JP2002290502A - Network system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To let the other party know without fail that he has been disconnected from a network system, when the power of the network system is cut off. SOLUTION: A control section 10 driven by a first power supply VDD1 outputs a signal S1, which is then converted into a level, and then is supplied as a signal 22 to an intermediate potential supply section 20 driven by a second power supply VDD2. In the intermediate potential supply section 20, a switch 22 receives a reset signal RST as a switching signal SW, and when the power is cut off, it outputs a ground potential instead of the signal S2 to a driver 23. Consequently, an intermediate potential SS, supplied to a second cable L2, is forcibly set to the ground potential.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to, for example, IEEE 1
The present invention relates to a network device for recognizing mutual connection by maintaining a cable at a predetermined intermediate potential, such as that used for an interface of the 394 standard, and particularly to a technique for compensating operation when power is cut off.

[0002]

2. Description of the Related Art A conventional network device recognizes an intermediate potential level supplied by a partner from a first cable, supplies an intermediate potential to a second cable, and allows the partner to recognize the level. A configuration for recognizing mutual connection is known (for example, Japanese Patent Application Laid-Open No.
161444). In this network device, when the supply of the intermediate potential of the other party is stopped while being connected to another network device, the connection is disconnected, and when the power supply of the other side is restarted, the connection is disconnected. reacknowledge.

[0003]

SUMMARY OF THE INVENTION However, from the study of the present inventors, it has been found that the conventional network device has the following problems.

A plurality of power supplies are provided in a network device, and supply an intermediate potential generated by another power supply in accordance with a control signal generated by one power supply. In this case, when the power of the entire device is cut off,
Depending on how the potential of each power supply drops, the control signal may become unstable even though the intermediate potential supply unit is still operating normally.

At this time, a current flows into the power supply of the intermediate potential supply unit via a cable to which the intermediate potential is supplied from the other party, and the power is supplied to the driver for supplying the intermediate potential from the power supply. . As a result, there occurs a state in which the level of the intermediate potential, which should be the ground potential, cannot be completely reduced by replenishment of power from the partner.

At this time, the network device on the other end is
Because the intermediate potential level of the cable is higher than the predetermined potential,
The connection is erroneously recognized as being continued. In other words, the connection state remains recognized even though the power supply of the network device is cut off.

[0007] Then, the network device on the other end determines that it is still active and tries to access the network device whose power is turned off, but cannot actually access it. For this reason, there has been a problem that the network is in an abnormal state.

In view of the above problems, an object of the present invention is to make a network device surely recognize that a connection has been cut off when power is cut off in a network device.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, a solution taken by the invention of claim 1 is to receive data via a first cable and to receive data via a second cable. A control unit that is operated by a first power supply and outputs a first signal, and that is operated by a second power supply and recognizes an intermediate potential level of the first cable and transmits the second signal to the second cable. An intermediate potential supply unit for supplying an intermediate potential to the cable, and a level conversion of the first signal in accordance with the voltage levels of the first and second power supplies, to the intermediate potential supply unit as a second signal. A level conversion unit for outputting the second signal and a ground potential, and a switch for selectively outputting one of the second signal and the ground potential according to a switch signal; In accordance with the output of the pitch is that a driver for generating an intermediate potential supplied to the second cable.

According to the first aspect of the present invention, when the switch is controlled to selectively output the ground potential by the switch signal, the ground potential is output from the driver, so that the intermediate potential supplied to the second cable is , Is forcibly set to the ground potential. For this reason, for example, by setting the switch signal so that the switch selectively outputs the ground potential when the power supply to the network device is cut off, the other network device can reliably recognize that the connection has been disconnected. Can be done.

A second aspect of the present invention provides a network device that receives data via a first cable and transmits data via a second cable, and operates on a first power supply. A control unit that outputs a first signal, an intermediate potential supply unit that is operated by a second power supply, recognizes an intermediate potential level of the first cable, and supplies an intermediate potential to the second cable. A level converter that converts the level of the first signal according to the voltage levels of the first and second power supplies and outputs the second signal as a second signal to the intermediate potential supply unit. Is, in response to the second signal,
A driver for generating a potential serving as an intermediate potential to be supplied to the second cable, a potential generated by the driver and a ground potential being input, and one of the potentials is switched to the second cable according to a switch signal. And a switch for selectively outputting as an intermediate potential to be supplied to the switch.

