Patent GB-143498-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/42
Filed: 1920-04-30
Abstract: 143,498. Western Electric Co., (Assignees of Lundell, A. E.). May 21, 1919, [Convention date]. Automatic exchange systems. - When a large group of lines is served by a large number of callfinders, the call-finders are divided into subgroups and an idle finder is pre-selected by the joint action of a master allotting-switch common to the whole group and an allotting-switch individual to a sub-group. By this means, when a preselected finder is taken into use, the allotment of another idle finder is accomplished in a minimum time. Successive calls are allotted to finders in different sub-groups, so as to distribute the work equally. The invention is described in connexion with finders having a capacity of 500 lines, each group of 50 finders being divided into five subgroups of 10. Fig. 1 shows the complete circuits of the first finder of the first sub-group, F1G1, and elements of the circuits of other finders of the same sub-group, F2G2, F3G1, &c., and of other sub-groups, F1G2, &c., F1G3, &c. Fig. 2 shows the master allotting-switch 100 and the sub-group allotting-switches GA1, GA2. When a finder is released at the termination of a call, its sequence switch 4 is moved to its 18tb or waiting position, but there is always one idle finder having its sequence switch in its 1st position, ready to deal with a call. The movement of a sequence switch from its 18th to its 1st position is controlled by a circuit including contacts of the master and subgroup allotting switches. For example, it will be assumed that the sequence switch 4 of the finder F1G1 has been moved to its 1st position by the closure of the circuit 109, 203, 204, 70, 5, 4. When a call arrives, the sequence switch 4 is moved to its 2nd position, and the finder F1G1 hunts for the calling line. As the sequence switch 4 passes from its 1st to its 2nd position, a contact 50 closes a circuit 102 to move the master allotting-switch 100 to its 2nd position, corresponding to the sub-group controlled by the allotting-switch GA2. If all the finders of this subgroup are busy, with their sequence switches in positions from 2 to 17, a circuit 104, 105 - - 108 is closed to keep the switch 100 moving until it reaches a sub-group, including an idle finder, having its sequence switch in its 18th position, and a circuit is then closed through contacts of the master switch 100 and of the selected sub-group allotting-switch to an idle finder of this sub-group, to move its sequence switch from its 18th to its 1st position, ready to deal with the next call. In the meantime, the switch GA1 has been moved forward to find the next idle finder in the first sub-group, the circuit 202 having been closed at contact 51 when the sequence switch 4 reached its 2nd position. The call-finders would in practice be two-co-ordinate switches with brush-choosing devices, but for simplicity they are represented as single co-ordinate switches. Operation of call-finder. The energization of the line relay 2 of a calling line closes a circuit 5, 4 to move the sequence switch of the pre-selected call-finder to its 2nd position, and puts a selectable potential on the test terminal of the calling line by shunting a resistance 14 across the cutoff relay 3. The finder is now actuated by the updrive magnet 7, which is de-energized by test relay 8 when the calling line is reached, the switch 4 being moved to its 3rd position, in which a busy potential is applied through a resistance 15. The cut-off relay 3 is energized at the same time as relay 8. Operation of connector. The two sets of digit impulses now operate the primary and secondary stepping magnets 24, 26 under control of relays 19, 20 in known manner, the relay 22 advancing the switch 4 at the beginning and end of each series of impulses, so that when the connector has been set on the wanted terminals, the switch 4 is moved to its 7th position. Busy test and release. If the wanted line is busy, relay 33 is energized over the test brush and locks itself up, completing a busy tone circuit and driving the switch 4 to its 10th position, in which the apparatus is restored. Supply of ringing current. If, on the other hand, the wanted line is idle, the relay 33 is not energized and the switch 4 moves only to its 8th position, in which ringing circuit is supplied through a relay 38. When the called subscriber answers, relay 38 is energized and moves the switch 4 to its 9th position, in which conversation takes place. Release. When the calling subscriber disconnects himself, the de-energization of relays 19, 20 moves the switch 4 to its 10th position, in which a circuit is closed for the down-drive magnet 42 of the call-finder, and when the finder regains its normal position, a normal contact 45 closes a cirouit to move the switch 4 to its 11th position, in which the connector release magnet 46 is energized. The release of the connector shifts the off-normal contact 47 so as to close a circuit for moving the switch 4 to its 18th position, where it waits until the finder F1G1 is again pre-selected for service, as already described.