CA2029138A1 - Ring trip detector for a solid state telephone line circuit - Google Patents

Abstract

A RING TRIP DETECTOR FOR A SOLID STATE TELEPHONE LINE
CIRCUIT

ABSTRACT
A ring trip detector for a solid state line circuit is disclosed that includes, a logic interface circuit connected to a central controller. The logic interface circuit is arranged to receive signals from the central controller for applying an ac ringing current to the subscriber loop and to transmit status signals from the ring trip detector to the central controller. A
resistor, connected to the subscriber loop through a ringing current conductor, detects and develops a differential voltage that is proportional to the ac ringing current. A voltage convertor circuit, connected to the resistor, divides down the differential voltage and converts the differential voltage to a ground referenced voltage signal. An active filter circuit, connected to the voltage convertor circuit, receives the ground referenced voltage signal and filters out an ac voltage component leaving a dc voltage component that is drawn into the subscriber loop from the central office battery, when a subscriber instrument goes "off-hook". A
comparator circuit, connected to the active filter circuit, develops and transmits to the logic interface a ring trip signal when the dc voltage component is less than the threshold voltage, terminating the ringing current and providing a status signal to the central controller indicating that a ring trip has occurred.

Description

~ 2~29~38 ..:

A RING TRIP DETECTOR FOR A SOLID STATE TELEPHONE LINE
CIRCUIT

CROSS REFERENCE TO RELATED APPLICATIONS
Cross Reference is made to the related Canadian Patent Applications entitled: ~A Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-032), ~A High Voltage Subscriber Line Interface Circuit,~ (Attorney Docket 89-1-034), nA Circuit For Synthesizing An Impedance Across The Tip And Ring Leads Of A Telephone Line Circuit,n (Attorney Docket 89-1-035), nA Tip-Ring Short Detector and Power Shut-Down Circuit For A
Telephone Line Circuit,~ (Attorney Docket 89-1-036), ~A
Thermal Protection Circuit For An Integrated Circuit Subscriber Line Interface,~ (Attorney Docket 89-1-037), nA Thermal Protection Arrangement For An Integrated Circuit Subscriber Line Interface,~ (Attorney Docket 8i9-1-038), and nA Control Circuit For A Solid State Telephone Line Circuit,~ (Attorney Docket 89-1-039), filed on the same date, and by the same assignee as this Application.

BACKGROUND OF THE INVENTION -~
1. Field of the Invention The present invention relates to the field of telecommunications and, more particularly, to a new and novel ring trip detector for a solid state telephone line circuit.

2. Description of the Prior Art Telephone line circuits are customarily found in the telephone switching system or central office of a telecommunications network. The telephone line circuit interfaces the central office, to a telephone or subscriber station found at a location remote from the central office. The telephone line circuit functions to supply power or battery feed to the subscriber station - 2~2~138 . ~
via a two wire transmission line or subscriber loop and to couple the intelligence or voice signal to and from the telephone switching system.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and or inductors which carry up to 12Oma dc. This passive circuit has a wide dynamic range, passing noise-free differential signals while not overloading with the 60Hz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths were the current fed to the subscriber station is more than necessary for equalization. Active line-feed circuits can be less bulky and require lower total power, but meeting dynamic range and precision balance requirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Devices such as high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL
OF SO~ID-STAT~ CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, ~A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface.~ These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system.
However, presently known solid state line circuits, still suffer from deficiencies in meeting good 2~29~38 transmission performance specifications. These deficiencies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy.
Other problems presently encountered are excessive power dissipation at short loops that consume prodigious amounts of central office power and 2 wire input impedance circuits that are complex and that exhibit poor return loss.
Ring trip detection is used to ascertain if a subscriber instrument has gone ~off-hook~ during or after a ringing period. A detected ~off-hook~ condition normally signals control circuitry on the telephone line circuit which disables or isolates a ringing generator from the tip and ring leads of the subscriber loop.
It is advantageous to have an efficient method in -~
detecting the instantaneous ring trip in order to avoid transmitting the ringing signal to the subscriber when he places the substation handset to his ear. -~
Accordingly, it is an object of the present invention to provide a new and more effective ring trip detector that will effectively and efficiently sense ring trip during a ringing cycle.
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DISCLOSURE OF THE INVENTION
The above and other objects, advantages, and -capabilities are realized in a ring trip detector used in solid state line circuit. The solid state line circuit includes a subscriber loop, having a tip lead and a ring lead. The subscriber loop is connected to a subscriber statiQn and to a!source,of dc loop current provided by a central office battery. A ringing current conductor connects the ring lead of the subscriber loop to a source of ringing current, which is disposed to inject an ac ringing current to the subscriber loop for signaling the subscriber station.

