Abstract:
A cable for transmitting signals between two systems is disclosed. The cable provides a connector, which encloses a circuit unit comprising at least one of a transmitter and a receiver and plugged with the system, and a co-axial cable pulled out from the connector. The circuit unit includes at least a transmitter and a receiver. The transmitter receives an input signal in the differential form from the system and outputs a transmitting signal in the single-ended form to the co-axial cable. The receiver receives the transmitting signal in the single-ended form from the co-axial cable and outputs an output signal in the differential form to the system.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present application relates to a cable for transmitting signals. 
     2. Background Arts 
     A Japanese Patent Application laid open No. 2005-135840A has disclosed a cable implemented with connectors in respective ends thereof. The connector disclosed therein includes a function to reshape a receiving signal and/or a transmitting signal. 
     Recently, one type of a cable for transmitting a signal, which includes in a connector attached to an end thereof a signal processing circuit, such as clock data recovery (CDR) circuit, is preferably applied to the connection between servers, storage, switches, and so on in the data center. Such application has used cables with the type of, the twisted-pair cable, the twin-Ax cable, which is often named as “twinax” cable, and so on with the connector in the end thereof. 
     SUMMARY OF THE INVENTION 
     An aspect of the present application relates to a cable for connecting between two apparatuses. The cable comprises a connector and a metal wire. The connector includes a circuit unit electrically connected to one of the apparatuses and plugged to the one of the apparatuses. The metal wire is electrically connected to the circuit unit in the connector. A feature of the cable of the present application is that the circuit unit provides at least one of a transmitter and a receiver. The transmitter receives an input signal in the differential form from the system and generates a transmitting signal in the single-ended form to the metal wire. The receiver receives the transmitting signal in the single-ended form from the metal wire and generates an output signal in the differential form to the apparatus. The connector may further include a circuit board that includes an area, an interface, and another interface. The area disposes the circuit unit thereon. The interface has an electrode electrically connected to the circuit unit and one of the apparatuses. The other interface has a pad electrically connected to the circuit unit and the metal wire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
         FIG. 1  schematically illustrates a cable for transmitting a signal according to an embodiment of the present application; 
         FIG. 2  shows a functional block diagram of a portion of the cable shown in  FIG. 1 ; 
         FIG. 3  shows a functional block diagram of a retiming circuit as an example of the circuit unit of the present application; 
         FIG. 4  is a side cross section of an example of the connector of the present application; 
         FIG. 5A  views a top surface and  FIG. 5B  views a back surface of the circuit board according to an embodiment of the present application; 
         FIGS. 6A and 6B  view top and back surfaces, respectively, of another circuit board modified from the circuit board of the embodiment; 
         FIGS. 7A and 7B  view top and back surfaces, respectively, of still another circuit board also modified from the embodiment shown in  FIGS. 5A and 5B ; 
         FIGS. 8A and 8B  view top and back surfaces, respectively, of still another circuit board modified from the embodiment shown in  FIGS. 5A and 5B ; 
         FIGS. 9A and 9B  view top and back surfaces, respectively, of still another circuit board modified from the embodiment shown in  FIGS. 5A and 5B ; and 
         FIGS. 10A and 10B  view top and back surfaces, respectively, of still another circuit board modified from the embodiment shown in  FIGS. 5A and 5B . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Some embodiments according to the present application will be described as referring to drawings. However, it is intended that the present invention is not limited to those particular embodiments and modification disclosed, but that the invention include all embodiments falling within the scope of the appended claims. In the description of the drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without duplicated explanations. 
       FIG. 1  schematically illustrates a cable  1 A according to an embodiment of the present invention. As shown in  FIG. 1 , the cable  1 A comprises a cable bundle  20  and two connectors  10  each attached to respective ends of the cable bundle  20 . The cable bundle  20  includes a plurality of co-axial cables  21 . One of connectors  10  is to be plugged with an external apparatus  101  and electrically connected to the external apparatus  101 . The other connector  10  is also to be plugged with other external apparatus  102  to constitute the electrical connection thereto. 
