Patent GB-206930-A (GB)
Abstract: 206,930. Relay Automatic Telephone Co., Ltd., and Johnson, B. B. Aug. 17, 1922. Automatic exchange systems.-A system in which connections are established entirely by relays is arranged to provide graded service, discriminating means being associated with the usual feed-relay connecting-circuit, which operate according to the facilities to which a calling subscriber is entitled to determine whether or not an attempted connection shall be completed. The system is divided into, for example, four groups of lines A, B, C, D, each consisting of, say, five lines, and, according to one arrangement, subscribers in group A have access to lines in any of the groups; those in group B to groups B, C, D only; those in group C, to groups C, D only; whilst those in group D are restricted to connections within their own group. In a second arrangement, the subscribers in each group are restricted to connections only within their own group, although all the groups in making such restricted connections have access to the same switching apparatus. The invention is described with reference to a system of known type in which a calling line 21, Fig. 2, is connected over an idle link circuit L1 and an associated outgoing trunk OT1 to a feed-relay group ABF1, Fig. 5, which by means of a connecting-relay group RC, seizes and prepares an idle impulse-responder IR. The latter, in response to impulses representing the two digits of the wanted number, operates in known manner to pick out group- and unitsmarking wires 5'S, MW, NMW, respectively, and the connection is completed over the relay group TCG, Fig. 3<a> an idle incoming trunk IT2, Fig. 2, and a link L2 to the wanted line 28 (Fig. 2, for convenience, being used to represent either the group containing the calling line or the group containing the called line). When a connection is extended over the trunk OT1 a relay CSD, energized in the usual holding circuit 6 prepares a circuit for a relay CSM, which is completed when the impulse-responder performs its marking operations; the relay CSM, one of which is provided for each group of calling lines, controlling contacts in the group-marking wires 5's MW so that the particular marked circuit 5MW will only be completed if the selected group is one to which the calling subscriber is entitled to obtain connection. Assuming now that the groups of subscribers have progressive facilities according to the first arrangement outlined above, and that subscriber 21, Fig. 2, in group A, desires connection with subscriber 28 in group B, when subscriber 21 removes his receiver, his line relay LE closes circuit 3 for relay OTT which completes a circuit 4 over the test wire T of the first idle outgoing trunk OT1. Relay OTC1 energizes in this circuit to connect the link L1 to the trunk OT1 whilst relays LM1, LE close circuit 5 in which relay LC1A operates to connect the calling line to the link L1 and thence over the trunk OT1 to feed-relay group ABF1. Relays LC1A, OTC1 close a holding circuit 6 in which relays CO, F of the calling line operate in series with a relay CSD.1A which prepares a circuit 21 for relay CSM.A. The impulse-responder IR selected by the relay group RC is operated by the two series of impulses to pick out a group-marking wire 23 and a units-marking wire 25, and at the same time a relay OTM1 is energized which marks the calling trunk OT1 in the relay group. TCG, Fig. 3<a>. Relay OTM1 also completes circuit 21 in which relay CSM.A energizes. Since subscribers in group A are entitled to calls to any other group, the contacts of relay CSM.A lead to all the group relays ITTA, ITTB, &c., so that the closing of group-marking circuit 23 energizes relay ITTB, Fig. 2, which selects the first idle link circuit of the circuits L1 - - L4. Assuming that link L1 is busy, relay ITTB completes circuit 24 over relays LM2, ITC2, ITB2, FIG 3<a>, OT1/IT2, to battery at the impulse-responder IR, so connecting the calling trunk OT1 to the incoming trunk IT2 associated with the next link L2. Relays ITTB, LM2 complete units-marking circuit 25 (unless the called line is busy, in which case the operation of relay F would prevent the completion of this circuit and a busy signal could be given in known manner), and relay LC2C operates to connect wanted line 28 to the link L2, so completing the connection. Relays LC2C, ITC2, ITB2, OT1/IT2, remain energized over the holding circuit 26 in which relays CO, F of the called line also operate. If a subscriber in group C attempted connection with a line in a prohibited group A, say, relay CSM.C would be operated and no completion of a groupmarking circuit would be possible, since the relay ITTA is not cross-connected to the contacts of relay CSM.C. In the modified system in which each group of subscribers is restricted to connection only within its own group, the contacts of relay CSM.A lead only to relay ITTA, those from relay CSM.B only to relay ITTB, and so on.