JPH1155426A - Line interface circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To deal with the special service using polarity inversion operation before forming a DC loop by permitting a feeble current to flow so as to detect the polarity inversion of a line by means of detecting elements which are connected to the line in parallel. SOLUTION: When a forward current is made to flow in the detecting element D4 and a reverse voltage is impressed on the detecting element D5, the outputs of the detecting elements D4 and D5 becomes 0 and 1. Then, a polarity is inverted by a line exchange network and the outputs of the elements D4 and D5 are changed-over from 0 and 1 into 1 and 0. CPU detects it, forms the DC loop by adopting switches SW1 and SW2 as the ones at a (b) side and executes first response. The line exchange network detects the first response of a callee terminal so as to report a call originator number, the switches SW1 and SW2 are connected to a (c) side so as to open the DC loopa after detecting the number by the callee terminal and first response is released. When the line exchange network detects it, an incoming call signal is transmitted, the callee terminal detects this so as to execute hook-up and the switches SW1 and SW2 are made to be at the (b) side so as to form the DC loop again.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a line interface circuit provided between a telephone or a facsimile machine and a line and for performing signal processing between the telephone and a facsimile apparatus and a switching connector.

[0002]

2. Description of the Related Art Telephones and facsimile machines connected to a communication line such as a public network can communicate between transmitting and receiving terminals through a predetermined calling process and a receiving process. A predetermined control signal is transmitted and received during such calling processing and incoming call processing.
A line interface circuit is provided for receiving these signals from the exchange and sending them to a telephone or facsimile machine. Therefore, the line interface circuit has various functions such as not only signal transmission / reception for communication but also incoming call detection and line polarity reversal detection.

[0003]

However, the conventional line interface circuit as described above has the following problems to be solved. One of the functions of the line interface circuit is a DC loop forming function. When the calling terminal sends the telephone number of the called terminal, the switching network sends an incoming signal to the called terminal in response to the calling request. The called terminal responds to the switching network by detecting this incoming signal and forming a DC loop. The switching network inverts the polarity of the line connected to the calling terminal in response to the response of the called terminal. The calling terminal can know that the line has been captured by detecting this polarity inversion. As described above, the DC loop forming operation of the called terminal is one of the important procedures until the connection between the terminals is completed.

On the other hand, a caller ID notification service for transmitting a caller ID to a receiver has been started in order to improve user services. When performing this service, the polarity of the line is once inverted before the DC loop is formed. In this case, the circuit for detecting the polarity inversion provided in the conventional line interface circuit cannot be operated as it is. If these are operated, a DC loop is formed, and the switching network may mistakenly recognize it as an off-hook operation. Therefore, a new circuit for detecting polarity inversion without forming a DC loop is required.

Further, the line interface circuit has a function of making the exchange recognize the terminal line capture by forming a DC loop in this way. However, in line standards in Europe and the like, there is a demand for having a function of detecting whether a predetermined DC voltage is supplied to a line before capturing the line. In order to satisfy such requirements, it is necessary to detect a DC voltage supplied to a line without forming a DC loop.

[0006]

The present invention employs the following structure to solve the above problems. <Configuration 1> A current limiter having a function of forming a DC loop when an incoming signal is detected at a called terminal, and allowing only a line current sufficiently smaller than the current flowing through the DC loop. A line interface circuit comprising: a resistor; and a detection element connected in parallel to the line via the current limiting resistor, and receiving the line current and detecting the polarity thereof.

<Structure 2> When a called terminal detects an incoming signal, it has a function of forming a DC loop,
A pair of detection elements that are connected in parallel to the line with opposite polarities, receive the line current and detect the polarity, and a switch inserted in a circuit that connects each of these detection elements to the line, Of the pair of detection elements, the detection element of the polarity that accepts the line current in the forward direction is disconnected from the line by turning off the switch, and the other detection element is connected in parallel to the line, and the polarity of the line is changed. A line interface circuit for monitoring reversal and detecting polarity reversal when a current flows in the forward direction to the other detection element due to reversal of the polarity of the line.

<Structure 3> In the circuit described in Structure 2,
In a state where the other detection element is connected in parallel to the line, the polarity inversion of the line is monitored. A line interface circuit forming a DC loop passing through a detection element.

<Structure 4> A function of forming a DC loop when a called terminal detects an incoming signal,
A current limiting resistor that allows only a line current sufficiently smaller than the current flowing in the DC loop, and is connected in parallel to the line via the current limiting resistor, receives the line current and is supplied by the line A detection element for detecting the presence or absence of a predetermined voltage to be supplied, and a signal control unit for performing control for issuing an alarm when the detection element detects that a predetermined voltage is not supplied to the line. A line interface circuit.

