Patent GB-720191-A (GB)
Title: A telephone system with a relay-less line circuit and circuits in co-operation therein for extending a call
IPC: H04Q3/00
Filed: 1952-01-07
Abstract: 720,191. Automatic exchange systems. AUTOMATIC ELECTRIC LABORATORIES, Inc. Jan. 7, 1952 [March 16, 1951], No. 504/52. Class 40 (4). In a system using two co-ordinate finder switches which are controlled by a call allotter, each line circuit is connected to a resistance network in which potential changes due to closure of a subscriber's loop cause the response of a common start circuit in the allotter. A 100-line exchange is described with the allotter and a line finder, and a final selector with camp-on-busy facilities, shown in Figs. 1 and 2 respectively. Arrangement of resistance networks and common start circuit. Each line circuit 11 forms part of a series network comprising resistors 14, 15 and 17 which are connected together in groups of ten lines to vertical test outlets 145 of the line finders 32 over decoupling rectifiers 18. The groups of lines are connected over decoupling rectifiers 20-22 to a common start circuit which comprises a normally conducting thermionic valve 26 and a relay 29 controlled by anode current. Call allotter. In addition to the start circuit, a valve 34 controlling a relay 70 is provided which functions as a detector for a free line finder and for a calling line. Normally the control grid is connected to wiper 40 of allotter switch 35, and when the associated outlet corresponds to an idle finder-final selector link, the valve is cut off by negative potential on wire c. If the link is not available, the outlet is open-circuited or earthed so that the valve conducts and pulls up relay 70. This completes a drive circuit for magnet 80 of the allotter switch which hunts until the valve cuts off and releases relay 70 when a free link is found. Calling condition. Closure of the subscriber's loop causes the potential at the junction of resistors 16, 17 to change from zero to about one third of the applied voltage, and valve 26 becomes cut off so that relay 29 falls back. Relay 50 operates and transfers valve 34 to the vertical test wiper 146 of the finder which, being unmarked, causes the valve to pull up relay 70. Relay 110 then operates, followed by line-finder relay 60, which busies the link by connecting earth to wire c. The vertical drive magnet 129 is energized by 110 and steps the finder by self-interruption until the test wiper 146 finds the calling-line group. The potential at the junction of resistors 16, 17 then cuts off valve 34 so that relay 70 falls back and stops the switch. Relay 120 pulls up and transfers valve 34 to wiper 151 of the finder. Valve 34 again becomes conducting and re-operation of relay 70 energizes rotary magnet 149 so that the switch steps over the selected level until the outlet characterized by the calling potential is found. Valve 34 against cuts off and disables relay 70, causing relays 100 and 160 to pull up in series over rotary off-normal contacts R.O.N. Relay 100 resets an alarm circuit by connecting battery to lead A.L.M. while relay 160 switches through the speech circuit and earths the junction of resistors 16, 17 so that valves 26, 34 both conduct and operate their relays 29, 70 (provided that no other calls are waiting). All the operated relays with the exception of 160 fall back and complete a drive circuit for the allotter switch 35 which hunts for the next free line finder. Valve 34 then cuts off and releases relay 70 to stop the switch, while relay 50 pulls up if other calls are waiting. Defective operations. If the finder fails to switch through in a certain time, timing pulses on leads TIME-1 and TIME-2 operate relays 130 and 140 in sequence so that switch 35 is advanced one step and relay 50 disabled. If valve 34 fails to operate relay 70 after operation of relay 50, the alarm circuit will not be reset over lead A.L.M. and an alarm will be given after a predetermined time interval. If valve 26 fails to hold relay 29 operated, a finder switch is started and tests the vertical finder outlets. At the tenth level the switch steps to the eleventh outlet where cam springs 169 operate and prevent resetting of the alarm circuit. All line finders busy. Normally operated relay 90 falls back and disables the allotter switch. Busy tone is also induced in all calling circuits over coil 170. Final selectors with camp-on-busy outlet facilities. Each line finder is permanently linked with a final selector, Fig. 2. When line finder relay 60 pulls up, a loop circuit is completed to the final selector and line relay 210 operates, followed by 230. Dialling relay 220 also pulls up over wire c and disconnects busy tone. The first impulse train is repeated by relay 210 to the vertical switch magnet 214. After the first vertical step, off-normal springs 213 disable slow-release relay 220 which falls back at the end of the impulse train. Relay 240 then pulls up over closed off-normal springs 215 and transfers the impulse-repeating circuit to the rotary switch magnet 218. Relay 220 re-operates until the second impulse train is received, when the relay is disabled by rotary off-normal springs 217. After completion of impulsing 220 falls back and completes a circuit to the lower winding of test relay 250. Called line busy. Conductor CN is earthed so that the lower winding of relay 220 is shortcircuited. Busy tone is transmitted to the calling party who may hold the connection. When the called line becomes free, busy earth is removed from conductor CN and relay 250 pulls up so that busy earth is re-applied to CN, and ringing generator GEN connected to the called line. Busy tone is disconnected by the release of relay 240. When the called party replies, ring cut-off and switch-through is effected by relay 260, and back bridge relay 270 pulls up over the line loop. When the calling party clears down, relay 210 releases, followed by relays 230, 250, 260, relay 230 closing a circuit to operate release magnet 216. Final selectors without camp-on-busy facilities, Fig. 4 (not shown). In this embodiment, which is otherwise similar to the arrangement shown in Fig. 2, a relay equivalent to relay 240 is de-energized after the second impulse train is received. The test relay can test the outlet only during the release time of 240 and if it fails to respond, busy tone is connected to the calling line.