Abstract:
A data communication system includes a terminal identification information adding unit generating a unit carrier wave including a predetermined plurality of periods of carrier waves and arranging the unit carrier wave in arrangement patterns differing at every transmission terminal to generate a unit carrier row signal shorter than one bit of the digital transmission data, a transmission timing determination unit detecting a binary inversion timing of the digital transmission data and causing the terminal identification information adding unit to transmit the unit carrier row signal every time the binary is inverted, a reception timing distinguishing unit generating a reception timing signal in synchronization with reception of the unit carrier row signal and collecting the reception timing signal of the unit carrier row signal of the same type, and a digital data restoring unit inverting the binary according to the collected reception timing signal, restoring the digital transmission data.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a data communication system and a data communication method modulating and demodulating binary multibit serial digital transmission data using carrier waves and transmitting and receiving the data between a plurality of transmission terminals and a plurality of reception terminals. 
         [0003]    2. Description of the Related Art 
         [0004]    A time sharing system (TSS) and a digital subscriber line (DSL) are known as conventional systems for data communication with a plurality of transmission terminals being discriminated. For example, JP-A-2000-284800 discloses such a conventional data communication system. JP-A-2003-116186 discloses another conventional data communication system. Furthermore, amplitude shift keying (ASK) and frequency shift keying (FSK) are conventionally known as methods of modulation and demodulation. For example, refer to “Glossary of Technical Terms in Japanese Industrial Standards” 5th edition, compiled and published by Japanese Standards Association, Tokyo on Mar. 30, 2001. 
         [0005]    Signal waves transmitted from a transmission terminal are sometimes multireflected on a boundary (such as wall) in a propagation space and a terminal of transmission path, producing a primary reflected wave, secondary reflected wave and so on each of which is shifted by reflective propagation. When modulation and demodulation are carried out by a conventional method of ASK or FSK using a conventional system of TSS or DSL, a regular signal wave is superposed on the primary reflected wave, secondary reflected wave and so on such that it is difficult to discriminate the regular signal wave from the primary reflected wave, secondary reflected wave and so on. As a result, communication failure sometimes occurs. In particular, the communication failure due to the aforesaid multireflection frequently occurs when data communication is carried out through a relatively shorter transmission path such as one in a vehicle. 
       SUMMARY OF THE INVENTION 
       [0006]    Therefore, an object of the present invention is to provide a data communication system and a data communication method which can prevent communication interference due to multireflection of signal waves so that better data communication can be performed. 
         [0007]    The present invention provides a data communication system modulating and demodulating binary multibit serial digital transmission data using a carrier wave and transmitting and receiving the data between a plurality of transmission terminals and a plurality of reception terminals. The system comprises a terminal identification information adding unit provided in each transmission terminal for generating a unit carrier wave including a predetermined plurality of periods of the carrier wave and arranging the unit carrier in arrangement patterns differing at every transmission terminal to generate a unit carrier row signal shorter than one bit of the digital transmission data, a transmission timing determination unit provided in each transmission terminal for detecting a binary inversion timing of the digital transmission data and causing the terminal identification information adding unit to transmit the unit carrier row signal every time the binary is inverted, a reception timing distinguishing unit provided in each reception terminal for generating a reception timing signal in synchronization with reception of the unit carrier row signal and collecting the reception timing signal of the unit carrier row signal of the same type, and a digital data restoring unit provided in each reception terminal for inverting the binary according to the collected reception timing signal of the same type, thereby restoring the digital transmission data. 
         [0008]    In the above-described system, each transmission terminal transmits the unit carrier row signal containing identification information thereof in synchronization with inversion of binary of the digital transmission data. In this case, when a plurality of transmission terminals simultaneously carry out data transmission, a plurality of types of unit carrier row signals coexist. On the other hand, the reception terminal generates a reception timing signal every reception timing of the unit carrier row signal, collecting the reception timing signals of the same type of unit carrier row signals. The reception terminal inverts binary according to the same type of reception timing signals collected to restore the digital transmission data. Consequently, the reception terminal can receive the digital transmission data transmitted from a plurality of transmission terminals substantially simultaneously while distinguishing the data for every transmission terminal. 
         [0009]    In the above-described arrangement, the digital transmission data is converted to a collection of unit carrier row signals to be transmitted in the communication between the transmission and reception terminals. Each unit carrier row signal is shorter than 1 bit of the digital transmission data. Furthermore, each unit carrier row signal is composed of rowed unit carrier waves. More specifically, the digital transmission data is decomposed into unit carrier waves each of which is highly shorter than 1 bit of transmission data, to be transmitted. Consequently, when the unit carrier waves are multireflected thereby resulting in generation of primary reflected wave, secondary reflected wave and so on, normal unit carrier waves can readily be distinguished from the reflected waves. Accordingly, the above-described arrangement can prevent communication interference due to multireflection of signal waves so that better data communication can be performed. 