<Structure 5> In the circuit described in Structure 4,
A line interface circuit characterized in that the detecting element comprises a bidirectional photocoupler for detecting the presence of an input current in any direction when the direction of the input current is equal to or greater than a predetermined value.

<Structure 6> In the circuit described in Structure 5,
A line interface circuit comprising a switch for connecting a line and a detection element only when a line voltage is detected.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below using specific examples. <First Embodiment> FIG. 1 is a connection diagram of a line interface circuit according to a first embodiment. In the figure, lines L1 and L2
Is connected to a modem circuit or the like of a facsimile apparatus (not shown) via a transformer TR1. Note that L1, L2
Denotes a pair of cables constituting one line, but these are hereinafter referred to as lines L1 and L2.
The lines L1 and L2 are branched and connected to external telephone circuits T1 and T2 by switches SW1 and SW2 operated by relays. The resistor R1, the capacitor C1, the detection element D1, and the pull-up resistor R2 constitute a circuit for detecting an on-hook of an external telephone (not shown) and sending an incoming detection bit to the facsimile apparatus.

A capacitor C inserted in series with the line L1
Detecting elements D2 and D3 are connected in parallel with opposite polarities to both ends of 2, respectively. Each of the detecting elements D2 and D3 is a photocoupler in which a light emitting diode that emits light by an input current and a light receiving element that detects the light are combined. Therefore, when a predetermined voltage is applied to the input side and a forward current flows through the light emitting diode, each of the detection elements D2 and D3
The output side conducts and the switch is turned on. The pull-up resistors R4 and R5 raise the output sides of the respective detection elements D2 and D3 to a state of "1" at a logic level. Therefore,
In the non-input state, the output is held at the level of "1", and this is input to the CPU input port of the facsimile apparatus.

On the other hand, when a forward current flows to the input side, the output side becomes conductive, and the output level becomes 0 level. Thereby, the CPU can detect the application of the predetermined voltage by the detection element D2 or D3. Since the detection elements D2 and D3 are connected in parallel so that the polarities of the light emitting diodes on the input side are opposite, if a forward current flows in one, a reverse voltage is always applied to the other, and the other does not operate. This makes it possible to determine the polarity of the voltage applied to both ends of the capacitor C2.

The detecting elements D4 and D5 are connected to the detecting elements D2 and D5.
3, and the output side thereof has pull-up resistors R6, R7 as well as the detection elements D2 and D3.
Is connected. The terminals of the detection elements D4 and D5 are directly connected in parallel to the lines L1 and L2, respectively. Further, a line current limiting resistor R3 is inserted in series in this circuit.

Normally, the lines L1 and L2 are constantly supplied with a DC voltage of, for example, 48 volts from an exchange. Since the detection elements D4 and D5 are connected in parallel to the line L1, a direct current always flows through them. If this current is not sufficiently small as compared with the case where a DC loop is formed by an external telephone or the switches SW1 and SW2, it causes a malfunction. Therefore, the resistor R3 is selected to have a high resistance of, for example, 1 M ohm or more.

Since the detecting elements D4 and D5 are connected in parallel to the lines L1 and L2 with opposite polarities, whether one of the detecting elements D4 and D5 is always turned on determines whether a predetermined DC voltage is applied to the line, The polarity of the voltage can be detected. For these purposes, detection elements D4, D5
Use a high CTR product that operates with a very small current. The above is the configuration of the line interface circuit according to the first embodiment of the present invention.
3 and a circuit including the detection elements D4 and D5 are newly provided.

FIG. 2 is an explanatory diagram of a caller ID notification service procedure. According to the line interface circuit as described above, it is possible to detect and respond to the polarity inversion before the DC loop is formed in the caller ID notification service as shown in FIG. This figure shows the contents of communication between the calling terminal 1, the circuit switching network 2, and the called terminal 3. The vertical axis indicates the passage of time. The line interface circuit as described above is incorporated in both the calling terminal 1 and the called terminal 3.

First, in step S1, the calling terminal 1 forms a DC loop by hooking up or the like.
Next, in step S2, when the circuit switching network 2 detects a DC loop, a dial tone is transmitted to the calling terminal 1. When the calling terminal 1 detects this dial tone, it sends a call by transmitting the telephone number of the called terminal (step S3). In response to the call request, the circuit switching network 2 inverts the polarity of the line connected to the called terminal 3 (step S4).

