JPH05300235A - Signal absence detector for isdn line - Google Patents

Abstract

PURPOSE:To automatically detect the interruption of a signal due to a hardware fault such as improper contact or a broken line in a signal line 3 connecting to a DSU 2 and each terminal equipment 4 in an ISDN. CONSTITUTION:The detector is provided with R line, T line voltage detection circuits 11, 12 detecting the presence of a pulse signal on an R line 3a and a T line 3b and with two kinds of noise detection circuits 13, 14, 15 detecting various noise signals on a signal line 3. When other voltage detection circuit does not detect a voltage yet after lapse of an allowable time after one voltage detection circuit detects a voltage and the noise detection circuit does not detect any noise, it is discriminated that the signal line corresponding to the other voltage element has no signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to ISDN, and more particularly, IS for detecting a state where a pulse signal transmitted on a signal line of ISDN is interrupted due to a cable failure or the like.
The present invention relates to a signal non-detection device for a DN line.

[0002]

2. Description of the Related Art ISDN (Integrated Services Digital Network) is adopted as a communication system for transmitting a large number and multiple digital data at high speed and efficiently. This ISD
In N, an ISDN exchange installed in the office of the telephone company, for example, D installed in the company via a data line.
A large number of terminals such as a telephone, a telex, a personal computer, and a facsimile are connected to the SU (line terminating device), and the DSU (line terminating device) is connected.

The interface between the DSU (circuit terminating equipment) and each terminal is composed of a [2B + D] basic interface. The signal line that connects the DSU and each terminal is roughly divided into DSU (line terminator)
From each terminal to the R line that transmits signals from each terminal to each terminal
It is composed of a T line that transmits a signal to the SU. Signals transmitted on the R and T lines are generally AMI (Alternat).
e Mark Inversion) Encoded digital pulse signal. The R line and the T line are each composed of a pair of signal lines.

When such an ISDN is newly constructed or at the time of regular inspection and maintenance, a disconnection failure of a cable or a contact failure of a connector is found in a signal line connecting a DSU and each terminal, and it is early. It is required to repair.

[0005] The most primitive method for detecting this cable disconnection or connector contact failure is to lay a signal line for testing separately in parallel with the cable and conduct a continuity test. However, since various devices are connected to both ends of the cable, the continuity test cannot be easily performed. So I
In the normal operating state as SDN, when the pulse signal is transmitted from the DSU to each terminal via the R line, it is necessary to use the fact that the pulse signal is always sent from the terminal to the DSU via the T line, A method of detecting cable disconnection or connector contact failure based on the presence or absence of signals on the R and T lines can be adopted.

[0006]

However, the R of the signal line is
Since pulse signals are not always transmitted on the T-line and T-line, only checking for the presence of a pulse signal transmitted on the R-line and T-line indicates that there is no signal due to cable disconnection failure or connector contact failure. It cannot be distinguished simply whether the pulse signal to be transmitted is in the state where it is not transmitted from the DSU or the terminal.

There is no device that automatically detects the absence of a signal due to a disconnection or a contact failure in consideration of the relationship between the signals transmitted on the R line and the T line. Therefore, conventionally, in order to detect the presence or absence of each pulse signal on the R line and the T line, an operator observes the presence or absence of the pulse signal transmitted on each signal line by connecting an oscilloscope or a logic analyzer to each signal line. Then, it was judged from the mutual relationship that there was no signal due to the failure. Therefore, a lot of trouble and expense were required.

[0008] Generally, a signal transmitted on a signal line wired in a building has a power source noise of 50 Hz or 60 Hz, a high frequency EMI noise output from various electronic devices installed in the vicinity, and an external signal. The noise due to the radio waves radiated from the antenna of the wireless device is superposed. Such noise that is input from the power supply, ground and antenna to the signal line is called common mode noise.

On the other hand, the noise generated between the signal lines in the R line or the T line of the signal line is called normal mode noise. This noise has a waveform and a frequency approximate to the signal waveform of the transmitted pulse signal. Have.

Therefore, since the above-mentioned common mode noise and normal mode noise are superimposed on the pulse signals transmitted on the R line and the T line, when observing no signal with an oscilloscope, the pulse caused by this noise is signaled. There is a concern to judge that there is. Therefore, in order to find the normal mode noise generated between the signal lines from the pulse signal waveform displayed on the oscilloscope and to identify the noise type, a high level of knowledge about ISDN and a skilled technique for measurement are required. ..

