Differential communication device

When a transmission signal is detected as having been changed from a high level to a low level, two transmission lines are connected for only a predetermined time through a diode by a first transistor and a second transistor. The diode is arranged such that its forward direction is from a high-side transmission line to a low-side transmission line. The diode turns on, when a potential of the high-side transmission line becomes higher than that of the low-side transmission line by ringing and a potential difference therebetween exceeds a forward drop voltage of the diode. As a result, a peak wave level of a positive side in the ringing is limited to the forward drop voltage of the diode.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2010-116612 filed on May 20, 2010.

FIELD OF THE INVENTION

The present invention relates to a differential communication device, which switches over a signal level of a transmission signal to either a high level or a low level in accordance with a potential difference between two transmission lines.

BACKGROUND OF THE INVENTION

One example of differential communication devices is known by the following patent document 1. The communication device disclosed in the patent document 1 has a terminal end portion and a communication completion detection portion, which are connected between two transmission lines. The terminal end portion includes a series circuit of a resistor and a capacitor. The communication completion detection portion detects completion of communication with other devices. When the communication completion detection portion detects the completion of communication, it connects the two transmission lines through the terminal end portion for a predetermined period from the completion of communication. Ringing is thus suppressed from arising on the transmission lines at the time of completion of communication.

According to a communication device disclosed in patent document 2, a diode is arranged between a positive-side output terminal and a negative-side output terminal of a transmission-side ECU, which are connected to two transmission lines, respectively. The diode is arranged so that its forward direction is from the negative-side output terminal to the positive-side output terminal. Ringing is thus reduced by shorting and suppressing a signal voltage changing in the negative direction by the diode.

PRIOR ART DOCUMENT

Although the communication device according to the patent document 1 suppresses the ringing by using the terminal end portion, which is formed of the series circuit of the resistor and the capacitor, the series circuit of the resistor and the capacitor cannot ensure suppression of ringing to a desired peak level.

The communication device according to the patent document 2 suppresses the first signal change in the negative direction of the ringing by the diode based on that the ringing starts first in the negative direction immediately after the signal transmission has been switched over from on-level to off-level. This communication device thus suppresses a signal change, which arises in the positive direction following the first signal change in the negative direction. However, limitation of the signal change in the negative direction of the ringing to a level corresponding to a forward voltage drop of the diode is not sufficient. That is, the signal change in the positive direction cannot be suppressed sufficiently. As shown in a waveform diagram of ringing vibration in the patent document 2, the signal change in the positive direction exceeds an output voltage level, at which signal transmission is turned on. If the change in the ringing in the positive direction thus becomes higher than the output voltage level, at which the transmission signal is turned on, a receiver side is likely to detect the signal level erroneously. This will cause communication error.

The patent document 2 further discloses another example, in which a Zener diode is connected in parallel to the diode in addition thereby to suppress the change in the positive direction. Even in this example, it is necessary to provide the Zener diode not to impede generation of the output voltage when the signal is transmitted. A Zener voltage of the Zener diode is therefore set to be greater than the output voltage, at which the transmission signal is turned on. It is thus impossible to suppress the signal change of ringing in the positive direction to be less than the output voltage, at which the transmission signal is turned on surely.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a differential communication device capable of suppressing positive amplitude of ringing, which arises after a transmission signal has switched over from a high level to a low level, to a predetermined peak level lower than the high level of the transmission signal.

According to one aspect of the present invention, a differential communication device is provided for switching over a level of a transmission signal to either a high level or a low level in accordance with a potential difference between two transmission lines. The differential communication device comprises a communication circuit and a ringing suppression circuit. The communication circuit is connected to the transmission lines and includes at least one of a transmitter circuit for transmitting the transmission signal and a receiver circuit for receiving the transmission signal. The ringing suppression circuit is connected to the transmission lines in parallel to the communication circuit.

