JP3718175B2 - Subscriber circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a test circuit and a method for a subscriber circuit, for example, a subscriber circuit system of an exchange, and more particularly to a subscriber circuit and a method capable of disconnecting a connection state of a subscriber terminal.
[0002]
[Prior art]
In a subscriber circuit system of an exchange, there is a demand for a simple and economical method for detecting a connection state of the system and identifying and identifying an abnormal part such as disconnection. Conventionally, as this method, a method of identifying a plurality of media having different time constants to identify the media to some extent and detecting the presence or absence of disconnection has been disclosed in the 1996 IEICE Communication Society Conference B-562. , Morie et al. "Study on a method of testing for disconnection of subscriber lines that can be separated into multiple disconnection points". A conventional test method according to this document will be described with reference to FIG. 3 and FIG.
[0003]
This test method utilizes the fact that the internal impedance characteristics, that is, the time constant of the medium on the subscriber terminal side connected to the switching system is different depending on the type of medium. The configuration of this proposal will be described with reference to FIG. 3. A subscriber terminal 400 is connected to a subscriber circuit system, which is generally equivalent to a two-terminal network 406 in which a resistor 402 and a capacitor 404 are connected in series as internal impedance. Can be considered. The two-terminal network 406 is a medium such as a telephone TEL, a modular jack CMJ with a capacity, and / or a protector 6PT with a separation function.
[0004]
For example, assume that the time constants of two of these media, namely media A and B, are τ1 and τ2. In the medium A or B, the potential of the connection point 408 of the equivalent circuit formed by the capacitor C and the resistor R is set to Vn1 or Vn2, respectively. These media A or B are connected across the line terminals 110AA and 110AB of the subscriber circuit 500 via the subscriber line 108. The power supply circuit (VBB) 504 of the subscriber circuit 500 is connected to the line terminals 110AA and 110AB, and has an internal resistance (R1) 506. A switch (SW1) 502 is connected between the subscriber circuit line terminals 110AA and 110AB. The terminal 110AB is connected to the ground potential 508.
[0005]
As shown in FIG. 4A, in the period Twc1 at the beginning of the test, since the switch 502 is opened, the connection point 408 between the capacitor C and the resistor F of the medium A or B that is the load of the subscriber circuit 500. Is maintained at the power supply voltage VBB of the subscriber circuit 500. Now, when the switch 502 is short-circuited, the ground potential at the connection point 408 decreases with the time constant τ1 or τ2 depending on the medium as the accumulated charge in the capacitor C is discharged. When the switch 502 is opened after the line terminals 110AA and 110AB are short-circuited for a certain time Twd1, the potential at the connection point 408 becomes a residual charge after discharging according to the time constant τ1 or τ2 of the corresponding medium during the discharge time Twd1. The electric potential (almost 0 volt) due to the initial value begins to rise again toward the power supply voltage VBB. That is, medium A or B and subscriber line 108 begin to be charged. The value of the potential due to this residual charge depends on the time constant τ1 or τ2 of the medium. The state of discharge charging at that time is as shown in “Voltage V” in FIG.
[0006]
In FIG. 4, symbols S1 and S2 conceptually indicate the integrated value of the charging voltage from the residual voltage after opening the switch to the power supply voltage VBB. As shown by the periods Twc1, Twd1, Twc2 and Twd2 in FIG. 4, the charging / short-circuiting of the switch 502 is repeated twice, and the values of the short-circuiting times Twd1 and Twd2 are determined according to the discharge time constant of the medium A or B to be tested. By setting the values, it is possible to make a difference between the values of the integrated charging voltages S1 and S2.
[0007]
Now, if the first short-circuit time Twd1 is set to 100 mS ≦ Twd1 ≦ 200 mS (milliseconds) and the second short-circuit period Twd2 is set to several milliseconds, the equivalent impedance of the subscriber line 108 itself is different from other load media. The time constant is about several microseconds. Therefore, when the subscriber line 108 is disconnected, the time constant is very small, so the integrated charging voltage is S1≈S2, and other media (telephone TEL, modular jack CMJ with capacity, protector 6 with disconnect function) And disconnection can be distinguished. In one example, the internal impedance of these media is 0-7.5 kΩ for the phone, 0.25-3 μF for the capacitance, and 100 kΩ for the capacitive jack, 0.27 μF for the capacitance, In the case of a protector with a separation function, the resistance is 510 kΩ and the capacitance is 0.68 μF. Each time constant τ is as follows. That is, in the case of a telephone set, τtel = 0 to 7.5 kΩ × 0.25 to 3 μF = 0 to 22.5 mS. In the case of a modular jack with a capacity, τcmj = 100kΩ × 0.27μF = 27mS, and in the case of a protector with a cutting function, τ6pt = 510kΩ × 0.68μS = 347mS.