Therefore, there is a concern that an unfamiliar worker may reduce the accuracy of the no-signal determination due to the above-mentioned hardware failure.

The present invention has been made in view of the above circumstances, and uses a predetermined logic to determine the mutual relationship between the presence / absence of a pulse signal on the R line, the presence / absence of a pulse signal on the T line, and the presence / absence of noise. Analysis, the signalless information due to the correct hardware failure without the influence of noise can be detected automatically, and the measurement work efficiency can be greatly improved. It is an object of the present invention to provide an ISDN line no signal detection device which enables an inexperienced worker who does not have the above technique to accurately grasp the no signal state.

[0013]

In order to solve the above problems, in the signal non-detection device of the ISDN line of the present invention, the AMI-encoded pulse signal transmitted on the R line constituting the ISDN signal line is detected. An R line voltage detection circuit for detecting a pulse voltage, a T line voltage detection circuit for detecting a pulse voltage of an AMI-coded pulse signal transmitted on a T line forming a signal line, and an external input generated in the signal line A common mode noise detection circuit for detecting the generated common mode noise, a normal mode noise detection circuit for detecting normal mode noise generated between the signal lines forming the R line and the T line, and an R line voltage detection circuit. An R line signal counter that measures the elapsed time after detecting the pulse voltage and a elapsed time after the T voltage detection circuit detects the pulse voltage The line signal counter and the other party time reading means for reading the elapsed time of the other party's signal counter at the time when each allowed time set individually is measured, and the other party time reading means When the elapsed time is zero and no noise is detected by each noise detection circuit, the line corresponding to the elapsed time of zero on the other side is provided with no signal determination means for determining no signal. ..

[0014]

In the signal non-detection device of the ISDN line constructed as described above, under the condition that there is no hardware failure such as disconnection or contact failure in the R line and the T line forming the signal line, the DSU transfers to the terminal. When an AMI-coded control pulse signal is transmitted via the R line,
A pulse signal for response is transmitted from the designated terminal to the DSU via the T line. Since the time difference between the control pulse signal transmitted on the R line and the response pulse signal on the T line is very small, for example, about 2 bits, the R line voltage detection circuit and the T line The elapsed time after each detection of the pulse voltage by the voltage detection circuit is the R line signal counter and T
If the line signal counters detect each of them and the elapsed time of the other party is read after the predetermined permissible time has elapsed, there is always a non-zero value.

On the other hand, when a hardware failure occurs in either the R line or the T line, the pulse signal does not exist in the line in which the failure occurs, so the above-described allowable time has elapsed. Later, when the elapsed time of the other party is read, it is zero.

When noise is present, each voltage detection circuit determines the noise as a regular pulse voltage, and when this noise occurs before the regular pulse signal, each signal counter starts counting elapsed time. .. Therefore, it is confirmed that noise is not detected by each noise detection circuit when it is finally determined that there is no signal. If noise is detected, repeat the measurement.

Therefore, the absence of a signal due to a hardware failure can be accurately grasped.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 is a schematic view showing the entire ISDN in which the signal absence detecting device of the embodiment is incorporated. For example, a plurality of DSUs (line terminators) 2 are connected to an ISDN exchange 1 installed in a telephone office of a telephone company via a data line. A large number of terminals 4 are connected to each DSU 2 via a signal line 3. The exchange of information between the DSU 2 and each terminal 4 is executed based on the basic interface of the [2B + D] configuration described above.

The ISDN exchange 1 has a digital DSU 2
In addition to the above, an existing analog line 5 and a center unit 7 including a computer for monitoring and controlling the operation of the ISDN as a whole are connected via a dedicated DSU 6.

The diagnostic device 9 incorporating the signal non-detection device 8 according to the embodiment of the present invention is connected to the signal line 3 to be tested as required. The diagnostic device 9 is also connected to the ISDN exchange 1 via a dedicated DSU 10. Therefore. The diagnostic result of the diagnostic device 9 can be directly transmitted to the center device 7 via the ISDN exchange 1 without passing through the signal line 3 to be tested. On the contrary, it is also possible to directly remotely operate the diagnostic device 9 from the center device 7.