At this time, the detecting elements D4 and D5 shown in FIG.
Changes the output signal and notifies the CPU. That is,
For example, a forward current initially flows through the detection element D4, and the detection element D5
, The output of the detection element D4 is "0" and the output of the detection element D5 is "1". Here, when the polarity is inverted by the circuit switching network, the output of the detection element D4 switches from “0” to “1”, and the output of the detection element D5 switches from “1” to “0”. The CPU detects the polarity inversion by monitoring this output, forms a DC loop by connecting the switches SW1 and SW2 to the b side, and performs a primary response (step S5 in FIG. 2).

Before the DC loop is formed, no DC voltage is applied to both ends of the capacitor C2. Therefore,
Each of the detection elements D2 and D3 is in a state where no signal is input, that is, both outputs are in a state of “1”. For this reason, the polarity inversion detection circuit conventionally provided is
At this stage it is not working at all.

In FIG. 2, the circuit switching network 2 executes step S
5, when the primary response by the called terminal 3 is detected, the calling number is notified to the called terminal 3 (step S5).
6). After detecting the caller number, the called terminal 3 releases the primary response by opening the DC loop by connecting the switches SW1 and SW2 to the c side (step S).
7). When detecting the release of the primary response, the circuit switching network 2 sends an incoming signal (step S8). Upon detecting this incoming signal, the called terminal 3 connects an external telephone by hooking up, or switches SW1 and SW2.
Is connected to the b side to form a DC loop again.

Here, the circuit switching network 2 inverts the polarity of the calling terminal 1. A line interface circuit similar to that shown in FIG. 1 is also connected to the calling terminal 1. Then, the detection elements D2 and D3 operate to detect the polarity inversion. As a result, in step S11, the line capture between the calling terminal 1 and the called terminal 3 ends, and communication between the terminals becomes possible.

<Effect of Specific Example 1> According to the above configuration,
Before forming the line loop, a minute current can be passed through the current limiting resistor using the detecting element connected in parallel to the line to detect the polarity reversal of the line. It is possible to respond to a special service using the polarity inversion operation.

FIG. 3 shows a connection diagram of a line interface circuit according to a second embodiment. This circuit is exactly the same as the circuit of the first embodiment in its basic parts. This circuit includes the detection elements D4 and D5 in addition to the circuit of the first embodiment.
Switches SW4 and SW for turning on and off the circuit current
5 is inserted.

In the case of the circuit shown in the specific example 1, the detecting element D
4 and D5 are always connected in parallel to the lines L1 and L2, and one of them detects the line current and always operates to notify the CPU of the state. Therefore, a detection element that operates by a small current is required. However, such high-sensitivity elements are relatively expensive and are not suitable for terminal devices that require the use of less expensive circuit elements. Therefore, in this specific example, the detection elements D4 and D5 are connected to the detection elements D2 and D3.
The device was devised so that it could be used with general-purpose products of the same level.

That is, first, when a facsimile apparatus or the like is connected to the line and the power is turned on, the polarity of the line is detected using the detecting elements D4 and D5. That is, in this case, when the switches SW4 and SW5 are closed, the detection elements D4 and D5 are closed.
5 are connected to the lines L1 and L2. As a result, the output of one of the detection elements becomes “1” and the other output becomes “0”, so that the polarity can be detected. When the polarity is detected in this manner, the switch SW4 on the input side of the detection element through which the forward current flows, for example, the detection element D4 is turned off.
In this case, the detection element D4 where a forward current has been flowing
No current flows through. Further, since the detection element D5 has the opposite polarity, a small amount of current flows even if the switch SW5 is turned on.

In this state, monitoring of the lines L1 and L2 is continued.
When voltages of opposite polarities are applied to the lines L1 and L2, the detection element D5 detects a forward current. CPU is the detection element D
5 only needs to be monitored, this output is "1".
It is determined that the polarity has been inverted when switching from "0" to "0". At the same time as the polarity inversion, the detection element D5 is connected to the
Since L1 and L2 are short-circuited, a DC loop is formed in the line.

Due to the above operation, the resistor R8 may have a relatively lower resistance value than the current limiting resistor R3 shown in FIG. 1, and if sufficient measures for circuit protection are taken, this resistor R8 may be used. The resistor R8 may be omitted.

