Patent GB-548724-A (GB)
Title: Improvements relating to telephone systems for railway signalling
IPC: H04Q5/22
Filed: 1939-10-26
Abstract: 548,724. Railway telephone systems. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd., and COLEY, J. P. Oct. 26, 1939. No. 28761. [Class 105] [Also in Group XXXIX] In a railway telephone system of the kind in which communication may be established with the cabin by a driver from any one of a number of wayside stations, connected by a single pair of lines or line and earth return, to the exclusion of the other stations, but in which communication between the stations is not possible nor can the cabin initiate a call, arrangement is made whereby the wayside operator can, if desired, retain communication with the cabin notwithstanding that the signalman has exercised his normal control to terminate the call. Fig. 1 shows the circuits and apparatus at one of the stations which are connected in series to the cabin through lines L1, L2 the other stations being similar, Figs 2, 3 show portions of the cabin circuit and apparatus. The Specification gives complete diagrams and traces in detail the operations outlined below. At the station a special signal P is mounted on a post and a track relay provided in an insulated rail section near the station, this relay dropping its contact TR in the usual way when the section is occupied. The remaining apparatus is housed in a cabinet the door of which, when opened, closes contacts DC in the circuit of a door-open relay DO. All relays shown shaded are of slowacting or retarded release type to give requisite timing. The station apparatus includes the telephone instrument TW (protected during the calling process by a shunt circuit through contacts ONI controlled by the dial), a dialling transmitter of known type having contactcontrolling cams A. B. C. D. and a "line-engaged" indicator E. The station relays SLR are included in series in line LI over back contacts 1 of relays STR, normally closed back contacts D03 of relays DO, and normally closed contacts D2. Each station relay STR, energization of which at the calling station is essential to establish communication, is connected across the lines L1, L2, through back contacts 2 of its own station relay SLR and contacts A3, normally open as shown. Relays SLR, STR have upper and lower windings in parallel through reversely connected rectifiers a, b, and c, d respectively, whilst the lines L1, L2 are remotely connected through a rectifier RE which prevents flow of current of normal polarity from a cabin battery CB, Fig. 2, so that relays SLR, STR are normally deenergized as shown. The cabin apparatus includes the telephone instrument TS, Fig. 2 normally hanging on a hook SH controlling switch-hook contacts SH1, and switch-hook repeater relay SHP, Fig. 3 and a rotary stepping selector with banks of contacts some of which are shown at F, G, J, K, the selector being moved one step by each energization and deenergization of a magnet SM in known manner. Other banks of contacts, not shown, selectively control indicator lamps indicating which station is calling according to the contact on which the arm comes to rest, and step proving relays the contacts SI, S2 &c. of which are shown in the circuit of a step registering relay R: To establish communication the station operator opens the cabinet door, thus closing contacts DC, and operates the dial. Contacts, A3 (closed twice in succession) close the line circuit through relay STR of the calling station, the SLR relays of stations between the calling station and the cabin, and the cabin line relay CLR. Contacts 2 of these SLR relays interrupt the corresponding STR relay circuits at these stations and prevent calls being effected therefrom, Contact 1 of relay STR at the calling station breaks the line circuit for all stations more remote from the cabin, By this arrangement interfering calls are prevented and, if two stations attempt to call simultaneously, the station nearer the cabin takes precedence and establishes communication with exclusion of the other station. The operations of relay CLR control, by contacts CLR1, CLR9, the operation of relays LC, LCP, LO to close and open the circuit of a relay SR, not shown, contact SR4 of which makes and breaks the circuit of magnet SM to move the, selector one step. A selector arm then close the circuit of pole-changer relay PC and its contacts PC2, PC3 reverse the battery line connections and cam B with contacts B1 sends a reverse current to line leading, inter alia, to the selector moving a second step. These two steps take place with a call from any station, and if the calling station be No. 1 no further line impulses are sent and a selector arm, on second contact, operates a corresponding calling station indicator in the cabin. Otherwise, disc C sends further impulses to line (of normal polarity, as movement of selector arm breaks the circuit of relay PC) in number characteristic of the calling station,the selector being correspondingly stepped round so that finally the calling station is indicated in the cabin. Cam C next sends a long impulse to line resulting in the "count complete" relay CC being energized so that its contact CC2 breaks the circuit of magnet SM. and prevents further movement of the selector. Cams C, D then open contacts C1, D1, which are in series in the line circuit, for short and long periods respectively, by the short and long gaps CN, DN. Contacts D1 are bridged by track relay contact TR, so that if the track section at the calling station be unoccupied contacts D1 are ineffective, and a short break occurs at contacts C1 only. If the section be occupied, contact TR is open, and contacts D1 effect the longer break. The duration of the break in the line circuit determines whether relay R shall become de-energized before or after relays CLR and SR are re-energized, thus determining which of two alternative cabin indicator circuits shall be closed to give a "track clear" or "track occupied" indication. Closure of contact LC6 has already rung the cabin telephone bell TB, and when the signalman removes his instrument from its hook, circuits are prepared whereby the signalman can, by operating a press button PB, send a reverse current through the line to energize relay SPR at the calling station, so that contact SPR2 closes the circuit from an alternating current source BX, NX, through the special wayside signal P. Current tapped from this circuit and passed through rectifier f energizes signal proving relay PK whose front contact 1 closes the circuits of relays Z1, Z2 operating alternately to send impulses to the cabin which result in energization of a signal indicator. Relay contact PK2 breaks the circuit of relay DO and terminates the call. If however the signalman hangs up his instrument without pressing button PB, the calling station retains connection with the cabin through a loop circuit over contacts DO2 and ON 2 so long as the cabinet door remains open, closure of the door breaking this connection by opening contacts DC. In initiating a call as above, a circuit is prepared whereby, should the dial at any other station be operated in attempt to communicate with the cabin, indicator E at that station is operated, When the call is terminated and cabinet door closed, the circuits are returned automatically to the normal condition shown. The Specification describes a slight modification using a condenser in place of rectifier RE and a further arrangement for use when the cabin, instead of being at the end of the line as above, occupies an intermediate position with wayside stations in both directions therefrom.