Patent GB-307389-A (GB)
Title: Improvements in circuit arrangement for automatic telephone exchanges
IPC: H04Q3/00
Filed: 1927-12-05
Abstract: 307,389. Merk, F. Dec. 5, 1927. Divided on and addition to 281,725, under which a Specification was laid open to inspection under Sect. 91 of the Acts, Dec. 5, 1927. Semi-automatic exchange systems.-Relates to a system as described in the parent Specification in which the same final selectors are used for local and for toll calls and in which in the case of a toll call the testing and breaking-in operations take place automatically under the control of a special signal transmitted from the toll operator's position. The present invention provides means whereby (1) a warning signal is transmitted to the parties conversing before a local connection is broken down, (2) the toll operator is informed as to the actual (busy or idle) condition of a called line, (3) the toll operator by repeating the special controlling signal may recall a party who has prematurely hung up, and (4) different frequencies of ringing-current are used for local and for toll calls. The invention is also applicable to arrangements for giving ordinary and preferred local calls. The invention is described with preference to a system in which the final selectors are constructionally of the type described in Specification 275,255 in which a first set of impulses sent to the magnet H effects the selection of a set of wipers which are then tripped by the magnet S, a second set of impulses through the magnet H effecting the selection of the wanted line. Toll call. When the toll operator, Fig. 1, plugs into the junction L1, relays A, B, A1 and lamp SL are operated, followed by relay R. Impulses over the a-wire generated at contacts 11 are repeated by relay A to magnet H and at the end of the digit an impulse transmitted over the b-lead from contacts 12 causes the operation of tripping magnet S, the contact s of which remains closed until the switch is released. (During dialling the a. and b-leads of the cord circuit are directly connected at contacts z to prevent clicks in the trunk L2). On the re-energization of B, relay V is operated and transfers the locking circuit of R to a front contact of B so that at the end of the units impulses R releases and connects the test relay PI to. the test wiper. If the wanted line is free relay PI operates, connects up ringing current, and also completes a circuit over other windings of relays A, B by which a " free signal " #s is transmitted to the operator. The controlling impulse over the b-lead at the end of the units impulses is sufficiently long for relay V to fall back so that when this occurs the righthand winding of relay PII is connected to the test wiper in parallel with relay PI and relay PII operates its left-hand armature only. At the end of the controlling impulse relays B, V operate again and relay PII is energized over its left-hand winding operating its right-hand armatures whereby positive battery is directly connected to the last wiper, the frequency of the ringing-current is changed to indicate that the toll operator is calling, and a local circuit over coil x is completed for P1. When the called subscriber replies, relay Q operates and switches through short-circuiting PI which releases. Relay Y establishes a holding circuit for relay Q over coil x and release relays A, A1, extinguishing lamp SL. If the wanted party is busy in a local call, relay PI does not energize but PII operates in two stages as before. During the interval a busy signal bs is transmitted to the toll operator and is also heard by the conversing subscribers warning them that the connection is to be interrupted. When relay PII operates its right-hand armatures, the direct connection of positive battery to the test wiper releases the existing connection, whilst relay Q operates over its lefthand winding and switches through. Relay Y operates immediately, opening the circuit of relay A so that the busy signal is terminated and lamp SL is extinguished. If the wanted party is busy in a toll call, neither of relays PI, PII operates, and a busy signal is sent back as before, but in this case it is continuous. When the existing call is finished the toll operator may complete the new call by repeating the special controlling impulse over the b-lead to release relays B, V as before. The operator can also rering a subscriber who has prematurely hung up, by repeating the special controlling impulse. When the operator takes down the plug, the selector is restored to normal by further impulses sent through the magnet H from an interrupter (not shown) over contacts wand back contacts of relays Q, B, A. Local call. The same final selectors are. used in a similar manner for local calls but the controlling impuse over the b-lead at the end of the units impulses is not long enough for relay V to relapse so that PII is not connected up and an existing call cannot be interrupted.