         [0010]    The reception terminals may be responsive to digital transmission data from a plurality of transmission terminals. Alternatively, the reception terminals may be responsive to the digital transmission data from only predetermined transmission terminals out of a plurality of transmission terminals, while the digital transmission data from the other transmission terminals may be ignored or discarded. 
         [0011]    Research made by the inventor reveals that when the unit carrier wave is propagated to a transmission path in a vehicle, the reflected wave can be attenuated to such a level that the reflected wave can be distinguished from the normal unit carrier wave when an interval between the unit carrier waves is 20 ns. In contrast, better data communication can be performed in the vehicle when each unit carrier wave has a length which is less than 20 ns and an interval between the unit carrier waves is not less than 20 ns. 
         [0012]    In a preferred embodiment, the transmission terminals and the reception terminals are mounted in a vehicle. Furthermore, a transmission side opposed conductive member is disposed in the transmission terminals so as to be opposed to a vehicle ground. A reception side opposed conductive member is disposed in the reception terminals so as to be opposed to the vehicle ground. The transmission terminal applies voltage between the transmission side opposed conductive member and the vehicle ground and changes the voltage according to information to be transmitted, thereby changing an electric field produced over a whole surface of the vehicle ground. This arrangement changes the potential difference between the reception side opposed conductive member and the vehicle ground. Each reception terminal can obtain information from the electric field based on the change in the potential difference. Thus, the above-described invention uses the vehicle ground as a transmission path but not electric waves in order to carry out data communication between the transmission terminal and the reception terminal both of which are located at two positions spaced away from each other in the vehicle ground respectively. Consequently, radio disturbance can be suppressed, and power consumption can be reduced. Furthermore, since radio waves need not be radiated into the air, data communication can be realized by use of lower frequencies than in the conventional arrangements. 
         [0013]    Furthermore, in another preferred embodiment, the unit carrier row signals may be transmitted and received between the transmission terminals and reception terminals by a TSS in order that interference may reliably be prevented between the unit carrier row signals. 
         [0014]    Additionally, in further another preferred embodiment, the unit carrier waves may be propagated to a power supply line in a vehicle so that the unit carrier waves are transmitted and received. Consequently, space can be saved in this case as compared with the case where a communication line is separately provided. In this case, one of output electrodes of a direct current power supply circuit provided in the vehicle is connected to a vehicle ground, and the other of the output electrodes of the DC power supply circuit is connected to the power supply line. Consequently, space saving can further be achieved as compared with the case where paired positive and negative output electrodes of a DC power supply circuit are connected to paired positive and negative power supply lines respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Other objects, features and advantages of the present invention will become clear upon review of the following description of the embodiments with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a conceptual rendering of a vehicle provided with a data communication system of a first embodiment in accordance with the present invention; 
           [0017]      FIG. 2  is a conceptual rendering of a transmission terminal; 
           [0018]      FIG. 3  is a conceptual rendering of a reception terminal; 
           [0019]      FIG. 4  is a conceptual rendering of a transmission and reception terminal; 
           [0020]      FIG. 5  is a timing diagram showing a structure of digital transmission data and the like; 
           [0021]      FIG. 6  is a timing diagram showing the case where a plurality of pieces of data are transmitted; 
           [0022]      FIG. 7  is a conceptual rendering of a vehicle provided with a data communication system of a second embodiment in accordance with the present invention; 
           [0023]      FIG. 8  is a conceptual rendering of a transmission terminal in the second embodiment; 
           [0024]      FIG. 9  is a conceptual rendering of a reception terminal in the second embodiment; 
           [0025]      FIG. 10  is a circuit diagram showing a high impedance filter; 
           [0026]      FIG. 11  is a conceptual rendering of a vehicle provided with a data communication system of a third embodiment in accordance with the present invention; 
           [0027]      FIG. 12  is a conceptual rendering of an electric field relay device; 
           [0028]      FIG. 13  is a waveform chart showing waveforms transmitted from the transmission terminal and measured in a first example; 
           [0029]      FIG. 14  is a waveform chart showing waveforms received by a reception terminal; and 
           [0030]      FIG. 15  is a timing diagram showing a structure of digital transmission data etc. in a modified form. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    A first embodiment of the present invention will be described with reference to  FIGS. 1 to 6 . A data communication system of the embodiment is provided in a vehicle. Referring to  FIG. 1 , a vehicle  10  includes a chassis  11 , an engine  12 , axles  13 , a vehicle housing body  14  and the like, all of which are conductive components. These conductive components are rendered conductive, thereby composing a vehicle ground GND. The vehicle ground GND is insulated from a ground  100  by tires  15 . 
