Patent GB-687197-A (GB)
Title: Improvements in or relating to telephone exchange systems
IPC: H04M3/54
Filed: 1948-10-01
Abstract: 687,197. Exchange systems. AUTOMATIC ELECTRIC LABORATORIES, Inc. Oct. 1, 1948 [Oct. 15, 1947], No. 25657/48. Class 40 (iv). Calls to unassigned directory numbers in an exchange 30 are routed over an intercepting circuit 500 and a junction 390 to a toll board 300X at a control office 20. The same junction 390 is used in the reverse direction when the operator verifies the identity of a calling station by setting up an independent connection to the calling station over switches 601, 603 and breaks in upon the original connection. The system shown in Fig. 1 is otherwise substantially the same as that described in Specification 688,197. Features common to the two systems are denoted by references bearing a suffix X. A splash of intercept tone is transmitted in both directions from the intercepting circuit 500 when the operator at 300X answers. If the call is a toll call a splash of toll intercept tone is also transmitted to 300X from toll line circuit 1100X. When a call from a toll board 220X in exchange 10 is intercepted the supervisory lamp at 220X is extinguished and re-lit to give intercept supervision. Answer, intercept and re-ring supervision at 220X are controlled by resistances terminating the calling loop in 500. As shown, a local connector 605 and a combination connector 1217X belong to a group connected to the fourth levels of selectors 1207X and 1215X. The number 400 is unassigned and the corresponding contacts (00) in switches 605, 1217X are connected to 500. Similarly, the numbers 9XX are unassigned and the first contacts of the ninth levels of switches 1215X and 1207X are connected to 500. The intercepting circuit is shown in Figs. 5, 6, 7, which should be arranged as shown in Fig. 8. The circuit comprises two line circuits, Fig. 6, for trunks 620, 610 and a common ring-down circuit, Fig. 5. Interception, line circuits. If the unallotted number 400 is dialled the trunk 620 is selected and a two-step relay 660 is operated by ringing current from a connector 1217X and at the second step, providing that the circuit is free and lock-out relay 590 is not energized, operates a two-step start relay 580 to extend the talking conductors to a ring-down relay 560 which also responds to the ringing current. Ringing current is cut off when an answer signal, described later, is returned to the connector. If a number 9XX is dialled the intercepting circuit is seized over trunk 610. A line relay 640 operates and is followed by relays 645, 670, 650, 690 but 670 falls back almost immediately. When relay 690 operates, it holds in series with start relay 580 and a conductor 675 is connected to project ringing current forward and to return ringing tone over 610. Ringing current and tone are both cut off by slow-to-operate relay 680 in response to the answer signal from the ring-down circuit. In the case of a toll call, conductor 614 is grounded at 1100X and 630 is operated and released to give an end of dial signal to 1100X before ringing current is projected from 675. Ring-down circuit. When start relay 580 and ring-down relay 560 operate, relays 590, 570, 530, 510 follow and 510 signals trunk 480 to seize a circuit 400. Battery is reversed in junction 390 (Fig. 1), a circuit 300 is seized and calling supervision is given at the toll board 300X. When the operator replies a marginal relay R440 (not shown) is operated over the junction 390 and battery is reversed in trunk 480 to operate answer relay 520. The talking circuit is completed when switch relay 540 operates. Tone, supervisory and other signals. When the operator at 300X replies and answer relay 520 is operated by battery reversal, relays 750, 740 follow. Switch relay 540 operates and 530 releases to complete the talking circuit to trunk 480, but, during the release period of 530, the calling loop is terminated through low resistances 537, 538. This answer signal cuts off ringing current which comes either from a connector or from the line circuit of trunk 610. In the latter case, ring trip relay 680 is operated. At the same time, intercept tone is connected at 547. When relay 530 releases, the calling loop is terminated by high resistances 718, 719 and a low impedance 717 equivalent to the low resistances 537, 538. When the operator at 300X disconnects, answer relay 520 restores and releases relay 750. In the interval between the relapse of 750 and the relapse of 740, flash relays 710, 720, 730 operate to cut out the high resistances 718, 719 and to close the calling loop through low impedance 717. When 740 falls back, the high resistances are reconnected. This signal is transmitted before slow-to-operate release relay 550 has time to release the intercepting circuit and has no effect when the call is local. In a toll call, the answer signal involving resistances 537, 538 controls circuit 1100X to extinguish the supervisory lamp at 230X (answer supervision) and to transmit toll intercept tone to 300X. The connection of high resistances 718, 719 controls 1100X to re-light the supervisory lamp at 220X (usually disconnect supervision). Disconnect supervision quickly following answer supervision when 530 falls back constitutes intercept supervision. The operator at 300X can recall the operator at 220X by operating and restoring the ring front key once, which gives intercept supervision twice. When the key is operated, ring relays 360, 340 (not shown) connect ground momentarily to one leg of junction 390 to operate a differential relay 445 (not shown) in the circuit 400. The signal is repeated as a momentary reversal of battery on trunk 480 and 520 relapses momentarily, giving release relay 550 insufficient time to operate. Flash relays 710, 720, 730 momentarily loop low impedance 717 in the calling line as described. A similar signal is sent when the ring front key is restored. In response, the circuit 1100X gives intercept supervision twice and thus signals a re-call. Verification calls. When a junction 390 is selected by an operator at 300X the connection is extended to a test distributer 601 and to a connector and the connection is set up in known manner, notwithstanding the busy condition of the line called by the operator.