Patent GB-1029479-A (GB)
Title: Electronic telephone exchanges, such as private branch exchanges
IPC: H04M7/10
Filed: 1963-10-11
Abstract: 1,029,479. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. Oct. 11, 1963 [Oct. 15, 1962], No. 40198/63. Heading H4K. In an exchange system comprising a current-controlled self-seeking switching network having a number of line circuits connected to its inlets and a number of links connected to its outlets, the links are enabled in time divided manner whereby in the event that one or more of the line circuits is in a calling condition then during the time position of the first free link, the latter is enabled and switching paths commence to be established from the line circuits to the link and wherein when one of these paths reaches the link that path is held and all the other partially-completed paths are disabled. The system which is of the same type as those disclosed in Specifications 953,895 and 1,021,816 is described in terms of a PBX having facilities for:-automatically or manually routing incoming calls to a particular subscriber; triangular or interim conversations involving two PBX subscribers and a distant exchange subscriber and call transfer. The PBX is completely electronic but it includes special arrangements for allowing it to exchange conventional loop signals with orthodox electromechanical exchanges. General operation (Figs. 1, 2).- A calling line A is initially connected through the PNPN switching network 20 to a link circuit, 22 say, during a time position allotted to this link. If the caller is entitled to a special class of service, link 22 marks a special features link 35 so that during the time slot allotted thereto a new path through the network to link 35 may be set up, the original path and link then being released. In either case dial tone is reverted and the number of the called party is recorded in a register in the link circuit. In the case of a local call, the called party's line circuit is marked via bus 40 and during the time slot of the link involved the connection between the calling and called parties is completed. In the case of a trunk call, the link marks bus 36 in order to seize the first free trunk circuit 38 which is similarly allotted in time divided manner by an allotter 52. The trunk circuit then marks one of the outlets T1 and T4 of the network and a new path fires thereto from the calling line circuit. The original path to link 22 at 35 is released. Dialling pulses are then repeated to the central office in a manner described below. Incoming calls may be routed through an operator 25 whose keyboard is enabled only during a particular time slot or they may be completed automatically. The latter case involves the use of " in dial circuits " 24 which are connected to a register 172 via an auxiliary switching network 171, again the register is only enabled during a particular time slot. The called party's line circuit is marked from the register via bus 40 and a path fires therefrom through network 20 to outlets T1 or T2 connected to the indial circuits. Interim conversation and call transfer (Fig. 6). - A PBX subscriber, e.g. 180, requiring such service dials the digit 1 so as to enable a " digit one register " 182 connected to the trunk circuit. This register applies a marking to point P1 at the outlet of an auxiliary self-seeking network 53. A free transfer circuit 51 which is selected by transfer scanner 183<SP>1</SP> (Fig. 2) during a time slot Ta allotted thereto then marks a point P2 of network 53 whereby the trunk circuit 55 is connected to the transfer line circuit 185. The latter marks an inlet to the main switching network 20 whereby an idle link, e.g. 186, is connected thereto. The link returns dial tone. The three digits of the interim or transfer party's number are recorded in the link's register, a sequence switch 190 in transfer circuit 51 stepping-on after each digit until it triggers bi-stable circuit 188 whereby the next digit dialled by subscriber 180 is recorded in register 192. The value of this digit indicates the particular facility required. Thus digit 2 signifies an interim call so that the trunk circuit 55-line circuit 180 connection must be held during this type of call and for this purpose bi-stable circuit 193 holds open AND gate 194 whereby transfer circuit 185 provides a suitable holding potential for the trunk circuit. In the latter a speech gate 183, controlled by transfer circuit 51, may be open or closed depending on whether a triangular or an interim call is required. In the case of call transfer the digit used is 7 whereby the setting of bi-stable circuit 198 upon recept of this digit leads to the connection of trunk circuit 55 to the transfer party, e.g. 181 and the disablement of the original path from subscriber 180. Semi-automatic two-way trunk circuit 38 (Fig. 4).-In response to an incoming call, transistor 91 turns-on and lights a warning lamp 93. The operator applies a negative potential to lead 101 whereby transistor 106 turns-on to switch-off 107 and switch-on 116. A saturable core transformer 102 then passes a switch-on pulse to silicon-controlled rectifier (SCR) 80 thereby completing the loop to the main exchange. The operator learns the identity of the called party and applies marking to bus 40 (Fig. 2) whereby during the time slot allotted to the operator a connection between the wanted subscriber and the trunk circuit is completed through network 20. For an outgoing call, the connection of the calling subscriber to a trunk circuit, e.g. 38, causes the application of a negative potential to lead 36 whereby SCR80 again turns-on to complete the main exchange loop. Each dialling pulse from the caller removes the potential on lead 36 thereby opening the exchange loop correspondingly, the turn-off pulse for SCR80 being supplied by transistor 117 which turns-on temporarily when transistors 106 and 107 revert to their original states, i.e. off and on, respectively. Automatic indialling trunk circuit 55 (Fig. 5, not shown).-This comprises three magnetic amplifiers suitably interconnected so that in response to an incoming call a very small current flows through the output winding of one amplifier and the series connected control winding of another amplifier thereby marking the trunk circuit busy. The last-mentioned amplifier then responds to incoming impulses and repeats them to a register. When all the digits are stored, the register causes the third amplifier to operate and thereby apply answer supervision in the form of battery reversal to the incoming trunk. An outgoing-call is set up in a similar manner to that described above for trunk circuit 38. Power supply (Fig. 4).-The output from a ringing current generator 70 is rectified by fullwave rectifier 75.