring makes relay 903 slow to release, so it remains attracted while sending the pulse train.



   When the switch brushes leave the normal position, the engage contacts 905 prepare the circuit for the release solenoid 912, the engage contacts 906 prepare the circuit for the rotation solenoid 911, the engage contacts 907 open the initial pull circuit of relay 903, leaving pull relay 903 under the control of its lower winding.



   At the end of the transmission of the digit, the relay 903 releases since no impulse is received any more. By releasing

  <Desc / Clms Page number 25>

 relay 903 removes the prompt connection from the lower winding of relay 901 to its upper armature and substitutes a direct positive; at its lower frame it disconnects the ascension electro 910; to its lower inner frame it connects the own controlled ascension electro 911. The electro 911 now operates through the engagement contacts 906 to drive the brushes 921 to 925 until the circuit of the rotation electro is open at the lower armature of relay 904.



   In the example proposed, the line requested is the line of station B, fig. 6, to which the number 1513 is assigned.



  The first digit emitted being therefore the number 1, the brushes of the BTS selector are brought in front of the first level of the bano contacts. The contacts shown are assumed to be the first group of first level contacts, ie those from where wires 931 to 935 go to the BPS bypass hundreds selector, fig. 11. Assuming that the BPS selector is free when brushes 921 through 925 are brought to the first group of first level bank contacts, brush 923 meets a negative on wire 933, through the contacts of the bank. key 1119, the engagement contacts 1108, and the upper coil of the relay 1104.

   The broom 923 being. connected through the top contacts of relay 902 to a terminal of the test winding of relay 904, this is energized through wire 854, contacts of relay 808, wire 853 and contacts of relay 902 Pulling the relay 904 opens the control circuit of the rotation electro 911 and the circuit of the relay 902 to its inner lower frame; it gives an additional earth connection to wire 855; to its inner upper armature it connects the blocking circuit of its polarization and blocking windings on wire 854;

   it applies a direct positive from the ground wire 853 to the brush 923 to mark

  <Desc / Clms Page number 26>

 the selector entered BPS at Unoccupied; it connects brushes 921, 922 and 925 at the same time as it disconnects relay 901.



   Relay 901 therefore releases and releases relay 902 which is released shortly after, but without any particular consequence at this time because before releasing relay 902 a positive was replaced on wire 933, coming from the BPS selector d. 'a way which will be described later. This positive is extended through the contacts of relay 904 to wire 853, providing a blocking circuit through the contacts of relay 808 and through wire 854 for the lower winding of relay 904 which now loops onto the relay. wire 854 to the top inner contacts of this relay.



   In the BPS bypass hundreds selector, fig.ll, when relay 1102 draws into the loop of the calling line and through wires 931 and 932 it closes the circuit of relay 1103. Relay 1103 draws and places at its upper middle armature, through the contacts of key 1119 a positive on wire 933 to provide the blocking circuit indicated above to the BTS bypass thousands selector. To its upper frame, the relay 1103 connects the brush 1116; it prepares the circuit of the polarizing winding of relay 1105 to its inner upper armature; on its lower frame it prepares the pulse circuit of the 1111 electro-electro; at its lower inner frame it places a positive on wire 1119.



   The bypass hundreds selector is therefore ready to operate when the second digit is emitted and wire 1119 is grounded to activate the second stage switch TS3.



   At this point it may be interesting to point out the fact that the BPS bypass hundreds selector is part of a group of identical selectors intended to serve

  <Desc / Clms Page number 27>

 lines of the first group of 1000 lines and that the individual switch TS3 to the BPS bypass selector is activated to enter a hundred selector used for conversation such as the PS selector, in the first group of 1000 lines, according to the position taken by the TS thousands selector, entered previously.



   When wire 1119 is grounded by relay 1103, a circuit is established through the contacts of relay 1007, and those of relay 1002, to relay 1001 associated with the control electro 1020 of switch TS3. The relay 1001 and the electronic control 1020 interact as described for the relay and the corresponding electronic, 801 and 820 in fig. 8, and the electro drives the brushes 1021 to 1030 in search of a free selector in the first associated thousand group.



   While testing a hundred selector switch TS3 searches for a reachable selector from the group including the TS thousands selector. All the selectors in this group have access to the same trunks and these are divided more or less equally between the various groups of 1000 lines. The common group wire 300 is assigned to the thousands selector group in which the thousands selector TS is included and this common group wire is therefore connected to the distributor DF5 to each of the wires such as 239 associated with the connection circuit through which a switch such as TS2 gains access to a group thousands selector.

   In an identical manner the common group wire 300 is connected to the DES distributor to a wire such as 334 of each of the accessible conversation circuits of the group of thousands selectors. It will be understood that the common wire 300 is part of a set of first stage common group wires since there is a separate common group wire for each group of 1000 switches used.

  <Desc / Clms Page number 28>

 for conversation.



   Common group wire 300 has been tagged at switch TS2 by brush 829 through wire 239.



  Switch TS3 by means of relay 1005 tests the common group wire marked by switch TS2. when an accessible junction line of the marked selector group is reached, relay 1005 draws through common group wire 300 through wire 334 from the junction including wires 331 to 334, a flying wire from the DES distributor, wire 431 and brush 1021. The other terminal of relay 1005 is connected through wire 934, brush 924 of selector BTS, contacts of relay 904, wire 856 and contacts of relay 805 to the left pin of the secondary winding of the individual transformer ell.



  The other terminal of this secondary winding is now connected to the common group wire 300 through the contacts of relay 805, the contacts of relay 810, brush 829 and wire 239. At its upper contacts it connects relay 1002 to the brush. 1025 to allow testing of individual connection circuits to determine if they are busy or free.