-3- ~-':

~29~38 The ring trip detector of the present invention includes a logic interface circuit connacted via a address/data bus and a control bus to a central controller. The logic interface circuit is arranged to receive signals from a central controller for applying the ac ringing current to the subscriber loop and to transmit status signals from the ring trip detector to the central controller.
A resistor, connected to the ringing current lo conductor, detects and develops across the resistor a differential voltage that is proportional to the ac ringing current.
A voltage convertor circuit is connected to the resistor. The voltage converter circuit divides down the differential voltage and also converts the differential voltage to a ground referenced voltage signal.
An active filter circuit is connected to the voltage converting circuit. The active filter circuit receives the ground referenced voltage signal and filters out an ac voltage component from the ground referenced voltage signal.
Finally, a comparator circuit is connected to a source of threshold voltage that is provided by a reference voltage circuit.
When the subscriber station goes noff-hook~, the active filter passes a dc voltage component from the ground referenced voltage signal to the comparator circuit. The dc voltage component is drawn into the subscriber loop from the central office battery, when a subscriber instrument goes ~off-hook~. The comparator circuit develops and transmits to the logic interface circuit, a ring trip signal, when the dc voltage is less than the threshold voltage. This terminates the ringing current and provides a status signal to the central controller that a ring trip has occurred. ~-2~29138 BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention may be had !`-from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:
Figure 1 is a broad level block diagram of a solid state telephone line circuit, in accordance with the present invention.
Figure 2 is a detailed block diagram of the control circuit, shown in Fig. 1.
Figure 3 is a schematic drawing showing the ring trip detector, in accordance with the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT
Directing attention first to Figure 1, a broad level block diagram of a solid state telephone line circuit is shown. The line circuit is shown driving a subscriber station or telephone 10, via a subscriber loop 20. The subscriber loop 20 is comprised of a twisted two wire (2W) loop pair having a tip and a ring lead. The 2W loop is connected from the subscriber station 10 to an interface circuit 30. The interface circuit 30 feeds a ~
48 V dc voltage to the subscriber loop across the tip and ring leads from a central office battery (not shown).
The interface circuit 30 further functions to superimpose a voice signal on the dc feed voltage and also feed ringing current to subscriber loop 20 for signalling.
The Interface circuit 30 further functions to provide the 2W to 4W (four wire) hybrid function of splitting the balanced signal on the tip and ring leads into separate transmit and receive paths that are ground referenced.
The control circuit 40, in accordance with the -present invention, works in conjunction with the -interface circuit 30 to provide the DC loop current shaping and the line balance impedance portion of the 2W
to 4W hvbrid function. The control circuit 40 further controls various detection functions, such as, ring trip 2~2~138 detection and loop sense detection, as well as, providing a logic interface to the central controller of the central office switching system.
Most modern digital telephone switching systems use Pulse Coded Modulation (PCM) digital data to convey voice traffic through the central office switching system.
Therefore, some method of signal translation is required to convert the analog voice signals received by the interface circuit 30 to PCM digital data. This is typically accomplished by a PCM codec and filter circuit such as shown at 50. These devices are commercially available as a so called CODEC/FILTER COMBO~ from various manufacturers. Such as the TP30XX family of COMBO~
devices manufactured by the National Semiconductor Company. Analog voice data from subscriber station 10 is processed by the PCM codec 50 and applied to the PCM bus of the central office switching system for transmission to its destination. Similarly, the codec 50 receives PCM
data from the switching system and converts the PCM data into analog signals which are superimposed on the dc feed voltage of the subscriber loop 20.
The three solid state circuits 30, 40 and 50 just described, in combination embody a complete line circuit adapted to connect a single subscriber station to a central office switch.
Referring now to Figure 2, a more detailed explanation of the functions of the control circuit 40 shown in Figure 1, will now be given. The control circuit 40 shown in Figure 2, is constructed as a bipolar integrated circuit. All signals requiring low voltages and currents are interfaced~by circuit 40 while those signals requiring high voltages, typically central office battery (-48v), are interfaced by device 30.
The control circuit 40 of the present invention includes in combination, a logic interface 410, a DC loop control circuit 401, a loop sense comparator circuit 409, .~ ~

