Patent GB-917108-A (GB)
Title: Improvements in or relating to private telephone systems
IPC: H04M3/58
Filed: 1960-02-22
Abstract: 917,108. Automatic exchange systems. GENERAL ELECTRIC CO. Ltd., JONES, A. F., and SMART, N. C. Feb. 14, 1961 [Feb. 22, 1960], No. 6157/60. Class 40 (4). Stations in a private telephone system: may connect themselves with a public exchange line by means of a manually operable switch, connection of the exchange line with a station lighting a lamp at each station to show the line busy, the arrangement being such that should a station attempt to intrude on a busy line an impedance, is switched in.and out of the line; and the busy lamps are flashed to signal the intrusion. A call to the public exchange may be held while an enquiry is made over the private exchange. A call from the public exchange may be transferred from a first station by offering the call over an enquiry path to a second station who accepts the call by intruding, intrusion signalling the first station to withdraw. As shown in Figs. 1 and 2, which should be arranged side-by-side with Fig. 1 to the left, five stations 1 to 5 have access to the automatic switching equipment of the private exchange over lines 6 to 10. Each station has a telephone set such as 16, 17, 18, with the normal cradle switch contacts, and impulse transmitter. Two exchange lines 12 and 13 are multipled over the stations to which they may be connected by means of a manual switch LA or LB, respectively, at each station. Each exchange line has a control relay AA or AB and an intrusion relay BA or BB, respectively, located with equipment common to the private system shown in Fig. 2. Should a station connect with the line 12 by operating a key LA it connects a resistor such as 32, 40 or 41, into circuit with relay AA which responds. Contacts of AA connect up lamps such as 27, 28 and 29, to show the line busy, and connect the resistor 32, 40 or 41, as one arm of a bridge circuit with other arms formed by resistors 38, 39 and the relay AA. When the station handset is lifted the public exchange line 12 is seized over terminals 14. The exchange line 13 is similarly provided for with the relays AB and BB and the exchange terminals 15. Should a station intrude by connecting to an engaged exchange line, such as 12, the connection of its resistor 32, 40 or 41, into the bridge arm with an already connected one of these resistors upsets the balance of this bridge and causes a transistor 33 to conduct and relay BA to operate. Relays AA and BA now interact causing a resistor 45 to be switched in and out of the exchange line producing a buzzing tone to warn the connection of intrusion. The line lamps such as 27, 28 and 29 also flicker. These conditions persist until one of the stations withdraws after which the bridge circuit balances and BA is unenergized. A call from the public exchange over terminals 14 or 15 causes a relay - CA or CB to respond to bursts of ringing current. With CA or CB operated by the first burst of ringing current a relay E operates and holds up between bursts. A relay D is also operated by the first burst and is held up by discharge current from a capacitor 61 for about ten seconds. During this time a bell-set 51 at station 1 is rung and, failing reply, after this time a bell-set 52 at station 2 is rung. The connection of bell-set 51 may be transferred to bellset 52 by means of a key RT. As well as the bell-sets at stations 1 and 2, all stations are signalled by connecting up lamps such as 27, 28 and 29 during each burst so that the calling exchange line may be identified. Reply may be made from any station. Having accepted an incoming call from the public exchange a station may transfer the line by, firstly, holding it with a hold key H to connect a resistor such as 31 across the line and to connect the telephone set across the private system line such as 6. A call may now be set up over the private system to a wanted station which will be asked to intrude on the calling public exchange line by throwing key LA. The original accepting station will detect intrusion from the buzzing tone set up by relays AA and BA and will retire by releasing keys LA and H, the wanted station now being left with the call and the buzzing tone being cut off. The stations may have a mechanical interlock which releases the exchange line keys when the handset is replaced on the cradle switch.