   Suppose the connection circuit comprising wires 431 to 440, assigned to the selector of hundreds PS, fig. 5, is the first free connection circuit among those which are accessible to the group having carried out the marking, this junction is put into service and the brushes 1021 to 1030 stop on the wires 431 to 440.



   Relay 1002 is connected through brush 1025, wire 435, key contacts 411, relay contacts 408, and resistor 412 to the pole that is not grounded to the desktop battery.



   By drawing in this circuit the relay 1002 opens the drive circuit and prepares the circuit of the relay 1007

  <Desc / Clms Page number 29>

 through wire 1119 and relay contacts 1003.



   A moment later the relay 1003 remains attracted during a series of operations of the relay 1001, releases and closes the circuit of the relay 1007, therefore the relay 1007 closes its own blocking circuit to the preliminary contact of its upper inner armature by the 'Through the grounded wire 1119, it places a direct positive on the brush 1025 to protect the connection circuit from being taken by another switch; finally it connects the negative and positive brushes 1024 and 1023.



   At its lower armature, relay 1007 connects relay 1006 in series with relay 810 through wire 935, brush 923 and wire 857 which attracts the two relays in series. Relay 1006 transfers wire 934 from brush 1021 (via group test relay 1005) to brush 1022 through the lower inner contacts of relay 1007, thus removing the marking potential of the secondary winding of transformer 811 to wire 432 and from there to wire 333 from the junction entering the hundred PS selector, fig. 5, and coming from the thousands selector TS, freezes 3. The hundred PS selector is therefore distinctly marked so that it is the one entered by the thousands selector TS when it operates.



   In the TS switch, fig. 8 when relay 810 attracts, it disconnects the pin associated with it from the secondary winding of transformer 811 of brush 825 and the connection to brush 820 through relay 809. In addition, relay 810 to its lower armature connects brush 830 to earth through the contacts of relay 808 and the contact of relay 809, thereby closing the blackout of the control electro 307 of the thousands TS selector via the wire 240. The control eleotro 307

  <Desc / Clms Page number 30>

 therefore operates like a buzzer to advance brushes 301 to 306 on the contact bank in search of the marked junction leading to the hundred PS selector.

   When this junction is reached, the marking potential of wire 333 is found by brush 303 which closes the circuit of relay 809 via wire 223, contacts of relay 208, wire 238 and brush 828. The relay 809 attracts under the action of the current supplied by the secondary winding of the transformer 811. By attracting the relay 809 opens the control circuit of the electro 307 of the thousands TS selector and closes the circuit of the. upper winding of relay 808.



   Relay 808 at its upper armature places an earth on brush 827, through armature 840 of relay 804, thereby closing, through wire 237, the circuit of relay 208 associated with the connection circuit made up of the seeker se- condaire SF and the thousands selector TS.



   When relay 208 draws, the thousands TS selector having been set on the wires leading to the hundreds PS selector, the calling line is extended through the contacts of switching relay 208, wires 221 and 222, brushes 301 and 302, to the incoming wires 331 and 332, fig. 4. From this point the calling line is extended through the contacts of relay 403, wires 434 and 433, brushes 1024 and 1023, contacts of relay 1007 of the second stage switch, to relay 1102 of the second stage switch. BPS hundred bypass selector. The establishment of this conversation circuit being carried out through the switch TS2, the bypass thousands selector BTS is no longer necessary and can be released.



   In addition to the operations performed by the relay 808 and which are explained above, the latter opens the control circuit at one point, at its lower middle armature; it cuts the transfer wire at its lower reinforcement to

  <Desc / Clms Page number 31>

 release relays 1006 and 810 and at its inner lower armature it locks on itself, in parallel. on the winding of relay 804 and through the contacts of relay 804, to wire 855 grounded to the contacts of relay 904. In addition, relay 808 separates wires 853 and 854 which releases relay 904 which has been held previously attracted through wire 933, brush 923, wire 853, relay contacts 808, wire 854, and inner top contacts of relay 904.

   By releasing relay 904 disconnects the brushes associated with it and removes the ground from wire 855.



   With wire 855 no longer connected to earth, relay 804 releases as well as relay 808 a little later. In addition, relay 805 drops due to removal of the earth on wire 855, so all relays of switch TS2 return to the normal position.



   When relay 904 releases it closes the circuit from the release electro to its inner lower armature through the contacts of the released relays 901 and 902 and through the engagement contacts 905. The electro 912 attracts to release the selector. and the circuit is open at engage contacts 905 when the selector reaches its normal position. Additionally, 907 engage contacts connect, wire 853 through the top winding of relay 903 to the pole that is not connected to the office battery ground to make the BPS selector accessible again by distributors. .



   When the calling subscriber issues the hundreds digit, the relay 1102 of the BPS bypass selector momentarily releases five times. Each time it releases, relay 1102 closes the attraction circuit of the ascension electro through the lower armature of the attracted relay 1103, through the contacts of relay 1012 / relay 1105 . The-

  <Desc / Clms Page number 32>

   Ascension Electro 1111 therefore draws five times to raise the brushes 1114 to 1118 by five steps, so that they come to face the fifth level of the bank contacts.



  The relay 1104 receives the pulses from the relay 1102 through the contacts of the relay 1103 so that it remains attracted while sending the pulse train, it does not immediately release due to the opening of the circuit of its in- upper bearing to the engagement contacts 1108 when the switch leaves its normal position.



   At the end of the ascent movement, relay 1104; releases and closes the circuit of the own control electro 1112 through the engagement contacts 1107 and the contacts of the relay 1105. The electro 1112 therefore operates and drives the brushes 1114 to 1118 in the contact bank on the level selected until the rotation electro circuit is opened by the attraction of relay 1105 via brush 1116.



