GB1495994A - Telephone systems - Google Patents

Abstract

1495994 Automatic exchange systems SIEMENS AG 13 March 1975 [29 March 1974] 10420/75 Heading H4K A ringing current detector comprises a counter which is arranged to count clock pulses for a predetermined number of cycles of an incoming A.C. signal. An incoming A.C. signal is converted to square pulses of the same frequency and the first pulse sets a binary counter EMP to count 50 Hz clock pulses, whilst counter JZ counts the incoming pulses. When the counter EMP reaches a predetermined count gate G5 produces an output signal and if counter JZ is simultaneously indicating a count between 3 and 8 inclusive the received signal is accepted as a ringing signal i.e. as a frequency lying between 18À2 and 61À5 Hz. A counter UPZ is used to count clock pulses between adjacent incoming signal pulses, this counter being reset by each incoming pulse. If the count exceeds a predetermined limit an inhibition signal is produced to prevent counters EMP and JZ from producing a ringing signal indication on output AVe/AR. This counter can also be used to indicate that the ringing signal has terminated. The whole circuit may be produced as a monolithic integrated circuit. The application is to detect ringing signals incoming to a PBX from a main exchange.