Patent GB-239374-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1924-09-24
Abstract: 239,374. Coventry Automatic Telephones, Ltd., and Amberg, V. Sept. 24, 1924. Automatic exchange systems.-In a districtoffice system in which a subscriber's line switch A hunts for an idle selector-repeater B having associated with it a secondary hunting switch C which selects an idle trunk to the main exchange, and in which the selector B is marked busy if the associated switch C has no idle outlet, this busy condition is removed from the selector B when every main-exchange trunk is busy so that local calls, in which no main.exchange trunk is employed, may still be made. A subscriber desiring to make a main-exchange call when every trunk is busy receives a busy tone. Local call. The line switch A operating as described in Specification 208,075 extends the connection to the selector-repeater B at which relays 51, 53 operate, the latter closing a circuit over normal contacts 62 for the rotary magnet 101 of secondary switch C in series with switch ing relay 102. The mid-point of this circuit is connected to the test wiper 105 so that the switch hunts in the usual manner and, when an idle trunk is found, relay 102 operates and locks up independently of normal contacts 62. Dialling tone having been transmitted from lead 56, the first series of impulses is repeated by relay 51 over the trunk to the main exchange and also to the vertical magnet 66 of the selector-repeater. Relay 63, energized during the digit, short-circuits the impedance 82 to improve the impulsing circuit and operates stepping relay 71 which locks up over the interrupter contacts 75 of the rotary magnet 74. On the relapse of relay 63, the wipers are rotated on to the first set of contacts and, if the first digit denotes a local connection, hunting takes place in the usual manner until a free local second selector is found, when relay 80 energizes, switches through to wipers 87, 88 and releases the main-exchange trunk. Subsequent digits are repeated by relay 51 to local switches. Call to main exchange. The switch C selects an idle trunk, and the selector-repeater responds to the first digit in the. manner described above for a local call. When the first rotary step is taken, however, an auxiliary wiper 76 completes a circuit for relay 79 which opens the circuit of relays 71, 80 and vertical magnet 66 so that the switch B does not hunt. and succeeding digits are repeated bv relay 51 to the main exchange. Busying arrangements. When all the trunks accessible from the switch C are busy, a relay 201, being no longer short-circuited, responds and connects grounds over contacts 206, 207, 152, 109, 108, 61 to mark the selector-repeater B and switch C busy to the test wiper 7 of switch A. When, however, all the trunks accessible, to all the switches C are busy, relay 205 operates in a chain circuit and opens this ground connection. A calling subscriber's line switch A may now seize the selector-repeater B if idle even though all the trunks leading from the switch C are busy. In this case, the switch C will hunt continuously until the arrival of the first impulse opens the hunting circuit at contacts 62. If the call is a local one it is completed as before; but if it is a main-exchange call, the operation of relay 79 connects up busy tone 89 over a back contact of switching relay 102 of switch C.