         [0032]    The vehicle  10  is provided with a direct current (DC) power supply circuit  17  having positive and negative output electrodes. The negative output electrode of the DC power supply circuit  17  is connected to the vehicle ground GND. The positive output electrode of the DC power supply circuit  17  is connected to a power supply line  23 . The DC power supply circuit  17  further has a pair of input electrodes to which positive and negative electrodes of a battery  16  are connected. The negative electrode of the battery  16  is also connected to the vehicle ground GND. The DC power supply circuit  17  transforms output of the battery  16  to a predetermined voltage, delivering the voltage between the power supply line  23  and the vehicle ground GND. 
         [0033]    Drive circuits  30 A,  31 A and  32 A are connected in parallel with one another between the power supply line  23  and the vehicle ground GND. The drive circuits  30 A,  31 A and  32 A are provided for actuating a wiper motor  30 , headlights  31 , a door lock  32  and the like, respectively. The wiper motor  30 , headlights  31 , the door lock  32  and the like are driven when an operation switch provided in a main controller  27  is operated or when the main controller  27  delivers control signals to the respective drive circuits  30 A,  31 A,  32 A according to results of detection by a raindrop sensor  34 , a luminous intensity sensor  35 , a vehicle speed sensor  36 , etc. Transmission terminals  51  are provided in signal processing circuits  34 A,  35 A and  36 A of the sensors  34 ,  35  and  36  in order that the signals may be transmitted or received, respectively. Reception circuits  60  are provided in the drive circuits  30 A,  31 A and  32 A respectively. A transmission and reception terminal  70  is provided in the main controller  27 . The transmission terminals  51 , the reception terminals  60  and the transmission and reception terminal  70  constitute a data communication system  50  in the invention. 
         [0034]    Referring to  FIG. 2 , each transmission terminal  51  includes a central processing unit (CPU)  52 , an inversion detector  53 , an identification pulse generator  54  and an amplitude modulator  56 . In the embodiment, the identification pulse generator  54  and the amplitude modulator  56  constitute a terminal identification information adding unit in the invention. The inversion detector  53  serves as a transmission timing determining section in the invention. 
         [0035]    The CPU  52  adds control information and the like to serial digital transmission data D 1  with a plurality of bits composed of binary of “1” and “0” as shown in  FIG. 5A , transmitting the data D 1 . In the embodiment, 1 bit length of the digital transmission data D 1  is set to about 100 μs. 
         [0036]    The inversion detector  53  of the transmission terminal  51  as shown in  FIG. 2  detects an inversion timing of “1” and “0” or an edge of the digital transmission data D 1  delivered by the CPU  52 , thereby delivering an inversion timing signal D 2  as shown in  FIG. 5B . 
         [0037]    The inversion timing signal D 2  as shown in  FIG. 5B  triggers the identification pulse generator  54  to deliver an identification pulse train D 3  as shown in  FIG. 5D . Each transmission terminal  51  includes a memory  52 M (see  FIG. 2 ) on which a terminal identification code for identifying each transmission terminal is stored. The CPU  52  transforms the terminal identification code to 10-bit serial identification data D 4  and supplies the data D 4  to the identification pulse generator  54  as shown in  FIG. 5C . In this case, the ID data D 4  has 1-bit length of 20 ns, for example, and an overall length of about 200 ns. When having received the ID data D 4 , the identification pulse generator  54  generates a detection pulse D 5  with a pulse width of, for example, 6 ns in synchronization with the inversion of “1” and “0” of ID data D 4  (or the edge), generating the aforesaid identification pulse train D 3  (see  FIG. 5D ) from a plurality of detection pulses D 5  corresponding to the whole ID data D 4 . 
         [0038]    A low-pass filter  54 F as shown in  FIG. 2  limits the identification pulse train D 3  to a band of 150 MHz, for example. As a result, each of the detection pulses D 5  composing the identification pulse train D 3  has a pulse width of 6 ns. 
         [0039]    The amplitude modulator  56  acquires the identification pulse train D 3  having passed through the low-pass filter  54 F, amplitude-modulating the identification pulse train D 3  using a carrier wave delivered by an oscillation circuit  56 A and having a predetermined frequency (520 to 680 MHz, for example). As a result, the carrier wave is divided into a plurality of unit carrier waves D 7  each of which has a pulse width of 6 ns corresponding to the width of detection pulse D 5  as shown in  FIG. 5E . The unit carrier waves D 7  are aligned to serve as a unit carrier row signal D 8 . The unit carrier row signal D 8  is delivered as a voltage signal between the power supply line  23  and the vehicle ground GND through a buffer circuit  57  and an output circuit  58 . 
