Patent GB-1365534-A (GB)
Title: Line equipment for telecommunication systems
IPC: H04Q3/00
Filed: 1971-10-15
Abstract: 1365534 Automatic exchange systems COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL 15 Oct 1971 [15 Oct 1970] 47997/71 Heading H4K Line equipment employing non contact making relays is arranged to mark a line Ap common to a plurality of such equipments with earth potential when an electronic scanner DT1 marks wire C 1  with earth potential subsequent to the subscriber line L 1 , L 2  being looped, thereby enabling the exchange equipment to select the calling line and to connect it via lines AB to the switching network, e.g. a crosspoint network employing cross-bar switches. In the event of a subscriber off-hook which is not followed by dialling or failure of the subscriber to replace his handset at the end of a call, a line FAp is marked enabling a "false call" scanner DT 2  to detect the subscriber in question. Outgoing call.-In the on-hook condition transistors T 1 , T 3 , T 4 , T 5  and a bi-stable circuit EB, which behaves as a thyristor, are non- conducting and transistor T 2  is conducting. When the subscriber off-hooks thereby connecting L 1  to L 2 , transistor T 1  turns on, thus turning off transistor T 2 ; transistor T 4  is therefore turned on and also a negative voltage is applied to the base of transistor T 3 . When next a scanner E 1  of electronic scanner DT 1  applies earth potential to wire C 1 , transistor T 3  will conduct and the point Ap will go to near earth giving an indication that one of the line equipments connected to that point is initiating a call. At this time earth potential is applied to wire C 2 , scanner E 1  is stopped at the wire C 1 , and hence transistor T 3  turns off. A second scanner E 2  is started which successively removes the earths from the C 2  wire of each line equipment. When this earth is removed from the equipment at which E 1  is stopped the corresponding transistor T 3  again conducts, Ap again goes to near earth potential and scanner Eg is stopped. The calling equipment is thereby identified. The exchange control equipment then completes a speech path through the switching stages in a conventional manner and when this is completed an earth potential is temporarily applied to a hold wire M. This causes circuit EB to switch on and results in the base of transistor T 3  going to near earth. Transistor T 3  is therefore turned off and the scanners E 1  and E 2  are freed. When the connection through to the called subscriber is completed a position hold voltage is applied to wire M. However circuit EB remains on thereby holding off transistor T 3  even if earth is applied to wire C 1 . When the subscriber replaces his handset transistors T 1 , T 2 , T 4  revert to their original states. EB is therefore switched off. The hold voltage on wire M is removed. Incoming call.-The exchange equipment automatically sets scanners E 1 , Eg to the C 1 , C 2  wires of the called subscriber. An earth potential is applied to the wire T, and if transistor T 2  is on i.e. called subscriber on-hook, this earth potential turns off transistor T 2  and transistor T 2  is turned on as the scanners are positioned. Transistor T 4  is also turned on as before. As transistor T 3  conducts wire Ap goes to earth and the exchange equipment notes that the line is free. The subscriber is rung and earth potential is applied to wire M to turn on circuit EB, thereby turning off transistor T 3  and freeing the scanners. When the called subscriber answers L 1 , L 2  is looped and transistor T 1  turns on thereby ensuring transistor T 2  holds off. Once the subscribers are connected, hold voltage replaces earth on wire M. If the line had been busy the application of earth to wire T would not have resulted in transistor T 3  turning on as it would be held off because of the conduction of circuit EB. Permanent loops.-If the subscriber off-hooks and fails to dial the earth potential on wire M is removed after a preset time. Similarly at the end of the call, the hold voltage is removed from the wire M. If the subscriber remains offhook there is a condition whereby circuit EB is conducting and wire M is floating. This results in transistor T 5  conducting. At regular intervals a scanner E 3  of an electronic false call scanner DT 2  scans wires C 3  applying earth potential pulses to them. If T 5  is conducting it is turned off by C 3  being earthed and hence the potential on wire FAp is altered and the scanner E 3  stops; at the end of the pulse transistor T 5  turns back on; a scanner E 4  is started which applies earth potential pulses to wires C 4  and is similarly stopped. Scanner E 5  operates similarly, stopping at the line equipment in question, and the exchange control equipment recognizes the permanently looped circuit from the positions of the three scanners. It is not disclosed what action is taken once the permanently looped circuit has been recognized. Protection against voltages surges:-A diode D 1  is provided in the emitter circuit of transistor T 1 ; this diode has a high reverse voltage capability and is intended to protect the line equipment against voltage overloads on the line L, e.g. due to high tension power cables falling on the line. Capacitor C is intended to prevent circuit EB being triggered by interference pulses on wire 17.