# US 4,398,064 A · Call-for-service and continuity sensor circuit > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,398,064 A - Document ID: US4398064A - Status: Granted - Kind: A - Application number: 06/335,004 - Filed: 1981-12-28 - Granted: 1983-08-09 - Earliest claimed: 1981-12-28 - Assignees: GTE Automatic Electric Labs Inc. - Inventors: Edward A. Formosa, Jr. - Web page: https://patentgrep.com/patent/US4398064A ## CPC - H04M 7/0096: {Trunk circuits} - H04M: TELEPHONIC COMMUNICATION (circuits for controlling other apparatus via a telephone cable and not involving telephone switching apparatus G08) ## Abstract A "call-for-service" and continuity sensor circuit for use with a trunk circuit of a digital PABX telephone system interfacing with a central office via ground start signaling providing for detection of a "call-for-service" signal to the PABX by the central office. This circuit also provides high and closely matched tip to ground and ring to ground impedances for all trunk conditions in order to prevent excess loading of the trunk circuit. This circuit provides a precise and stable threshold detection level necessary for accurate sensing of the trunk circuit conditions mentioned above despite parasitic leakage current, induced longitudinal power line voltages, and earth potentials sometimes found to exist on the line connecting the central office to the PABX. ## Claims ### Claim 1 A call-for-service and continuity sensor circuit for use in a trunk circuit of a telephone switching system including at least first and second switching centers connected via said trunk circuit in ground start configuration, said trunk circuit having first and second leads connecting said switching centers, said call-for-service and continuity sensor circuit comprising: circuit input means including signal conditioning means connected to said first and said second leads of said trunk circuit; circuit sensing means including comparator means connected via included inputs to said signal conditioning means; said input means cooperating with said signal conditioning means to adapt signals sensed on said trunk circuit leads for use by said comparator means while protecting said trunk circuit from excessive loading and protecting said comparator means from excessive induced potentials; and circuit output means connected to an output of said comparator means and having an output connected to said second switching center, said circuit output means operated to convert signals from said comparator means to signals suitable for use by said second switching center and to couple said suitable signals to said second switching center via an output terminal; whereby, in response to the application of negative potential to said second lead and an open circuit to said first lead at said first switching center, said sensor is operated to produce a first output signal to said second switching center, and in response to the application of negative potential to said second lead and earth ground to said first lead at said first switching center, said sensor is further operated to produce a second output signal, and in response to the application of an open circuit to both of said leads at said first switching center, said sensor is still further operated to produce said second signal. ### Claim 2 (depends on 1) A call-for-service and continuity sensor as claimed in claim 1, wherein: said circuit input means includes line bridging means comprising a plurality of resistors connected across said first and second leads, providing access to signals on said leads while protecting said leads from excessive loading. ### Claim 3 (depends on 1) A call-for-service and continuity sensor as claimed in claim 1, wherein: said circuit input means includes filter means comprising a plurality of three-stage RC filters each connected to a different common point of said bridging means resistors, said filters operated to prevent signals induced on said trunk circuit leads from interfering with the operation of said comparator means. ### Claim 4 (depends on 1) A call-for-service and continuity sensor as claimed in claim 1, wherein: said signal conditioning means comprises a biasing network connected between said trunk circuit leads and said comparator means, said signal conditioning means operated to adapt said lead signals for sensing by said comparator means. ### Claim 5 (depends on 1) A call-for-service and continuity sensor as claimed in claim 1, wherein: said circuit sensing means includes feedback means connected between said comparator means output and an input of said comparator means, said feedback means operated to prevent fluctuations on said comparator means inputs from causing changes of said comparator means output. ### Claim 6 (depends on 1) A call-for-service and continuity sensor as claimed in claim 1, wherein: said sensor means further includes voltage regulation means comprising a resistor and regulator diode connected in series, said voltage regulation means operating to supply a controlled potential to said comparator means. ### Claim 7 (depends on 1) A call-for-service and continuity sensor as claimed in claim 1, wherein: said circuit output means further comprises bias and clamping means, and level shifting means connected between said comparator means output and said bias and clamping means, said level shifting means operated to shift the level of said comparator means output signal and said bias and clamping means operated to restrict said level shifted comparator means output signals within predefined limits suitable for use by said second switching center. ### Claim 8 (depends on 7) A call-for-service and continuity sensor as claimed in claim 7, wherein: said level shifting means comprises a diode. ### Claim 9 (depends on 7) A call-for-service and continuity sensor as claimed in claim 7, wherein: said bias and clamping means comprises a resistor and diode connected in series.