Patent GB-1342921-A (GB)
Title: Telephone switching system
IPC: H04M9/00
Filed: 1971-05-07
Abstract: 1342921 Automatic exchange systems WESTERN ELECTRIC CO Inc 7 May 1971 [7 May 1970] 13755/71 Heading H4K In a key telephone system wherein each station normally has access via a switching network to some only of a plurality of exchange lines, an additional switching network is provided for allowing access to the remaining lines when necessary. The system is concerned with minimizing installation cost per station without denying it full access to a main exchange on an as-needed basis. The crosspoints used are SCR's of shorted-emitter design formed as integrated circuits on ceramic substrates. Each station is normally given access to some but not all the exchange lines via a package of such crosspoint package. The networks as described follow remote concentrator practice in that inter station connections are not provided thereby. A stored programme controlled or wired logic processor effects supervision of the stations using polling and last-look scanning techniques. Each station comprises a 6-key set 200 (Fig. 1) (consisting of one hold key and five line keys) which is connected by a two-wire speech line and a 4-wire synchronous bit serial data line to an individual station package. The latter couples the speech line to an integrated circuit pack 12 containing SCR crosspoints 1316 that permit selective access to two exchange lines via their respective line packages. Indirect access to other lines is afforded by the junctions 25, 26, 27 and crosspoints 25-1 to 27-4 which are controlled via trunk packages 1 ... K. Fig. 2B depicts the details of one line and one station package. In operation, key actuation and/or off-hook in a calling station is recognized by a data transceiver 201 in its station package 216 as described in Specification 1,264,821 and the processor is informed accordingly during its cyclic scan of the packages. Data in respect of each station is stored in a multiple diode integrated circuit chip arranged on a printed circuit board. The processor reads the state of each line key in the station and on finding the calling condition causes the line package corresponding thereto as well as the calling station package to be marked. (This consists in placing a "1" on each of the line select, line connect, station select and station connect leads.) Lamp and ring control signals are also supplied by the processor to a store so that the keys may be illuminated in a manner corresponding to the current state of the call. Bi-stable D on becoming set is effective to apply priming potential to the row of crosspoints 1316 pertaining to the marked line package while bi-stable SM causes transistor Q1 to saturate. Consequently the crosspoint fires whereby Q1 enters its constant current zone of operation, Q2 turns-on to indicate successful operation to the processor over MM and a single wire speech path from line transformer T1 to station transformer T2 is completed, Q3 being turned-on with Q1. Additionally flip-flop D is reset so as to remove the priming potential. Relay C provides at its contact C1, a D.C. loop to the exchange and relay D via detector 224 informs the processor of the busy condition of the line. Every line has a word in memory indicating its free/busy status. This word is accessed by the processor during a station calling condition and is updated if a new connection is cut through. At the end of a call though it is not further updated until after the occurrence of a special logic sequence during which every station is interrogated as to whether or not it has a key corresponding to this line depressed. This ensures that conference and transfer type calls are not prematurely broken-down, e.g. during a hold condition of the exchange line or when one party withdraws from the call, c.f. Specification 1,342,922. Disconnection is effected by marking the appropriate leads so as to reset the bi-stables and thereby turn-off the transistors Q1 &c. If a path could not be set up for any reason, e.g. because there is no direct crosspoint, the processor seizes a trunk package (Fig. 1) so as to mark a row of crosspoints such as 25-4 whereby, in conjunction with the previous markings, a path is set up via an "overflow" junction such as 25. If the first trunk package selected fails to establish the connection, e.g. because its trunk is already busy, the processor selects another trunk package on its next scan of the calling station. The setting-up of incoming calls is not described except in so far as a statement that incoming ringing is detected by relay D and reported to the processor during its scan of the line packages.