The ringing suppression circuit includes a first control circuit, a switch circuit and a drive signal output circuit. The first control circuit is provided in a connection line connecting the two transmission lines and includes a first diode, which is arranged such that a forward direction thereof is from a high-side transmission line of the two transmission lines to a low-side transmission line of the two transmission lines. The high-side transmission line and the low-side transmission line provide a relatively high potential and a relatively low potential in case of transmission of a high level signal, respectively. The switch circuit is provided in the connection line in a series relation with the first diode. The drive signal output circuit outputs a drive signal, which turns on the switch circuit for only a predetermined time, upon detection of a change of the transmission signal from the high level to the low level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A differential communication device according to embodiments of the present invention will be described below with reference to the drawings.

First Embodiment

A differential communication device according to a first embodiment is applied to a communication network, which is provided to exchange data among a plurality of in-vehicle ECUs mounted in a vehicle, for example.

As shown inFIG. 1, a number of transmission/reception nodes30are provided as differential communication devices in a communication network. Each transmission/reception node30is connected to be capable of communication mutually through junction connectors10and transmission lines20. When one node of a plurality of transmission/reception nodes30starts transmission of a signal by a transmitter circuit, the other transmission/reception nodes30receive by respective receiver circuits the transmission signal transmitted through the junction connectors10and the transmission lines20.

Each transmission/reception node30is configured as shown inFIG. 2. The differential communication device is configured to switch over the transmission signal to either a high level or a low level in accordance with a potential difference between two transmission lines, a high-potential side transmission line (hereinafter high-side transmission line) BP and a low-potential side transmission line (herein after low-side transmission line) BM. The high-side transmission line BP and the low-side transmission line BM are a positive potential-side transmission line and a negative potential-side transmission line. A transmitter circuit32, which is one example of a communication circuit, therefore has a high potential-side MOS transistor (hereinafter high-side transistor)34and a low potential-side MOS transistor (hereinafter low-side transistor)42. The high-side transistor34switches over a potential of the high-side transmission line BP to a relatively high potential when a high level signal is transmitted. The low-side transistor42switches over a potential of the low-side transmission line BM to a relatively low potential, which is lower than the high potential, when the high level signal is transmitted. For setting the transmission signal to the high level, both the high-side transistor34and the low-side transistor42are turned on. For setting the transmission signal to the low level, both the high-side transistor34and the low-side transistor42are turned off. The high-side transistor34and the low-side transistor42are turned on or off by an ECU (not shown), which transmits data.

An output terminal of the high-side transistor34is connected to the high-side transmission line BP through a rectifier diode36. By turning on or off the high-side transistor34, the potential of the high-side transmission line BP is switched to the high potential, for example Vcc, or other potentials close to Vcc. An output terminal of the low-side transistor42is connected to the low-side transmission line BM through a rectifier diode40. By turning on or off the low-side transistor42, the potential of the low-side transmission line BM is switched to the low potential, for example 0, or other potentials close to 0. When the potential of the high-side transmission line BP becomes high and the potential of the low-side transmission line BM becomes low, the difference between the potentials of the transmission lines BP and BM increases and the transmission signal level (level of signal transmitted by the transmission lines BP and BM) becomes high.

The high-side transmission line BP and the low-side transmission line BM are connected via a resistor38. When the high-side transistor34and the low-side transistor42are both turned off, the difference between the potentials of the high-side transmission line BP and the low-side transmission line BM decreases to 0 or close to 0. The signal level of the transmission signal thus becomes low.

A receiver circuit44, which is also an example of the communication circuit, is connected between the high-side transmission line BP and the low-side transmission line BM to receive signals, which are transmitted from other transmitter circuits32in the other transmission/reception nodes30. That is, the receiver circuit44detects the potential difference between the high-side transmission line BP and the low-side transmission line BM and detects the level of the transmission signal.

