Patent GB-309670-A (GB)
Title: Improvements in or relating to automatic or semi-automatic telephone systems
IPC: H04Q3/58
Filed: 1928-02-16
Abstract: 309,670. Coventry Automatic Telephones, Ltd., and Wilman, C. W Feb. 16, 1928. Automatic exchange systems.-A Strowger switch at a satellite hunts on its first level for a junction to the main exchange, where the digits are received until it is determined whether the call is local or not, and if the call is local a signal (open junction) is sent back to release) the switch which then responds to a further digit or digits to extend the call over another level. The digit 1 is excluded as the first digit of local numbers, and if accidentally dialled causes a tone signal to be returned. For local calls the switch may act as a group selector, Fig. 1, or as a final selector, Fig. 2. In the latter case it is assumed that only one digit is used for discrimination, and if all junctions are busy the switch releases itself and modifies its circuits so that if the dis. criminating digit is local the switch again releases itself and completes the call in response, to the two final digits. Seizure of junction, Fig. 1. Relays 13, 16, 17 pull up, the vertical magnet 4 and relay 15 are energized over 31, 30, 24, and relays 14, 18, 20 pull up. When 15 falls back, the rotary magnet 5 is energized and opens 10 to interact in the usual way with 18 to find an idle junction, whereupon the switching relay 19 is energized and closes a loop through 20. Digits are repeated over the junction at 45, the relay 20 being shortcircuited by 15 during each digit. From the moment when 14 was energized 16 is locked under control of 20 or 15. Local call. Opening of the junction at the main exchange causes 20 to relapse and unlock 16, 17 and the release magnet 6 is energized over 25, 50. Relay 14 remains locked. The next digit is repeated to vertical magnet 4 over 31, 24 and the switch operates as a group selector in the usual way. First local digit = 1. On its first level the switch operates springs 29 (connecting ground 41 to relay 18) and hunts to the 11th row, when contacts 11, 12 are shifted and tone is sent back from 51. Seizure of junction, Fig. 2. Relays 116, 118 pull up, the vertical magnet 105 and relav 123 are energized (122, 121, 117) and the following relays pull up :-125 (124, 117), 131 (134, 132, 129, 110, 128), 138 (139, 138, 137, 134, etc) 144 (107, 144, 143, 142, 128). When 123 falls back, the rotary magnet 106 is energized (148, 147, 142, 128) and opens 111 to inter-act with 144 (which is connected over 143, 150, 149 to 103) to find an idle junction, whereupon relay 151 is energized (128, 151, 143, 144, 107) and energizes 165 which closes a loop through 138. Digits are repeated over the junction at 137, the relay 138 being short-circuited by 152 during each digit. From the moment when 125 was energized 118 is locked under control of 123, 152. or 138. Local call. Opening of the junction causes 138 to relapse. and unlock 118, so that 159 pulls up (158, 156A, 157, 104) and energizes the release magnet 107, which keeps itself energized over a contact 183, controlled by the double dog, until release is complete. When wiper 104 leaves the bank, relay 162 becomes locked in series with 159. Relay 131 is unlocked at the off-normal contact 110. The next digit is repeated to 105 (117, 122) and the final digit to 106 (117, 122, 108, 154, 166, 148). If the line is busy, 144 is energized (169, 168, 167, 103) and returns tone from 172. Otherwise ringing current is sent over 131 until the subscriber replies. The called line is fed from 138. All junctions busy. In this case the release magnet 107 is energized over 182, 104 and then over 183. Relay 118 is unlocked at 110, dial tone is sent back from 184, and the switch responds to the discriminating digit. If this raises it to a local level, springs 112 are shifted and on the relapse of 123 relay 159 is energized (158, 112, 185) to release, the switch, which responds as before to the two following digits to complete the call. For a non-local call 112 is not shifted, and 144 is energized (143, 186, 112, 185) and sends back busy tone.