JPH0155425B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for detecting faults that occur in paired cables when transmitting signals using paired cables constituting a communication line, and in particular to a circuit for detecting faults that occur in paired cables that constitute a communication line. The present invention relates to a fault detection circuit that quickly detects disconnection of a subscriber line or short circuit between lines from the line end when performing signal transmission.

The main faults that occur in subscriber lines are wire breaks, line-to-line shorts, and ground faults, and among these, wire breaks and short circuits are particularly common, and detection of these faults is most important. Therefore, the fault detection circuit must be able to reliably detect each of the above faults. In addition to the above requirements, the fault detection circuit must not have a negative effect on normal communication, the circuit must be as simple as possible, and it itself is unlikely to cause a problem. An important required characteristic is that if a fault occurs in the circuit, it can be detected from the line end.

Conventionally, a method as shown in FIG. 1 has been devised as this type of cable failure detection circuit. In the figure, 1 and 2 are paired cables for downlink communication and paired cables for uplink communication, respectively, which constitute a communication line, 3 and 4 are communication devices that perform communication using the communication cables 1 and 2, 5 is an oscillator, and 6 is a DC superimposition circuit,
7 is a relay winding; 8, 9 are relay contacts driven by the current flowing through the relay winding 7; 10, 11,
12 and 13 are line transformers, 14 is a detection circuit for detecting the signal from the oscillator 5, and 15 is a switch for operating this circuit. In this circuit, when the switch 15 is closed, a DC current flows to the relay winding 7 via the upper and lower lines (pair cable), and the relay contacts 8 and 9 close, and the output signal of the oscillator 5 is transferred to the upper and lower lines and the line transformer 10, 11 and 13 and the relay contacts 8 and 9, it is recognized that the line is normal, and when it is not detected by the detection circuit 14, it is recognized that there is a fault in the line.

However, although the above-mentioned conventional system is applicable to a four-wire communication system, it is not applicable to a two-wire communication system, and has the disadvantage that the circuit becomes complicated.

The present invention has been made in order to eliminate the above-mentioned drawbacks of the conventional technology, and an object of the present invention is to be able to apply it to a two-wire communication system, and to accurately detect the presence or absence of line faults using an extremely simple circuit configuration. The object of the present invention is to provide a new fault detection device that can detect faults.

The above object of the present invention is to connect an oscillation circuit and a low-pass filter to one end of the paired cable, connect a voltage detection circuit to the output terminal of the low-pass filter, and connect the oscillation circuit to the other end of the paired cable. This is achieved by a cable failure detection circuit characterized by connecting a two-terminal circuit that provides impedance to the output signal and the DC signal and a series circuit of diodes.

That is, in the present invention, by using a configuration in which an oscillation circuit and a low-pass filter are connected to one end of the paired cable, and a rectifier circuit that rectifies the signal from the oscillator is connected to the other end, it is possible to Due to the action of the rectifier circuit, a signal with a DC component is detected at the output of the low-pass filter, and if a line fault occurs, the rectification action of the rectifier circuit no longer appears at the measurement end, so a DC component is detected at the output of the low-pass filter. signals with components are no longer detected. Therefore, according to the present invention, there is provided a fault detection device using a two-wire communication system that can accurately detect the presence or absence of a line fault with an extremely simple circuit configuration.

Hereinafter, a preferred embodiment of the present invention will be explained in detail with reference to the drawings.

FIG. 2 is a configuration diagram showing an embodiment of the present invention. In the figure, reference number 16 is a telephone subscriber line (pair cable), 17 and 18 are communication devices that perform some kind of signal transmission using the telephone subscriber line 16, 19,
20, 21, and 22 are capacitors; 23 is a variable impedance circuit (low-pass filter) that has low impedance with respect to the oscillation frequency of the oscillator 31 and the DC signal; ), 24 is a diode, 26, 26, 27 is a capacitor, 28, 2
9 and 30 are resistors, 31 is an oscillator, 32 is a reference voltage generator, and 33 is a voltage comparator.

In FIG. 2, capacitors 19, 20, 21, 2
2 has a sufficiently low impedance in the frequency band used by the communication devices 17 and 18 to the extent that it does not adversely affect signal transmission between the communication devices 17 and 18, but in the frequency band used for fault measurement, i.e.
The capacitance value is selected so as to provide a sufficiently high impedance for the frequency generated by the oscillator 31. Further, although the variable impedance circuit 23 has a low impedance with respect to signals having the frequency of the oscillator 31 and DC signals,
It is designed to be in a high impedance state for signals in the frequency band used in 8, so as not to adversely affect the characteristics of the communication devices 17 and 18. In addition, the resistors 28 and 29 include capacitors 25, 26, 27, a resistor 30, a reference voltage generator 32, and a voltage comparator 33.
The failure detection circuit 34 consisting of the communication devices 17 and 18
It has the function of preventing the properties of the material from being affected. Furthermore, the capacitor 27 and the resistor 30 constitute a kind of low-pass filter, and have the function of detecting a DC signal generated in the subscriber line. Furthermore, the voltage comparator 33 outputs a "high" level signal when the DC voltage generated across the capacitor 27 exceeds the reference voltage generated by the reference voltage generator 32, and outputs a "low" level signal when the DC voltage does not exceed the reference voltage generated by the reference voltage generator 32. Outputs a level signal.

Next, the operation of the circuit according to the present invention will be explained separately for the case where the telephone subscriber line 16 is normal, the case where there is a disconnection fault, and the case where there is a line fault. As is clear from the above description of the functions of each part, the capacitors 19, 20, 21, 22 and the variable impedance circuit 23 have the purpose of not affecting the characteristics of the communication devices 17 and 18, and Since it is not related to the operation during line fault search, it will be omitted in the following explanation.