The signal line 3 connecting the DSU 2 and each terminal 4 is, as shown in FIG. 1, an R line 3a consisting of a pair of signal lines for transmitting a signal from the DSU 2 to each terminal 4, and each terminal 4 To the DSU 2 and a T line 3b composed of a pair of signal lines. The no-signal detecting device 8 incorporated in the diagnostic device 9 is connected to the signal line 3
Is connected to the R line 3a and the T line 3b.

As shown in FIG. 1, the no-signal detecting device 8 is roughly divided into R connected to the R line 3a and detecting the pulse voltage of the AMI-coded pulse signal transmitted on the R line 3a. A line voltage detection circuit 11, which is also connected to the T line 3b, detects a pulse voltage of an AMI-coded pulse signal transmitted on the T line 3b.
2, R line normal mode noise detection circuit 13 for detecting normal mode noise generated on the R line 3a, same T line 3b
T-line normal mode noise detection circuit 14 for detecting normal mode noise generated above, common mode noise detection circuit 1 for detecting common mode noise generated on T line 3b
5, and a signal absence determination processing unit 16 configured by, for example, a microcomputer that determines the signal absence state based on the detection signals of the detection circuits 11 to 15 and displays the result on the display unit 17. ..

The R-line voltage detection circuit 11 is composed of a pair of comparators 11a and 11b as shown in the figure. And
One comparator 11a has a lower limit voltage of 0.
23 v is set as a threshold value, and the upper limit voltage 1.2 v in pulse voltage is set to the other comparator 11b. That is, when the voltage of the pulse signal on the R line 3a is lower than the lower limit voltage 0.23v, it is determined that there is no voltage, and the output signal C _RA of [0] is sent to the signal presence / absence determination processing unit 16. On the contrary, when the pulse voltage exceeds the upper limit voltage 1.2 v, it is determined that the pulse voltage is a voltage caused by noise, and the output signal C _RB of [1] is sent to the signal presence / absence determination processing unit 16. Therefore, the signal presence / absence determination processing unit 16 can determine that the regular pulse voltage exists on the R line 3a when the combination of the output signals C _RB and C _RA is [01].

The T-line voltage detection circuit 12 has the same configuration as that of the R-line voltage detection circuit 11 described above, and a pair of comparators for sending the output signals C _TA and C _TB to the signal presence / absence determination processing section 16. It is composed of 12a and 12b.

The common mode noise detection circuit 15 is constructed as shown in FIG. The signal input through the impedance matching pseudo communication circuit 15a connected between the pair of signal lines forming the T line 3b is input to the high pass filter 15b. The high pass filter 15b extracts a high frequency noise component included in the input pulse signal. The signal level of the extracted high frequency noise component is measured by the level detector 15c. The assumption circuit 15d outputs the noise detection signal N _C of [1] when the level measurement value or a predetermined threshold value is exceeded.

Further, the pulse signal inputted through the pseudo communication circuit 15a has a low frequency component removed by another high pass filter 15e, and then the pulse interval measuring circuit 15f.
Thus the pulse interval is measured. If the measured pulse interval is the interval (cycle) corresponding to the power supply frequency of 50 Hz or 60 Hz, for example, the determination circuit 15g outputs the noise detection signal N _C of [1] because the power supply noise is input.

Since the common mode noise appears commonly on the R line 3a and the T line 3b, only the noise on one of the T lines 3b is detected.

FIG. 4 is a block diagram showing the configuration of the T-line normal mode noise detection circuit 14. The pulse signal input from the T line 3b through the impedance matching transformer 14a is input to the level detection circuit 14b. The level detection circuit 14b measures the pulse voltage of the input pulse signal. Then, when the measured pulse voltage is out of the allowable level range, the determination circuit 14c determines that the input pulse is a noise pulse, and outputs the noise detection signal N _NT of [1].

The pulse signal input through the transformer 14a is input to the pulse width measuring circuit 14d. The pulse width measuring circuit 14d measures the pulse width of the input pulse signal. Then, when the measured pulse width is out of the allowable level range, the determination circuit 14e determines that the input pulse is a noise pulse and outputs the noise detection signal N _NT of [1].

The R line normal mode noise type circuit 13 is T
It has the same configuration as the line normal mode noise type circuit 14. And. When the normal mode noise described above is detected on the R line 3a, the noise detection signal N _NR of [1] is output.

