Patent GB-864728-A (GB)
Title: Improvements in or relating to electric telegraph systems
IPC: H04L12/00
Filed: 1958-06-10
Abstract: 864,728. Telegraph exchange systems. GENERAL ELECTRIC CO. Ltd., and GIBSON, G. A. May 27, 1959 [June 10, 1958], No. 18515/58. Class 40(3) In an electric telegraph system comprising a number of switching centres which are linked together and in which some, but not necessarily all, of said centres are individually connected to one or more terminal stations, the switching centres are arranged to operate automatically in response to a calling signal originated at any one of the terminal stations so as to interconnect all the terminal stations of the system so that electric message signals from the calling terminal station are received by all the other stations. The calling signal is a long mark or space maintained for a period greater than the duration of the respective element during the transmission of message signals. The system is arranged so that a calling signal from a special priority terminal station can interrupt an existing connection and modify the circuit so that transmission takes place from the priority station to all other terminal stations. For a call from terminal station 11 of switching centre 4 shown in Fig. 2 a calling signal constituted by a long mark on line 19 operates a relay S/2 which at contact S1 connects the calling potential to outgoing lines 12, 14, 16 leading to switching centres 1, 2, 3 respectively. Also the potential on line 19 operates relay M/4 closing contact M1 completing a locking circuit for relay S/2 before opening contacts M2, M3, M4. The calling potential on line 16 leading to switching centre 3, shown in Fig. 3, operates relay B/2 which at contact Bl applies the calling potential to lines 20, 22 laeding to terminal stations 9, 10. The calling potential on line 16 via actuated contact B1 also operates relay A/3 which at contact A1 completes a locking circuit for relay B/2 and at contacts A2, A3 prevents the operation of relays C/2, D/2 from incoming lines 21, 23. Transmission between switching centres is by voice frequency signals, and is effected by D.C. pulses to terminal stations from the centres. Priority operation. If, for example, the terminal station 9 is transmitting to all other terminal stations, and the terminal station 5, which is assumed to have special priority, breaks in by sending a calling signal, the calling potential on line 25 is applied to a relay corresponding to relay B/2 of Fig. 3, but not associated with an arrangement such as indicated by 46 for transforming D.C. pulses to voice frequency signals and vice versa. The relay B/2 operates and at contact B1 connects the incoming potential to relay B/2 which operates, breaks the locking circuit of relays C/2, D/2 and applies calling potential to the outgoing line corresponding to line 13 leading to the switching centre 4, and to relay A/3 which at contact A1 completes the locking circuit of relay B/2. The outgoing line corresponding to the line 26 leading to the terminal station 6 will not be changed since it was already engaged in reception. When station 9 was transmitting, a calling potential was present on lines 21 and 17, and relay P , Fig. 2 2, of the switching centre 4 was operated and locked up. The potential on the line 13 operates relay N/2 closing a locking circuit at contact N2 and breaking the holding circuit of relay P/2 which releases. At operated contact N1 the potential on line 13 is applied over released contact P1 to line 16 and to lines 14, 18 as previously. The potential on line 16 operates relay B/2 opening the locking circuit of relay C/2 which was operated when terminal station 9   was transmitting. The release of relay C/2 applies potential from the line 16 over operated contact B1 and released contact C1 to line 20 which conditions terminal station 9 for reception. Terminal station 10 remains conditioned for reception 1. The potentials on lines 14, 18 are not changed since stations 7, 8 of switching centre 2 and station 11 of switching centre 4 are to remain in the receiving condition. If calling signals are received over two lines, e.g. 15, 17 simultaneously, relays O, P are both operated, but 2 2 break contact 02 opens the possible locking circuit for relay P/2 which releases when contact M3 opens. Relays N/2 and B/2 take precedence and operate even if one of the relays O, P, S, (Fig. 2) or C, D (Fig. 3) at their respective switching centres has already been operated.