Patent GB-1268151-A (GB)
Title: Improvements to selection systems for electrical circuits
IPC: H04Q3/00
Filed: 1970-02-11
Abstract: 1,268,151. Automatic exchange systems. INTERNATIONAL STANDARD ELECTRIC CORP. 11 Feb., 1970 [28 March, 1969], No., 6643/70. Heading H4K. In an exchange having a PBX line group connected thereto, failure of.a marker to find, within a preset time, a path between a junctor and a PBX line selected during a first PBX hunt causes another line to be selected by a second PBX hunt whose starting point within the group is different from that of the first PBX hunt. The procedure may be repeated until either a preset number of repeated hunts has taken place or a predetermined period of time has elapsed since the start of hunting. If no free PBX lines are available, a caller of sufficient priority, e.g. an operator, may be allowed to break-in to one of those lines which has no other third party camping thereon. General operation.-A calling line similar to lg is routed through cross-bar network RC to the calling side la of a junctor AL and thereafter dialled digits are received in a register EN. The latter transmits the number in binary form to a decoding relay tree RE in a marker MQ. If the number corresponds to a PBX group interrogating device MA transmits an interrogating battery/earth to all the lines in the group whereby those that are free repeat the battery/ earth to a device CSI in which initially a relay in respect of every free line comes up. A priority determining circuit CDM marks a relay contact chain so that only that energized relay nearest the insertion point of the marking remains up. The marker reverts an earth marking via the register and junctor to device DI, wherein a pair of relays come up to identify the calledside appearance le of the junetor on the network. Control circuit CS then proceeds to set up a connection between le and the selected line using the information stored on relays in CS, RE and DI. If a connection is set-up the marker and the register are cleared at once. If a connection is not set up, timer TP (which comprises a relay whose release time is controlled inter alia by a capacitor) increments counter CP and instructs CSI to select another line. If counter CP or timer TP reach a preset count or a predetermined overall time, the marker is force released and busy tone is reverted to the caller. 'If all the lines (as opposed to the network) are busy, their line circuits supply a negative potential to corresponding transistors in CSI, whereby on a priority call one of the relays in CSI can be brought up in series with the transistor instead of with MA via the line circuit. Once such a relay has been selected in this manner the setting up of a connection to the busy line takes place as outlined above. A line that is brokeninto in this manner, however, removes the negative potential so that it cannot be seized by a further priority caller. The details of the marker are disclosed in Fig. 3 (not shown) while those of the priority determining circuit CDM are in Fig. 4 (not shown). Essentially CDM comprises a relay contact chain which is operated by the relay in CSI corresponding to a selected line on a previous hunt so that .this relay is last choice in the next hunt, the direction of hunting always being the same.