According to the second aspect of the present invention, when the switch is controlled to selectively output the ground potential by the switch signal, the intermediate potential supplied to the second cable is forcibly set to the ground potential. . For this reason, for example, by setting the switch signal so that the switch selectively outputs the ground potential when the power supply to the network device is cut off, the other network device can reliably recognize that the connection has been disconnected. Can be done.

According to a third aspect of the present invention, there is provided a network device that receives data via a first cable and transmits data via a second cable, and operates on a first power supply. A control unit that outputs a first signal, an intermediate potential supply unit that is operated by a second power supply, recognizes an intermediate potential level of the first cable, and supplies an intermediate potential to the second cable. A level converter that converts the level of the first signal according to the voltage levels of the first and second power supplies and outputs the second signal as a second signal to the intermediate potential supply unit. Is, in response to the second signal,
A driver for generating an intermediate potential to be supplied to the second cable; and a switch for switching whether or not power is supplied to the driver in accordance with a switch signal.

According to the third aspect of the present invention, when the switch is controlled to stop the power supply to the driver by the switch signal, the ground potential is output from the driver, so that the ground potential is supplied to the second cable. The intermediate potential is forcibly set to the ground potential. Therefore, for example, by setting the switch signal so that the switch stops supplying power to the driver when the power supply to the network device is cut off, it is possible to ensure that the connection to the other network device is disconnected. Can be recognized.

According to a fourth aspect of the present invention, a network device that receives data via a first cable and transmits data via a second cable operates on a first power supply. A control unit that outputs a first signal, an intermediate potential supply unit that is operated by a second power supply, recognizes an intermediate potential level of the first cable, and supplies an intermediate potential to the second cable. , The level of the first signal is converted in accordance with the voltage levels of the first and second power supplies, and the intermediate potential supply unit is provided as a second signal serving as a basis of the intermediate potential supplied to the second cable. And a level conversion unit for outputting to
The level conversion unit includes a switch that switches whether or not the second signal is at a ground potential according to a switch signal.

According to the fourth aspect of the present invention, when the switch controls the second signal to the ground potential by the switch signal, the ground potential is output from the driver.
The intermediate potential supplied to the second cable is forcibly set to the ground potential. Therefore, for example, by setting the switch signal so that the switch sets the second signal to the ground potential when the power supply to the network device is cut off, the disconnection of the connection to the network device on the other side is established. It can be surely recognized.

According to a fifth aspect of the present invention, the switch according to any one of the first to fourth aspects receives a reset signal supplied from outside the network device as the switch signal.

Further, in the invention of claim 6, the first aspect is provided.
In any one of 1 to 4, the intermediate potential supply section may include:
When a potential supplied from the second power supply is lower than a predetermined potential, a potential detection circuit that makes a potential detection signal effective is provided, and the switch receives the potential detection signal as the switch signal.

[0019] According to another aspect of the present invention, a network device that receives data via a first cable and transmits data via a second cable operates on a first power supply. A control unit that outputs a first signal; and an intermediate potential supply unit that is operated by a second power supply, recognizes an intermediate potential level of the first cable, and supplies an intermediate potential to the second cable. The level of the first signal is converted in accordance with the voltage levels of the first and second power supplies, and the second signal is supplied to the intermediate potential supply unit as a second signal serving as a basis of an intermediate potential supplied to the second cable. An output level converter, and the second
And a diode for coupling the first power supply and the first power supply, and a resistor provided between the first power supply and the ground.

According to the present invention, the second cable is connected to the first power supply by the diode.
For this reason, when the power supply to the network device is cut off, the potential of the first power supply drops, so that electric charge flows from the second cable into the first power supply. Since the first power supply is connected to the ground via the resistor, the floating of the potential is suppressed. Therefore, the potential of the second cable eventually becomes the ground potential, so that the other network device can reliably recognize that the connection has been disconnected.