       FIG. 2  schematically shows a functional block diagram of the cable  1 A. Two connectors  10  each provides a circuit unit  12 , which may include a signal shaper such as a clock data recovery (CDR), a repeater, and so on, having a type of an integrated circuit (IC)  121 . The circuit unit  12  in one of the connector  10  is electrically connected to the external apparatus  102  by plugging the connector  10  with the external apparatus  101 , and the circuit unit  12  in the other of the connector  10  is also coupled with the external apparatus  101  by plugging the other connector  10  with the external apparatus  102 . 
     The circuit unit  12  includes a transmitter  122  and a receiver  123 , where the transmitter  122  constitutes a part of the IC  121 , receives an input signal S 1  in a pair of input terminals,  121   a  and  121   b . The input signal S 1  has the arrangement of the differential signal comprising a positive phase signal Sa 1  and a negative phase signal Sb 1 . The transmitter  122  also provides a terminator  124  which may be a combined circuit of a capacitor and a resistor with resistance of 50 ohm, connected between one of output terminals,  121   c  and  121   d , and the ground GND. The terminator  124  may be connected from the one of the output terminals,  121   c  and  121   d , and a power line. 
     The transmitter  122  reshapes the input signal S 1  and outputs the reshaped signals to the paired output terminals,  121   c  and  121   d , differentially. Because one of the output terminals  121   d  is terminated by the terminator  124 , one of the positive phase signal and the negative phase signal is not transmitted. The other of the output terminals  121   c  is connected to one of co-axial cables  21  in the cable bundle  20 . The one of the output terminals  121   c  may be electrically connected to the metal wire  21   a  of the co-axial cable through a coupling capacitor. The transmitted signal S 2  is the single-ended signal including one of the positive phase signal Sa 1  and the negating phase signal Sb 1 . The transmitted signal S 2  is carried to the other connector  10  through the co-axial cable  21 . 
     The receiver  123 , which is also involved within the IC  121 , provides a pair of input terminals,  121   e  and  121   f . One of the input terminals  121   e  is coupled with the metal wire  21   a  of the co-axial cable  21  through a coupling capacitor not illustrated in the figures. The other of input terminals  121   f  is grounded through the terminator  125  that includes a resistor and a capacitor. The input terminal  121   f  may be terminated to the power line. Thus, the IC  121  receives in one of the input terminals  121   e  the transmitted signal S 2 , which is the single-ended signal, from the other connector  10  through the co-axial cable  21 . 
     The receiver  123  in the IC  121  re-shapes the transmitted signal S 2  and provides an output signal S 3  in the pair of output terminals,  121   g  and  121   h , to the external apparatuses,  101  or  102 . The output signal S 3  has the arrangement of the differential signal including the positive phase signal Sa 2  and the negative phase signal Sb 2 .  FIG. 2  illustrates an arrangement of the circuit unit  12  including both of the transmitter  122  and the receiver  123 . However, the circuit unit  12  may provide only one the transmitter  122  and the receiver  123 . For instance, the arrangement, where the circuit unit  12  in one of the connectors  10  provides only the transmitter  122 , while, the circuit unit  12  in the other connector  10  provides only the receiver  123 , may be applicable to the cable  1 A. 
       FIG. 3  shows an exemplary block diagram of the IC  121  implemented within the cable  1 A. The transmitter  122  in the IC  121  may provide an equalizer  122   a , a clock data recovery (CDR)  122   b , and a driver  122   c , where all of them are connected in series and have the differential arrangement. The differential input of the equalizer  122   a  is connected to the input terminals,  121   a  and  121   b , of the IC  121 , and the differential output of the equalizer  122   a  is connected to the differential input of the CDR  122   b , the differential output of the CDR  122   b  is connected to the differential input of the driver  122   c , and the differential output of the driver  122   c  is connected to the pair of output terminals,  121   c  and  121   d.    
     The receiver  123  in the IC  121  also comprises an equalizer  123   a , a CDR  123   b , and a driver  123   c , each having the differential arrangement and connected in series between the pair of input terminals,  121   e  and  121   f , and the pair of the output terminals,  121   g  and  121   h . In an example shown in  FIG. 3 , the IC has the arrangement of, what is called, the re-timing circuit. However, the IC may have other types of the signal re-shaping circuit such as a repeater. When the IC has the arrangement of the repeater, the CDRs implemented in the re-timing circuit shown in  FIG. 3  may be removed. 