   We can assume that the BFS bypass final selector is the first that has been found free and that it is therefore entered.



  This selector is taken by means of the brush 1116 when it encounters a negative on the wire 1136 through the engagement contacts 1203 and the associated resistance.



  The test winding of relay 1105 is grounded to the upper middle frame of relay 1103, and its other terminal is connected through the upper contacts of relay 1103 to brush 1116. Relay 1105 therefore attracts and disconnects. the rotation electro 1112 at its lower inner frame to stop the movement of the brushes; it places a direct positive on brush 1116 at its inner top frame through the middle top frame of relay 1103; at its lower armature it disconnects the pulse wire of the electro-ascension 1111 and extends it, through the relay contacts

  <Desc / Clms Page number 33>

 1101, to brush 1118, now connected through wire 1130 to the BFS selector pulse circuit.



   In the BFS bypass final selector, relay 1201 draws through wire 1136 and at one point opens the circuit of release electro 1208.



   As soon as the BPS bypass selector has been put in position the group of 100 lines for which the call is intended is determined, and the selector of the hundreds PS, fig. 5, selects a free junction in one of the ten groups of 100 lines indicated according to the level to which the BPS bypass selector has been directed. There are ten common level wires such as wire 1138 each associated with a different level of the hundreds bypass selectors. The wire 1158 is the common wire associated with the fifth level of the contact banks, it is connected to the distributor DF3 to a wire such as the wire 522 of each of the junctions ending in the corresponding group of final selectors.

   The common wire of the fifth level 1138 is now marked by the brush 1114 of the BPS bypass selector, connected to the upper middle armature of the relay 1105 to the lower terminal of the secondary winding of the individual transformer 1013 of the switch TS3. The other terminal of this secondary winding of transformer 1013 is connected through test relay 1011 to brush lo27 which is on wire 437, which extends through the contacts of relays 407 and 408 to brush 502. and 507 of the hundreds PS selector used for the conversation.



   Relays 1007 of the switch and 1105 of the selector
B of hundreds bypass / PS being attracted the earth of the wire is extended through the contacts of relay 1007 to brush 1030 through relays 1008 and 1009 and the upper contacts of relay 1105. Brush 1030 is now in position on wire 440 it results that the control earth is extended through the contacts of the

  <Desc / Clms Page number 34>

 relay 408 to the own control electro 511 of the PS selector. Brushes 501 to 510 rotate on the bench contacts looking for a free junction in the group marked by the connection made to wire 1138 by brush 1114, as noted above.



   As the brushes rotate, whenever one of brushes 502 or 507 encounters a fifth group junction, relay 1011 draws into the circuit comprising the secondary winding of transformer 1013 and the fifth level common wire marked 1138. By attracting the relay 1011 applies a positive through the contacts of relay 1009 to one of the pins of relay 1008, the other terminal is connected through the contacts of relay 1012 to brush 1029 of switch TS3. The connection is extended via wire 439 and through the contacts of relay 408 to brushes 505 and 510. Therefore, as soon as a free junction is found in the group marked, relay 1008 attracts to through either of the junction test brushes marked in the TS switch, to stop the search.



   We can assume that the junction going to the final selector FS is the first that has been found free; in this case the relay 1008 attracts through the brush 505 and the wire 525 via the potential marking the free junction brought through the key 611 and the resistor 612. By pulling the relay 1008 opens the control circuit and closes the relay circuit 1009.

   Relay 1009 at another point opens the control circuit at its upper middle armature, it connects the lower winding of relay 1012 to its upper armature, it opens the circuit of relay 1010 and closes its blocking circuit to wire 1119 at its upper inner frame, at its lower frame it places a guard potential through brush 1029 of switch TS3

  <Desc / Clms Page number 35>

 and through the brush 505 of the PS switch on the wire 525; at its inner lower armature, relay 1009 disconnects relay 1008 and prepares the circuit for introducing the relays of fig. 4.



   A moment later the lolo relay releases and closes at another point the introductory circuit mentioned above.



   After having therefore sent the thousands and hundreds digits 1 and 5, the calling subscriber now sends the tens and ones digits 1 and 3.



   When the tens digit 1 is emitted the relay 1102 releases once momentarily. It closes the circuit by momentarily re-attracting relay 1104 :, through the contacts of the released release relay 1103. At the same time a branch of this circuit closes through the contacts of relays 1012, 1105 and 1101 and by means of the brush. 1118 to operate the ascent electro 1206 of the BFS switch.



  The initial 1206 electro circuit consists of wire 1130, part of the engagement contacts 1205, and relay 1202. Relay 1202 draws in series with the ascension electro 1206 to maintain the ascender electro. ascension connected to the pulse wire and to keep the rotation electro disconnected after the engagement contacts 1205 have worked as it does when the upward movement of the switch begins.



   When the digit has been sent, relay 1104 of the BPS selector releases. The same is true for relay 1202 of the BFS selector which transfers the pulse wire from the ascension electro 1206 to the rotation electro 1207.



   When the last digit 3 is emitted, three pulses are transmitted to the rotation electro 1207 and the brushes 1221 to 1226, having been raised by one step, rotate on the con-

  <Desc / Clms Page number 36>

 tacts of the first level until they are placed on the group of contacts number 13. Among the brushes 1220 to 1225 of the BFS selector, the brush 1220 marks, in the example above, the line number 13 in each of the five groups of one hundred lines to which the BFS selector has access, while brushes 1221 to 1225 each connect to the test lead (such as 623) of line number 13 in the corresponding group of the five groups of 100 lines accessible by the BSF selector.