A ringing suppression circuit46is provided between the high-side transmission line BP and the low-side transmission line BM in parallel to the transmitter circuit32and the receiver circuit44. The ringing suppression circuit46is provided to suppress ringing of potential, that is, a cyclic variation or fluctuation in positive and negative directions in the potential difference, which is caused by impedance mismatching of the transmission lines BP and BM when the level of the transmission signal is switched from the high level to the low level. The ringing suppression circuit46may be provided in all of the transmission/reception nodes30or only in specified transmission/reception nodes30.

The ringing suppression circuit46is configured as shown inFIG. 3. The ringing suppression circuit46includes a drive signal output circuit, which includes a comparator circuit48, an inverter circuit50and a resistor54. The comparator circuit48compares the potential of the high-side transmission line BP and the potential of the low-side transmission line BM. The comparator circuit48outputs a high level signal as a drive signal when the potential of the high-side transmission line BP is higher than that of the low-side transmission line BM. The comparator circuit48outputs a low level signal as a drive signal when the potential of the low-side transmission line BM is higher than that of the high-side transmission line BP.

Normally, the potential of the low-side transmission line BM does not exceed the potential of the high-side transmission line BP. However, when the level of the transmission signal changes from the high level to the low level, ringing may arise as exemplified inFIG. 5. When the ringing develops a change in the negative direction, the potential of the low-side transmission line BM transiently rises to be higher than that of the high-side transmission line BP. Thus, change in the output signal of the comparator circuit48from a high level signal to a low level signal indicates that the level of the transmission signal has changed from the high level to the low level and the ringing arose. The comparator circuit48thus operates as a detection circuit for detecting ringing.

When the comparator circuit48once generates the low level signal by determining that the potential of the low-side transmission line BM exceeded the potential of the high-side transmission line BP, the comparator circuit48maintains outputting the low level signal unless the potential of the high-side transmission line BP becomes higher than the potential of the low-side transmission line BM by more than a predetermined voltage. The comparator circuit48thus has hysteresis function in comparing the potential of the high-side transmission line BP and the potential of the low-side transmission line BM. The magnitude of the hysteresis (magnitude of the predetermined voltage) is set to be greater than a forward drop voltage of a diode60of a first impedance control circuit (hereinafter first control circuit)58.

The output signal of the comparator circuit48is applied to a gate of a first MOS transistor (hereinafter first transistor)52, which is a first switching element, through the inverter circuit50. The output signal of the comparator circuit48is also applied to a gate of a second MOS transistor (hereinafter second transistor)56, which is a second switching element, through a resistor54. The first transistor52and the second transistor56are connected in series in a connection line BC, which connects the high-side transmission line BP and the low-side transmission line BM. The first transistor52and the second transistor56form a first switching circuit and a second switching circuit, respectively.

When the high level signal is being outputted from the comparator circuit48, the output level is inverted to the low level signal by an inverter circuit50. As a result, the first transistor52turns off and only the second transistor56turns on. When the output signal of the comparator circuit48changes from the high level to the low level, the high level signal, which is inverted from the low level signal by the inverter circuit50, is applied instantly to the gate of the first transistor52. As a result, the first transistor52turns on. The low level signal produced from the comparator circuit48is applied to a gate of the second transistor56with a predetermined time delay by a delay operation of the resistor54. The resistor54thus operates as a delay circuit. As the delay circuit, an even number of series-connected inverters, a RC circuit or a LC circuit may be used in place of the resistor54.

While the low level signal, which is to be applied to the gate of the second transistor56, is being delayed, the gate potential of the second transistor56is maintained at the high level. When the output signal of the comparator circuit48is switched from the high level to the low level, that is, when the ringing occurs, the first transistor52and the second transistor56are turned on.

The first control circuit58is connected in series with the first transistor52and the second transistor56in the connection line BC connecting the high-side transmission line BP and the low-side transmission line BM. Therefore, when the first transistor52and the second transistor56are turned on at the same time because of the ringing, the high-side transmission line BP and the low-side transmission line BM are connected through the first control circuit58.

