Improvements in or relating to Signalling Systems

1,159,251. Automatic exchange systems. H.M. POSTMASTER GENERAL. 29 July, 1966 [29 July, 1965; 24 March, 1966], Nos. 32557/65 and 13101/66. Heading H4K. In a switching system control signals from a control register 15, Fig. 1 to a stage register 102 are sent as a unique combination of a number of discrete current or voltage valves applied one to each conductor of the transmission path. A receiver in the stage register signals to the transmitter when it is ready to receive a combination compares the incoming current or voltage valves with reference valves in order to identify the received values, and signals to the transmitter when a combination has been identified. D.C. voltage sender and receiver is shown in Figs. 2, 3, 4, and 5, in which the control register seizes the D.C. sender by operation of relay SA, Fig. 2, and transmits the first digit stored in the register as energization of one line out of each of the groups of four lines L25-28 and L211 to 214. Contact SA1 puts an earth on signal wire L23, which connects to L33 on Fig. 3, and operates relay BB which via BB1 puts 12V+ve signal on lines L31 and 32 which signal, via rectifiers MN35 and 36 and transistors VT32 and 33, operates relay CP to put the receiver in signal receiving condition, and is reverted to the transmitter as a "proceed to send" signal. The 12V signal received on lines L21-22 Fig. 2, turns on transistors VT21 and 22 which, via transistors VT23 and 24 operate relay SC, which holds over contacts SC4, and applies D.C. voltage signals to lines L21 and 22 over contacts SC1 and 2, the actual voltages applied to the lines depending on which of the relays A to D and W to Z are operated to switch through potentials from the Zener diode chains RZ22 to RZ211. The resulting D.C. potentials received on lines L31 and 32 are applied over operated contacts CP2 and CP3 to lines L41 Fig. 4, and L51 Fig. 5, for evaluation. In Fig. 4, the voltage received from the transmitter line L21 and applied via lines L31 L311 and L41 is compared with reference potentials of earth, V5, V6, and V7 in transistors VT41-VT44 and causes all the transistors to conduct whose reference potential, applied to the emitter, is less than the signal potential. The diode matrix MR411 to 420 is arranged to enable only that transistor of transistors VT45 to VT48 which is associated with the highest numbered operated one of the transistors VT41 to VT44, to cause operation and self hold of one of the relays AA to DA. In a similar fas