The signal presence / absence determination processing unit 16 is shown in FIG.
It is configured as shown in. The bus line 18 includes a CPU 19 that executes various information processes, a ROM 20 that stores a control program, and a RAM 2 that stores various variable data.
1. Output signals C _RA , C _RB , C _TA , C _TB from the various detection circuits 11 to 15 and noise detection signals N _NR , N _NT ,
N _C and the input port 22, the processing cycle Tm (=
A timer 23 for sending a time interrupt signal to the CPU 19 every 100 ms), the display unit 17, an input / output interface 24 for exchanging information with an external device, etc. are connected.

In the RAM 21, a T-line signal counter 21a for counting the number of times a valid pulse voltage is detected on the T-line 3b, that is, an elapsed time T _T from the voltage detection time, and also an effective line on the R-line 3a. R pulse signal counter 21b that counts the number of times that various pulse voltages are detected, that is, the elapsed time T _R from the voltage detection time, a delay time T _D of 2 seconds after data detection to confirm the validity of the data.
A delay counter 21c for counting the time is formed. Further, in the RAM 21, the flag memories 21d and 21d for storing the analysis end flag E _A and the delay end flag E _D are stored.
e and an output memory 21f are formed.

Then, the CPU 19 controls the timer 23
6 and 7 when the time interrupt signal is input every 100ms
Execute the flow chart shown in.

When the flow chart is started, during the measurement period of the pulse voltage and pulse width of the input pulse signal in each of the normal mode noise detection circuits 13 and 14 (P1),
The counters 21a to 21e and the flag memory 21e are cleared. That is, this time interrupt process is not executed during the measurement period.

If it is not during the pulse voltage and pulse width measurement period (P1), it is confirmed that a monitor request is input from the diagnostic device 9 main body. Then, it is confirmed in P2 that the analysis end flag E _A is not set to 1 and that the delay end flag E _D is not set to 1 (P3).

When a normal voltage between 0.23 and 1.2 v is detected as the pulse voltage in the pulse signal of the T line 3b, it is detected that T
It is judged by the combined value [01] of the output signals _CTB and _CTA from the line voltage detection circuit 12 (P4, P5). When the presence of the pulse voltage on the T-line 3b is confirmed, each comparator 12a, 1
2b is cleared, and the elapsed time T of the T-line signal counter 21a
Add 1 to _T. When the elapsed time T _T after addition is 300, that is, the allowable time of 30 seconds has not been reached (P6),
At P7 and P8, it is determined whether or not a regular pulse voltage is present on the R line 3a based on the combined value [01] of the output signals C _RB and C _RA from the R line voltage detection circuit 11. R line 3a
When it is confirmed that the regular pulse voltage exists, the comparators 11a and 11b of the R line voltage detection circuit 11 are cleared and 1 is added to the elapsed time T _R of the R line signal counter 21b. Then, in P9, when the elapsed time T _R after the addition is 2, that is, when it does not reach the allowable time of 0.2 seconds, the current time interrupt process is ended.

If the elapsed time T _R after addition has exceeded the permissible time of 0.2 seconds at P9, the value of the elapsed time T _T of the T-line signal counter 21a of the partner T-line 3b is obtained. Read. If the elapsed time T _T is a value other than 0 (P10), the correct pulse voltage exists on the R line 3a and the T line 3b at the same time during the allowable time (0.2 seconds).
The counters 21a and 21c and the flag memories 21d,
After clearing 21e (P13), delay completion flag E
Set _D to 1.

Further, at P10, if the elapsed time T _{T of} the partner T-line signal counter 21a remains 0, the permissible time (0.2 seconds) after the pulse voltage is detected on the R-line 3a.
It is determined that the pulse voltage is not detected on the T line 3b until the time elapses. Each noise detection circuit 13.14.1 at P11
After confirming that all the noise detection signals N _NR , N _NT and N _{C from 5} are all [0], T is stored in the output memory 21f.
Information indicating no line signal is set (P12). And P13
Go to.