The first control circuit58includes a first diode60and a first resistor62. The diode60is arranged such that its forward direction is from the high-side transmission line BP to the low-side transmission line BM. That is, the diode60is forward-biased in a direction from the high-side transmission line BP to the low-side transmission line BM to allow current flow only in the forward direction from the high-side transmission line BP to the low-side transmission line BM. The resistor62is connected in parallel to the diode60. Even when a change arises in the negative direction of the ringing, that is, the potential of the low-side transmission line BM becomes higher than that of the high-side transmission line BP, and a change in the positive direction of the ringing arises, the impedance of the first control circuit58becomes equal to the matching impedance of the resistor62as shown inFIG. 4as long as the potential difference between the potential of the high-side transmission line BP and the potential of the low-side transmission line BM is less than the voltage drop of the diode60in the forward direction.

When the potential of the high-side transmission line BP becomes higher than the potential of the low-side transmission line BM by more than a potential difference, which corresponds to the forward drop voltage of the diode60, the diode60turns on. As a result, the impedance of the first control circuit58becomes as low as near zero. At this time, the potential of the high-side transmission line BP is clamped at a level, which is higher than the potential of the low-side transmission line BM by the forward drop voltage of the diode60.FIG. 5shows comparison of ringing, which are generated when the level of the transmission signal is switched over from the high level (H) to the low level (L), between cases, in which the ringing suppression circuit46is provided and not provided.

The function and advantage of the ringing suppression circuit46configured as above is described below. In the ringing suppression circuit46, the first and the second transistors52and56connected in series are turned on at the same time for the predetermined time when the comparator circuit48detects the ringing. When the change of the ringing in the positive direction increases and the diode60of the first impedance control circuit turns on, the potential of the high-side transmission line BP is clamped at a level, which corresponds to the forward drop voltage of the diode60. As a result, the peak wave level of the ringing in the positive direction is limited to the forward drop voltage of the diode60. By thus setting the forward drop voltage of the diode60to be less than a level, at which the transmission signal of the high level is outputted, the change of the ringing in the positive direction can be suppressed to the predetermined wave peak level, which is less than the voltage level of the transmission signal of the high level.

The first control circuit58is connected operatively between the high-side transmission line BP and the low-side transmission line BM for only the predetermined time, when the ringing arises at the time of change of the transmission signal from the high level to the low level. Therefore, the limitation to the peak wave level in the positive direction of the ringing by the diode60is limited to be within a time period, in which the diode60will not impede generation of a high level signal when the high level signal is transmitted.

The resistor62is connected in parallel to the diode60in the first control circuit58of the ringing suppression circuit46. By the resistor62, the high-side transmission line BP and the low-side transmission line BM is operatively connected before the diode60turns on, that is, when the ringing has a change in the negative direction and a change of the ringing in the positive direction is less than the forward drop voltage of the diode60. The matching impedance of the resistor62suppresses a large potential difference from developing between the high-side transmission line BP and the low-side transmission line BM. The ringing is thus reduced by the matching impedance as well.

The comparator circuit48in the ringing suppression circuit46continues to output the low level signal unless the potential of the high-side transmission line BP becomes more than the predetermined voltage higher than that of the low-side transmission line BM, once the potential of the low-side transmission line BM becomes higher than the potential of the high-side transmission line BP and outputs the low level signal. This predetermined voltage is set to be greater than the forward drop voltage of the diode60. As a result, the first transistor52can be maintained in the on-state by the diode60while the peak wave level of the ringing in the positive direction is being limited. Further, the drive signal for turning off the second transistor56can be generated after the predetermined time delay. That is, it is prevented by the diode60of the first control circuit58that the switch circuit formed of the first and the second transistors52and56is turned off in the period of limiting the peak wave level in the positive direction of the ringing.

