Patent GB-355740-A (GB)
Title: Improved switching arrangement for telephone and like transmission systems
IPC: H04B3/20
Filed: 1930-05-28
Abstract: 355,740. Two way transmission. STANDARD TELEPHONES & CABLES, Ltd. and THOMAS, P. R., Columbia House, Aldwych, London. May 28,1930, No. 16457. [Class 40 (iv).] In a cable system with four-wire terminal stations, in order to prevent lock up when speech is initiated simultaneously at each end, one terminal station is arranged to have preference, a control current being transmitted therefrom to operate relays at the other terminal station and at an intermediate station to effect the necessary connections. When the cable is used also for telegraphy, the terminal station having preference may be arranged to change as the direction of telegraphy changes. In Fig. 1 the receiving paths L2, L3 at the terminal stations A, B are normally connected to the cable, and the station A has preference. Speech initiated at B operates relay S2 to connect the transmitting path L1 to the cable, and on arrival at A disables the transmitting path L2. Speech initiated at A operates relays S3, S4, connecting the transmitting path L4 to the cable and sending direct current over the cable to operate relay S1, which releases relay S2 if the latter has meanwhile been operated by speech initiated at B. Delay networks D1, D2 are provided. Fig. 2 shows an intermediate station in which relay S2 responds to speech currents from B while relays S1, S4 respond to control current from A, relay S4 repeating the control current to B. Fig. 3 shows a modification of the station B of Fig. 1 in which the transmitting path L1 is normally connected to the cable and the switching relay S2 is operated by control current from A, while Fig. 4 shows an intermediate station modified in a similar manner. Fig. 5 shows a terminal station for a system used also for telegraphy, the speech and telegraph currents being separated by filters F1, F2 and the speech receiving path L2 being normally connected to the cable. The station has preference when it is transmitting telegraph currents. Assuming speech is initiated at this time, relay S1 connects the transmitting path L1 to the cable, while S4 sends control current, which is in the telegraph range, over the cable by way of operated contact S3 to operate the relay similar to S2 at the distant station. If on the other hand the station is receiving telegraph currents, relay S4 operates without effect. At the intermediate station, Fig. 6, the control current passes through the telegraph channel in the appropriate direction and operates one or other of relays Sl, S2. In Fig. 7, the speech paths are normally connected for transmission in the direction opposite to that of telegraphy. As shown, telegraphy is from A to B, and no switching occurs when speech is initiated at B. If speech is initiated at A however, switching relay S2 energizes and relay S7 sends control current to B, where relay S5 responds and releases switching relay S1, which is normally energized over the telegraph switching contacts TS1. When telegraphy is from B to A, relay S2 instead of relay S1 is normally energized. Fig. 8 shows an intermediate station in which switching relays S1, S2 are both normally energized or not according to the direction of telegraphy.