~29~38 , a switched capacitor filter 405, an AC summing amplifier 408, a ring trip circuit 403 and a Vref circuit 411.
The logic interface 410 functions to provide a digital interface between the control circuit 40 and a central controller of a central office switching system.
The control circuit operating states are defined by a bi-directional 4-bit address and one bit data bus NDATA 413 transmitted to control circuit 40. The reading back to the central controller is also accomplished using the NDATA 413 bus. Write (WR), read (RD), and chip select signals (CEN) are transmitted to logic interface 410 via lead 412.
The DC loop control circuit 401 is used to generate a control voltage to drive phase splitter amplifier 309 on interface circuit 30. The feedback control voltage generated by loop control circuit 401 controls loop current allowing for predetermined and smoothly ,,;, decreasing current curve for increasing loop resistance ' ' up to 2000 ohms.
The loop sense comparator 409, is used to sense the condition of the subscriber loop 20, such as loop closure, due to an i~off-hook~ condition, or rotary dial pulses. , The ring trip circuit 403, is used to detect if the subscriber has gone ~off-hook~ during a ringing period.
AC summing amplifier 408, functions to amplify the ac voice signals transmitted from the subscriber and to cancel out the receive voice signals that are fed back to the line interface. The transmit voice signals are output to the PCM codec 50 via the XMT lead for transmission to"the central office switch.
The switch capacitor filter 405, functions to reshape the ac voice signals received from the PCM codec 50 on lead RCV providing echo cancelation and line ' balance usually required in 2-4 wire conversions. The capacitors are selectable under control of the logic ~29138 interface, via leads L and NL, to provide loaded (L) or non-loaded (NL) network lines.
Vref 411 generates a 750mV voltage used by the loop sense comparator 409 and a -36mV reference voltage used by the ring trip circuit 403.
Since control circuit 40 works in association with ;
interface circuit 30 some of the functions of interface circuit 30 will be explained in combination with those of control circuit 40 in order to aid in the understanding of the present invention. A more comp~ete explanation of the functionality of interface circuit 30 may be had by reference to applicant's copending Application titled, "A
High Voltage Subscriber Line Interface Circuit~, (Attorney Docket No. 89-1-034), filed concurrently with the present Application.
Current on either the tip or ring lead of the subscriber loop 20 is sensed by a network of parallel resistors 305 and 306 whose value is approximately 50.0 ohms net on each side of the loop 20. The voltage drop across each resistor 305 and 306 is fed back to a tip drive and a ring drive amplifier 307 and 308, respectively. A phase splitter amplifier 309, couples input voltage SUMB to the tip drive amplifier 307 and the ring drive amplifier 308 and together with the feedback voltage from resistors 305 and 306 create a voltage to current converter. Input voltage SUMB controls the loop feed current, such that, a given voltage at SUMB results in a given current in the subscriber loop 20 flowing from the tip lead to the ring lead. The feed circuit of the present invention is designed to provide a typical gain of 20mA/Volt at SUMB. , It should be noted that the current feed in subscriber loop 20 is not a constant current, as the aforementioned discussion may imply. Rather, the current -is shaped to provide sufficient variation of current versus loop resistance to ensure efficient power usage.
This is accomplished by providing a feedback loop between 2 0 ~d 9 ~ 3 8 the interface circuit 30 and a DC loop control circuit 401. As loop resistance becomes less than 2K Ohm, a resistor network comprising resistors 310-311 apply the voltage drop sensed across subscriber loop 20 to a XMT
Differential amplifier 314. A voltage that is approximately half, but proportional to the voltage sensed by resistors 310-311 then appears at XMTB. The voltage at XMTB is fed to DC loop control circuit 401, where it is properly shaped and output to circuit 30.
This shaped voltage is fed back to the phase splitter amplifier 309 as the SUMB input voltage.
The current shaping just described, generates feed current characteristics that exhibit smoothly decreasing current with loop resistance, allowing for power savings at short loops, while still providing sufficient variation of current versus loop resistance to ensure proper transmit levels from the subscriber station. As the total external resistance becomes greater than 2K
Ohms, the battery feed reverts to a constant voltage feeding scheme.
The control circuit of the present invention is arranged to provide either loaded or non-loaded line balance, under control of logic interface 410. Switched capacitor filter 405, includes three switched capacitor 2~ filters, one for non-loaded lines, one for loaded lines and one for 9:2 load. The individual filters are selected by signal lines NL and L from logic interface 410. The capacitor filters appear between the receive side speech signal at RCV of circuit 40, and phase splitter amplifier 309 on interface circuit 30. The switched capacitor filter 405 combines the filtered signal with the transmit speech signals passed on XMTA.
The filters have a response which models the gain phase of the selected (loaded or non-loaded) line and the interface circuit 30 plus external components. The signal output from the filters 405 is 180 degrees out of phase with the transmit speech signals. The net result ~` 2~2~138 is that the ~echo~ from the receive side voice path, is sent to the ac summing amplifier 40~ via lead -RCV. The receive voice signals are then canceled on the transmit side voice path before the transmit voice signals are output via XMT for PCM conversion by the PCM codec 50.