         [0040]    The data communication system  50  of the embodiment is designed to transmit the unit carrier row signals D 8  from a plurality of transmission terminals  51  by the time sharing system (TSS) in synchronization with the terminals  51 ,  60  and  70 . More specifically, as shown in  FIGS. 6A ,  6 C and  6 E in contrast to one another, the CPUs  52  of a plurality of transmission terminals  51  deliver digital transmission data D 1  with output timing shifted for more than a predetermined time of 200 ns. Accordingly, the binary inversion sections (edges) of digital transmission data D 1   a , D 1   b  and D 1   c  delivered by the respective first to third transmission terminals  51  also shift from one another. Consequently, as shown in contrast by  FIGS. 6B ,  6 D and  6 F, the unit carrier row signals D 8   a  to D 8   c  delivered by the first to third transmission terminals  51  are shifted from one another. As shown in  FIG. 6G , even if the first to third transmission terminals  51  simultaneously deliver the unit carrier row signals D 8   a  to D 8   c  respectively, the unit carrier row signals D 8   a  to D 8   c  can reliably be prevented from superimposition. 
         [0041]    In the transmission terminal  51  of the embodiment, the digital transmission data D 1  having 1-bit length of about 100 us delivered by the CPU  52  is rendered a group of unit carrier row signals D 8 . The group has a data length of 200 ns. Since the output timing of each unit carrier row signal D 8  is an integral multiple of 100 μs, the time occupancy of each unit carrier row signal D 8  relative to the whole data length of the digital transmission data D 1  can be reduced to 1/500. As a result, when for example, 50 pairs of transmission and reception terminals communicate simultaneously without use of the TSS, the unit carrier row signal D 8  of one terminal identification code is detected (Carrier Detection, CD) while the power supply line  23  is being monitored. Subsequently, the unit carrier row signal D 8  of self terminal identification code is declared so that interference of transmission data by a plurality of transmission terminals  51  can be avoided. 
         [0042]      FIG. 3  shows the arrangement of the reception terminal  60 . The reception terminal  60  includes an amplitude demodulator  63 , an identification code decoder  64 , a code discriminator  65 , a timing signal generator  66 , a digital data restoring unit  67  and a central processing unit (CPU)  68 . The identification code decoder  64 , the code discriminator  65  and the timing signal generator  66  constitute a reception timing distinguishing unit in the invention. 
         [0043]    In the reception terminal  60 , the unit carrier row signal D 8  propagating to the power supply line  23  is matched by a transformer (not shown) of a receiving circuit  61 . A frequency band of the unit carrier row signal D 8  is limited by a band-pass filter  61 F and the signal D 8  is amplified by an amplifier  62 , thereafter being loaded into the amplitude demodulator  63 . The amplitude demodulator  63  detects a phase of the unit carrier wave D 7  at a frequency of amplitude modulation (520 to 680 MHz) contained in the loaded unit carrier row signal D 8 . The amplitude demodulator  63  amplitude-demodulates the unit carrier row signal D 8  based on the detected phase, generating the identification pulse train D 3 . 
         [0044]    The amplitude demodulator  63  amplitude-demodulates the loaded unit carrier row signal D 8  as a normal unit carrier wave D 7  when an amplitude level and data length are at or above respective predetermined reference values. Otherwise, the loaded unit carrier row signal D 8  is discarded as reflected waves or noise. The unit carrier wave D 7  may be synchronized between predetermined one of the transmission terminals  51  and predetermined one of the reception terminals  60 . In this case, the phase of the unit carrier wave D 7  normally synchronized may be compared with the phase of the actually detected unit carrier wave D 7 . Data may be discarded when the phases vary from predetermined reference values to a large degree. 
         [0045]    The identification code decoder  64  decodes an identification code contained in the identification pulse train D 3 . Suppose now the case where the reception terminal  60  is set to process only signals delivered from predetermined one of the transmission terminals  51 . In this case, when the identification code decoder  64  of the reception terminal  60  confirms that the identification code is information directed to own reception terminal  60  and that a time interval is normal (the time interval is 14.2857 μs when the ID data D 4  of the aforesaid identification code is 70 kbps, for example.) When these conditions are met, the received data is supplied to the code discriminator  65 . When the conditions are not met, the data is discarded. On the other hand, when the reception terminal  60  is set so as to process signals delivered from a plurality of transmission terminals  51 , the identification code decoder  64  of such reception terminal  60  supplies all the received data of a plurality of types of identification codes to the code discriminators  65 . 