According to another aspect of the present invention, a network device that receives data via a first cable and transmits data via a second cable operates on a first power supply. A control unit that outputs a first signal, an intermediate potential supply unit that is operated by a second power supply, recognizes an intermediate potential level of the first cable, and supplies an intermediate potential to the second cable. The level of the first signal is converted according to the voltage levels of the first and second power supplies, and the second signal is supplied to the intermediate potential supply unit as a second signal that is a basis of an intermediate potential supplied to the second cable. A level conversion unit for outputting the signal; and a loop cutoff unit for forcibly setting the intermediate potential to the ground potential when the power supply to the network device is cut off.

According to the invention of claim 8, when the power supply to the network device is cut off, the intermediate potential supplied to the second cable is forcibly set to the ground potential by the loop cutoff means. As a result, it is possible to make the other network device surely recognize that the connection has been disconnected.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, components having the same basic functions are denoted by the same reference numerals,
The detailed description may be omitted.

(First Embodiment) FIG. 1 is a diagram showing a configuration of a network device according to a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a first network device corresponding to the network device according to the present embodiment, and 3 denotes a first network device 1 and first and second cables L1, L.
2 is a second network device connected through the second network device.
The network device 1 receives data via the first cable L1 and transmits data via the second cable L2.

The first network device 1 includes a control unit 10, a level conversion unit 15, and an intermediate potential supply unit 20.
And The control unit 10 has a first power supply VD
D1 operates, and the control circuit 11
A control signal S1 as a first signal is output.
The intermediate potential supply section 20 is operated by the second power supply VDD2, and performs recognition of the intermediate potential in the first cable L1 and supply of the intermediate potential to the second cable L2. The level converter 15 converts the control signal S1 output from the controller 10 into the first and second power supplies VDD1 and V1.
The level is converted according to the voltage level of DD2, and is output to the intermediate potential supply unit 20 as an output control signal S2 as a second signal that is a basis of the intermediate potential SS supplied to the second cable L2.

In the specification of the present application, reference characters VDD1, V1
DD2 may represent a power supply itself or a power supply voltage. Further, here, the power supply voltage of the first power supply VDD1 is 2.5V, and the power supply voltage of the second power supply VDD2 is 3.3V.

The intermediate potential supply section 20 includes the protection diode 2
1, a switch 22, and a driver 23.
The protection diode 21 connects the first network device 1 to the first
From surges from the cable L1. The switch 22 receives the output control signal S2 output from the level converter 15 and the ground potential as inputs, and
Depending on W, one of them is selectively output as the intermediate potential control signal S3. The driver 23 generates an intermediate potential SS (for example, 1.8 V) to be supplied to the second cable L2 according to the output of the switch 22, that is, the intermediate potential control signal S3. FIG. 2 is a circuit diagram showing an example of the internal configuration of the switch 22 and the driver 23.

Here, a reset signal RST is provided from outside the first network device 1 as a switch signal SW for controlling the switching of the switch 22.
This reset signal RST is, for example, a reset IC provided in a system including the first network device 1.
Supplied from

The second network device 3 includes a driver 3
1 and a connection detection circuit 32. Driver 31
Generates an intermediate potential to be supplied to the first cable L1.
The connection detection circuit 32 recognizes the connection based on the intermediate potential level (for example, 0.8 V or more) of the second cable L2 and outputs a connection detection signal SC.

Here, when the power is turned on to both the first and second network devices 1 and 3 and a network is connected, the first network device 1
The driver 23 maintains the intermediate potential of the second cable L2. The driver 31 of the second network device 3 maintains the intermediate potential of the first cable L1. Then, the intermediate potential of the second cable L2 is changed to the second potential.
The connection with the first network device 1 can be recognized by the detection by the connection detection circuit 11 of the network device 3. When the intermediate potential of the second cable L2 becomes, for example, 0.8 V or more, the connection detection circuit 11 validates the connection detection signal SC.

Hereinafter, the operation of the network device 1 shown in FIG. 1 will be described.

Now, it is assumed that the power supply to the first network device 1 has been cut off. At this time, first, the first power supply V
The control signal S1 becomes unstable due to the cutoff of DD1, and therefore, the output control signal S2 output from the level converter 15 also becomes unstable. At this time, the undefined output control signal S2 is converted into an intermediate potential control signal S3 via the switch 22 by the driver 23.
, The intermediate potential SS generated by the driver 23 also becomes unstable. In this case, the second network device 3 on the partner side cannot recognize that the cable connection has been disconnected, and the loop of power supply via the first cable L1 as described in the section of the problem occurs. Occurs and is not preferred.