[0008]
On the other hand, if the first short-circuit period Twd1 ≧ 1S (seconds) is set, the difference between the integrated voltages S1 and S2 when the medium is a protector with a separation function is Compared with the case of a modular jack, it is large and can be distinguished from others, that is, a protector with a disconnection and disconnection function of a subscriber line and a telephone jack and a modular jack with capacity. In the case of disconnection, of course, it may be considered that the resistance is ∞ and the capacitance is 0. Therefore, by repeating the above measurement for the two types of short circuit time Twd1 of the switch 502, it is possible to distinguish between a disconnection of a subscriber line, a protector with a disconnecting function, and a telephone jack or a modular jack with a capacity. It is.
[0009]
[Problems to be solved by the invention]
However, in such a configuration for detecting the connection state of the conventional subscriber line, the size of the internal resistance 506 in the power supply circuit 504 of the subscriber circuit is about 100 kΩ. On the other hand, when the protector with the separation function is disconnected by the protector control, the internal resistance is 20 kΩ and the capacitance is about 5.4 μF. For this reason, there is almost no difference between the time constant when the protector with the separation function is disconnected and the time constant when the modular jack with capacity or the telephone is connected. Therefore, in the prior art, it is difficult to distinguish between a protector with a separation function (disconnected state) and a telephone or a modular jack with a capacity based on the difference between the voltage integration values S1 and S2.
[0010]
The present invention is difficult in the past even when the difference in time constant between the protector with disconnection function (disconnected state) connected to the subscriber line and the telephone or the modular jack with capacity is not large. It is an object of the present invention to provide a subscriber circuit, particularly a test circuit and method for a subscriber circuit system, which can identify them.
[0011]
[Means for Solving the Problems]
According to the present invention, a subscriber circuit having a pair of line terminals to which a subscriber line is connected and detecting a state of a terminal device connected to the subscriber line is connected to the pair of line terminals and connected. A first switch means for short-circuiting or opening a pair of line terminals of a subscriber line, a constant voltage supply means for supplying a constant voltage to a subscriber line through a pair of line terminals, and a pair of constant voltage supply means. The second switch means selectively connected to the line terminal, inserted between one of the pair of line terminals and the constant voltage supply means, monitors the current flowing through the subscriber line and differentiates the monitored current. Current monitoring means for calculating a value, and control means for controlling the first and second switch means. The control means controls the first and second switch means to establish a gap between a pair of line terminals. After short-circuiting, it is opened and the constant voltage supply means is The current monitoring means detects the difference between the differential value at the rising edge and the differential value at the falling edge after the constant voltage supply means is connected to the pair of line terminals. The state of the terminal device is determined.
[0012]
According to the present invention, the subscriber circuit further includes power supply means for supplying power to the subscriber line through the pair of line terminals, and voltage monitoring means for detecting a voltage between the pair of line terminals, the control means. Is configured to control the first and second switch means to short-circuit between the pair of line terminals for a first predetermined time, and then to open, and again between the pair of line terminals, The voltage monitoring means integrates the voltage monitored in the opening after the first predetermined time short circuit, and then opens the second predetermined time shorter than the first predetermined time. Integrating the monitored voltage at the opening after the short circuit of the predetermined time of 2 to detect the difference between the two integrated values, obtaining the difference for the two first predetermined times of different lengths by the difference The state of the terminal device is determined.
[0013]
According to the present invention, a method for detecting a state of a terminal device connected to a pair of subscriber lines short-circuits between a pair of subscriber lines, then opens and opens a constant voltage to a pair. A step of detecting a difference between a differential value at a rising edge and a differential value at a falling edge after a constant voltage is supplied to a pair of line terminals; And determining the state of the terminal device.