   As mentioned above, BFS selector relay 1104 attracts during transmission of each of the last three pulse trains. While sending the pulses to the electro-rotation 1207 of the last digit train, the brush 1117 is connected to the brush 1225 via wire 1135 and the brush 1225 scans the contacts connected to the test leads, such as 623, of the lines. of subscribers. Each time the brush passes over the test contact of a busy line, the positive on the test lead of that line causes relay 1101 to attract. But relay 1101 does not attract when brush 1220 engages on the line. test pin of a free line, because the test lead of such a line is not connected to earth.

   From this we can see that the relay 1101 does not attract at the end of the rotation movement of the selector BFS if the line of station B, fig. 6, is free. is
If it is assumed that the line / free, the relay 1101 does not attract when the relay 1104: releases at the end of the sending of the last pulses when the brushes 1220 to 1225 are in position on the group of contacts 13. So , when relay 1104 releases it closes the circuit of the test winding (lower winding) of relay 1012 in the event that the hundreds PS selector has been set, as it has been said, at the time considered ( it is usually

  <Desc / Clms Page number 37>

 thus) and the relay 1009 is therefore drawn.

   If relay 1009 is not engaged, relay 1012 is not switched on at the time in question, but it will be as soon as the relay
1009 draws to indicate that the selector of the hundreds PS has been set to a free final selector of the desired group of 100 lines. When relay 1012 attracts, its power circuit is as follows: positive to the lower coil of the relay, contacts of relay 1009, contacts of relays 1101 and 1104 of the BPS selector, brush 1117, wire 1135, brush 1225 of the BFS selector , individual wire 1245, a jumper wire from the DF10 distributor, individual test lead 623 from the line of station B, contacts of the cut-off line relay of the called line circuit, negative; the line circuit can be the same as the LC line circuit in fig. 1 of station A it is therefore not shown.



   By attracting relay 1002, its upper inner frame locks on itself to the grounded wire 933, at its lower middle frame it prepares the grounding circuit to the brush 1029 under the control of the relay 1011; at its lower armature it opens the pulse circuit to prevent any subsequent movement of the selector brushes. final BFS bypass in the event that recalling still acts on its disk; at its upper frame it disconnects the broom
1114 and connects the brush 1115 of the BFS selector; on its inner lower armature it places a positive through the contacts of relay 1011 on brush 1028.



   The preceding operations have the effect of prolonging the marking potential of the lower terminal of the upper bearing of transformer 1013 to the brush 1115 of the BPS selector through the operated upper armature of the relay 1012 and then by the '' intermediate wire 1137 and brush 1220 from the BFS selector to wire 1231. From wire 1231

  <Desc / Clms Page number 38>

 this labeling potential continues until. individual wire 624 to the called line, via a flying wire from the DF10 distributor. The wire 624 is therefore marked such that the final selector FS will stop on the pins of the called line.



   The fact that the earth is extended to brush 1028 at the lower internal contacts of relay 1012, closes the circuit of relay 408, fig. 4, through wire 438. Relay 408 attracts and at its upper armature again extends positive through armature 441, negative brush 501 of switch TS, wire 521, contacts of selector relay 610 final FS up to control electro 609. Electro 609 therefore operates and drives the final selector in search of the wire marked 624.



   The relay 408 at its second upper armature locks on itself to the wire 333 now grounded through the brush 1022 of the switch TS3, at the same time it causes the opening of its original attraction circuit; it disconnects the wire 439, which was used previously to constitute the junction test circuit, of the brushes 505 and 510 of the selector of the hundreds PS and connects it to the upper winding of the relay 402; it places a positive, through the contacts controlled by the armature 444 and through the brush 505, on the wire 525 to replace the positive previously applied at this point, through the brush 1029, by the relay 1009;

   it disconnects the electro control circuit 511 of the TS selector from wire 440; it / connects the wire 435 to the potential marking the selector as free and which was previously supplied via the resistor 412, so that the selector PS is kept after the release of the switch TS3; to his. upper inner frame it disconnects wire 457 from brushes 502 and 507 of selector PS and extends it through brush 503,

  <Desc / Clms Page number 39>

 from the wire 523 and the contacts of the relay 610 to the brushes 604 and 608 of the final selector ES, so that it allows these brushes to cooperate with the relay 1011 to perform the test of the called line.



   When the brush 604 of the final selector FS reaches the wire marked 624 the circuit of the relay 1011 which serves as the final test relay, closes through the positive wire of the connection established between the final selector and the hundred selector, l The upper inner frame of relay 408, the contacts of relay 407, wire 437 and brush 1027. When it attracts relay 1011 opens the control circuit of elec- tro 609 which stops the final selector FS on the line of station B, it closes the circuit of relay 402 via the contacts of the operated relay 1009, the contacts of the relays 1010 and 1012, the brush 1029 and the contacts of the relay 408.



   At this point it can be explained that the function of relay 1010 is to prevent the premature establishment of the circuit described above in the event that a bypass selector such as BFS has already been placed in position on a free line ( and the attraction of the relay 1012 therefore simply waits for the attraction of the relay 1009) before the relay 1009 attracts as a result of the attraction of the relay 1011 when the hundreds selector has found the level marked by the hundreds selector bypass . Without the delay introduced by slow relay on release 1010, relay 1012 could have immediately closed the circuit of relay 402 described above and brought about a premature attraction of the line relay of FIG. 4 without giving relay 1011 time to release and then re-attract when the final test is complete.



   At this moment the final selector, FS has been placed on the pins of the called line and the bypass selectors (BPS fig. 11, and BFS fig. 12) can be released; the con-

  <Desc / Clms Page number 40>

 nexion established through switch TS3 can be cut.