Second Embodiment

A differential communication device according to a second embodiment will be described next with reference toFIGS. 6 and 7. The differential communication device according to the second embodiment is different from that of the first embodiment only in a part of configuration of the ringing suppression circuits46and46a.

As shown inFIG. 6, a ringing suppression circuit46ahas a second impedance control circuit (hereinafter second control circuit)64in series with the first control circuit58in the connection line BC, which connects the high-side transmission line BP and the low-side transmission line BM. The second control circuit64has a second diode66and a second resistor68. The diode66is arranged such that its forward direction is from the low-side transmission line BM and the high-side transmission line BP. That is, the diode66is reverse-biased. The resistor68is connected in parallel to the diode66.

The second control circuit64can thus suppress not only changes of ringing in the positive direction but also changes of ringing in the negative direction. As a result, the ringing can be suppressed advantageously.

More specifically, when a change arises in the negative direction of ringing, a current flows from the low-side transmission line BM to the high-side transmission line BP through the resistor62of the first control circuit58and the resistor68of the second control circuit64before the potential difference between the low-side transmission line BM and the high-side transmission line BP becomes higher than the forward drop voltage of the diode66of the second control circuit64. This current varies in accordance with the potential difference. As a result, a large potential difference is suppressed by the matching impedances of the resistor62and the resistor68from developing between the low-side transmission line BM and the high-side transmission line BP. Generation of ringing is thus reduced.

The diode66turns on, when the potential difference between the low-side transmission line BM and the high-side transmission line BP becomes higher than the forward drop voltage of the diode66of the second control circuit64. The current flows at this time through the resistor62of the first impedance control circuit and the diode66of the second control circuit64. The potential of the low-side transmission line BM is clamped at a level, which is higher than the potential of the high-side transmission line BP by the forward drop voltage of the diode66and the voltage of the resistor62. Thus, the peak wave level of the change in the negative direction of the ringing is limited to a level, which corresponds to the forward drop voltage of the diode66and the voltage of the resistor62.

The configuration for suppressing the change in the positive direction of the ringing is generally the same as in the first embodiment. However, it is different in that the current flows through the resistor68of the second control circuit64when it flows from the high-side transmission line BP to the low-side transmission line BM.

Third Embodiment

A differential communication device according to a third embodiment is described next with reference toFIG. 8.

A comparator circuit48ashown inFIG. 8is different from the comparator circuit48in that it has no hysteresis function. That is, the comparator circuit48asimply outputs a high level output signal and a low level output signal, when the potential of the high-side transmission line BP is higher than that of the low-side transmission line BM and the potential of the low-side transmission line BM is higher than that of the high-side transmission line BP, respectively.

The comparator circuit48athus outputs the high level signal when the change arises in the positive direction in the ringing. An inverter circuit50ais configured not to turn off the first transistor52immediately in response to the high level signal. That is, the inverter circuit50ais formed as a CMOS inverter circuit of a pMOS70and a nMOS72and has a resistor74between the nMOS72and the ground.

When a change arises in the positive direction in the ringing and the comparator circuit48aoutputs the high level signal, the pMOS70and the nMOS72in the inverter circuit50aare turned off and on, respectively. As a result, the gate of the first transistor52is grounded through the nMOS72and the resistor74.

However, the resistor74is provided in a path, by which the gate of the first transistor52is grounded. The gate potential of the first transistor52therefore does not fall to the ground potential immediately because of a delay operation of the resistor74and remains high for a predetermined time. This predetermined time is set to be longer than a ringing period (½ of one cycle of ringing) by the resistor74. Thus, even when the magnitude of ringing changes from the negative side to the positive side at the time of development of ringing and the output signal of the comparator circuit48achanges to the high level, the output signal of the comparator circuit48achanges to the low level before the high level signal turns off the first transistor52. Therefore, it is prevented that the first transistor52is turned off within a period, in which the peak wave level in the positive direction in the ringing need be limited.