[0014]
Furthermore, the method according to the present invention further comprises a step of short-circuiting between the pair of subscriber lines for a first predetermined time and then opening, and after this opening, the voltage between the pair of subscriber lines. Monitoring and integrating, a step of short-circuiting again between the pair of subscriber lines for a second predetermined time shorter than the first predetermined time, and then opening, And then monitoring and integrating the voltage between the pair of subscriber lines and detecting the difference between the two integrated values, and two first predetermined times of different lengths. The state of the terminal device is determined by executing the above step and obtaining the difference between the integral values.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of a circuit for identifying a state of a medium connected to a subscriber line according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing the configuration of a test circuit for a subscriber circuit system in an embodiment of the present invention. FIG. 2 is a time chart for explaining on / off timing control of switches in the subscriber circuit of the embodiment shown in FIG.
[0016]
In the subscriber circuit system test system of this embodiment, for example, in a subscriber circuit 200 of a telephone exchange, a pair of line terminals 110A and 110B are connected to a subscriber line 108, and further on the far end side of the latter. It is connected to the arranged terminal device, that is, the subscriber terminal 400. The subscriber terminal 400 is generally considered equivalent to a two-terminal network 406 in which a resistor 402 and a capacitor 404 are connected in series as internal impedance. The two-terminal network 406 is a medium such as a telephone TEL, a modular jack CMJ with a capacity, and / or a protector 6PT with a separation function. Of course, the present invention is not limited to these specific media and numbers.
[0017]
The subscriber circuit 200 includes a power supply circuit 204 that supplies a DC power supply VBB to the subscriber line 108 and the subscriber terminal 400. The power supply circuit 204 can be equivalently regarded as a series connection of a DC voltage source 206 and an internal resistor 208. Further, a constant voltage circuit 230 is connected in parallel to the power supply circuit 204 via a switch (SW2) 210. In this embodiment, the constant voltage circuit 230 is provided separately, but a power feeding circuit that is originally provided in the subscriber circuit 200 may be used. By doing so, the test function according to the present invention can be realized without increasing the circuit configuration.
[0018]
The switch SW2 is composed of two on / off switches 260 and 262 as shown, and the switches 260 and 262 are respectively connected in series between the terminals 264 and 266 of the constant voltage circuit 230 and both terminals of the power supply circuit 204. Has been inserted. In other words, the constant voltage circuit 230 is connected in parallel to the power supply circuit 204 via the switch 210. Normally, the positive terminal 240 of the DC power supply 206 is grounded, and a negative supply voltage is output from the line terminal 110A to the subscriber line 108. In this embodiment, the terminal 110B of the subscriber circuit 200 is grounded. However, the terminal 110B is not grounded and does not restrict use in a floating state.
[0019]
Between the line terminals 110A and 110B of the subscriber circuit 200, a switch (SW1) 202 and a voltage monitor (V) 222 are connected in parallel as shown in the figure. The switch 202 is an on / off switch. When the switch 202 is turned on (closed), the line terminals 110A and 110B of the subscriber circuit 200 are short-circuited, and the subscriber terminal 400 connected to the subscriber circuit 200 via the subscriber line 108 or a subscriber A discharge current flows into the subscriber circuit 200 according to the discharge time constant of the subscriber line 108. The voltage monitor 222 monitors the line voltage between the line terminals 110A and 110B of the subscriber circuit 200. The voltage monitor 222 also has a function of integrating the voltage, and stores the integrated value.
[0020]
Further, a current monitor (A) 220 is connected between the ground-side terminal 240 and the line terminal 110B of the power supply circuit 204, and this monitors the load current of the subscriber circuit 200. Although the current monitor 220 is not specifically illustrated, the current monitor 220 has a current differentiation circuit, detects and stores the differential value of the current flowing through the current monitor 220, and stores the stored differential value as will be described later. Compared with a preset threshold value, it has a function of determining its magnitude. In this embodiment, the current monitor 220 is inserted on the ground line side as described above, but may be connected to the terminal 110A on the high voltage side without doing so.
[0021]
The subscriber circuit 200 of this embodiment includes a control circuit 250, which is a switch control circuit that provides control signals 252 and 254 to the switches 202 and 210, respectively. The control signals 252 and 254 are control signals for controlling on / off of the switches 202 and 210, respectively, and the on / off time width is set to a predetermined value according to the type of a specific medium of interest to be detected in the subscriber terminal 400. Is done. This set value will be described later in connection with the description of the operation of this apparatus.
[0022]
In the present embodiment, the constant voltage circuit 230 is turned on / off by the switch 210. However, without providing such a switch circuit, for example, the control circuit 250 controls the output state of the constant voltage circuit 230 to increase it. / By making the impedance low, the switch 210 may function equivalently. In that case, the switch 210 may be omitted.