         [0046]    The code discriminator  65  collects identification pulse train D 3  for every same identification code, generating a pulse train group composed of the same type of identification pulse trains D 3  aligned. The timing signal generator  66  generates a reception timing signal D 9  (see  FIG. 3 ) which corresponds to, for example, a starting position of all the identification pulse trains D 3  contained in the pulse train group. The reception timing signal D 9  is the same as the inversion timing signal D 2  as shown in  FIG. 5B . The digital data restoring unit  67  inverts the binary of “1” and “0” for every reception timing signal D 9 , thereby restoring the digital transmission data D 1  (see  FIG. 5A ). 
         [0047]    The digital transmission data D 1  thus restored is loaded into the CPU  68  together with the terminal identification code decoded by the identification code decoder  64 . As a result, the CPU  68  of the reception terminal  60  can obtain control information together with the terminal identification information as to which transmission terminal  51  or transmission and reception terminal  70  the digital transmission data D 1  came from and carry out processing for response to the information. 
         [0048]    The transmission and reception terminal  70  comprises a transmission processing circuit  51 X having the same structure as the above-described transmission terminal  51  and a reception processing circuit  60 X having the same structure as the above-described reception terminal  60  as shown in  FIG. 4 . A CPU  71  serves both for the transmission processing circuit  51 X and for the reception processing circuit  60 X. 
         [0049]    The following will describe the operation and advantages of the data communication system  50  of the embodiment. Upon drive of the vehicle  10 , the vehicle speed sensor  36  constantly detects a running speed of the vehicle. The results of detection are indicated by a speedometer (not shown) of the vehicle. Furthermore, when the headlights  31  are set so as to be automatically turned on, the luminous intensity sensor  35  detects ambient darkness, turning on the headlights  31  automatically. In these cases, the results of detection by the vehicle speed sensor  36  and the luminous intensity sensor  35  are to be transmitted by the signal processing circuits  36 A and  35 A respectively. 
         [0050]    More specifically, the CPU  52  of the transmission terminal  51  related to the vehicle speed sensor  36  delivers digital transmission data D 1  containing information about vehicle speed. The unit carrier row signal D 8  containing an identification code of the transmission terminal  51  is delivered by the transmission terminal  51  to the power supply line  23  in synchronization with inversion of binary composing the digital transmission data D 1 . Furthermore, the CPU  52  of the transmission terminal  51  related to the luminous intensity sensor  35  delivers digital transmission data D 1  containing information about luminance. The unit carrier row signal D 8  containing an identification code of the transmission terminal  51  is delivered by the transmission terminal  51  to the power supply line  23  in synchronization with inversion of binary composing the digital transmission data D 1 . Other transmission terminals  51  including the transmission and reception terminal  70  functioning as transmission terminal  51  also constantly deliver the unit carrier row signals D 8  containing necessary information. 
         [0051]    The reception terminals  60  (including the transmission and reception terminal  70  functioning as the reception terminal  60 ) constantly load with signals transmitted to the power supply line  23 . When a plurality of transmission terminals  51  simultaneously carry out signal transmission, a plurality of types of unit carrier row signals D 8   a , D 8   b , D 8   c , . . . coexist as shown in  FIG. 6G . On the other hand, after reception of the unit carrier row signal D 8 , each reception terminal  60  determines the reception timing for every same type of unit carrier row signal D 8 . In other words, each reception terminal  60  collects the reception timing signal D 9  for every same type of unit carrier row signal D 8 . The binary is inverted according to the reception timing so that each digital transmission data D 1  is restored. As a result, each reception terminal  60  can distinctly process information from each of a plurality of transmission terminals  51 . 
         [0052]    The transmission and reception terminal  70  of the main controller  27  functioning as the reception terminal  60  receives the digital transmission data D 1  from the luminous intensity sensor  35  and the digital transmission data D 1  from the vehicle speed sensor  36  simultaneously, for example. The main controller  27  controls a speedometer (not shown) so that the vehicle speed is displayed and delivers a command to turn on the headlights  31 . Digital transmission data D 1  containing information about turn-on command is then converted to the unit carrier row signal D 8 , which is transmitted. The reception terminal  60  provided in a drive circuit  31 A for the headlights  31  receives the transmitted unit carrier row signal D 8 . A switch (not shown) of the drive circuit  31 A is turned on so that the headlights  31  are lighted. 