  As a result of these operations and also for the purpose of transferring control of the connection established to the conversation train and sending the inrush current. towards the requested line, the relay 402 attracts in the circuit described above and closes the circuit of the relay 403. The relay 403 places the wire 450 to the ground, it causes the relay 405 to attract through the contacts of the relay. relay 406; it establishes an additional guard connection on the incoming junction release wire 333 and on its inner upper armature through the contacts of relay 405 and relay 406; it disconnects the incoming conversation wires 331 and 332 from the wires 434 and 433 terminating at the bank of the switch TS3 and connects them to the windings of the relay 402 via the contacts of the relay 404.



  The relay 402 is therefore kept attracted in the circuit of the calling line, independently of the initial attraction circuit in the switch TS3.



   Grounding wire 450 through relay 403 also attracts relay 407; to armature 444 and its associated contacts it disconnects the earth of the junction test brushes 505 and 510 and consequently of the junction test leads 525 of the final selector FS and applies this earth to the brushes 504 and 509 and by Consequently to the switch wire 524, which ends at the relay 610 of the final selector FS.

   As a result, relay 610 attracts immediately and that at its lower armature it disconnects resistance 612 from wire 525 so that it keeps the final selector guarded; at its inner lower frame, it disconnects wire 523 from brushes 604 and 608; its inner upper armature, it applies a positive, through the brush 603, to the wire 623 of the called line to keep the called line guarded and to keep the cut-off relay (. not shown) of the latter drawn;

  <Desc / Clms Page number 41>

 it connects the wires 521 and 523 through the brushes 601 and 602 to the wires 621 and 622 of the called line, at the same time it disconnects the wire 521 from the control electro 609.



   In the relays of the equipment of fig. 4, the relay 407 to the armatures 443 and 441 closes the circuit of the inrush current to the called line at the same time it disconnects the wires 437 and 438. The inrush current is supplied by the wire 410, usually it s 'acts of inrush current on which one superimposes the negative potential of the battery of the office. The inrush current passes through the upper winding of relay 406 and the contacts of this relay, going to the called line through the negative talk wire, it returns to earth through the positive conversation wire through the contacts of relay 406.



  The called subscriber is therefore alerted by the inrush current in the usual manner. A small portion of this current passes through armature 442 and capacitors 413 and goes to the calling line to give auditory signaling of the operation.



   Pulling relay 403 and connecting wires 433 and 434 to incoming line wires 331 and 332 removes the circuit keeping relay 1102 drawn into the BPS bypass selector. Relay 1102 therefore releases, by incidentally attracting relay 1104 for a moment, at the same time it opened the circuit of relay 1103. After a short interval, relay 1103 releases and opens the circuit of relay 1105 to its inner upper armature which allows relay 1105 to release and disconnect associated brushes. At its lower inner frame, relay 1103 removes the positive from wire 1119, allowing the release of relays 1007 and 1009. Relay 1112 can release when the positive is removed from wire 933 at the upper middle armature of relay 1003.

  <Desc / Clms Page number 42>

 



     The release electro 1113 is operated through the engagement contacts 1106 and through the contacts of the relays 1102 and 1103 to return the BPS selector to normal condition, therefore the release electro circuit is open at the contacts. 1106. Switch TS3 and BPS bypass selector are released.



   In the final bypass selector, relay 1201 releases when brush 1116 is disconnected at the contacts of relay 1103. The release electro 1208 therefore attracts through the engagement contacts 1204 to release BFS, the circuit of the release electro is open at the engagement contacts.



   The bypass train is completely clear at this moment and the connection established by the conversation train is under the action of the inrush current which is sent intermittently on the called line.



   When the called subscriber answers by picking up his receiver (not shown), the usual direct current circuit closes through the wires of his line and through the upper winding of relay 406 (this relay as it should be. note, does not attract under the action of the alternating current flowing through its upper winding) this relay attracts enough to close its own blocking circuit to its slightly adjusted lower armor. By pulling the relay 406 fully under the control of its lower winding to its upper middle armature, removes the return-to-earth connection of the positive wire and connects it to the lower coil of relay 404, while at its upper armature it disconnects the negative wire from the inrush sending circuit and connects it to the top winding of relay 404.



  The two lines are now interconnected by telephone through the capacitors which separate the two sides.

  <Desc / Clms Page number 43>

 of the circuit. Relay 404 draws into the called line circuit and reverses the connections made between relay 402 and the calling line wires, which reverses the direction of current in the calling line circuit, this operation can be used to give supervision or perform an identical operation if the call comes from an operating table.



   When relay 406 pulls as described above, it disconnects relay 405 from wire 450 at the normally closed contacts controlled by the inner bottom armature to release relay 405 after a short time interval. At the same time it connects the pole of the booster battery which is not connected to earth, through resistor 409, the upper armature of relay 405 and the contacts of relay 403 to wire 333. This connection increases the value of the current flowing through the marginal adjustment counter 106 of the LC line circuit so that the call can be counted as usual.



   A moment later, relay 405 releases and replaces a positive on wire 333.



   It should be noted that each bypass train test circuit, which comprises at all the stages of the conversation circuit a wire corresponding to wire 343, pin 4, also includes a polarized relay such as relays 709 to 711 , fig. 7, the result is that a test carried out when the connection to the booster battery is established to operate the meter, such as 106, does not cause the selector to tap unexpectedly, because the direction of the current flowing in the test relay is reversed with respect to the direction of the current obtained when the circuit is free.