[0023]
Various media as described above may be connected to the subscriber line 108 depending on the situation. Each medium has an internal equivalent impedance that determines a charge / discharge time constant, as described in the related art. The subscriber line 108 itself has its own internal impedance, but this equivalent impedance is negligibly small compared to the medium 406 associated with the subscriber terminal 400, for example, a line length of 1 km and a time constant of 15 μS (microseconds). Second).
[0024]
The operation of the embodiment configured as described above will be described with reference to the control time chart shown in FIG. FIG. 2 illustrates the operation timing for a specific medium among various media. In FIG. 6A, a waveform SW 1 indicates the on / off state of the switch 202, and is controlled by the control signal 252 according to the setting in the control circuit 250. A waveform SW2 in FIG. 5B shows the on / off state of the switch 210, and is on / off controlled in accordance with the control signal 254 set in the control circuit 250 in advance, like the switch 202.
[0025]
The horizontal axis in the figure, that is, the periods Twd1, Twd2, and Twd3 on the time axis indicate the time when the switch 202 is on, and at other times, that is, Twc1, Twc2, Twc3, and Twc4, the switch 202 is off (open). is there. At time Twc4, the switch 210 is also turned on. In FIG. 5C, the waveform “monitor voltage” shows the voltage waveform between the line terminals 110A and 110B detected by the voltage monitor 222 when the internal equivalent capacitor C is charged / discharged with respect to the medium 406 described above. It is. In these drawings, in order to avoid complication of explanation, the transient state of the waveform is omitted in the drawing and is drawn in an ideal form. Further, the line is provided with a transient suppression circuit (not shown) such as spark extinction.
[0026]
The period Twc1 is a state at the start of the test. In the subscriber circuit 200, both the switches 202 and 210 are off, and power is supplied from the subscriber circuit 200 to the terminal 400 via the subscriber line 108. As a result, electric charges are charged in the internal capacitor C in the medium 406 of the terminal 400, and in a steady state, the voltage across the capacitor C is substantially equal to the power supply voltage VBB.
[0027]
When the control circuit 250 turns on the switch 202 at time t1, the charge of the capacitor C is discharged according to the discharge time constant of the medium 406 in the terminal 400 at that time. At time Twd1, the voltage detection output of the voltage monitor 222 is 0 volts. After the short circuit continues for time Twd1, control circuit 250 turns off switch 202 at time t2. At this time, the charge of the capacitor C of the terminal 400 is discharged, and the voltage across the capacitor C is also approximately 0 volts. When the switch 202 is turned off at time t2, charging to the medium 406 of the terminal 400 is started from the power supply circuit 204 of the subscriber circuit 200, but the rising characteristic depends on the time constant of the medium 406 as described above. different. In FIG. 2, the target medium 406 is charged again to the power supply voltage VBB at time Twc2.
[0028]
Next, when the control circuit 250 turns on the switch 202 again at time t3, the detection voltage of the voltage monitor 222 becomes 0 volts again. At this time, as in the period Twd1, the charge of the medium 406 is discharged according to the time constant of the medium.
[0029]
When the control circuit 250 turns off the switch 202 again at time t4, the medium 406 starts charging again. In this embodiment, the time Twd2 during which the switch 202 is on is set shorter than the time Twd1 described above. Therefore, if the switch 202 is turned off before the discharge from the target medium 406 is sufficiently completed, the capacitor C may have a value other than 0 due to the residual charge. In that case, the voltage value detected by the voltage monitor 222 is at a certain level. At time Twc3, the medium 406 is charged again, and the measured voltage of the monitor 202 is restored to the voltage value VBB.
[0030]
In FIG. 2, the charge integration voltage value of the target medium 406 at time Twc2 is conceptually indicated by S1, and the charge integration voltage value at time Twc3 is indicated by S2. Accordingly, as described in the conventional example, by appropriately setting the values of the periods Twd1 and Twd2, the medium 406 is either a protector with a separation function or a telephone or a modular jack with a capacity, or a subscriber. It is possible to identify whether the line 108 is broken.
[0031]
More specifically, the first short-circuit time Twd1 is set to 100 mS ≦ Twd1 ≦ 200 mS, and the second short-circuit period Twd2 is set to about several mS. Therefore, when the subscriber line 108 is disconnected, the time constant is very small, so the integrated charging voltage is S1≈S2, and other media (telephone TEL, modular jack CMJ with capacity, protector 6 with disconnect function) And disconnection can be distinguished.