       FIGS. 4 and 5  show the arrangement of the connector  10 , where  FIG. 4  is a side cross section of the connector  10 , while  FIGS. 5A and 5B  are top and bottom views of the connector  10 . 
     The connector  10  of the present embodiment further includes a circuit board  13  having a rectangular substrate made of dielectric material and interconnections provided on both of a top surface  13   a  and a back surface  13   b  of the dielectric substrate. The circuit board  13  demarcates an area to which the circuit unit  12  is assembled from other areas to which an interface  14  to the external apparatuses,  101  or  102 , is provided. 
     The present embodiment shown in  FIGS. 5A and 5B  provides the circuit units in both of the top surface  13   a  and the back surface  13   b  of the circuit board  13 . Specifically, a part of the circuit unit  12 A is mounted on the top surface  13   a , while, another part of the circuit unit  12 B is mounted on the back surface  13   b . As shown in  FIG. 5A , the part of the circuit unit  12 A constitutes one IC  121  that includes two transmitters  122  and two receivers  123 . On the other hand, another part of the circuit unit  12 B constitutes another IC  121  that also includes two transmitters  122  and two receivers  123 .  FIGS. 5A and 5B  omit the terminators  124  and  125  attributed to the circuit units,  12 A and  12 B. 
     The interface  14  includes a plurality of electrodes,  14   a  to  14   d , in both of the top surface  13   a  and the back surface  13   b . Specifically, the top surface  13   a  provides two pairs of the electrodes,  14   a  and  14   b , and two pairs of another electrodes,  14   c  and  14   d . The former pairs of the electrodes,  14   a  and  14   b , may be those for the input signals S 1  and connected to the input terminals,  121   a  and  121   b , of the transmitter  121  corresponding thereto. The later pairs of the electrodes,  14   c  and  14   d , may be those for the output signals S 3  and connected to the output terminals,  121   g  and  121   h , of the receiver  123  corresponding thereto.  FIGS. 5A and 5B  only illustrate the electrodes for the input and output signals, and omit other electrodes for the ground and the power lines. 
     The other interface  15 , which is disposed on the top surface  13   a  of the circuit board  13 , provides four pads  15   a  and another four pads  15   b . These pads,  15   a  and  15   b , which are electrically connected with respective ends of the metal wire  21   a  of the co-axial cable  21 , have a width 1.2 to 2.0 times greater than a diameter of the metal wire  21   a  and a gap to the next pads greater than the width thereof. 
     The former pads  15   a  may be those for the transmitter and two of them are connected by the AC mode to the output terminal  121   c  of the transmitter  122  in the part  12 A of the circuit unit  12 , while, other two are connected also in the AC mode to the output terminal  121   c  of the transmitter in another part  12 B of the circuit unit  12  as interposing via holes  16   a . The latter pads  15   b  may be those for the receiver. Two of the latter pads  15   b  are connected in the AC mode to the input terminal  121   e  in the part  12 A of the circuit unit  12 , while, other two of the latter pads  15   b  are connected also in the AC mode to the input terminal  121   e  in another part  12 B of the circuit unit  12  through via holes  16   b.    
     Referring to  FIG. 4 , the connector  10  further provides a housing  17  to enclose the circuit unit  12  and the circuit board  13  therein. The housing  17  has a parallelepiped arrangement along the longitudinal direction of the circuit board  13  and having a lid  17   a  facing the top surface  13   a  of the circuit board  13  and a bottom  17   b  facing the back surface  13   b  of the circuit board  13 . The housing  17  provides a front opening to constitute an electrical connector accompanied with the interface  14 . The co-axial cables  21  are guided from the rear of the housing  17 . In the embodiment shown in  FIG. 4 , a distance from the bottom  17   b  of the housing  17  to the back surface  13   b  of the circuit board is shorter than a distance from the lid  17   a  of the housing  17  to the top surface  13   a  of the circuit board  13 . That is, the circuit board  13  is set in the housing  17  offset from the center thereof in the vertical direction. 