   If we assume that the called subscriber is the first to hang up at the end of the conversation, relay 404 releases

  <Desc / Clms Page number 44>

 and reverses again the direction of the current in the calling line, it is reestablished in the normal direction, the relay to its lower armature removes the positive at a point of the wire 450. This one remains however connected to the earth to the contaets interiors of relay 403. The circuits remain in this state until the calling subscriber hangs up, the relay 402 then releases and opens the circuit of the relay 405. The relay 403 releases after a short time and removes the positive of. wire 333 at its inner upper frame, while at its lower frame it removes the positive from wire 450, Relays 406, 407 and 408 then release.



   FS final selector relay 610 releases when relay 407 at armature 444 removes positive from brush 504 and lead 524. Relay 105 and relays 119 and 208 release when relay 403 removes positive from lead 333 .



  The relay 119 replaces the potential indicating that the machine is free, via the resistor 118, on the wire 136 to make the finder PF accessible to the reinforcements such as TS1; the relay 208 replaces the potential indicating that the machine is free ,, via the resistor 210, on the wire 236 to make the secondary finder SF accessible again to the reinforcements such as TS2.



   The entire connection is released at this time and the switching equipment kept busy during the call is again available for common use.



   When the calling subscriber is the first to hang up the relay 404 remaining attracted maintains a positive on the wire 450. As a result, when the relay 403 releases it only causes the release of the portion of the circuit which precedes it but does not release. not relays 406, 407 and 408 to release the connection between fig. 4 and the called line. The connection remains maintained by the relays of the equipment in fig. 4 through

  <Desc / Clms Page number 45>

 the selectors PS and FS to the line of station B fig. 6 as long as the receiver is not hung up at the station of the called line, where the direct current bridge placed across the line of station B would operate the line circuit as for a call waiting, if the called line was clear before the receiver hung up.



   When the receiver is hung up at the called station, relay 404 releases and finally removes the positive at a last point of wire 450 which allows relays 406, 407 and 408 to be released. Relay 610 and the cut-off relay (not shown) associated with the called line release as a result of removing the ground from brush 504 and wire 524 to armature 444 of relay 407.



   Returning now to the point where the final BFS bypass selector drives brushes 1220 to 1225 in rotation on contact groups 11 and 12 and finally engages them on group 13, it will be remembered that the relay 1104 of the BPS is kept attracted during the rotation movement of the final BFS selector and that the relay 1104, when it is attracted, keeps the brush 1117 connected to the relay 1101 and disconnected from the relay 1012. If the called line is occupied there is has a positive on wire 623 which closes the draw circuit of relay 1101 through wire 1245, brush 1225, wire 1135, brush 1117 and the upper armature of relay 1104.

   Relay 1101 therefore attracts when the brushes of the BFS switch engage on group 13 of contacts and relay 1104 releases. Under these conditions, releasing relay 1104 closes the circuit of relay 1101 by means of brush 1117, through the upper inner armature of relay 1101 and through the normally closed contacts controlled by the upper armature of relay 1104.

   The relay 1101 being blocked in the attracted position, the lower winding of the relay

  <Desc / Clms Page number 46>

   1012 remains disconnected, at the upper middle armature of relay 1101, of brush 1117 and the positive found by brush ll17 is extended through the upper contacts of relay 1101, through brush 1026 and wire 435 to the winding top of relay 402, through relay 401.



  The relay 401 attracts in series with the relay 402 and prepares the blocking circuit of its lower winding to its lower armature, at the same time it disconnects the relays 407 and 408 of the wire 450. When the relay 402 attracts and causes the attraction- tion of relay 403 the positive is placed on wire 450 to close the blocking circuit of the lower winding of relay 401 which therefore remains attracted until the called subscriber hangs up his receiver.



   At the upper contacts of the relay 401 a connection is established between the common wire of the busy tone and the conversation wire 331 which causes the transmission of this tone to the calling line.



   Following the attraction of relay 403 and the disconnection of wires 331 and 532 from wires 434 and 433, relay 1102 of the BPS selector releases and causes the release of the BPS and BFS selectors as well as that of the switch TS3. as explained above. A final selector, such as FS, has ordinarily been entered at the time in question but is returned to common service when the positive previously applied to wire 525 through brushes 505 and 1029 is removed as a result of releasing the relay. 1009 associated with switch TS3.



   When the calling subscriber hangs up his receiver, the relay 402 releases and causes the release of the relay 403, and the relay 401 being unblocked releases as does the relay 405. The connection established from the calling line to the final selector PS through the researchers PF and SF and the selector of the thousands TS is therefore cut as it was explained

  <Desc / Clms Page number 47>

 upper.



   The operations which take place when all the junctions are occupied at the various stages of the selection will be described below. It is considered unlikely that a total occupation can often present itself for a sufficiently long time to primary or secondary researchers, for this reason no special precautions have been taken to take this fact into account. The ordinary precautions taken in automatic systems in which communications are established by two-motion switches are taken here also at thousands selectors, they come into play when a thousands selector, such as BTS, fig. 9 cannot find a free junction to a hundreds bypass selector.

   In addition, special precautions have been taken to account for the factual situation encountered when no free junction in the desired group can be found to a thousands selector of the talk circuit, such as TS, and identical precautions have been taken. in the hundreds bypass selector such as BPF, which comes into play in the event that the bypass hundreds selector cannot find a free final bypass selector in the desired group of 500 lines.



   Suppose now that the BTS bypass thousands selector has been put into operation as described above, that it has been put into position by sending a digit and that it has turned to the desired level at the search for a free junction, but none were found available. In this case, the rotation continues until the brushes 921 to 925 have passed through all the contacts of the selected level and the usual ineffective search contacts 908 and 909 operate.