[0032]
On the other hand, if the first short-circuit period Twd1 ≧ 1S is set, the difference between the integrated voltages S1 and S2 when the medium is a protector with a separation function is the difference between the medium and another telephone or a capacitive modular jack. Larger than in some cases. Therefore, it is possible to distinguish between the protector with disconnection and disconnection function of the subscriber line, the telephone and the modular jack with capacity. In this way, by closing the switch 202 for the two types of short circuit time Twd1 described above, it is possible to distinguish between a disconnection of a subscriber line, a protector with a disconnecting function, and a telephone jack or a modular jack with a capacity. It is.
[0033]
Now, the control circuit 250 turns on the switch 202 again at time t5. At this time, the current flowing through the current monitor 220 is substantially equal to a value VBB / R1 obtained by dividing the power supply voltage VBB of the power supply 204 by its internal resistance R1 in a steady state, as shown in FIG. 2 (D). In this embodiment, the period during which the switch 202 is closed, that is, the value of the time Twd3 is set to a time during which the target medium 406 is sufficiently discharged, for example, 1S or more. Therefore, after the elapse of time Twd3, control circuit 250 turns off switch 202 again at time t6 and simultaneously turns on switch 210. In this state, the capacity C of the medium 406 is charged from the power supply circuit 204 as well as the constant voltage circuit 230. In this embodiment, since the internal impedance of the constant voltage circuit 230 is suppressed to be lower than that of the power supply circuit 204, the constant voltage circuit 230 mainly dominates the rise of charging at this time.
[0034]
The current I flowing through the current monitor 220 when the switch 202 is turned off and the switch 210 is turned on is viewed from the subscriber circuit 200 toward the terminal medium 406 as shown by the waveform I1 or I2 in FIG. The exponential change according to the value of the equivalent resistance R and the equivalent capacitance C is shown.
[0035]
For example, when the medium 406 is a telephone, the resistance R = 0 to 7.5 kΩ and the capacity C = 0.25 to 3 μF, and when the medium jack with a capacity, the resistance R = 100 kΩ and the capacity C = 0.27 μF. . In addition, when the medium 406 is a protector with a separation function, when this is in the connected state, the resistance component R = 510 kΩ, the capacitance component C is about 0.68 μF, on the other hand, in the separated state by the protector control The internal resistance R is 20 kΩ and the capacitance C is about 5.4 μF. Therefore, if the medium 406 is a telephone or a modular jack with a capacity, the monitor current attenuates relatively quickly as shown by the waveform I1, and if the medium 406 is a detached protective protector, The monitor current decays relatively slowly as shown by waveform I2.
[0036]
The current monitor 220 differentiates the monitor current I and observes the differential value S. More specifically, a rising differential value S3 and a falling differential value S4 of the monitor current I are derived, and a difference S3-S4 between them is calculated. As illustrated in the waveforms I3 and I4, if the medium 406 is a telephone or a modular jack with a capacity, the difference S31-S41 between the rising differential value S31 and the falling differential value S41 of the monitor current I is large. In the case of the protector with the separation function 406 in the disconnected state, the difference S32-S42 between the rising differential value S32 and the falling differential value S42 of the monitor current I becomes small. A certain threshold value is set for the output of the differentiation circuit (not shown) of the current monitor 220, and when the differential current value difference S3-S4 exceeds this threshold value, the medium 406 is a protector with a separation function (detaching function). State), and if it is equal to or less than the threshold value, it is determined that the telephone is a modular jack with capacity.
[0037]
As described above, according to the present embodiment, by the on / off control of the switch 202, first, the disconnection of the subscriber line in the subscriber circuit system, the telephone or the modular jack with the capacity, and the protector with the separation function are identified. By distinguishing the differential value between the rise and fall of the current by detecting the current differential value by turning on / off the switch 210, the telephone or the modular jack with capacity and the protector with disconnection function (disconnect) are identified. As a result, it has become possible to identify all disconnections, disconnections, telephones, modular jacks with capacities, and disconnection protectors that have been difficult in the past.
[0038]
In the above-described embodiment, the identification is made between the telephone or the modular jack with capacity and the separated state of the protector with the separation function. In general, it can be widely applied to identification of
[0039]
Although not shown in the figure, the voltage monitor 222 and the current monitor 220 are configured to be controlled by the control circuit 250, and the control circuit 250 controls the above-described voltage integration, current differentiation and calculation of the difference between them. May be configured to perform automatically.