     Advantages of the cable  1 A will be described. The external apparatuses,  101  or  102 , generally processes information by differential signals and outputs these differential signals. The cable  1 A of the embodiment in the transmitter  122  receives the input signal S 1  as the differential signal from the external apparatuses,  101  or  102 , and carries the differential signal as the transmitted signal S 2  by the single-ended signal on the co-axial cable  21 . The cable  1 A in the receiver  123  receives thus carried transmitted signal S 2  as the single-ended signal and generates the output signal S 3  as the differential signal. Thus, the transmitted signal S 2  is transmitted on the signal metal line as the single-ended signal; accordingly, this arrangement becomes free from the skew between two signals constituting the differential signal. The skew between two signals results in the transmission loss. 
     Also, the single metal line like the co-axial cable  21  comparing with, for instance, a twisted pair cable and/or a twin-Ax cable, makes it possible to form the cable  1 A in further flexible and slim. Accordingly, the cable  1 A between two external apparatuses,  101  and  102 , may enhance the workability of the installation. Moreover, when the signal metal cable has a core cross section substantially equal to sum of core cross sections of the twisted pair cable, or the twin-Ax cable, the transmission loss of the transmitted signal S 2  may be reduced because of the reduction of the series resistance. A communication with a longer haul becomes available. 
     The metal wire  21   a  of the co-axial cable  21 , like the present arrangement, may be directly connected to respective output terminals,  121   c  and  121   e  of the circuit unit  12  without interposition other electrical components, such as a balun, a common mode choke, and so on to transmit the differential signal. Accordingly, the cable  1 A of the embodiment not only becomes free from the loss due to those electronic components, but the connector may be formed in compact. 
     The transmitter  122  of the present embodiment provides the pair of output terminals,  121   c  and  121   d , one of which is terminated to the ground GND through the terminator  124  and the other is connected to the metal wire  21   a  of the co-axial cable  21 . This arrangement effectively brings the function to convert the input signal S 1  with the differential mode into the transmitted signal S 2  of the single-ended arrangement. 
     Also, the receiver  123  of the present embodiment provides the pair of input terminals,  121   e  and  121   f , one of which receives the transmitted signal S 2 , while, the other is grounded. This arrangement preferably realizes the function to convert the transmitted signal S 2  with the single-ended arrangement into the output signal S 3  of the differential arrangement. 
     The connector  10  provides the circuit board  13  that demarcates the area for mounting the circuit unit  12  from the interfaces,  14  and  15 , the latter of which provides the pads,  15   a  and  15   b . This arrangement of the connector  10  preferably realizes the mechanism to electrically connect the external apparatuses,  101  and  102 , the circuit unit  12 , and the co-axial cable  21 . 
     The part  12 A of the circuit unit  12  is mounted on one of the surface, namely, the top surface  13   a  of the circuit board, while, the other part  12 B of the circuit unit  12  is mounted on another of the surface, namely, the back surface  13   b  of the circuit board  13 . Thus, the circuit board  13  may provide enough room to mount the circuit unit  12 , which enables the interconnections on the circuit board  13  to wire with widened spaces and to reduce the crosstalk between the interconnections. 
     The interface  14  may provide electrodes on the top surface  13   a  of the circuit board  13  and other electrodes on the other surface, namely, the back surface  13   b  of the circuit board  13 . The former electrodes are electrically connected to the part  12 A of the circuit unit  12 , while, the latter electrodes are electrically connected to the other part  12 B of the circuit unit  12 . Thus, the electrodes in the interface  14  may be connected to the circuit unit  12  in respective surfaces,  13   a  and  13   b , of the circuit board  13  without interposing via holes. 
     The pads,  15   a  and  15   b , may be provided in only one of surfaces,  13   a  or  13   b , of the circuit board  13 . This arrangement of the pads,  15   a  and  15   b , enables the co-axial cable  21  in the metal wires thereof to be fixed to respective pads,  15   a  and  15   b , by the single side soldering, which may enhance the efficiency of the soldering process. 
     The circuit board  13  is set within the housing such that a distance from the back surface  13   b  thereof to the housing  17  is shorter than a distance from the top surface  13   a  to the housing  17 ; that is the circuit board  13  is offset in the vertical direction from the middle of the housing. In a conventional arrangement of the cable, in particular, the conventional arrangement of the circuit board in the connector, the co-axial cables are fixed to the circuit board in both of the top and back surfaces. When the circuit board  13  is offset form the vertical midway of the housing  17 , the surface closer to the housing possibly becomes hard to assemble electronic components thereon. The present embodiment, however, the co-axial cables  21  are fixed only to the top surface  13   a  of the circuit board  13 . Accordingly, the back surface  13   b , which is closer to the housing  17 , may be left to mount electronic components thereon. 
