Patent GB-766563-A (GB)
Title: Improvements in or relating to subscribers' line circuits for use in automatic telephone systems
Inventor: ALLUM ROBIN DEVENISH
IPC: H04Q3/00
Filed: 1954-02-20
Abstract: 766,563. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Feb. 2, 1955 [Feb. 20, 1954], No. 5049/54. Class 40 (4). A subscriber's line circuit comprises a gate circuit adapted, on receiving potential over two leads from a line scanning circuit, to pass current to a further gate circuit which is thereupon opened if the subscriber's loop is closed to pass a signal to initiate the setting up of a connection from the calling subscriber's line. In a first embodiment, Fig. 1, the scanning potentials are applied from a common control circuit CC over the leads 10 and 11 which are intersecting leads in a co-ordinate array of such leads. These potentials are applied to a rectifier gate MR3, coincidence between these potentials causing this gate to pass a pulse to condenser C2. When the incoming line SL is looped the potential of the right hand plate of C2 is raised sufficiently that the pulse from rectifier MR3 is able to overcome the back bias on rectifier MR1 to cause a pulse to pass to the common control circuit CC over lead 12, thereby stopping the scanning. The common control circuit thereupon applies pulses over leads 13 and 14 of a second co-ordinate array to fire tube VK1. A pulse is thereby applied to tube VK2 which fires to provide a co-ordinate marking of the subscriber's line circuit to enable the finder LF to be set thereto. Tube VN1 also fires to provide a class of service indication over lead 15. The rise in potential at the top of resistance R9 backs off rectifier MR3 to render ineffective the subsequent scanning pulses which re-commence as soon as the control circuit has initiated the setting of the line-finder LF. Removal of H.T. from lead 18 extinguishes VK2 and VN1 at this time. Tube VK1 remains conducting until contacts BM3 of the line-finder operate to pass a negative pulse over condenser C8. Contacts BM4 maintain the backing off potential for MR3. In the case of an outgoing call the line scan is stopped at the wanted subscriber's line circuit and pulses are applied to leads 13 and 14. These are without effect if VK1 is already fired or if contacts BM4 are already operated in which case VK1 is backed off. If the line is free however, VK1 fires, followed by VK2 which returns potential over leads 16, 17 to initiate completion of the connection. In a second embodiment, Fig. 2, the completion of the calling subscriber's loop enables the scanning pulse derived by coincidence of pulses on leads 21, 22, to pass through gate MR6 to lead 20, thereby causing the stopping of the line scan. The control circuit thereupon pulses leads 26, 27 to fire tube VK3 to provide a marking on lead 28 for the line-finder LF. When the latter finds the line circuit it applies negative potential to lead 29 to back bias rectifier MR8 to render further scanning pulses ineffective. Tube VK3 is extinguished on release of the control equipment CC. On an outgoing call the line scan is stopped in the position of the wanted line, pulses being applied to co-ordinate leads 23, 24. If the line is busy, MR9 is back-biased over lead 29. If, however, the line is free the pulse is passed over condenser C11 to lead 20 which is now biased negatively to cause MR6 to pass the pulse even with the subscriber's loop open. Tube VK3 is now fired as previously described and the connection completed. The Provisional Specification describes a third arrangement in which a saturable magnetic core type gate is used. Coil L2 is normally provided with biasing current of such value as to saturate the core and maintain it saturated in the face of opposing fields due to scanning currents in coils L3, L4. If, however, the line loop is closed, coil L1 provides further opposing field so that coincidence of current in coils L3, L4 now provides a flux change so causing coil L5 to deliver a pulse to the control circuit to stop the scanning. Tube VK4 is then struck over leads 35, 36 to provide a marking on lead 38 for the finder. When the line is formed coil L6 is energized over lead 37 in such a direction as to assist the bias in L2 and further scanning signals are again ineffective.