[0040]
【The invention's effect】
Thus, according to the present invention, first, the integrated value of the voltage between the two line terminals is detected by short-circuiting and opening the two line terminals in the subscriber circuit system. This short-circuit is performed for two periods of different lengths, and the difference between the voltage integral values is detected for each of them. Identify In addition, a constant voltage is supplied between both line terminals and opened to detect the difference in the differential value between the rise and fall of the current. This makes it possible to identify all disconnections, telephones, modular jacks with capacities, and protectors with a disconnecting function, which have been difficult in the past.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration example of a medium identification circuit of a subscriber system according to an embodiment of the present invention.
FIG. 2 is a waveform diagram illustrating a control operation of the embodiment shown in FIG.
FIG. 3 is a functional block diagram showing a configuration related to medium identification of a conventional subscriber circuit.
4 is a waveform diagram illustrating a control operation of the conventional example shown in FIG.
[Explanation of symbols]
108 subscriber lines
110A, 110B subscriber circuit line terminals
200 subscriber circuit
202, 210 switch
204 Power supply voltage supply circuit
206 Power supply VBB
220 Current monitor A
222 Voltage monitor V
230 Constant voltage circuit
240 Ground
402 Internal equivalent resistance
406 Medium

Claims (6)

In a subscriber circuit having a pair of line terminals to which a subscriber line is connected and detecting a state of a terminal device connected to the subscriber line, the circuit includes:
A first switch means connected to the pair of line terminals and short-circuiting or opening between the pair of line terminals of the subscriber line;
Constant voltage supply means for supplying a constant voltage to the subscriber line through the pair of line terminals;
Second switch means for selectively connecting the constant voltage supply means to the pair of line terminals;
Current monitoring means inserted between one of the pair of line terminals and the constant voltage supply means, monitoring a current flowing through the subscriber line, and calculating a differential value of the monitored current;
Control means for controlling the first and second switch means,
The control means controls the first and second switch means to short-circuit between the pair of line terminals, and then opens and connects the constant voltage supply means to the pair of line terminals. ,
The current monitoring means detects a difference between a differential value at a rising edge and a differential value at a falling edge after the constant voltage supply means is connected to the pair of line terminals, and based on the difference, A subscriber circuit for determining a state. The circuit of claim 1, further comprising:
Power supply means for supplying power to the subscriber line through the pair of line terminals;
Voltage monitoring means for detecting a voltage between the pair of line terminals,
The control means controls the first and second switch means to open a short circuit between the pair of line terminals for a first predetermined time, and then open the pair of line terminals again. After a short circuit for a second predetermined time shorter than the first predetermined time,
The voltage monitoring means integrates the voltage monitored in the opening after the short circuit for the first predetermined time, further integrates the monitored voltage in the opening after the short circuit for the second predetermined time, A subscriber circuit, wherein a state of the terminal device is determined by detecting a difference between values and obtaining the difference for two first predetermined times having different lengths. 2. The subscriber circuit according to claim 1, wherein the state of the terminal device to be determined is a telephone or a modular jack with a capacity and a protector with a separation function of a separation state. 3. The circuit according to claim 2, wherein the first predetermined time is between 100 milliseconds and 200 milliseconds, and 1 second or more, and the second predetermined time is several milliseconds or less, and the determination The subscriber circuit is characterized in that the state of the terminal device is a disconnection of the subscriber line, a modular jack with capacity, a telephone or a protector with a disconnection function. In a method for detecting a state of a terminal device connected to a pair of subscriber lines, the method includes:
A first step of short-circuiting between the pair of subscriber lines, then opening and supplying a constant voltage to the pair of subscriber lines;
Detecting a difference between a differential value at a rising edge and a differential value at a falling edge of the current after the constant voltage is supplied to the pair of line terminals;
And determining the state of the terminal device based on the difference. A method for detecting the state of the terminal device. The method of claim 5, wherein the method further comprises:
A step of short-circuiting between the pair of subscriber lines for a first predetermined time, and then opening,
Monitoring and integrating the voltage between the pair of subscriber lines after the opening;
Between the pair of subscriber lines, after short-circuiting again for a second predetermined time shorter than the first predetermined time, and then opening,
After the reopening, monitoring and integrating the voltage between the pair of subscriber lines;
Detecting a difference between the two integrated values,
A method of detecting a state of a terminal device, wherein the state of the terminal device is determined by executing the step for two first predetermined times having different lengths and determining a difference between the integral values.

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