The AC sum amplifier 408 is used to amplify the voice signals received from the subscriber station and to cancel out the receive voice signals fed back from the subscribers loop, before they are converted to PCM
representations by PCM codec filter 50.
Differential tip/ring speech signals transmitted from the subscriber station 10 over the subscribers line 20, are initially coupled to the interface 30 via resistors 310 and 311. The received differential speech signals are applied to XMT differential amplifier 314 which converts the speech signals to a single ended signal that appears at XMTA of interface circuit 30. The speech signals at XMTA are coupled to control circuit 40 were a high impedance amplifier in circuit 408 provides a gain of 2.536. The amplified transmit speech signals then have a ~reflection replican of the receive side -signal subtracted from them, as explained above in the description of the switched capacitor filter circuit 405, the net signal is output from XMT to PCM codec filter 50.
Loop sensing, is the detection of on/off hook status of the subscriber and the replication of dial pulsing for dial pulsing equipped subscriber stations. All loop sensing detection in the present invention occurs within the control circuit 40. This detection ls accomplished by sensing an analog representation of the loop voltage and comparing the analog representation to a fixed ;~
threshold with a comparator. This is accomplished in the following manner. As explained earlier for the battery feed function of the present invention, a voltage that is proportional to the voltage across the subscribers line 20 normally appears at lead XMTB. This voltage is processed by the DC loop control circuit 401 and an 202913~
output signal SUMB is generated which is proportional to loop curren~. The generated SUMB signal is also passed to the loop sense comparator circuit 409 where it is compared to a 750mY reference voltage generated by circuit VREF 411. Upon detection of a sensed loop condition, such as for example, a ring trip, loop sense comparator 409 sends an output signal to input LS of logic interface 410. A latch internal to logic circuit `;
A10 then is set and its data read by the switching system central controller via bus 413.
The control circuit 40, is further disposed to provide a means of applying ringing voltage to the subscriber loop 20 for signaling the subscriber instrument 10. A ringing relay RR associated with the subscriber loop 20 is operated or released to provide the desired on/off ringing period, under control of the central office central controller. To ring the subscribers instrument, the central controller writes to the RR control point of the logic interface 410. This sets an internal RR latch in interface 410. The now set control point, through the RR output lead, pulls the RR
relay via its NPN transistor driver. When the relay is activated RR contacts 318 and 315 are closed allowing ac ringing voltage and current from a ringing generator (not shown), to be applied in series with the dc battery potential via leads RBB and RBA.
With reference to Fig. 2 and Fig. 3 of the present invention an explanation of the ring trip detector of the present invention will now be given. To detect a ring trip during ringing, such as when the substation instrument is placed ~off-hook'l, a resistor 317 connected in series with the RING lead develops a voltage drop ' !
which is proportional to the applied ringing current.
The differential voltage developed by resistor 317 is applied to a resistor network 319, comprising resistors 520, 521 and 522. The differential signal is divided down by resistor network 319 and converted to a ground 2~2~138 referenced signal by op amp 523. The signal from op amp 523 is then applied to an active two pole ac filter 539.
The ac filter 539 includes resistors 530, 531, capacitor 532 connected to ground and an op amp 535 connected between resistors 530 and 531 via capacitor 534.
Circuit 539 filters out the ac ringing component of the signal from op amp 523 and keeps any dc component.
Normally, when a substation is non-hook~ during a ringing cycle, ringing current has only an ac component.
However, when the subscriber goes ~off-hook~, dc loop current from the battery biased ringing generator is drawn into the loop. The dc voltage output by filter 539 is fed to a comparator circuit comprising op amp 536. The dc voltage applied to the negative input of op amp 536 is compared against a -36mV reference voltage developed by Vref circuit 411. If the dc signal sensed by circuit op a~p 536 is lower than the -36mV reference, a signal is generated and sent to the RT sense point of logic interface 410. This resets the RR latch, deactivating relay RR and removing the ringing generator from the loop. The RT sense point is also monitored by the central controller to detect if a ring trip has occurred. ~ ;
The control circuit 40 just described, can be manufactured as a large scale integrated circuit suitable for mounting on a hybrid assembly. The integrated circuit can thus provide the capabilities of a line circuit control device, which in the past, occupied a complete circuit card, to a control circuit having greater functional capability and occupying one sixteenth the same space. The control circuit of the present invention also benefitslfrom the increased reliability inherent in integrated circuit construction as well the economies in labor cost and manufacture which are enjoyed by such devices.
It will be obvious to those skilled in the art that numerous modifications to the present invention can be made without departing from the scope of the invention as : ,., ~.