The polarity inversion detecting operation according to the second embodiment will be described with reference to FIG. First, in step S1, when a facsimile apparatus or the like is connected to a line,
Open the switches SW4 and SW5 using a relay, set the input voltages of the detection elements D4 and D5 to "0", and confirm that the polarity reversal detection port is initialized. Next, in step S2, the CPU closes the switch SW4 by the relay, and determines whether a forward voltage is applied to the detection element D4. If a forward voltage is applied, the output of the detection element D4 becomes "0".
If this state is expressed that the loop detection port is turned on, the process proceeds to step S4 when the loop detection port c is turned on in step S3.

Here, the CPU memorizes the polarities of the lines L1 and L2, closes the switch SW5 by a relay, and shifts to a state of monitoring the polarity reversal. Further, the switch SW4 is opened by the relay (step S6). on the other hand,
If it is not detected in step S3 that the loop detection port c is turned on, it is determined that a reverse voltage is applied to the detection element D4.
At 7, the switch SW4 is opened using the relay,
At step S8, the switch SW5 is
Close. At this time, if the loop detection port d of the detection element D5 has not been turned on, it is determined that the line is not connected, so the process proceeds to step S22, and a warning display is performed.

If the loop detection port d is turned on, the process proceeds to step S10, where the polarity is stored, and the switch SW4 is closed using a relay (step S11). Further, in step S12, the switch SW5 is opened using the relay. Thus, the operation proceeds to the line polarity reversal detection operation by the detection element D4. Steps S1 to S1
The polarity reversal detection preprocessing of 2 is only performed when the power is turned on.
It may be executed to check the connection.

When the polarity reversal detection processing is started from step S14, it is continuously monitored in step S15 whether the loop detection port c or d is on. If the switch SW4 is turned on, the detecting element D
4. If the switch SW5 is turned on, only the port of the detection element D5 is monitored. If any of the ports are turned on, the process proceeds to step S16, where a primary response guard timer is set. That is, a DC loop is formed by the operation of the detection element D4 or D5 simultaneously with the polarity inversion detection. The time until the caller ID is notified after the DC loop is formed does not exceed a predetermined time. Accordingly, this prevents erroneous detection of polarity inversion.

In step S17, the switches SW1 and SW2 shown in FIG. 3 are connected to the b side, and the lines L1 and L2
And the primary side of the transformer TR1. by this,
Enables detection of caller ID notification signal by modem.
In step S17, by opening the switch that is closed among the switches SW4 or SW5, no current flows through the detection elements D4 and D5 during detection of the caller ID notification signal and during subsequent communication. To do. If there is a notification of the caller number within the monitoring time by the timer, the process proceeds from step S18 to step S19. After detecting this, the process proceeds to step S20, and the switches SW1 and SW2 are opened by the relay. Thus, the operation shifts to a standby operation for detecting an incoming signal from the switching network. After the termination of the communication after the detection of the incoming signal, the switch SW4 or SW5 is turned on, and the process proceeds to the polarity inversion detection processing from step S14.

After the termination of the communication after the detection of the incoming signal, the switch SW4 or SW5 is turned on, and the flow proceeds to the polarity inversion detection processing from step S14. That is, step S1
If it is detected in step S5 that the loop detection port c has been turned on, the switch SW4 is opened in step S17, the switch SW5 is turned on after the detection of the calling number notification signal, the detection of the incoming signal, and the end of communication, and the processing proceeds to step S15. Shift to monitoring of loop detection port d. If it is detected in step S15 that the loop detection port d is on, the switch SW5 is opened in step S17, and the switch SW4 is turned on after the detection of the calling number notification signal, the detection of the incoming signal, and the end of the communication. The process shifts to monitoring of the loop detection port c in step S15.

On the other hand, if there is no notification of the caller number even after the monitoring time by the timer has elapsed, the process proceeds from step S18 to step S21, where the switches SW1 and SW1 are connected by the relay.
At the same time that SW2 is released, the process proceeds to polarity reversal error processing.
That is, it is determined that the polarity inversion has been detected by mistake, and SW4
Alternatively, after the switch SW5 is turned on, the process proceeds to the polarity inversion detection processing from step S14.

<Effect of Specific Example 2> Specific Example 2 described above.
According to the method described above, by providing the switch SW4 or SW5, only the detecting element having the opposite polarity is connected to the line when the line is being monitored. Performance can be improved. Further, since it is not necessary to use a detection element having a very high sensitivity, the cost can be reduced. Furthermore, if a forward current flows through the detection element being monitored due to the polarity inversion and a DC loop is automatically formed, control for forming the DC loop can be simplified.

