Patent GB-786821-A (GB)
Title: Circuit arrangements for markers in exchange systems, particularly telephone systems
IPC: H04Q3/00
Filed: 1955-10-21
Abstract: 786,821. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Oct. 21, 1955 [Oct. 29,1954], No. 30162/55. Drawings to Specification. Class 40 (4). The subscribers' lines are divided into groups; a calling line is extended to a free link circuit and the wanted number is then stored in one of a plurality of registers, an associated discriminating means determining the group to which the wanted line belongs and causing the seizure of a control circuit individual to that group, information relating to the wanted line then being transferred from the register to the control circuit which thereafter controls the establishment of the complete connection from the link circuit to the wanted line. The embodiment described is a private automatic branch exchange with a three-digit numbering scheme and containing a plurality of link circuits. Each link is connected at one end to a line-finder and at the other end to a group selector and contains switches on which dialled digits may be registered. A calling line is extended through a line-finder, or, if all the accessible line-finders are busy, through a final selector acting as a line-finder and through a further line-finder, to a link. On receiving dial tone the calling subscriber dials the wanted number. Each group of links is served by a control circuit known as an " internal marker " which includes so-called direction relays," one for each possible value of the first digit of a dialled number. Assuming that the associated internal marker is free, the registering of the first dialled digit causes the operation of the relevant direction relay. If the first digit indicates an external call, the direction relay causes the immediate seizure of an outgoing junction to a nearly public exchange. The connection of the calling line is transferred from the seized link to the junction, over which subsequently-dialled digits pass directly to the public exchange. Alternatively, a calling subscriber may release a seized link, in order to set up an external call directly, by depressing an earth key at his substation. In an internal call, the first digit indicates the group to which the wanted line belongs, and the direction relay causes the testing and seizure, if free, of a marker serving this group. When the link circuit has received and registered the complete wanted number (three digits), the second and third digits are transmitted to the seized marker over leads selected by the setting of the register switches. The marker then applies a marking to the wanted line and sets it to a " called " condition if it is free. In the present system, cross-bar switches are used, and the operation of the switch magnets corresponding to the wanted line brings about the completion of a test connection back from the wanted line through the various switching stages to the seized link circuit. This backward connection identifies an available route through the exchange, and the relevant switches thereupon operate substantially simultaneously, completing the connection between the calling and called lines. Ringing tone and ringing current are applied, and the markers are released. An incoming call seizes a link circuit and is extended to the wanted line in a similar manner to an internal call. Arrangements suitable for exchangers of different sizes are described.