Therefore, a reset signal RST is input from the outside as a switch signal SW applied to the switch 22. This reset signal RST is, for example, a reset IC
Generated when the power supply voltage falls below a certain level.

When the reset signal RST becomes valid, the switch 22 as the loop breaking means selectively outputs the ground potential instead of the intermediate potential control signal S2. As a result, the input signal of the driver 23 is fixed at the ground potential, and its output also becomes the ground potential. Therefore, the second
The intermediate potential SS of the cable L2 is forcibly set to the ground potential.

In the second network device 3, when the intermediate potential SS of the second cable L2 drops to the ground potential, the connection detection circuit 11 invalidates the connection detection signal SC and the cable is disconnected. Is recognized.

The return procedure when the reset is released is as follows:
It looks like this: First, the reset signal RST is released, the switch 22 operates normally, and selectively outputs the signal S2. Then, the control unit 10 checks whether or not the cables L1 and L2 are connected. If the cables L1 and L2 are connected, the control unit 10 raises the signal S1 and raises the intermediate potential SS. After that, initial settings are made with the connection partner, and the connection returns to the normal connection state.

As described above, according to the present embodiment, when the power supply to the first network device 1 is cut off, the switch 22 as the loop cut-off means replaces the output control signal S2, which becomes undefined, with the ground potential. Is selected and output, the intermediate potential SS supplied to the second cable L2 is forcibly set to the ground potential. This allows
It is possible to make the second network device 3 on the other end recognize that the connection has been disconnected.

(Second Embodiment) FIG. 3 is a diagram showing a configuration of a network device according to a second embodiment of the present invention. Compared with FIG. 1, in the configuration of FIG. 3, the intermediate potential supply unit 20A includes a switch 24 provided at the subsequent stage of the driver 23 that generates a potential serving as an intermediate potential, instead of the switch 22.

The switch 24 receives the potential generated by the driver 23 and the ground potential as inputs and outputs a switch signal S
Depending on W, one of them is selectively output as the intermediate potential SS to be supplied to the second cable L2. As the switch signal SW, a reset signal RST is externally supplied as in the first embodiment. When the reset signal RST becomes valid, the switch 24 as a loop breaking means
A ground potential is output instead of the output of the driver 23.
Therefore, the intermediate potential SS of the second cable L2 is set to the ground potential.

That is, when the power supply to the first network device 1A is cut off, the ground potential is selected and output instead of the output of the driver 23 which becomes indefinite by the switch 24 as the loop cutoff means. The intermediate potential SS of the second cable L2 is forcibly set to the ground potential. As a result, according to the present embodiment, the second
Can be surely recognized as disconnected by the network device 3.

The present invention can also be realized by a configuration in which the power supply to the driver that generates the intermediate potential is switched. FIG. 4 is a circuit diagram around a driver of another configuration example of the network device according to the present invention.
In the configuration of FIG. 4, the presence or absence of power supply to the driver 23 is determined by
Switching can be performed according to the switch signal SW. First, in the configuration of FIG. 4A, the power supply VDD2 can be prevented from being supplied to the driver 23 by the switch 25A as a loop cutoff unit. Further, in the configuration of FIG. 4B, the power supply VDD2 itself can be dropped to the ground by the switch 25B as the loop cutoff means. The switch signal SW is normally “H”, and is “L” at power-off or reset.

Further, instead of providing a switch in the intermediate potential supply section, a switch may be provided in the level conversion section. FIG. 5 is a circuit diagram of a level converter of another configuration example of the network device according to the present invention. In the configuration of FIG. 5, the output control signal S output from the level converter 15A is output.
Whether or not 2 is set to the ground potential can be switched according to the switch signal SW by the switches 16a and 16b as loop breaking means. Switches 16a, 16
b may have a circuit configuration similar to that of the switch 22 shown in FIG. Note that only one of the switches 16a and 16b may be provided.

(Third Embodiment) In the above embodiment,
An external reset signal RST is used as the switch signal SW.
However, in the present embodiment, a potential detection circuit for detecting the potential of the second power supply VDD2 is provided in the intermediate potential supply unit, and the potential detection signal output from the potential detection circuit is changed to a switch signal. It shall be used as SW.

