Patent GB-210601-A (GB)
Title: Improvements in private branch exchanges
IPC: H04M5/10
Filed: 1923-01-02
Abstract: 210,601. Western Electric Co., Ltd., (Deakin, G.). Jan. 2, 1933. Exchange systems.-A Private Branch Exchange having lines to an automatic central office and tie lines i.e. direct lines to other P.B.X.'s in addition to local lines uses the same cord circuit for making any connection. This circuit is of split cord type, (i.e. of the type in which when an extension hangs up after a connection through the main exchange, the cord circuit is opened so that the same trunk may be seized again before the P.B.X. operator has taken down the cord, without ringing the extension) and has the following features :-(1) The operator cannot listen to a conversation, (2) certain lines, for example tie lines, cannot be connected to the main exchange, (3) the insertion of a plug into a junction jack automatically disconnects the ringing key. (4) slight circuit alterations can be made to render features (1) or (2) inoperative. The cord has a separate ringing key for each plug, a listening key and a through service key. Connecting extension to extension. Extension lines terminate in the jack with grounded sleeve shown in Fig. 5. The operator answers with the plug 10, Fig. 1. The line lamp goes out and relay 4 operates, cutting out lamp 2, and lamp 1 lights. Listening key 11 is thrown, completes the operator's microphone circuit through relay 13 and energizes listening relay 12. Relay 13 cuts out the buzzer and relav 12 connects the operator's set to the cord circuit. Plug 15 is inserted in the wanted jack and ringing key 16 is thrown. On reply of the called subscriber, relay 3 extinguishes lamp 1 and de-energizes relay 12 so that the operator cannot listen. The extension lines are fed with current through the bridged relays 3, 4. When the subscribers hang up, relays 3 and 4 fall back, light lamps 1, 2 and energize relay 17 which operates the buzzer. Relay 17 has three windings so as to serve three cord circuits. Connecting junction to extension. A final selector at the central office seizes an idle trunk and connects battery to the T side and ringing current to the R side as shown at C, Fig. 2. The trunk terminates in the jack shown in Fig. 4 and relay 19 operates, locks up, lights the line lamp, and grounds the T side of the trunk thus operating relay 9 and causing the central office circuits to change to the conditions shown at B, Fig. 2. The call is answered with plug 15. Relay 19 falls backs and extinguishes the line lamp and relay 7 energizes (the sleeve of the junction line jack being connected to battery), disconnects relays 3, 4, connects the two plugs together, the tips directly and the rings through a low resistance relay 5, and puts the coil 23 across the cord circuit between relay 5 and the central office thus operating relay 24, B, Fig. 2, and causing the automatic circuits to advance to a talking position. The listening key 11 is then thrown. While the listening key 11 is thrown to energize relay 12 the shunt 23 is replaced by a similar shunt 26. Plug 10 is inserted in the wanted jack and ringing key 27 thrown. Lamps 1, 2 glow and listening key is restored to normal. The reply of the called subscriber energizes relay 5 which extinguishes lamps 1, 2 open-circuits relay 12 and energizes relay 8 which removes the shunt 23. When the extension hangs up, relay 5 falls back, lights lamps 1, 2 and removes a short circuit from right hand winding of the relay 6 which energizes in series with relay 8 and splits the cord circuit. allowing the central office switches to release and connecting battery and ground to the conductors of the extension side of the cord. If the trunk is seized again before the connection is taken down, relay 19 will light the line lamp intermittently, but the central office ringing cut-off relay will not be operated until the call is answered by removing the plug 10 and throwing the listening key, thus releasing relays 6, 8. Connecting extension to junction. Attendant dials. The call is answered with plug 10. If plug 15 is now inserted in a. trunk jack relay 7 operates and energizes relay 5 which cuts out listening relay 12 and thus prevents the attendant from dialling until the extension hangs up so as to release relay 5. Normally therefore the attendant moves the plug 15 of another cord in a junction jack, throws the listening key and on receiving dialling tone works her impulse sender. Off normal springs of the latter energize relay 28 which disconnects the talking set and energizes relay 29. Relay 29 removes the impedance 26 and energizes relay 25 which bridges the junction through the impulse springs. At the end of each digit off-normal springs release relays 28, 29 the impedance 26 being again put across the line. The call is completed by means of the plug 10 of the cord used in calling the main office. When the listening key is returned to normal relay 25 falls back. The supervisory signals at the P.B.X do not alter during the progress of a connection, supervision being effected by listening in. At the end of the conversation relav 5 falls back, lights lamps 1, 2 and energizes relay 6 to release the central office switches. As an alternative the subscriber may hang up until rung by the operator after the connection has been set up. Connecting extension to junction. Subscriber dials. Operator throws through service key 22 to disconnect the bridged impedances 3, 4, 23 and the contacts of relays 12 and to break the circuits of relays 7, 8. Relay 5 is connected in series in the cord circuit, operates and opens the circuit of relays 12. Lamps 1, 2 may flicker during dialling but burn steadily on release of relay 5 when the central office subscriber hangs up. When the extension hangs up the cord circuit is not split so that he may be rung again if the trunk is seized for a new call. This condition is indicated bv flickering of lamps 1, 2, 20. Connecting tie lines to extension. Restricted service. The lines terminate in the circuit shown in Fig. 3 and must always be answered or called by plug 15. Insertion of the plug energizes highresistance relay 32 which connects the line to the jack. Relay 7 is in series with relay 32 but does not operate. When plug 10 is inserted in the extension jack lamp 2 burns until the extension answers and relights when he hangs up. Relay 3 is energized through impedance 33 and prevents lamp 1 from lighting. If the attendant tries to connect the tie line to the central office, relay 6 is operated by its left hand winding from battery at sleeve 21, and splits the cord. She cannot use two cord circuits as the listening keys are all wired in series so that only one listening relay can operate at a time. The tie line cannot be answered with plug 10 since there would be no current to operate relay 32. Nullifying secret or restricted service. Secret service is nullified by connecting relay 12 direct to battery and cutting the dotted lead at the cross shown. Restricted service is nullified bv breaking the lead connecting relay 6 to the left-hand working contact of relay 3.