GB1430429A - Telephone system with ringing-path detector - Google Patents

Abstract

1430429 Automatic exchange systems PLESSEY CO Ltd 22 April 1974 [18 May 1973] 23768/73 Heading H4K A called subscriber line is tested for a valid ringing loop during the first cycle of ringing current and if such a loop is absent a fault signal is given. A valid loop is considered to be a capacitor and magneto bell in series. In operation a relay X closes contacts X1, X2 to initially connect continuous ringing current from a supply CR, -50 v. via a balanced bridge network R1-R6, C1, C2, and a switching network path to the called subscriber line, a relay INT closing a contact INT1 to supply interrupted ringing current after a short period of time, e.g. 300 ms. If the line terminates for a valid ringing loop, the point VA remains positive with respect to the point VB throughout the first positive cycle of ringing current. Transistors T1 and T2 are switched to be ON and OFF, respectively and relay RD is operated to prepare a ring trip circuit of the kind disclosed in Specification 1,396,768, by connecting resistor R7 in series with resistor R6, resistors R1, R6, R7 constituting part of the ring trip circuitry. Transistor T5 therefore conducts to give a valid ringing loop indicator to the central processor on wire DOK, the processor responding to cause ringing tone to be supplied to the calling line. If a valid ringing loop is not present point VA never goes more positive than point VB during positive ringing half-cycles. Relay RD therefore remains unoperated and transistor T5 remains OFF to give a non-valid loop indication to the central processor. Ringing tone is not supplied to the calling line and the ringing current to the faulty called line is disabled. A second attempt to set up the connection may then be made as the fault could be in the switching network speech path and not on the subscriber's line. A fault indication tone may be supplied to the calling line. The arrangement may be incorporated in the exchange of Specification 1,393,156.