FIG. 6 is a diagram showing a configuration of a network device according to the third embodiment of the present invention. In the configuration of FIG.
The difference from FIG. 1 is that the intermediate potential supply unit 20B includes a potential detection circuit 26 that detects the potential of the second power supply VDD2. The potential detection circuit 26 enables the potential detection signal DT when the potential of the second power supply VDD2 becomes lower than a predetermined potential, for example, 1.8 V. Then, this potential detection signal DT is provided to the switch 22 as the switch signal SW instead of the reset signal RST.

According to such a configuration, the second power supply VD
When the potential of D2 drops, the potential detection signal DT becomes valid, and the selected output of the switch 22 switches from the output control signal S2 to the ground potential. Therefore,
The ground potential is supplied from the driver 23 to the second cable L2 as the intermediate potential SS. As a result, similarly to the first and second embodiments, the second network device 3 on the partner side can reliably recognize that the connection has been disconnected.

It is needless to say that the potential detection signal DT is switched to the switch signal S in the configuration other than FIG.
Needless to say, it may be given as W. FIG. 7 shows a configuration in which this embodiment is applied to the configuration of FIG. As in FIG. 6, the intermediate potential supply unit 20C includes a potential detection circuit 26 that detects the potential of the second power supply VDD2, and the potential detection signal DT output from the potential detection circuit 26 is used instead of the reset signal RST. , As a switch signal SW.

As described above, according to the present embodiment, there is no need to supply a reset signal indicating that power supply has been cut off to the network device, so that the system configuration can be simplified. However, a configuration in which a reset signal is supplied from the outside makes it easier to test the network device. Therefore, in consideration of the easiness of the test, both the potential detection signal generated internally and the reset signal supplied from the outside may be selectively used as the switch signal.

(Fourth Embodiment) FIG. 8 is a diagram showing a configuration of a network device according to a fourth embodiment of the present invention. In FIG. 8, reference numeral 2 denotes a first network device corresponding to the network device according to the present embodiment; 41, a Schottky diode as a diode for coupling the second cable L2 and the first power supply VDD1; 1 is a resistor provided between the power supply VDD1 and the ground. The Schottky diode 41 and the resistor 42 constitute a loop blocking unit.

When the power supply of the first network device 2 is cut off, the intermediate potential supply unit 20D malfunctions, and the electric charge of the first cable L1 is supplied to the second cable L2. But Schottky diode 4
When the electric potential of the first power supply VDD1 decreases due to the presence of 1, the electric charge of the second cable L2 is released to the first power supply VDD1. Thereby, the electric charge of the second cable L2 is consumed, and finally, the second cable L2
, The intermediate potential SS is lowered to the ground potential.
In addition, the resistor 42 prevents the first power supply VDD1 from floating due to charge supply resulting from a malfunction of the intermediate potential supply unit 20D.

As described above, according to the present embodiment, when the power supply to the network device 2 is cut off, the intermediate potential SS of the second cable L2 is forcibly reduced to the ground potential. Thereby, the second network device 3
It is possible to reliably recognize that the connection has been disconnected.

[0051]

As described above, according to the present invention, when the power supply to the network device is cut off, the intermediate potential supplied to the second cable is forcibly set to the ground potential. As a result, the network device on the other end can be made to surely recognize that the connection has been cut off, and the power supply loop can be cut off.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a network device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of an internal configuration of a switch and a driver in FIG.

FIG. 3 is a diagram illustrating a configuration of a network device according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram showing a main part of another configuration example of the network device according to the present invention.

FIG. 5 is a circuit diagram showing a main part of another configuration example of the network device according to the present invention.

FIG. 6 is a diagram illustrating a configuration of a network device according to a third embodiment of the present invention.

FIG. 7 is a diagram illustrating another configuration of the network device according to the third embodiment of the present invention.

FIG. 8 is a diagram illustrating a configuration of a network device according to a fourth embodiment of the present invention.

