Patent GB-579455-A (GB)
Title: Improvements relating to telephone systems
IPC: H04Q3/00
Filed: 1944-04-15
Abstract: 579,455. Automatic and semi-automatic exchange systems. SIEMENS BROS. & CO., Ltd., and HENSLER, B. A. April 15, 1944, No. 6985. [Class 40 (iv)] In an automatic tandem exchange forming part of a trunking system which employs alternating current for impulsing and supervision, a trunk selector failing to find an idle outlet in the group denoted by the incoming digit, stops in its last position and circuit changes take place to cause the selector to hunt over an alternative group. An additional routing digit is inserted at the tandem exchange and an A.C. signal is reverted if necessary to hold up impulse transmission from the originating exchange meanwhile. As described, the A.C. signals are of voice-frequency, but they would be of carrier frequency in a system employing carrier working. The trunks are two-way, and a 4-wire connection is shown, wires -, + being connected to the line windings and wires N-, N + to the balance windings of a hybrid transformer. The trunks may be switched through on a 4-wire or on a 2-wire basis depending on whether repeaters are included or not, as described in Specification 499,832. Specifications 462,773 and 499,820 also are referred to. Incoming trunk. A calling signal of X frequency energizes relays B, Z, CB and relays A, C pull up at the end of the signal, after which X frequency impulses are repeated by relay A to set the marking switch M. The selector may provide for 19 groups of outlets denoted by the digits 1 ... 9 and 01 ... 00, in which case the first digit 0 steps the switch M to a mid-normal position in which dialling relay C is held. Relay C on falling back makes a circuit over wiper M7 and terminal a or b for the latch magnet LM of the selector and if necessary for wiper-switching relays WS, WT, and hunting takes place over the group marked over wiper M2. Test relay T and switching relay H operate in the usual way when an idle outlet is found, and subsequent signals pass directly through the exchange. Relay A responds to X frequency impulses, but without effect since relay C cannot operate with the switch M off-normal, and Y frequency signals while operating relay P are not long enough to release sluggish relay Z until the clear forward signal of 1500 ms. minimum is received. The release of Z is followed by that of B, A and at the end of the signal, relay PA comes up and disconnects P. A Y-frequency clear back signal should now be received to operate relay R, which brings about release by disconnecting H at the end of the signal, the incoming trunk being guarded by relay CB while the selector restores. If the clear back is not received, release is brought about by the successive relapse of P, PA. If the clear forward signal consists of two impulses and the first is not received at the distant end, relay P will reoperate on the second impulse and the sequence will be repeated, relay H being lagged to hold between the release of PA and the re-operation of P. Busy condition and alternative routing. If the group is one for which no alternative route is provided, the penultimate position of the selector is marked over wiper M4 and, if no idle outlet is found, relay T energizes in this position and relay H switches through to a source of busy tone. When there is an alternative route, however, the penultimate position is unmarked and relay T energizes in the last position in series with relay AR, which immediately short-circuits T to restart the selector and substitutes wiper M3 for M2 to mark an alternative group of outlets. Relay TN energizes if terminals g, h are strapped and sends back X frequency to hold up impulsing at the originating exchange, this action being unnecessary if no more digits are to follow. The operation of switching relay H disconnects AR, TN, but if an additional digit is to be inserted, these relays remain held over wiper M5 and terminal f and impulsing relay YT also operates over this circuit and steps the sending switch IC, a circuit controlled by YT being made at the first step over wire aw for relay A of the outgoing trunk for sending X frequency impulses thereover as described below. The impulse train is terminated when the switch IC picks up a marking over wiper M6 by the operation of relay SZ which disconnects AR, TN, whereupon impulsing from the originating exchange is resumed if necessary. If there is no idle outlet in the alternative group, the selector stops in the penultimate position since AR is up and busy tone is sent. In the case of release after a busy condition, relay CN operates and connects up TN to return a Y-frequency signal. Outgoing trunk. Upon seizure, relay GS energizes in the test circuit and through the medium of relay CN connects up relays CB, H, relay TN being operative momentarily to send a calling signal of X frequency. Numerical impulses from the originating exchange pass directly over the trunk, but locally generated impulses are repeated by relay A which is controlled over wire aw, TN being operative meanwhile over wire tnw. If the trunk is busied at the distant end, Y frequency therefrom operates relay P which connects up guard relay CB.