     (First Modification) 
       FIGS. 6A and 6B  are plan views of the circuit board  13 A modified from the aforementioned circuit board  13 , where  FIG. 6A  shows the top surface  13   a , while,  FIG. 6B  shows the back surface  13   b  thereof. The description below is only for features distinguishable from those of the aforementioned embodiment, and portions not described below are substantially same with those shown in  FIG. 5 . 
     The circuit board  13 A of the modified embodiment provides the area to mount the circuit unit  12  only in one of the top surface  13   a  and the back surface  13   b . That is, the whole circuit unit  12  is mounted in the top surface  13   a . As illustrated in  FIG. 6A , the circuit unit  12 , which is constituted by a unique IC  121  that includes four transmitters  122  and four receivers  123 .  FIGS. 6A and 6B  omit the terminators,  124  and  125 . 
     The electrodes,  14   a  and  14   b , provided on the top surface  13   a  in the interface  14  are connected to the input terminals,  121   a  and  121   b , of the transmitter  122  corresponding thereto. Similarly, the electrodes,  14   c  and  14   d , formed on the top surface  13   a  in the interface  14  are coupled with the output terminals,  121   g  and  121   h , of the receiver  123  corresponding thereto. The electrodes,  14   a  and  14   b , provided in the back surface  13   b  in the interface  14  are connected to the input terminals,  121   a  and  121   b , of the transmitter  122  corresponding thereto through the via holes  16   c . Similarly, the electrodes,  14   c  and  14   d , formed on the back surface  13   b  in the interface  14  are connected to the output terminals,  121   g  and  121   h , of the receiver  123  through the via holes  16   d . The pads  15   a  for the transmission are connected in the AC mode to the output terminals  121   c  of the transmitter  122  only in the top surface  13   a , while, the pads  15   b  for the reception are connected in the AC mode to the input terminals  121   e  of the receiver  123  only in the topo surface  13   a.    
     The arrangement of the modified embodiment shown in  FIGS. 6A and 6B  may also enhance the workability of the installation of the cable connecting two external apparatuses,  101  and  102 , but may suppress the transmission loss due to the skew between two signals whose phases are opposite to each other. Moreover, the embodiment shown in  FIGS. 6A and 6B  installs the whole circuit unit  12  only in one of surfaces  13   a  of the circuit board  13 A, which not only makes the assembly for mounting the circuit unit  12  on the circuit board  13 A simple and shortens the process thereof, but also widens the area to mount other electrical components on the back surface  13   b.    
     The electrodes,  14   a  to  14   d , in the interface  14  may be formed in each surfaces,  13   a  and  13   b , of the circuit board  13 A, and those electrodes,  14   a  to  14   d , are electrically connected to the circuit unit  12  mounted on the top surface  13   a . Even in such an arrangement of the electrodes,  14   a  to  14   d , and the circuit unit  12  may show the advantages described above. 
     (Second Modification) 
       FIGS. 7A and 7B  are plan views of the circuit board  13 B according to the second modification of the first embodiment shown in shown in  FIGS. 5A and 5B , where  FIG. 7A  illustrates the top surface  13   a , while,  FIG. 7B  shows the back surface  13   b  of the circuit board  13 B. Explanations below concentrate on points distinguishable from former embodiment and modification, and arrangements not explained are substantially same with those shown in  FIGS. 5A to 6B . 
     The circuit board  13 B of the present modification provides interfaces,  15 A and  15 B, substituted from the interface  15 . The former interface  15 A, which is provided on the top surface  13   a , has two pads  15   a  and other two pads  15   b . Also, the interface  15 B provided in the back surface  13   b , has two pads  15   a  for the transmission and other two pads  15   b  for the reception. Two pads  15   a  in the first interface  15 A are connected in the AC mode to the output terminals  121   c  of the transmitter  122  corresponding thereto through the interconnection of the top surface  13   a . Two pads  15   a  in the second interface  15 B are connected in the AC mode to the output terminals  121   c  of the transmitter  122  corresponding thereto through the interconnection on the back surface  13   b  and the via holes  16   b . Two pads  15   b  in the top interface  15 A are connected in the AC mode to the input terminals  121   e  of the receiver  123  corresponding thereto through the interconnection on the top surface  13   a , while, two pads  15   b  in the back interface  15 B are connected also in the AC mode to the input terminals  121   e  of the receiver  123  corresponding thereto through the interconnection on the back surface  13   b  and the via holes  16   a.    
