Patent GB-328206-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q1/453
Filed: 1929-01-18
Abstract: 328,206. Standard Telephones & Cables, Ltd., and Skillman, T. S. Jan. 18, 1929. Semi-automatic exchange systems.-Each digit of a wanted number is transmitted over a longdistance junction in the form of a single pulse of alternating current comprising one or more voice frequencies A - - D, the incoming end of the junction having means whereby the coded pulses are adapted to control the setting of a registercontroller which completes the connection in known manner. When a toll operator plugs into jack J, Fig. 1, relay 15 operates and relay 20 sends a calling pulse of frequency-combination A, B, C whereby tuned relays A1, B1, C1 and direct current relays E1, F1, G1 are operated, and relay 27 is released. Relay 29 earths starting lead 11 and when an idle register-controller is seized, earth sent back over terminal 2 energizes relay 33 which transmits back over the junction a pulse A, B, C. The response of relays AO, BO, CO causes the release of relay 5 and the operation of relay 9 which earths the cord sleeve circuit to light the supervisory lamp. The operator now transmits from her keyboard, Fig. 3, over the leads a, b, pulses comprising various combinations of the frequencies A - - D, the pulses being clipped by means of relays 41, 42 so that they are not long enough to cause the effective response of the signalling relays at the outgoing end of the junction. Relays A1 - - D1, E1 - - H1, in responding to the pulses, connect earth to terminals 3 - - 6 leading to the register-controller, in various combinations. When the complete number has been recorded earth is bent back over terminal 7 to operate relays 22, 38, the former of which connects terminals 5, 6 to the outgoing leads 9, 10 to the local selector, whilst relay 38 operates relays 34, 31 whereby a pulse A, B, D is sent back over the junction. The release of relay 6 operates relay 10 which unlocks relay 9 and connects up interrupters 12, 13 to give an intermittent flicker to the supervisory lamp. When the register has completed its operations, the release of relay 38 causes relay 30 to send back a pulse A, C, D which releases relay 7 and operates relay 11 so that the supervisory lamp is flickered continuously. The operator now transmits a pulse of low-frequency ringing current, which, by means of relays 2, 18, is transmitted over the junction as a pulse of voice-frequencies A, B, D. The response of relays A1, B1, D1 releases relay 28 so that relay 23 transmits lowfrequency current over conductors 9, 10. Relay 2 also operates relay 43 which locks up relay 9 to light the supervisory lamp continuously. If the wanted line is busy, relay 24 is intermittently operated over the third wire 8 and, through relays 40, 36, 37, 32, 33, causes alternate pulses B, C, D and A, B, C to be sent back. These operate relays 8, 9 intermittently so that the supervisory lamp is flashed. A characteristic tone is also sent back over the talking wires and if this indicates that the wanted line is busy in a local call, the operator rings a second time to offer the toll call and, if this is accepted, rings a third time to release the local connection. If the wanted line is free, the second and third rings are made quickly. When the called subscriber replies relay 24 operates to extinguish the supervisory lamp. When the subscriber hangs up, the release of relay 24 relights the lamp and when the plug is taken down, the release of relay 15 sends forward a pulse B, C, D which releases relays 26, 29 and 25 to open the third wire 8. In order to prevent a signal in one direction being commenced whilst a signal in the other direction is being transmitted, relays 14, 39 are provided which are operated whenever a signal is being received, these relays controlling respectively relays 15, 40 in such a way that the latter are maintained in their (existing operated or unoperated) condition until the previous signal is terminated. Relay 17 is given a very long time of release so that the transmission of the release signal will be ensured even if a signal in the reverse direction is simultaneously initiated.