   Contacts 909 disconnect the invitation to transmit tone transformer 951 and replace it with busy tone transformer 952, to warn the calling subscriber, while contacts 908 close the circuit making

  <Desc / Clms Page number 48>

 re-attract relay 903 to cause the circuit of the electro-rotation 911 to open and to reconnect contacts 909 to the lower winding of relay 901 to allow the sending of the busy tone, which it just been question, on the calling line.



   When the calling subscriber hangs up under these conditions, the release of relay 901 is followed by that of relays 902 and 903. By releasing relay 902, the release electro operates which returns the switch to the normal position, thereby the release circuit. release is open at engage contacts 905. Contacts 908 and 909 are returned to the position shown on the drawing as soon as release operations begin, and the polarity indicating that the machine is free is returned through engagement contacts 907. on wire 853 when the switch reaches its normal position.

   The release of the relays associated with the switch TS2 occurs when the positive is removed from the starter wire 855 by the relay 902 - and the release of the distributor DS (in the case where it / has not yet occurred as described previously) is effected when the positive is removed from the incoming junction release wire 743 to the contacts of relay 902.



   For the purpose of explaining the precautions taken to deal with the total occupancy which arises when there is no free junction line between a selector of thousands, such as TS, and the group of thousands determined by the position of the thousands bypass selector, such as BTS fig. 9, it will be assumed that the BPS selector, fig.ll, and the TS3 switch, fig. 10, have been entered by the BTS bypass thousands selector and the second stage switch TS3 is unable to find a free junction to the TS thousands selector.



   It will be recalled that the group test relay 1005 of the TS3 / switch operates via the group test brush 1021 and the common group wire 300 of the thousands group in

  <Desc / Clms Page number 49>

 service and the upper winding of the transformer 811.



   When the brushes of the switch TS3 arrive on the first free junction having access to the group leading to the selectors of the hundreds, in the first group of the thousands of the group of selectors comprising the selector of the thousands TS, the relay 1005 attracts. Relay 1005 remains attracted through wires such as 431 and 434 of the successive group junctions as long as the brushes of switch TS3 advance over the junctions of the group in question. Since according to the assumption made all these junctions are occupied, the relay 1002 does not attract to stop the search movement of the switch TS3 and to bring about the attraction of the relay 1007.

   In this case, relay 1004 attracts at the second time as follows:
When relay 1005 pulls when brush 1021 arrives at the first junction assigned to the wanted group, it closes a circuit through the relatively inactive lower winding of relay 1004 which pulls relay 1004 only by an amount sufficient to close. its contacts, preliminary to the first attraction time, are those which are immediately above the relay, but the attraction is not sufficient to attract its two upper interior reinforcements. Closing of the upper blocking contacts of relay 1004; closes the circuit of the two windings of the relay in series, through the contacts of relay 1007, to wire 1119.

   This blocking circuit does not become active as long as the initial circuit of the relay remains closed, but as soon as the relay 1005 releases after the group has been exceeded, the two windings of the relay 1004 are supplied in series and the relay draws completely. Usually the complete attraction of relay 1004; is not carried out because the blocking circuit thereof remains open at the contacts of the switching relay 1007 before the relay 1005 can release.

  <Desc / Clms Page number 50>

 



   Pulling relay 1004 fully pulls relays 1006 and 810 to its lower contacts, while at its lower inner frame it disconnects the incoming group test and line mark wire 934 from the lower frame of relay 1006 and The extension to the common ineffective wire 1305, this wire is common to the entire second stage of switches such as TS3 and is also common to the first stage of ineffective circuits, such as that shown in fig. 13.

   The pull of relay 1006 is of no particular use at this time, but relay 810 of switch TS2 disconnects brush 829 and extends a connection through relay 809 to brush 828, at the same time a positive is brought through. the drive brush 830 and wire 240 to the control electro 307 of the thousands TS selector.



   Thousand TS selector brushes 301 to 306 rotate until brush 305 or brush 306 encounters a marking polarity which will not occur as long as an auxiliary circuit, such as that of FIG. 13, will not have been reached, at this moment the marking polarity placed on the wire 1305 is encountered by the brush 306 and allows the relay 809 to attract to stop the rotation of the selector brushes of the thousands by opening the circuit of the control unit 307.



  Relay 809 attracts relay 808 as already described, relay 808 closes the attraction circuits of relay 208, thereby the calling line is extended through brushes 304 and 305 to relay 1301 of the auxiliary circuit of fig. 13. The switch TS2, the selector of the thousands bypass BTS, and the selector of the hundreds bypass BPS, as well as the switch TS3 release in principle in the manner described above.
In the auxiliary circuit, fig. 13, relay 1301 attracts relay 1302 to disconnect the test lead in-

  <Desc / Clms Page number 51>

 trant of wire 1305 and to connect it to the positive to form a blocking circuit of the connection established.

   At its inner frame, relay 1302 connects the busy tone wire 1304, through capacitor 1303, to the positive talk wire to give the busy signal to the calling line.



   The connection established to the auxiliary circuit of fig. 13 is released when recalling hangs up its receiver and allows the release of relays 1301 and 1302.



   It may be noted that 2 or 3 adjacent pins in the bans of thousands selectors, such as TS, may be provided for the use of auxiliary circuits, such as that shown in fig. 13; these auxiliary circuits can be. common to all thousands selectors in the office.



   It will be noted that this precaution is used to immediately release the bypass selectors, which are the most expensive, and to allow the busy tone to be sent at the start of the conversation train.