[Explanation of symbols]

L1 first cable L2 second cable VDD1 first power supply VDD2 second power supply S1 control signal (first signal) S2 output control signal (second signal) SW switch signal SS intermediate potential RST reset signal DT potential Detection signal 1, 1A, 1B, 1C Network device 2 Network device 10 Control unit 15, 15A Level conversion unit 16a, 16b Switch (loop cutoff unit) 20, 20A, 20B, 20C, 20D Intermediate potential supply unit 22 Switch (loop cutoff) Means) 23 Driver 24 Switch (Loop Breaking Means) 25A, 25B Switch (Loop Breaking Means) 26 Potential Detection Circuit 41 Schottky Diode (Diode) 42 Resistance

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hironori Akamatsu 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Inventor Koichi Sugimoto 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd.

Claims (8)

[Claims] 1. A network device for receiving data via a first cable and transmitting data via a second cable, the network device being operated by a first power supply and outputting a first signal. A control unit, operated by a second power supply, recognizing an intermediate potential level of the first cable, and supplying an intermediate potential to the second cable; and A level conversion unit that performs level conversion in accordance with the voltage levels of the first and second power supplies and outputs the second signal as a second signal to the intermediate potential supply unit; And a switch that receives and inputs a ground potential and selects and outputs one of them according to a switch signal; and generates an intermediate potential to be supplied to the second cable according to an output of the switch. A network device comprising: 2. A network device for receiving data via a first cable and transmitting data via a second cable, the network device being operated by a first power supply and outputting a first signal. A control unit, operated by a second power supply, recognizing an intermediate potential level of the first cable, and supplying an intermediate potential to the second cable; and A level conversion unit that performs level conversion in accordance with the voltage levels of the first and second power supplies and outputs the second signal as a second signal to the intermediate potential supply unit; A driver for generating a potential serving as an intermediate potential to be supplied to the second cable, a potential generated by the driver and a ground potential being input, and A switch for selectively outputting either one of them as an intermediate potential to be supplied to the second cable. 3. A network device for receiving data via a first cable and transmitting data via a second cable, the network device being operated by a first power supply and outputting a first signal. A control unit, operated by a second power supply, recognizing an intermediate potential level of the first cable, and supplying an intermediate potential to the second cable; and A level conversion unit that performs level conversion in accordance with the voltage levels of the first and second power supplies and outputs the second signal as a second signal to the intermediate potential supply unit; A driver for generating an intermediate potential to be supplied to the second cable, and a switch for switching whether or not power is supplied to the driver in accordance with a switch signal. A network device, comprising: 4. A network device for receiving data via a first cable and transmitting data via a second cable, the network device being operated by a first power supply and outputting a first signal. A control unit, operated by a second power supply, recognizing an intermediate potential level of the first cable, and supplying an intermediate potential to the second cable; and A level conversion unit that performs level conversion according to the voltage levels of the first and second power supplies and outputs the second signal as a base of the intermediate potential supplied to the second cable to the intermediate potential supply unit; Wherein the level conversion unit includes a switch for switching whether or not the second signal is set to the ground potential according to a switch signal. . 5. The network device according to claim 1, wherein the switch receives a reset signal supplied from outside the network device as the switch signal. 6. The intermediate potential supply unit according to claim 1, wherein the intermediate potential supply unit activates a potential detection signal when a potential supplied from the second power supply is lower than a predetermined potential. A network device, comprising: a potential detection circuit; wherein the switch receives the potential detection signal as the switch signal. 7. A network device for receiving data via a first cable and transmitting data via a second cable, the network device being operated by a first power supply and outputting a first signal. A control unit, operated by a second power supply, recognizing an intermediate potential level of the first cable, and supplying an intermediate potential to the second cable; and A level conversion unit that performs level conversion according to the voltage levels of the first and second power supplies and outputs the second signal as a base of the intermediate potential supplied to the second cable to the intermediate potential supply unit; A network device comprising: a diode for coupling the second cable; the first power supply; and a resistor provided between the first power supply and a ground. 8. A network device for receiving data via a first cable and transmitting data via a second cable, the network device being operated by a first power supply and outputting a first signal. A control unit, operated by a second power supply, recognizing an intermediate potential level of the first cable, and supplying an intermediate potential to the second cable; and A level conversion unit that performs level conversion according to the voltage levels of the first and second power supplies and outputs the second signal as a base of the intermediate potential supplied to the second cable to the intermediate potential supply unit; When the power supply to the network device is cut off,
A network breaker for forcibly setting the intermediate potential to the ground potential.