When the elapsed time T _T of the T-line signal counter 21a after addition reaches the allowable time of 30 seconds at P6, the process proceeds to P14 of FIG. 7 and the elapsed time T _{R of the} partner R-line signal counter 21b. Read. The elapsed time T _R is 0
If it is a value other than, the correct pulse voltage exists on the T line 3b and the R line 3a at the same time during the allowable time (30 seconds). Therefore, the counters 21b and 21c and each flag memory 21
After clearing d and 21e (P15), the delay completion flag E _D is set to 1.

The output signal C of the comparator 12a is set at P5.
_{When TA} is [0], it can be determined that the regular pulse voltage is not yet detected on the T line 3b, so the elapsed time T _T of the T line signal counter 21a is not added. When the output signal C _RA of the comparator 11a is [0] at P8, the R line 3
Since the regular pulse voltage is not yet detected at a, the elapsed time T _R of the R line signal counter 21b is not added.

[0042] Further, at P14, the elapsed time T _R of the mating of the R line signal counter 21b is remains 0 arepa, permissible time (30 seconds) from the detection of the pulse voltage to the T line 3b until passage Then, it is determined that the pulse voltage is not detected on the R line 3a. Then, at P16, each noise detection circuit 13,
After confirming that all the noise detection signals N _NR , N _NT and N _C from 14 and 15 are all [0], the output memory 21f
The R line signal absence information is set to (P17). And P
Proceed to 15.

If the delay flag E _D of the flag memory 21e is set to 1 in P3 of FIG.
In P18, the count value of the delay counter 21c is 2
If 0, that is, the delay time T _D has not reached 2 seconds, 0.1 second is added to the delay time T _D. Then, the current time interruption processing is ended.

When the delay time T _D of the delay counter 21c reaches 2 seconds at P18, each of the counters 21a, 21a
If both elapsed times T _T and T _{R of} b are 0, all counters 21a, 21b. 21c and each flag memory 21
Clear d and 21e.

At least one of the elapsed times T _T and T _R is 0
If not, the noise detection circuits 13, 14,
It is confirmed that the noise detection signals N _NR , N _NT and N _C from 15 are 0. Then, in P21, the data validity information is set in the output memory 21f. After that, all the counters 21a, 21b, 21c are cleared, and the flag memories 21d, 21e are cleared. Finally, the analysis end flake E _A is set to 1.

Then, each signal absence information set in the output memory 21f is displayed on the display unit 17.

In P11, P16 and P20,
If any one of the noise detection signals N _NR , N _NT , and N _C is [1], it can be determined that a noise pulse is mixed during the measurement. In this case, there is a concern that noise will be determined to be a normal pulse, so there is a concern that no signal information will not be output even if a hardware failure has occurred in the R line 3a and T line 3b. Therefore, in this case, each counter is cleared and the measurement is restarted from the beginning.

In the signal non-detection device having the above structure, if there is no failure in the R line 3a and T line 3b, FIG.
Pulse signal 25a from DUS2 to R line 3a as shown in
Is output, the pulse signal 25b is output from the terminal 4 to the T line 3b with a delay of about 2 bits.

Therefore, normally, P4 to P5 and P
In 7 to P8, if one detects the pulse voltage and the other also detects the pulse voltage, the counter 21
Since a and 21b are reset, no signal information is not output.

However, as shown in FIG.
When the pulse signal 25b being output to the line 3b is interrupted midway due to the convenience of the terminal 3, the DSU 2 delays the pulse signal 25a in response to this, and the delay time is within 30 seconds at maximum. Stipulated in. Therefore, assuming this state, in the embodiment apparatus,
If the pulse voltage is not detected on the R line 3a after the maximum allowable time of 30 seconds has elapsed after detecting the pulse voltage on the T line 3b, it is determined in P17 that the R line 3a has no signal. ing.

On the contrary, when the pulse signal from the terminal 4 is not immediately output after the DSU 2 has transmitted the pulse signal 25a, the maximum allowable waiting time is 0.1 second (100 ms). Therefore, even in the apparatus of the embodiment, it is not necessary to wait any longer. However, considering the margin, it is set to 0.2 seconds. That is, the counter 21b of the T line 3a is detected after a lapse of 0 or 2 seconds from the detection of the pulse voltage on the R line 3a.
If the elapsed time T _T of 0 is 0 (P10), then T
It is determined that there is no signal for the line 3b (P1
2).