   A total occupancy occurring at a bypass hundreds selector, such as BPS, is treated in the same way as that exposed for the BTS bypass thousands selector, ie the springs of ineffective search 1109 and 1110 are operated when the brushes 1114 to 1118 are brought beyond the last contact of the selected level. Contact 1110 causes relay 1109 to be re-attracted by means of the internal upper contacts of relay 1103 to open the baked air of the rotation electro 1112, to prevent any unnecessary putting into service of the latter, contacts 1109 transfer the lower terminal of the line relay from direct positive to a positive taken through the secondary winding of a common busy tone transformer.



   When the calling subscriber hangs up his receiver when

  <Desc / Clms Page number 52>

 that he hears the busy tone given by the BPS selector, the release of the relay 1102 causes the release of the circuits in service as has been described previously.



    CLAIMS.



  1. A telephony system, having successive stages of main switches through which a conversation is established under the control of corresponding stages of bypass switches in which the bypass switches are temporarily associated with certain stages with the switches main switches via a finder which is designed to check the position of the main switches in accordance with the positions taken by the bypass switches.



  2. A telephone system using digital switches for the temporary establishment of a call to a called line and non-digital switches for the flow of the conversation which resume the connection established by the digital switches, in the - which digital switches and non-digital switches of a selection stage have access to different groups of digital and non-digital switches, respectively, of the next selection stage,

   a digital switch of one stage being associated with the non-digital switch of the next stage via a finder who controls the establishment of the connection by means of the non-digital switch to resume a temporarily established connection by digital switches.

** ATTENTION ** end of DESC field can contain start of CLMS **.


    

Claims (1)

3. A telephone system having a group of lines and a group of main connection circuits: by means of which a communication is established under the control of a bypass circuit, in which when a line is pending <Desc / Clms Page number 53> call a connection is established to the bypass circuit which is connected to a free main connexian circuit, which therefore establishes a connection between the calling line and the main connection circuit. 4. A telephone system in which the trunks entering a selection stage are divided into groups, each group being associated with a group of outgoing trunks, the seizure of one of the incoming lines being used to activate a searcher who has access. to all outgoing trunks to make the connection with an outgoing trunk in the appropriate group, the operation of the finder being controlled by a group wire common to the group of inbound trunks to which the calling line is connected. 5. A telephony system in which a group of trunks entering a digital selection stage has access to a number of outgoing trunks and in which a pending incoming trunk connects to one of the trunks of outgoing groups under the control of a common switchgear which is temporarily associated with the calling junction, the common switchgear being placed according to the designation assigned to a called junction in the group of outgoing junctions, thereby a free junction in the desired group is selected and marked under the control of the common switchgear and a connection is made between the incoming line and the marked outgoing line. 6. A telephony system using researchers who are looking for a calling line in which the lines are digested in groups, each of them being accessible to a certain number of researchers and having a starting relay associated with them, the operation of 'a dear- <Desc / Clms Page number 54> chor when a line of one of the groups is in the process of being called, being controlled jointly by the starting relay and the equipment common to all the lines and having access to researchers accessible by all the groups of lines. 7. A telephone system in which two groups of switches each have access to a group of lines and in which a connection is established by means of one of the groups of lines by first establishing a bypass connection by connecting a line from one of these groups to any of the switches having access to them under the control of test units comprising a common group wire, common to the group of lines in question. 8. A telephone system as claimed in claim 2 in which when the connection has been taken over by the non-digital switches the connection established by the digital switches is released. 9. A telephone system as claimed in claim 2 or 8 in which each digital switch has access to a number of connection circuits and selects a free connection circuit from a predetermined group, thereby a free trunk circuit to which the non-digital switch has access is marked and the non-digital switch searches for the marked connection circuit, under the control of devices associated with the digital selection stages. 10. A telephone system as claimed in claim 3 wherein when the connection is established between the calling line and the main connection circuit, the initial connection between the calling line and the bypass connection circuit and the connection between the bypass connection circuit and the connection circuit princil are released. <Desc / Clms Page number 55> 11. A telephone system as claimed in claim 3 or 10 in which the connection between the calling line and the connection circuit is made by a main finder and the connection between the bypass connection circuit and the main connection circuit is made by an auxiliary finder. 12. A telephone system as claimed in claim 5 in which the connection between the first order junction and the second order junction is effected by means of an automatic switch which searches for the marked second order junction. 13. A telephone system as claimed in claim 5 or 12 in which the marking is effected by means of a switch which operates according to the value of one of the digits sent by the calling party. 14. A telephone system as claimed in claim 6 in which the common apparatus makes the following connection with the finder and comprises two relays, one of which is operated when the common apparatus is in communication with a finder of the group containing the calling line and the other when a free seeker is found. 15. A telephone system as claimed in claim 14 wherein the test circuit of one of the relays comprises an individual current source not connected to earth and the test circuit of the other relay the office battery which is earthed. 16. A telephone system as claimed in claim 6 or 14, in which the common apparatus includes a finder who has access to several groups of researchers and who, activated by the operation of the starting relay, selects a free primary finder in the corresponding groups so that he performs the search. search for the calling line. <Desc / Clms Page number 56> 17. A telephony system as claimed in claim 6, 14 or 15 in which groups of secondary researchers are provided, each group having access to a number of different primary researchers, the operation of the secondary researchers being carried out under the control. control of a second researcher whose operation is controlled by two relays. 18. A telephone system as claimed in claim 7 in which after the bypass connection has been established the bypassed switch is put into service to establish the connection with the calling line through the switch with the bypass connection released, 19. A telephony system as claimed in claim 7 or 18 in which when a line in one of the groups is put into service a distinctive polarity is placed on the associated common group wire and a finder activated to select one of the free switches in the group whose Common group wire is marked in this way, and the selected switch then searches for the calling line. 20. A telephone system in principle as described and shown in the attached plans.