2~29~38 defined by the appended claims. In this context, it should be recognized that the essence of the invention resides in a control circuit for a solid state telephone line circuit having the advantages and capabilities described herein. .
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Claims (5)

1. A ring trip detector used in solid state line circuit, said solid state line circuit including a subscriber loop having a tip lead and a ring lead, said subscriber loop connected to a subscriber station and to a source of dc loop current, a ringing current conductor connected to said ring lead, and a source of ringing current connected to said ringing current conductor disposed to inject an ac ringing current to said subscriber loop for signaling said subscriber station, said ring trip detector comprising:
a logic interface circuit connected via a address/data bus and a control bus to a central controller, said logic interface circuit arranged to receive signals from a central controller for applying said ringing current to said subscriber loop and to transmit status signals from said ring trip detector to said central controller;
a resistor connected to said ringing current conductor for developing a differential voltage proportional to said ac ringing current;
a voltage convertor circuit connected to said resistor, said voltage convertor circuit dividing down said differential voltage and converting said voltage to a ground referenced voltage signal;
an active filter connected to said voltage convertor circuit, said active filter receiving said ground referenced voltage signal and filtering an ac voltage component from said ground referenced voltage signal; and a comparator circuit connected to a source of threshold voltage and to said logic interface circuit, and responsive to said subscriber station going "off-hook", said active filter passes a dc voltage component from said ground referenced voltage signal to said comparator circuit, whereby, said comparator circuit develops and transmits to said logic interface circuit a ring trip signal when said dc voltage is less than said threshold voltage, terminating said ringing current and providing a status signal to said central controller that a ring trip has occurred.

2. The ring trip detector as claimed in claim 1 wherein, said ring trip detector further including:
a ringing relay connected to said logic interface circuit and to said source of ac ringing current, and responsive to signals from said central controller, said ringing relay is rendered operated injecting said ac ringing current to said subscriber loop and alternatively, responsive to said ring trip signal from said comparator circuit said ringing relay is rendered non-operated, disconnecting said source of ac ringing current from said subscriber line.

3. The control circuit as claimed in claim 1 wherein, said control circuit further includes:
a voltage reference generator connected to said ring trip circuit, said voltage reference generator arranged to develop said threshold voltage used by said ring trip circuit for detecting said ring trip.

4. A ring trip detector used in solid state line circuit, said solid state line circuit including a subscriber loop having a tip lead and a ring lead, said subscriber loop connected to a subscriber station and to a source of dc loop current, a ringing current conductor connected to said ring lead, and a source of ringing current connected to said ringing current conductor disposed to inject an ac ringing current to said subscriber loop for signaling said subscriber station, said ring trip detector comprising:
logic interface means connected via an address/data bus and a control bus to a central controller, said logic interface means arranged to receive signals from a central controller for applying said ringing current to said subscriber loop and to transmit status signals from said ring trip detector to said central controller;
detector means connected to said ringing current conductor for developing a voltage signal in response to said ac ringing current;
converter means connected to said detector means, said converter means converting said detected voltage signal to ground referenced voltage signal;
filter means connected to said converter means, said filter means receiving said ground referenced voltage signal and filtering an ac component from said ground referenced voltage signal; and comparator means connected to a source of threshold voltage and to said logic interface means, and responsive to said subscriber station going "off-hook", said filter means passes a dc voltage component from said ground referenced voltage signal to said comparator means, whereby, said comparator means develops and transmits to said logic interface means a ring trip signal when said dc voltage is less than said threshold voltage, terminating said ringing current and providing a status signal to said central controller that a ring trip has occurred.

5. In combination:
a subscriber loop connected to a subscriber station and to a source of dc loop current;
a ringing current conductor connected to said subscriber loop and to a source of ac ringing current, said ac ringing current for signaling said subscriber station;
means connected to a central controller, receiving signals from said central controller for applying said ac ringing current to said subscriber loop and to transmit a ring trip signal to said central controller;
means connected to said ringing current conductor for developing a voltage signal in response to said ac ringing current;
means for converting said detected voltage signal to a ground referenced voltage signal;
means for filtering out an ac component from said ground referenced voltage signal; and means for comparing connected to a source of threshold voltage, and responsive to said subscriber station going "off-hook", said ground referenced voltage signal includes a dc voltage component, whereby, said comparator means develops and transmits said ring trip signal when said dc voltage component is less than said threshold voltage, terminating said ac ringing current and providing said ring trip signal.