<Embodiment 3> Each of the above embodiments has a function of detecting polarity reversal before forming a DC loop when capturing a line. On the other hand, in some European line standards, it is required to detect whether a DC voltage is supplied before capturing a line when a ring signal arrives from the line. In this case, the circuit of the above specific example can sufficiently cope with this, but it is not always necessary to detect even the polarity inversion. That is, it is only necessary to monitor whether a voltage is applied to the line. Thus, the function can be realized by a circuit as in this specific example.

In this case, if a line current similar to that at the time of forming the DC loop flows to detect the line voltage, it is regarded that hookup has been performed on the switching network side, and the switching network detects the ring signal. Might be stopped. Therefore, a circuit configuration for limiting such a current is employed.

FIG. 5 shows a connection diagram of the line interface circuit according to the third embodiment. In the figure, lines L1 and L2
, An external telephone 20 and the facsimile communication circuit 6 are connected in parallel via the switch CML1. By switching the switch CML1, one of a telephone call and a facsimile communication is selected. The switch CML1 and the facsimile communication circuit 6
And a DC holding circuit 5 is connected in parallel. Note that a switch DP3 is inserted in this circuit.

On the other hand, on the line side of the switch CML1, a ring signal detection circuit 4 is connected in parallel via a capacitor CC1. The lines L1 and L2 are connected to a detection element D6 composed of a bidirectional high-conductivity photocoupler.
Are connected to the switch DCDET2 via the resistor RD1. RD1 plays a role similar to that of the current limiting resistor R3 shown in FIG. 1 for limiting the current flowing to the detection element D6. A resistor RD2 that supplies a voltage corresponding to the output current to the comparator 8 is connected to the output side of the detection element D6. The output of the comparator 8 is input to a signal control unit 7 provided with a CPU and the like. The output of the ring signal detection circuit 4 is also input to the signal control unit 7. The signal control unit 7
Controls the facsimile communication circuit 6 and the buzzer 10 and further controls the relay drive unit 9. The relay drive section 9 is for controlling the switch CML, DCDET or DP.

FIG. 6 is a flowchart showing the operation of the circuit according to the third embodiment. First, in step S1, when a ring signal is input to the circuit shown in FIG. 5, this ring signal is detected by the ring signal detection circuit 4 through the capacitor CC1 because it is an alternating current (step S2). The ring signal detection circuit 4 notifies the signal control unit 7 that a ring signal has been detected, and upon receiving the notification, the signal control unit 7 monitors the output (step S3).

When the signal control unit 7 detects that the output of the ring signal detection circuit 4 has disappeared (step S).
4) The process proceeds to step S5 to control the relay drive unit 9. Then, an instruction is given to turn on the switch DCDET2. The reason for providing this switch DCDET2 is that
This is to prevent the detection of the ring signal by the detection element D6 and erroneous detection of the DC supply. Switch DCDET
2 is turned on, a DC loop is formed on the line by the resistor RD1 and the input side of the detection element D6 (step S).
6).

Normally, when a DC loop is formed, the exchange considers that the terminal has hooked up.
The switch is not considered a hook-up because its direct current is set sufficiently low by D1. Note that a high-sensitivity one is used so that the detection element D6 is turned on even with such a current value. When a predetermined voltage is applied to the line, a current corresponding to the input side of the detection element D6 flows, and the output side is turned on. Therefore, a predetermined detection voltage is formed at both ends of the resistor RD2.

The comparator 8 monitors the input voltage in advance by using a threshold value corresponding to an intermediate point between the case where the detection element D6 is on and the case where the detection element D6 is off. To level. If it is lower than this, the output is set to low level. Thus, a signal of, for example, a TTL level is supplied to the signal control unit 7, and the signal control unit 7 detects that a predetermined voltage is supplied to the line (step S7).

If it is determined in step S8 that the output of the comparator is at the high level, the operation shifts to a normal receiving operation. If it is determined in step S8 that the predetermined voltage is not supplied from the line, the buzzer 10 sounds and a warning is given to the user. In this case, the user checks, for example, the connection of the modular cable for line connection of the facsimile or determines whether the facsimile is operating normally.

<Effect of Specific Example 3> According to the above configuration,
It is possible to determine whether or not a predetermined voltage is supplied to the line before the DC loop is formed, and to give a function of notifying the result to the line interface circuit.

