Patent GB-239314-A (GB)
Title: Improvements in or relating to automatic and semi-automatic telephone systems
IPC: H04Q3/00
Filed: 1924-06-25
Abstract: 239,314. Automatic Telephone Manufacturing Co.., Ltd., (Automatic Electric Co.). June 25, 1924. Automatic exchange systems.-Forward-hunting switches are arranged to take one or more steps as soon as they are taken into use independently of the busy or idle condition of the trunks thus encountered in order to prevent a calling subscriber from getting repeated access to the same defective trunk. The preliminary circuit for the driving magnet includes a back contact of the line relay so that the preliminary steps take place during the interval before this relay, which is slightly slow-to-pull-up, operates. In a secondary hunting switch, accessible from a primary hunting switch which may be a group selector, the preliminary circuit for the stepping magnet is completed by an earth connection sent forward over the incoming test conductor when the switch is seized. In a subscriber's individual lineswitch, the circuit includes a front contact of one (slow-release) line relay and a back contact of a second (slow-to-pull-up) line relay. The stepping magnets of a group of switches have a common battery connection which is disconnected to prevent useless hunting when all the trunks to which the group has access are busy, and, in a secondary switch, is replaced by a guarding earth to prevent the seizure of such a switch by a primary switch. In order that, in switches in which the cut-off relay energizes in series with the driving magnet, the disconnection of the common battery connection may not release the cut-off relays of busy switches, these relays are provided with locking circuits independent of their stepping magnets. In a secondary switch, a relay operated when all outgoing trunks are busy disconnects the normal earth connection to one of the speaking conductors and replaces it by a busy tone, which is transmitted to the unsuccessful subscriber in the event of two switches hunting simultaneously for the last available trunk. Meters may record the number of times all records are busy, and the number of times simultaneous hunting occurs for the last available trunk. Subscriber's individual line switch LS, Fig. 1. When subscriber A removes his receiver, the response of the slow-release line relay 13 completes a circuit for a second (slow-to-pull-up) line relay 14, and, during the interval before back contact 19 is opened, stepping magnet 28 operates over contacts 70, 25, 26, 17, 19. When relay 14 pulls up, the de-energization of magnet 28 advances the wipers one step, whilst hunting takes place in the usual manner, the test wiper 30 being connected to the mid-point between cut-off relay 15 and magnet 28. When an idle trunk leading to a first selector S is found, relay 15 operates in series with magnet 28 and locks up independently thereof over resistance 36. The selector S then operates in known manner under the control of the first digit and selects an idle trunk 64 - - 66 terminating in a secondary hunting switch SLS, Fig. 2. By transferring the jumper 27 to connect terminals 24, 26 instead of terminals 25, 26, the interrupter contacts 39 may be included in the preliminary stepping circuit and the switch will then take more than one preliminary step when a call is initiated. All trunks busy. If all the selector trunks accessible to the switch LS are busy, earth is removed at all points from conductor 68, so that relay 69 de-energizes and disconnects battery from all the stepping magnets 28. The absence of dialling tone notifies the calling subscriber that a busy condition has been reached. Secondary line switch SLS, Fig. 2. The operation of switching relay 45 of selector S extends earth formed over test conductor 65 and back contact 109 of the slow-to-pull-up line relay 101 to stepping magnet 103, which thus advances the wipers 117 - - 119 one or more steps before the line relay 101 has time to pull up. Hunting then takes place in the usual manner and, when an idle repeater R is found, switching relay 102. operates in series with magnet 103 and locks up over resistance 104, whilst relays 141, 142 operate and earth the test conductor 121 to hold up the connection. All trunks busy. If all the repeater trunks accessible from the switch SLS are. busy, relay 123 de-energizes, disconnects the battery connection to the stepping magnets 103 and replaces it by earth. This earth connection is extended over the circuit 103, 109, to the test conductors 65 of all idle switches SLS to prevent the selection by the selector S of a switch which is unable to extend the call. Modified secondary line switch SLS<2>, Fig. 4. In this arrangement the switching relay B is initially energized in series, not with the stepping magnet 203, but with a resistance 202, the junction between relay B and resistance 202 being connected to the test wiper over armature 211, whilst the stepping magnet is independently connected to the test wiper over armature 212. All trunks busy; statistical metering. When the last idle trunk is taken into use., a chain circuit is completed for relay D which earths circuit M to operate a meter and de-energizes a normallyoperated relay E. The normal common battery connection to the stepping magnets 203 over the parallel armatures 226 is now replaced by a lowresistance earth over armatures 225, 226, this earth marking busy all idle switches SLS<2>. Relay D also replaces the normal ground connection to the lower talking conductor 266 by a busy tone connection BT so that, if two switches have been simultaneously hunting for the last idle trunk, the unsuccessful subscriber will receive a busy tone. Relay F will also operate over the lower talking conductor to operate a further meter OM.