Therefore, in this embodiment, if the pulse signal is not detected, it is judged that there is no signal after waiting until the limit time during which the pulse signal is not transmitted by software. Therefore, it is possible to reliably detect only a hardware failure.

Further, an R line normal mode noise detection circuit 13, a T line normal mode noise detection circuit 14 and a common mode noise detection circuit 15 are provided, and the R line 3a and the T line 3 are provided.
The noise generated in b is constantly monitored, and the R line 3a described above is monitored.
At the time when the signal absence information for the T line 3b and the signal absence is detected, only when there is no noise, it is determined that there is no signal. Therefore, it is possible to prevent an erroneous determination that the no-signal information is not output due to noise even if the cable is broken or the connector is not properly connected, and the reliability of the entire apparatus can be improved.

As described above, since the no-signal state caused by a hardware failure is automatically detected and displayed, even an inexperienced worker can easily and reliably perform the signal line 3 operation.
It is possible to grasp a hardware failure such as disconnection or contact failure in the.

[0055]

As described above, according to the signal absence detecting apparatus of the present invention, the mutual relationship between the presence / absence state of the pulse signal on the R line, the presence / absence state of the pulse signal on the T line, and the presence / absence state of noise is determined by a predetermined logic. Is used to determine the no signal state on the R line and the T line. Therefore, the signalless information due to the correct hardware failure without the influence of noise is automatically output, and the measurement work efficiency can be significantly improved, and even if the ISDN has a high level of knowledge and a skilled technique for measurement. Even an inexperienced worker who does not do so can accurately grasp the no signal state.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an ISDN line non-signal detector according to an embodiment of the present invention;

FIG. 2 is a schematic diagram showing an overall ISDN configuration in which the apparatus of the embodiment is incorporated,

FIG. 3 is a block diagram showing a common mode noise detection circuit of the device of the embodiment.

FIG. 4 is a block diagram showing a normal mode noise detection circuit of the apparatus of the embodiment.

FIG. 5 is a block diagram showing a signal presence / absence determination processing unit of the apparatus of the embodiment.

FIG. 6 is a flow chart showing the operation of the apparatus of the embodiment,

FIG. 7 is a flowchart showing the operation of the apparatus of the embodiment,

FIG. 8 is a diagram showing a mutual relationship of signals in a general T line and R line,

FIG. 9 is a diagram showing a mutual relationship of signals in a T line and an R line when a software abnormality occurs.

FIG. 10 is a diagram showing a mutual relationship between signals on a T line and an R line when a software abnormality occurs.

[Explanation of symbols]

1 ... ISDN exchange, 2 ... DSU (line terminating equipment), 3
... Signal line, 3a ... R line, 3b ... T line, 4 ... Terminal, 8 ...
No signal detection device, 9 ... Diagnostic device, 11 ... R line voltage detection circuit, 12 ... T line voltage detection circuit, 13 ... R line normal mode noise detection circuit, 14 ... T line normal mode noise detection circuit, 15 ... Common mode Noise detection circuit, 16 ... Signal presence / absence determination processing unit, 17 ... Display unit, 21a ... T line signal counter, 21b ... R line signal counter.

Front page continuation (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04M 11/00 302 8627-5K H04Q 11/04 9076-5K H04Q 11/04 L

Claims (1)

[Claims] 1. An R-line voltage detection circuit (11) for detecting a pulse voltage of an AMI-encoded pulse signal transmitted on an R line forming an ISDN signal line, and a T line forming the signal line. A T-line voltage detection circuit (12) for detecting the pulse voltage of the transmitted AMI-encoded pulse signal, and a common mode noise detection circuit (15) for detecting the externally input common mode noise generated in the signal line. And the R
And a normal mode noise detecting circuit (13, 14) for detecting normal mode noise generated between the respective signal lines constituting the T line and the T line, and an elapsed time after the R line voltage detecting circuit detects a pulse voltage. R line signal counter that keeps time
(21b), a T-line signal counter (21a) for measuring the elapsed time after the T-voltage detection circuit detects the pulse voltage,
The other party time reading means (P10, P14) for reading the elapsed time of the other party's signal counters at the time when each of the signal counters has independently counted the respective allowable times, and this other party time reading means When the elapsed time read is zero, and when each of the noise detection circuits has not detected noise, the signal absence determination means (P11, P12, P16, P17) and
SDN line no signal detector.