         [0053]    According to the data communication system  50  of the embodiment, data communication can be carried out simultaneously between a plurality of transmission terminals  51  and the reception terminal  60 . The digital transmission data D 1  is composed of a group of unit carrier row signals D 8  on the transmission path (power supply line  23 ) between each transmission terminal  51  and the reception terminal  60 . Each unit carrier row signal D 8  is shorter than 1 bit of the digital transmission data D 1 . Furthermore, each unit carrier row signal D 8  is composed of rowed unit carrier waves D 7  each of which is shorter than the row signal D 8 . More specifically, the digital transmission data D 1  is decomposed into unit carrier waves D 7  each of which is highly shorter than 1 bit of transmission data and which are to be transmitted. Consequently, when the unit carrier waves D 7  are multireflected thereby resulting in generation of primary reflected wave, secondary reflected wave and so on, normal unit carrier waves D 7  can readily be distinguished from the reflected waves. As also can be confirmed in another embodiment described below, each unit carrier wave D 7  has a width which is less than or equal to 20 ns and an interval between the unit carrier waves D 7  is not less than 20 ns. As a result, the reflected wave can be distinguished from the normal unit carrier wave D 7 . Accordingly, the above-described system  50  can prevent communication interference due to multireflection of signal waves so that better data communication can be performed. 
         [0054]    Furthermore, in the foregoing embodiment, the detection pulse D 5  of 6 ns, which can be regarded as an impulse signal, is further amplitude-modulated. It can be considered that the detection pulse D 5  would be delivered to the power supply line  23  as a single-shot impulse signal. However, since frequency components contained in the single-shot impulse signal of 6 ns range from DC to 150 MHz, multireflection due to discontinuity of the chassis  11  unavoidably results in frequency dip. Furthermore, in a relatively lower frequency range between DC to 150 MHz, propagation loss is hard to reduce, and reflected waves return forcefully thereby being added to normal transmission waves. On the other hand, in the embodiment, the 6-ns detection pulse D 5 , which can be regarded as an impulse signal, is amplitude-modulated, for example, in a range from 520 to 680 MHz. Accordingly, attenuation of reflected waves can be promoted at an early time. Consequently, frequency dip due to multireflection can be reduced. With this effect, the vehicle ground GND can be used as the transmission path. Furthermore, when the power supply line  23  and the vehicle ground GND are used as a transmission path as in the embodiment, space saving can be achieved as compared with the case where a communication line is separately provided. Additionally, the result of an experiment can confirm that data communication can desirably be carried out in the frequency range from 520 to 680 MHz in the case of general vehicles. 
         [0055]    Jamming impulse is contained in metal wires including the power supply line  23  and control line of the vehicle body  10  and the vehicle housing body  14 . The jamming impulse results from repeated on-off operation of an electric motor, ignition system, solenoid, relay or the like through each of which large current flows. The jamming impulse is mostly a ringing impulse with a width ranging from 0.5 to several us and has a long repetition frequency ranging from several tens to several hundreds μs (or merely one impulse). However, the jamming impulse superimposed on the power supply line  23  of the vehicle  10  is distributed in a broad frequency range. This means that a frequency filtering function cannot avoid communication error no matter what frequency band is employed in a transmission system using communication through the vehicle body. It has conventionally been considered that data should be transmitted at a frequency rate lower than a lowest frequency contained in the jamming impulse for the purpose of avoidance of transmission error. However, this results in a bottleneck that the data transmission speed needs to be equal to or below 1 kbps. 
         [0056]    According to the above-described data communication system  50  of the embodiment, a plurality of unit carrier waves D 7  are arranged in the predetermined arrangement pattern to serve as the unit carrier row signal D 8 . Consequently, the waveform of the unit carrier row signal D 8  can readily be distinguished from waveforms of jamming impulse, whereupon the transmission error can be rendered minimum. Furthermore, generation of the unit carrier row signal D 8  is desynchronized with respect to occurrence of jamming impulse. Accordingly, when phases of the unit carrier row signals D 8  are synchronized and phases of carrier frequencies in the modulation of the unit carrier waves D 7  are synchronized, the unit carrier row signal D 8  can readily be distinguished from the jamming impulse. 
         [0057]    Additionally, an adverse effect of the jamming impulse can further be reduced when data transmission is established between the transmission terminals  51  and the reception terminal  60  while the master and the slave handshake by bidirectional protocol. 
         [0058]      FIGS. 7 to 10  illustrate a data transmission system  50 R of a second embodiment of the invention. The second embodiment differs from the first embodiment in the principle of transmission of the unit carrier row signal D 8 . More specifically, each transmission terminal  51 R generates an electric field in the vicinity of the surface of the vehicle ground GND so as to change according to the unit carrier row signal D 8 . A reception terminal  60 R receives the unit carrier row signal D 8  based on the change in the electric field. 
         [0059]    Identical or similar parts are labeled by the same reference symbols in the second embodiment as those in the first embodiment. The description of these parts will be eliminated and only the difference of the second embodiment from the first embodiment will be described. 