FIG. 3 is an equivalent circuit of the present configuration shown in FIG. 2 when the subscriber line 16 is normal. In the figure, resistors 35 and 36 are equivalent representations of the telephone subscriber line 16, and other numbers correspond to those in FIG. In FIG. 3, the oscillation signal of the oscillator 31 is transmitted through the resistors 28 and 2.
It is applied to the diode 24 via 9, 35, and 36. The diode 24 repeats the "on" state and "off" state by reversing the polarity of the output signal of the oscillator 31, and current flows in the direction of the arrow in FIG. 3 only when the diode 24 is in the "on" state. Flowing charges move. The transferred charges are accumulated in the capacitor 27, and a DC potential difference is generated across the capacitor 27. As long as diode 24 repeats "on" and "off", the potential difference across capacitor 27 increases.

When the potential difference across the capacitor 27 reaches 1/2 of the output signal amplitude (PP value) of the oscillator 31, the state equivalently shown in FIG. 4a is reached. In the figure, Vo is the output signal amplitude value of the oscillator 31 (P-P
value). In the state shown in FIG. 4a, the potentials at points A and B are equal, so even if A and B are shorted as shown by the broken line, there will be no difference in the currents and potentials flowing through each part of the circuit. That is, FIG. 4a can be further equivalently transformed as shown in FIG. 4b. It is clear that no current flows through diode 24 in the circuit of FIG. 4b.

From the above explanation, in FIG.
It can be seen that the potential across both ends increases to Vo/2.
Therefore, the reference level of the voltage comparator 33 is set to Vo/2.
By setting as follows, the output of the voltage comparator 33 becomes a "high" level regardless of the magnitude of the subscriber line resistance.

Next, the operation when a short circuit fault or a disconnection fault occurs will be explained. FIG. 5 and FIG. 6 show equivalent circuits at the time of a line fault and a disconnection fault, respectively. It is clear that in either case, since the rectification effect by the diode does not work, no direct current is generated across the capacitor 27, and the output of the voltage comparator 33 becomes a "low" level.

As can be seen from the above description, the circuit according to the present invention operates so that a "high" level output is obtained during normal operation regardless of the resistance value of the line, and a "low" level output is obtained when a subscriber line fault occurs.

Next, a method for detecting a failure in the circuit itself will be explained. A particular problem is the variable impedance circuit 2 installed at the far end of the line.
3. The diode 24 is faulty. First, regarding a failure in the variable impedance circuit 23, possible types of failure in the variable impedance circuit or low-pass filter include a failure resulting in an open state and a failure resulting in a short circuit state. In the case of an open failure, it can be detected because no abnormality occurs in normal communication and when a failure search is performed, a failure state is indicated. Furthermore, in the case of a short-circuit failure, it is thought that some kind of failure will occur in normal communication due to the influence of the diode 24, but a normal state will be shown when searching for the failure. However, in this case, even if the fault is searched for at a frequency outside the passband of the variable impedance circuit 23, it can be detected because the same output as in the normal state is obtained. However, even in this case, in the case of a failure of the diode 24, which may be a short-circuit failure or an open failure, due to the function of the variable impedance circuit 23, normal communication is not affected in either case. However, when a fault search is performed, the fault can be detected because the result is similar to that of an abnormal state. Further, even if both the variable impedance circuit 23 and the diode 24 are faulty, it can be estimated from whether normal communication is possible or not and the results of the fault search.

As is clear from the above description, according to the present invention, when performing signal transmission using a paired cable, the oscillation circuit and low-pass filter at one end of the paired cable are connected, and the oscillator is connected to the other end of the paired cable. By using a simple configuration in which a rectifier circuit for rectifying signals is connected, it is possible to easily detect disconnections and short circuits in the paired cable, as well as failures in the faulty circuit itself.

Although the present invention has been described above with reference to one preferred embodiment thereof, this is merely an example, and the present invention is not limited to the embodiment described herein, and there may be cases where it deviates from its scope. It is easy to make various modifications and changes without any trouble. For example, in this embodiment, the reference voltage generator 32 and the voltage comparator 33 are used as the fault voltage detection circuit, but instead of these, a buffer amplifier or the like is used.
Of course, it is also possible to adopt a configuration in which the buffer amplifier is controlled to be "on" or "off" by a change in the terminal voltage of the capacitor 27 caused by the occurrence of a fault.

[Brief explanation of drawings]

Fig. 1 is a block diagram of main parts of a conventional fault detection circuit, Fig. 2 is a block diagram of main parts showing an embodiment of a cable fault detection circuit according to the present invention, and Figs. FIG. 3 is a circuit diagram for explaining the operation. 1, 2, 16...pair cable (telephone subscriber line),
3, 4... Four-wire communication device, 5, 31... Oscillator, 6... DC superimposition circuit, 7... Relay winding,
8, 9...Relay contact, 10-13...Line transformer, 14...Detection circuit, 15...Switch,
17, 18...Two-wire communication device, 19-22,
25-27...Electricity capacitor, 23...Variable impedance circuit (two-terminal circuit with low impedance at low frequency), 24...Diode, 28-30,3
5, 36...Resistor, 32...Reference voltage generator,
33...Voltage comparator, 34...Fault detection circuit, 3
7, 38...DC voltage source.

Claims (1)

[Claims] 1 Connect an oscillation circuit and a low-pass filter to one end of the pair cable, connect a voltage detection circuit to the output terminal of the low-pass filter, and connect the output signal of the oscillation circuit and the DC signal to the other end of the pair cable. A cable failure detection circuit characterized by connecting a two-terminal circuit with a low impedance and a series circuit of a diode.