<Embodiment 4> FIG. 7 is a connection diagram of a line interface circuit according to Embodiment 4. This circuit is different from the line interface circuit according to the third embodiment in that a mechanism for displaying whether or not a predetermined voltage is supplied to the line is provided. That is, the signal control unit 7 of FIG. 7 includes an LCD control unit 12, a memory 13, and an LCD display unit 11. The memory 13 has an LCD
(Liquid crystal display device) Data such as characters to be displayed on the display unit 11 is stored. Further, the LCD control unit 12 reads out the data stored in the memory 13 and
1 is a part for performing control to display the information on the screen. Since these functions and circuit configurations are exactly the same as those conventionally used for various display devices, the description will be omitted.

FIG. 8 is a flowchart showing the operation of the circuit according to the fourth embodiment. Using this flowchart, FIG.
The operation of the circuit of FIG. The operations from step S1 to step S8 in FIG. 8 are exactly the same as those in the apparatus in FIG. Therefore, the description up to this point is omitted, and the description after step S9 will be performed. First, when the output of the comparator is determined to be normal, that is, high level in step S8, it is determined that a predetermined DC voltage is supplied to the line, and the operation proceeds to the receiving operation.

On the other hand, if it is determined that the DC voltage is not supplied, the process proceeds to step S9, and the signal control unit 7 shown in FIG. The signal of is output. In step S10, the LCD controller 12 reads message data stored in advance from the memory 13. This message data is assumed to include a document such as "Please check the connection of the line".

In step S 11, LC
An error message is displayed on the D display unit 11. Further, the signal control unit 7 drives the buzzer 10 to sound the buzzer. This requires reading the warning display for the owner.

<Effect of Specific Example 4> With the above-described method, the owner can be notified of the content of the error, and promptly can take measures such as line connection.

[Brief description of the drawings]

FIG. 1 is a connection diagram of a line interface circuit according to a specific example 1.

FIG. 2 is a sequence chart showing a caller ID notification service procedure.

FIG. 3 is a connection diagram of a line interface circuit according to a specific example 2.

FIG. 4 is a flowchart of a polarity inversion detection operation.

FIG. 5 is a connection diagram of a line interface circuit according to Example 3;

FIG. 6 is an operation flowchart of a circuit according to a specific example 3.

FIG. 7 is a connection diagram of a line interface circuit according to a specific example 4.

FIG. 8 is an operation flowchart of a circuit according to a specific example 4.

[Explanation of symbols]

 C1, C2 Capacitor D2 to D5 Detection element L1, L2 Line R1, R2 Resistor R3 Current limiting resistor R4 to R7 Resistor SW1 to SW3 Switch TR1 Transformer

Claims (6)

[Claims] 1. A current having a function of forming a DC loop when an incoming signal is detected at a called terminal, and allowing only a line current sufficiently smaller than a current flowing through the DC loop. A limiting resistor, connected in parallel to the line via the current limiting resistor,
A detection element for receiving the line current and detecting its polarity. 2. A terminal having a function of forming a DC loop when an incoming signal is detected at a called terminal, wherein the DC loop is connected in parallel to the lines with opposite polarities, and the line current is received and the polarity thereof is changed. A pair of detection elements for detecting, and a switch inserted in a circuit connecting each of these detection elements to the line, of the pair of detection elements, a detection element of a polarity for receiving the line current in a forward direction. Is disconnected from the line by turning off the switch, and in a state where the other detecting element is connected in parallel to the line, the polarity inversion of the line is monitored. A line interface circuit for detecting polarity inversion when a current flows through the line interface circuit. 3. The circuit according to claim 2, wherein inversion of the polarity of the line is monitored in a state where the other detection element is connected in parallel to the line, and the polarity of the line is inverted to forward the other detection element. A line interface circuit for forming a direct current loop through the other detection element when a current flows through the line. 4. A current having a function of forming a DC loop when an incoming signal is detected at a called terminal, and allowing only a line current sufficiently smaller than a current flowing through the DC loop. A limiting resistor, connected in parallel to the line via the current limiting resistor,
A detection element that receives the line current and detects the presence or absence of a predetermined voltage supplied by the line; and, when the detection element detects that a predetermined voltage is not supplied to the line, issues an alarm. And a signal control unit for performing control of the line interface circuit. 5. The circuit according to claim 4, wherein the detection element comprises a bidirectional photocoupler for detecting the presence of an input current in any direction when the input current is equal to or greater than a predetermined value. Line interface circuit. 6. The circuit according to claim 5, further comprising a switch for connecting the line and the detecting element only when the line voltage is detected.