         [0060]    Referring to  FIG. 7 , each transmission terminal  51 R is also provided with an opposed electrode plate  51 T which serves as a transmission side opposed conductive member in the invention. The opposed electrode plate  51 T is connected via a low-pass filter  57 F to an output of a buffer circuit  57  of the transmission terminal  51 R as shown in  FIG. 8 . The opposed electrode plate  51 T is disposed so as to be opposed to the vehicle ground GND in an insulated relation to the vehicle ground GND. Upon actuation of the transmission terminal  51 R, voltage according to the amplitude of the unit carrier row signal D 8  is applied between the opposed electrode plate  51 T and the vehicle ground GND. 
         [0061]    Referring further to  FIG. 7 , each reception terminal  60 R is provided with an opposed electrode plate  60 T which serves as a reception side opposed conductive member in the invention. The opposed electrode plate  60 T is connected via a high-impedance filter  80  to an input of an amplifier  62  of the reception terminal  60 R as shown in  FIG. 9 . The opposed electrode plate  60 T is also disposed so as to be opposed to the vehicle ground GND in an insulated relation to the vehicle ground GND. 
         [0062]    The high-impedance filter  80  includes a voltage follower circuit  81  provided at the input side as shown in  FIG. 10 . This voltage follower circuit  81  serves a source follower circuit of a junction field effect transistor  82  (hereinafter referring as FET  82 ). The FET  82  has a source to which the vehicle ground GND is connected and a gate to which the opposed electrode plate  60 T is connected. In other words, a potential difference between the opposed electrode plate  60 T and the vehicle ground GND is supplied to the voltage follower circuit  81 . An input impedance of the reception terminal  60 R is raised as the result of provision of the voltage follower circuit  81 . Accordingly, even when a potential difference occurs between the opposed electrode plate  60 T and the vehicle ground GND, current which possibly flows therebetween becomes extremely small. As a result, information can be obtained from the electric field without reduction in the potential difference produced by the electric field. Furthermore, output of the voltage follower circuit  81  is supplied via an amplifier  62  to an amplitude demodulator  63  as shown in  FIG. 9 . 
         [0063]    Additionally, a transmission and reception terminal  70 R is structured so as to have a transmission circuit corresponding to the transmission terminal  51 R and a reception circuit corresponding to the reception terminal  60 R. 
         [0064]    According to the arrangement of the second embodiment, each transmission terminal  51 R applies voltage between the vehicle ground GND and the opposed electrode plate  51 T. The applied voltage is based on the amplitude of the unit carrier row signal D 8 . As the result of the aforesaid voltage application, an electric field is established around the whole vehicle ground GND. The reception terminal  60 R can receive the unit carrier row signal D 8  based on the change in the electric field. Thus, since data communication is carried out by utilizing the electric field established around the whole vehicle ground GND, data communication can be realized by employment of lower current as compared with the case where electric wave propagated in the space is used, whereupon electric power consumption can be reduced. Furthermore, since the communicable coverage is limited to the inside of the inner peripheral wall of the vehicle ground GND, occurrence of radio disturbance can be prevented without provision of electromagnetic shield. Furthermore, since the vehicle ground GND of the vehicle  10  is utilized as the propagation path, new communication cables need not be provided and the data communication system can be installed readily. 
         [0065]      FIGS. 11 and 12  illustrate a data communication system  50 R of a third embodiment. In the third embodiment, the data communication system of the second embodiment is improved according to the structure of the vehicle  10 . 
         [0066]    A vehicle housing body  14  of the vehicle  10  is divided into an engine compartment  14 A, a passenger compartment  14 B and a trunk  14 C by a metal wall  14 W as shown in  FIG. 11 . In particular, the whole engine compartment  14 A and the whole trunk  14 C are covered with the metal wall  14 W such that the interior of each compartment is shielded. Accordingly, it is difficult to transmit an electric field signal (serving as a wireless signal in the invention) generated in the passenger compartment  14 B into the engine compartment  14 A and the trunk  14 C. In view of this problem, two electric field relay devices  73  are provided between the engine compartment  14 A and the passenger compartment  14 B and between the passenger compartment  14 B and the trunk  14 C respectively. Each electric field relay device  73  serves as a radio relay device in the invention. As the result of provision of the electric field relay devices  73 , data communication can be carried out between two locations in any compartment in the vehicle housing body  14  using the electric field. 
         [0067]    More specifically, as shown in  FIG. 12 , each electric field relay device  73  comprises a pair of relay terminals  72  connected to each other by a relay cable  73 C. Each relay terminal  72  includes a transmission processing circuit  51 Y having the same structure as the transmission terminal  51 Y described in the second embodiment and a reception processing circuit  60 Y having the same structure as the reception terminal  60 R described in the second embodiment. The relay cable  73 C connects the transmission processing circuit  51 Y of the first relay terminal  72  and the reception processing circuit  60 Y of the second relay terminal  72 . The relay cable  73 C further connects the transmission processing circuit  51 Y of the second relay terminal  72  and the reception processing circuit  60 Y of the first relay terminal  72  to each other. As a result, the electric field signal received by the reception processing circuit  60 Y of the first relay terminal  72  can be transmitted from the transmission processing circuit  51 Y of the second relay terminal  72 . Furthermore, the electric field signal received by the reception processing circuit  60 Y of the second relay terminal  72  can be transmitted from the transmission processing circuit  51 Y of the first relay terminal  72 . 
