Telephone line circuit

1,052,420. Automatic-exchange systems. WESTERN ELECTRIC CO. Inc. May 28, 1964 [June 4. 1963], No. 22046/64. Heading H4K. In a time division telephone system ringing control signals are fed through the time division switching network, and the ringing signal is returned to the switching network when the call is answered. The invention is described with reference to a party line system and the circuitry, in addition to ringing the right party for incoming calls, identifies the party originating an outgoing call for metering purposes. Incoming calls.-Positive or negative ringing control pulses, according as tip T or ring R parties are to be rung, are applied by signal source 65 via the sampling switch 42 every other time it is opened and are smoothed by filter 41. One of transistors 45, 46 conducts to operate relay 10 or 20 and connect ringing current from a source 21 to T1 or R1 respectively and disconnect the line relay 30. The corresponding bell 6, 11 is thus rung. When the called party goes "off-hook" 7-1 and 7-2 close and ringing current is connected to one of primaries 27-2; 27-1, but does not energize the second bell because of the impedance 8 of the switch-hook. Ringing current thus induced in secondary 27-3 and limited by Zener diodes 12, 13 is returned to bus 81 and is detected by 66 every other opening of switch 42 (i.e. those when ringing control pulses are not being applied) to indicate that the call has been answered. Since either 45 or 46 only conducts during the ringing interval and not during the silent interval, ringing current cannot be returned if the called party goes off-hook in a silent period. If this happens relay 30 operates and bias source 51 overcomes source 58 to forward bias diode 57 and return pulses 60 from control 43 over condenser 56 and an auxiliary bus 91 to the scanner 64, which, knowing that a party is being rung realises that the party has answered. For a four-party line either positive or negative ringing current may be applied to either conductor T1 or R1. Outgoing calls.-Idle condition of the line is sensed once every 500 cycles of the time division switching network by pulses 60 from control 43 closing sampling switch 42. When either party goes off-hook contacts 7-1, 7-2 are closed, completing the line loop and operating line relay 30 to return pulses 60 to the control via bus 91. A time slot is allotted to the line and corresponding pulses 60 are applied to sampling switch 42 every cycle and returned via bus 91 for t