Patent GB-253619-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/42
Filed: 1925-03-20
Abstract: 253,619. Automatic Telephone Manufacturing Co., Ltd., and Ostline, J. E. March 20, 1925. Automatic exchange systems.-Where it is required to remove at a switch or repeater the holding potential trom a release wire for a sufficient time to allow the preceding switches to restore and then to replace it to guard the switch or repeater again, it is usual to open the holding circuit during the relapse of a slow-release relay. This method has the disadvantage that during the period that the switch is unguarded it may be again seized. According to this invention, direct guarding earth is removed for the shortest possible time necessary for ensuring that the release is effected, by arranging that immediately it is removed it is replaced by earth through a relay of sufficiently high resistance to cause the relapse of the switching relays at the preceding switches, and when these have all relapsed the circuit of the high resistance relay is opened to cause the re-application of the direct guarding earth. The invention is described as applied to a call indicator system of the kind described in Specification 229,371 in which a repeater repeats the first series of impulses received by it to an associated group selector which selects an idle trunk to a required exchange, and during this operation becomes associated with an idle coder over a hunting switch. When the repeater is taken into use, relays 4, 7 energize and a circuit is completed for the line relay 39 of the coder finder, which hunts for an idle coder in the usual way. Relay 4 closes a circuit 25, 6, 27, 63, for the line relay of the associated selector which operates in the manner described in Specification 250,694, in response to the next impulse series repeated at armature 6. Relay 7 energizes the right-hand winding of polarized relay 17, the lower winding of relay 12 which operates only its armature 14, and relay 37. Relay 37 energizes relav 19 which grounds the release wire 3 over armatures 8, 20. During the hunting of the group selector. the wire 64 is earthed over circuit 38, 56, 21 and when an idle trunk is found, relay 21 energizes over the circuit, locks up, directly earths wire 64, and extends the impulse circuit to the coder. If all the coders are busy, the switching relay 42 does not respond, and on the energization of relay 21 a circuit 67. 31, 22, 10 is completed for relay 55, which reverses the current sent back to the register sender and holds up impulse transmission as described in Specification 236,497. When the decoder at the incoming end of the seized trunk becomes free, the code impulses are transmitted, and on the completion of this operation the lead 52 is earthed and the switch-through relay 29 is energized. Relay 29 locks up, releases the coder selector, and bridges the supervisory relays 15, 17 across the trunk 62, 63. The wanted number is displayed, the connection set up by the operator, and on the reply of the called party the polarized relay 17 completes a circuit for the supervisory reversing relay 55. If the wanted line is busy, busy tone and flash are connected to the lower conductor 63, relays 15, 59 responding to the connection of the flashing current and repeating it over conductor 2. If the group selector finds all trunks busy, the flashing current is connected over 11th step springs to an additional wire 65, causing the switch-through relay 29 to energize, so that the busy tone and flash can be relayed by the repeater as before. If the operator delays to set up the required connection and the calling party hangs up, it is desirable that the preceding switches should be restored but that the repeater should not be left unguarded for longer than is absolutely necessary. On the relapse of relay 7, direct ground at armature 8 is replaced by ground through relay 19. the resistance of which is so high that the switching relays of the preceding switches are unable to hold up. Immediately the last of these relays becomes disconnected from the release wire 3, the relav 19 falls back and direct earth over armatures 38, 20 is again applied. Relays 37, 21 remain operated until the operator attends to the call, whereupon she receives a disconnect signal. The invention is also applicable in a system in which a connector is held until both parties of a. connection release, while the preceding switches are released by the calling party hanging up.