         [0068]    The paired relay terminals  72  constituting one electric field relay device  73  are disposed in the engine compartment  14 A and the passenger compartment  14 B respectively. The electric field relay device  73  is inserted through a communication hole  14 D formed through the metal wall  14 W dividing the engine and passenger compartments  14 A and  14 B. In the same manner, the paired relay terminals  72  constituting the other electric field relay device  73  are disposed in the passenger compartment  14 B and the trunk  14 C respectively. The electric field relay device  73  is inserted through a communication hole  14 D formed through the metal wall  14 W dividing the passenger compartment  14 B and the trunk  14 C. As a result, data communication can be carried out using the electric field signal between the transmission terminal  51 R and the reception terminal  60 R, between the transmission terminal  51 R and the transmission and reception terminal  70 R or between the reception terminal  60 R and the transmission and reception terminal  70 R between any two of the engine compartment  14 A, the passenger compartment  14 B and the trunk  14 C. 
       EXAMPLE 1 
       [0069]    The transmission terminal  51  of the first embodiment was made, and digital serial data “0011110000” with 1-bit length of 20 ns was generated as the ID data D 4  in the first embodiment. The detection pulse D 5  with the width of 6 ns was generated twice in synchronization with inversion of “0” and “1” of the ID data D 4 . A pulse train comprising two detection pulses D 5  was generated as the identification pulse train D 3  of the first embodiment. The pulse train D 3  was amplitude-modulated at 550 MHz so that the unit carrier row signal D 8  comprising two unit carrier waves D 7 . The waveform of the unit carrier row signal D 8  was measured at an output of the transmission terminal  51 .  FIG. 13  shows the results of the measurement. 
         [0070]    The aforesaid unit carrier row signal D 8  was applied as a voltage signal between a power supply line and a vehicle ground of an actual vehicle (passenger motor car). The waveform of voltage applied between the power supply line and the vehicle ground was measured at a distance.  FIG. 14  shows the results of the measurement. 
         [0071]    Since the above-mentioned ID data D 4 , “0011110000” has the 1-bit length of 20 ns, an interval between the edges is 80 ns and appears as an interval of 80 ns between a pair of unit carrier waves D 7  composing the unit carrier row signal D 8  as shown in  FIG. 13 . The width of the unit carrier wave D 7  is about 6 ns as obvious from  FIG. 13 . When the waveform of  FIG. 13  is checked against the waveform of  FIG. 14 , the normal unit carrier waves D 7  appear as the largest waveforms (as shown by W 1  in  FIG. 14 ). It is understood that primary reflected waves W 2  and secondary reflected waves W 3  appear in the vicinity of the largest waveforms with time shift respectively. Furthermore, it is understood that a noise wave W 4  superimposes over the whole voltage waveform. Moreover, it is understood that the unit carrier waves D 7  can readily be distinguished from the reflected waves W 2  and W 3  and noise waves W 4 . 
         [0072]    Furthermore, it is understood that when the time interval is equal to or more than 20 ns, amplitude levels of subsequent reflected waves are sufficiently deteriorated in the unit carrier waves D 7  with the frequency of 550 MHz as in the embodiment (that is, about 3 to 4 m in consideration of reciprocation), as obvious from  FIG. 14 . In the case of the vehicle  1   b  with the general size, an identification code of 50 Mbps can be transmitted. 
         [0073]    The present invention should not be limited to the foregoing embodiments. Modified forms as described below are within the technical scope of the invention. Furthermore, the invention can be practiced in modified forms other than those described below without departing from the gist of the invention. 
         [0074]    In the first embodiment, each transmission terminal  51  generates the identification pulse train D 3 , which is then amplitude-modulated so that the unit carrier row signal D 8  is generated. However, as shown in  FIG. 15 , an identification wave output circuit may be provided for outputting a unit carrier row signal D 8  specific to each transmission terminal  51 . In this case, the identification wave output circuit is driven in synchronization with the binary inversion of digital transmission data D 1  to output the unit carrier row signal D 8 . 
         [0075]    The data communication systems  50 ,  50 R are provided in the vehicle  10  in the first to third embodiments. However, the data communication system of the invention may be provided in a machine, equipment, etc. other than the vehicles. 
         [0076]    The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the scope of the invention as defined by the appended claims.