In addition, a delay circuit54ais provided in a ringing suppression circuit46band configured such that the second transistor56is turned off after a predetermined time from first detection of development of ringing by the comparator circuit48a.

Specifically, the delay circuit54ais formed of two-stages of inverter circuits and the resistor54. A resistor80is provided between a NMOS78of the first stage inverter circuit and the ground.

As a result, when the ringing develops a change in the positive direction and the comparator circuit48aoutputs the high level signal, the pMOS76and the nMOS78of the first stage inverter is turned off and turned on, respectively. The gates of the pMOS82and the nMOS84of the second stage inverter circuit are grounded through the nMOS78and the resistor80.

However, the gate potentials of the pMOS82and the nMOS84of the second stage inverter circuit does not fall to the ground potential immediately because of a delay operation of the resistor80and remains high for a predetermined time. This predetermined time is set to be longer than a period of the ringing by a resistance of the resistor80. Thus, even when the magnitude of ringing changes from the negative side to the positive side at the time of development of ringing and the output signal of the comparator circuit48achanges to the high level, the output signal of the comparator circuit48achanges to the low level before the high level signal changes the output of the second stage inverter.

As a result, the output of the second stage inverter is maintained at the low level from the time when the output signal of the comparator circuit48ais changed to the low level for the first time because of development of a change in the negative direction in the ringing. The second transistor56accordingly turns off after the predetermined time, which is determined by the delay operation of the resistor54.

It is noted that the comparator circuit48ahaving no hysteresis function may be formed of a pMOS transistor48band a resistor49as shown inFIG. 9. A ringing suppression circuit46cis configured as a modified example of the ringing suppression circuit46b.

According to the example shown inFIG. 8, the delay circuit54ahas the two-stage inverter circuit. However, the number of stages of the inverter circuit is not limited to be two as far as it is plural. In this case, by connecting the resistor80to a stage of the inverter circuit other than the final stage, the output of the final stage of the inverter circuit can be maintained even when the level of the output signal of the comparator circuit48ais changed.

Fourth Embodiment

A differential communication device according to a fourth embodiment will be described with reference toFIG. 10, which shows a ringing suppression circuit46din the differential communication device.

The ringing suppression circuit46dis configured to detect that the transmission signal, which is transmitted by the two transmission lines BP and BM, is changed from the high level to the low level only after detection of transmission of the high level signal by the two transmission lines BP and BM.

Specifically, the ringing suppression circuit46dis connected to the transmitter circuit32and the receiver circuit44through a controller90provided as a high level signal detection circuit, a first switch92and a second switch94as shown inFIG. 10. The controller90is provided to detect that the high level signal has been transmitted by the two transmission lines BP and BM based on transmission of the high level signal by the transmitter circuit32and reception of the high level signal by the receiver circuit44.

Upon detection of transmission of the high level signal by the two transmission lines BP and BM, the controller90turns on both the first switch92, which is provided between the high-side transmission line BP and the comparator circuit48, and the second switch94, which are provided between the low-side transmission line BM and the comparator circuit48. Thus, the potential of the high-side transmission line BP and the potential of the low-side transmission line BM are applied to the comparator circuit48. As a result, it becomes possible to detect the ringing by the comparator circuit48and operate the ringing suppression circuit46d.

According to the fourth embodiment, the ringing suppression circuit46dis operated only after detection of the transmission of the high level signal by the two transmission lines BP and BM. As a result, the ringing suppression circuit46dis prevented from operating erroneously in response to disturbance in a differential voltage, which is caused by external noise, or ringing, which is caused when the transmission signal rises high.

The present invention described with reference to the preferred embodiments is not limited to such embodiments, but may be implemented in different modified forms.

For example, the second control circuit64in the second embodiment may be provided in the third and the fourth embodiments. The ringing suppression circuit, which is operated only when the high level transmission signal is detected as described in the fourth embodiment, may be provided in the other embodiments as well.

The first control circuit58may be formed of only a resistor.