     The arrangement thus described, similar to the aforementioned arrangements, not only enhance the workability of the installation of the cable  10  connecting two external apparatuses,  101  and  102 , but may suppress the transmission loss due to the skew between two signals whose phases are opposite to each other. Also, the arrangement shown in  FIGS. 7A and 7B  provides the pads,  15   a  and  15   b , for the transmission and the reception in both of top and back surfaces,  13   a  and  13   b , of the circuit board  13 B, which enables to widen a space between pads and metal wires  21   a  and to reduce the crosstalk between the pads and/or metal wires. 
     Two types of the crosstalk should be considered, one of which is between the input signals S 1  or between the output signals S 3 , and another is between the input signal S 1  and the output signal S 3 . The former crosstalk is often called as the far end crosstalk (FEXT), and the latter is called as the near end crosstalk (NEXT). The arrangement to widen the space between the pads and the metal wires is effective to reduce both types of the crosstalk. 
     (Third Modification) 
       FIGS. 8A and 8B  are views of the top surface and the back surface of the circuit board  13 C according to the third modification. Similar to the description for the former modifications, the description below concentrates on points distinguishable from those of the aforementioned modifications, and the arrangements not explained are substantially same with or similar to those of the aforementioned modifications. 
     The circuit board  13 C of the present modification provides, instead of the interface  15  in the side of the co-axial cable  21 , the top interface  15 A in the top surface  13   a  and the back interface  15 B in the back surface  13   b . Each interfaces,  15 A and  15 B, provides two types of pads,  15   a  and  15   b . Two pads  15   a  for the transmission in the top interface  15 A are connected in the AC mode to the output terminals  121   c  of the transmitter  122  corresponding thereto in the top surface  13   a  of the circuit board  13 C; also, the pads  15   a  for the transmission in the back interface  15 B are connected in the AC mode to the transmitter  122  in the back surface  13   b . Two pads  15   b  for the reception on the top interface  15 A are connected in the AC mode to the input terminals  121   e  of the receiver  123  in the top surface  13   a , while, the pads  15   b  for the reception in the back interface  15 B are connected in the AC mode to the input terminals  121   e  of the receiver  123  but in the back surface  13   b  of the circuit board. 
     The arrangement thus described as referring to  FIGS. 8A and 8B , similar to the aforementioned arrangements, not only enhance the workability of the installation of the cable  10  connecting two external apparatuses,  101  and  102 , but may suppress the transmission loss due to the skew between two signals whose phases are opposite to each other. Also, two types of pads,  15   a  and  15   b , are provided in both surfaces,  13   a  and  13   b , of the circuit board  13 C, which may widen the space between the pads and metal wires  21   a  to reduce the crosstalk between the pads and the metal wires  21   a.    
     (Fourth Modification) 
       FIG. 9A  is a plan view of the top surface  13   a  and  FIG. 9B  is a plan view of the back surface  13   b  of the circuit board  13 D according to the fourth modification of the invention. Similar to the aforementioned modifications, the explanation below concentrates on points different from those of the aforementioned one, and left points not explained are substantially same with those of the former embodiments. 
     The circuit board  13 D of the present modification provides, substituted from the interfaces,  15 A and  15 B, a top interface  15 C and a back interface  15 D. The top interface  15 C provides two pads  15   a  for the transmission and other two pads  15   b  for the reception. A feature of the top interface  15 C of the present modification is that these pads,  15   a  and  15   b , are arranged close to one edge of the circuit board  13 D to form a room along the other edge opposite to the one edge, that is, the pads,  15   a  and  15   b , are unevenly arranged on the top surface  13   a . Accordingly, these pads,  15   a  and  15   b , in the top interface  15 C have spaces to the next pads narrower than those of the former modification  13 C shown in  FIG. 8A . 
