Patent GB-1077472-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: BROWELL LESLIE JAMES
IPC: H04Q3/00
Filed: 1964-09-02
Abstract: 1,077,472. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Aug. 19, 1965 [Sept. 2, 1964], No. 35838/64. Heading H4K. A switching arrangement, using first and second arrays of cross-bar switches in which two cross-points are operated on the same bridge after selection by a select and an auxiliary select magnet to connect an outlet to a bridge inlet, sets up a connection between an outlet of a first switch to a free bridge inlet of a second switch by extending marks, corresponding to idle bridge inlets of the first switch, over paths to an outlet in each switch in the second stage all such outlets being connectible by operation of the same auxiliary select magnet in each switch of the second array, means being provided to select one of 'the switches of the second array having at least one free bridge inlet and also to select one such free inlet where more than one is free in the selected switch. The switches are those described in Specification 983, 507 in which the bridges are split, one part being served by eight select magnets 1SM to 8SM to route a group of three out of 24 outlets to the other parts of the bridges where one of the group of three is selected by an auxiliary magnet 1AM, 2AM or 3AM. The other parts of the bridges also have select magnet 9SM serving a pair of outlets multipled over this part of the bridges, auxiliary magnet 3AM similarly serving a second such pair of outlets, selection of one or other of these pairs being made by auxiliary select magnets 1AM and 2AM. Auxiliary magnet 3AM is, therefore, in the case of two particular outlets, employed as a tenth select magnet 10SM. The outlets indicated above total 28 and have access to 10 bridge inlets. The first array of switches comprises twenty switches, DSB1 to DSB20, each providing a group of 26 subscriber's line circuits with access to 10 links to the second array of switches comprising eight switches DSA1 to DSA8. The first eight bridge inlets of each DSB switch are linked to corresponding outlets of the eight DSA switches according to a normal trunking pattern. The remaining two bridge inlets of each DSB switch (totalling 40 in all) are distributed over the remaining 48 outlets of the DSA switches in such a manner that the links from each DSB switch all reach outlets in DSA switches accessed by the same auxiliary magnet. With this trunking for each DSB switch there are two DSA switches each liable to receive markings over two paths. To prevent more than one mark being extended to each DSB switch, the marker provides a set of 10 marking potentials, two pairs of which are supplied over relay windings such that if a first one of the pair draws current the second marking potential is disconnected. The marking potentials are strapped to each DSB switch so that the pairs liable to mark a DSA switch twice are appropriately connected to the relay conditioned potentials. The marker has a relay chain 1LM to 26LM to identify the outlet associated with a calling condition for either originating or terminating calls. A relay chain LA to LF indicates from which group of DSB switches the calling condition emanates and a further relay chain GA to GD indicates which member of that group is involved. Having determined by this means which DSB switch is calling, that switch is connected to select magnet operating circuits prepared in the marker by the relay chain 1LM to 26LM. The DSB switch is also connected over strapping peculiar to that switch so that the ten marking terminals in the marker are presented to ten marking wires from the DSB switch to select magnets in the array of DSA switches. As explained above the markings are limited to eight at most by two relay pairs in the marker which ensures that where two links are idle from the DSB switch to the same DSA switch, only one of these links is marked. Since the identity of the DSB switch is known to the marker contacts of the relay chain LA to LF operate a relay AM, BM or CM, to prepare circuits for the same corresponding auxiliary select magnet in all the DSA switches. Relay CM in this respect also connects the auxiliary select magnets 3AM in the DSA switches so that they pursue an auxiliary select function as they normally stand connected in the manner of a tenth select magnet which is operated from a test access switch connected to the otherwise unused 27th and 28th outlets of each DSA switch. The bridge inlets of each DSA switch are divided into two groups of five, one group giving access to trunk relay groups for originating calls and the other group being accessible from a router for terminating calls. A common set of test relays TA to TE in each DSA switch serves both groups of bridge inlets and is connected to the control wires of one or other group as required by the marker. For each originating call dealt with by the marker a start-shift circuit steps once to vary the order of priority inherent in the relay chains LA to LF and GA to GD, the marker thereby equalizing its accessibility to DSB switches. The startshift circuit also directs a holding earth to a particular DSA switch so that when the marker connects the test relays TA to TE to indicate free bridge inlets connected to idle trunk relay groups, or to indicate idle bridge inlets being sought by the router, the test relays in that DSA switch are given preference. If no test relays in that DSA switch respond the test relays extend the holding earth to the next DSA switch and so on. When the start shift circuit steps at the end of an originating marker function a different DSA switch is given preference. Within each DSA switch the test relays TA to TE have a fixed chain of precedence. Operation of a test relay in a DSA switch used for an originating call completes the circuits of the select and auxiliary select magnets marked by and on behalf of the calling DSB switch. Operation of both select magnets completes a circuit for a DSA bridge magnet and the extenscion of a path, and particularly the P-wire from the trunk relay group over the DSA switch to the bridge inlet linking the DSB switch to the DSA switch, causes that DSB bridge magnet to operate and the marker can retire. In the case of call termination a mark is applied to the wanted line circuit in simulation of a calling condition. If the line is busy the cut-off relay diverts the calling condition to an access relay group which supervises the return of busy tone. Where a PABX group is concerned the cut-off relays produce a chain circuit to route the calling condition to the first idle line circuit or to the access relay group if all the line circuits are occupied. Given a free line circuit the calling condition recruits the marker which sets up a connection over the DSA and DSB switches in much the same way as for a call origination, the test relays being connected to DSA bridge outlets to the router and both DSA and DSB bridge magnets being operated from the P-wire. The start shift circuit does not step when the marker completes a call terminating function. Trunk offering can be effected by an operator by operation of a relay ToA in the marker which deflects the simulated calling condition at a line circuit back to the marker despite its deflection to the access relay group by the cutoff relay. Class-of-service is indicated according to strapping, to one of four terminals in the line circuit, from a terminal selectively connected to earth in the DSB switch over off-normal select and auxiliary select contacts. The four class-of-service indicating terminals from each line circuit are commoned back to the marker where the one connected by strapping extends earth, or resistance earth, over one of two wires which are multipled over corresponding control wires associated with the trunk relay group bridge inlets of the DSA switches so as to be extended to the relay group when the test relay for a bridge inlet is operated. Permanent loop conditions result in the release of the line relay with the cut-off relay held up. Prior to this release which connects the alarm circuit, a period of NU tone is supplied. To increase the reliability of the marker relays may be double wound with a separate fuse for each winding. Contacts may also be duplicated.