Patent GB-821630-A (GB)
Title: Signalling network
IPC: H04L25/22
Filed: 1956-03-16
Abstract: 821,630. Telegraph exchange systems. STANDARD TELEPHONES & CABLES Ltd. March 16, 1956 [March 22, 1955], No. 8307/56. Class 40 (3). In a signalling network for repeating telegraph signals between a two-wire line and a four-wire line, the receiving and sending pairs of lines forming the said four-wire line are electrically associated with conjugate arms of a bridge network, another arm of said network is associated with said two-wire line and the said network is balanced for one electrical condition of said two-wire line. The input terminals of the two-wire line L1 are connected through a quadripole denoted by its impedance Z3 in the arm of the bridge whilst of the outgoing and incoming pairs s, r forming the four-wire line L2, the pair s is connected to a diagonal denoted by Z5. The pair r is connected to a quadripole in which lines f, h include the receiving relay Rr and lines e, g connected in the conjugate diagonal arm are connected to earth and to the armature Rc of relay Rr engaging contacts connected to voltages +V, - V. The impedances Z1, Z2, Z3, Z4 are calculated so that the signals received at f, h from the pair r are not transferred to the sending leg a, b and the line s. When the teleprinter contacts Tpc are closed, the bridge is balanced by the relation (RL+Rp+R3)R2=R1.R4, so that no current due to the voltage +V2 flows through resistor R5 in which the only current, il, is that produced by battery V3 in the teleprinter loop. When the key Tpc opens, the bridge is unbalanced and current i2 opposite in sign to il flows through R5. When the contact Rc oscillates to apply + V and - V to the line g the current through R5 remains equal to il. When Rc connects +V, the major part of the current flows through R1+R2+Rp+R3 across .which there is a P.D. of 2V volts, whilst minor currents flow via resistors R2, R4 and resistors R1, R5, R4. When the contact Rc connects the voltage - V the current through R4 from earth divides in a major part via R2 to negative battery, and a minor part via resistor R5 large compared with R2, and this minor part further divides into current via R1 and RL+Rp+R3, large relative to R1, in parallel, so that the current through the teleprinter is negligible to provide " no current " impulse or element. Control of motors at subscriber stations.-The initiation of transmission at station STP by closure of key K1 effectively closes the teleprinter loop and reverses the current through R5 of the sending pair s connected to a similar network SN at the distant exchange where the contact Rc of the receiving relay changes over and operates relay D. At the calling subscriber station a small reversed current flows through polarized relay P, but is insufficient to effect its operation. At the distant exchange, the reversal of relay contact Rc, by cutting out a relatively high resistor R7, operates the polarized relay P of the called subscriber which at contact p operates motor relay M starting the motor and completing a signal circuit for the teleprinter. Also the completion of the D.C. circuit for the distant teleprinter reverses the current through the resistor R5 of the distant network SN thereby effecting the operation of the receiving relay of the network SN so that contact Rc is changed over, relay D is operated and the calling subscriber's polarized relay P operates so that his motor is started and thus the signalling circuit is completed. Termination of signal transmission.-At the end of communication, a subscriber opens a key K2 reversing the current in the resistor R5 of the associated network SN which at the distant network SN reverses the receiving relay so that its armature Rc is connected to - V. Relay D releases and at contact d effects charging of capacitor C2, producing a current in the opposite or reversed direction through polarized relay P which releases and cuts out the motor. Also D.C. is cut off from the distant teleprinter loop so that the current through the resistor R5 is reversed and the receiving relay at the near network SN restores its contact Rc to the position shown in which relay D is de-energized and capacitor C2 provides a current de-energizing polarized relay P which is effective to stop the motor and by the change-over of contact m, to restore this subscriber station to its idle condition.