     Also, the back interface  15 D provides two pads  15   a  for the transmission and the other two pads  15   b  for the reception. A feature of the arrangement of these pads,  15   a  and  15   b , in the back interface  15 D is that a portion of the pads, namely the pads  15   a  for the transmission, are arranged along one edge of the circuit board  13 D, while, another portion of the pads, namely the pads  15   b  for the reception, are disposed along another edge of the circuit board  13 D, namely, the pads,  15   a  and  15   b , are also unevenly arranged in the back surface  13   b  of the circuit board. Accordingly, a wide room is left between the pads in a center portion of the circuit board  13 D. The room between the pads may dispose other electronic components. 
     (Fifth Modification) 
       FIG. 10A  is a plan view of the top surface  13   a ; while,  FIG. 10B  is a plan view of the back surface  13   b  of the circuit board  13 E according to the fifth modification of the invention. Similar to the aforementioned modifications, the explanation below concentrates on points different from those of the aforementioned one, and left points not explained are substantially same with those of the former embodiments. 
     The circuit board  13 E of the present modification provides, instead of the parts,  12 A and  12 B, of the circuit unit  12 , other parts,  12 C and  12 D, of the circuit unit  12 . The part  12 C is mounted on the top surface  13   a  of the circuit board  13 E, while, another part  12 D is provided in the back surface  13   b  of the circuit board  13 E. Each parts,  12 C and  12 D, constitutes respective ICs. Specifically, the former part  12 C includes four transmitters  122 , while, the latter part  12 D includes four receivers  123 . 
     The electrodes,  14   a  and  14   b , in the interface  14  of the top surface  13   a  are connected to the input terminals,  121   a  and  121   b , of the transmitter corresponding thereto through the interconnections on the top surface  13   a . The electrodes,  14   a  and  14   b , in the interface  14 , which are provided on the back surface  13   b , are connected to the input terminals,  121   a  and  121   b , of the transmitter  122  in the top surface  13   a  through the via holes  16   c . The electrodes,  14   c  and  14   d , in the interface  14  of the top surface  13   a  are connected to the output terminals,  121   g  and  121   h , of the receiver  123  provided in the back surface  13   b  through the via holes  16   d , and the electrodes,  14   c  and  14   d , in the interface  14  of the back surface  13   b  are connected to the output terminals,  121   g  and  121   h , of the receiver  123  in the back surface  13   b.    
     For the interface  15  to the co-axial cables  21 , the circuit board  13 E of the present modification provides the interfaces,  15 E and  15 F, the former of which is disposed in the top surface  13   a  and the latter is in the back surface  13   b . The interface  15 E includes four pads  15   a  for the transmission and the other interface  15 F also provides four pads  15   b  but for the reception. The pads  15   a  are connected in the AC mode to the output terminals  121   c  of the transmitters  122  in the topo surface  13   a . The pads  15   b  in the interface  15 F are connected in the AC mode to the input terminals  121   e  of the receivers  123  in the other part  12 D of the circuit unit  12 . 
     In the present modification, the whole transmitters  122  are provided in one of the surfaces  13   a  of the circuit board  13 E, and the whole receivers  123  are disposed in another of the surfaces  13   b  of the circuit board  13 E. Even for such an arrangement, the advantages same as those of the aforementioned examples may be obtained. Moreover, the components for the transmission and those for the reception are fully isolated by the circuit board  13 E. Specifically, the pads  15   a  for the transmission and the transmitters  122  are arranged in one surface  13   a  of the circuit board  13 E, while, the pads  15   b  for the reception and the receivers  123  are disposed in another surface  13   b . Such an arrangement may effectively suppress the near end crosstalk (NEXT). Moreover, when the electrodes,  14   a  and  14   b , for the transmission are arranged on the one surface  13   a  and the other electrodes,  14   c  and  14   d , for the reception are arranged in the other surface  13   b , which may remove the via holes,  16   c  and  16   d , may further suppress the NEXT. 
     The cable according to the present invention thus described is not restricted to those embodiments or modifications, and may further modify in various ways. For instance, the transmitter and/or the receiver in the circuit unit terminate one of the outputs or one of the inputs thereof to the ground or the power line through the terminator. However, the method to convert a differential signal into a single-ended signal is not restricted to this arrangement. An active circuit, such as the single-ended push-pull, or the like may be also utilized in the conversion. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.