Abstract:
A communication system is capable of determining, at a reception side of a communication circuit, whether a communication method connected is of optical communication or electric communication. When the communication system is activated, a communication circuit transmission unit transmits a differential high fixed output. When a communication circuit reception unit receives a differential high fixed signal, it is determined that electric communication is used. When the communication circuit reception unit receives a differential low fixed signal, it is determined that optical communication is used.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a communication system, in particular, to a communication system capable of determining, at a reception side of a communication circuit, whether a communication method connected is of optical communication or electric communication. 
         [0003]    2. Description of the Related Art 
         [0004]    As communication methods in communications performed via a cable between electronic devices, with which either of an optical signal or an electrical signal can be optionally selected for data transmission, the following methods are available: 
         [0005]    (1) a method for connecting an electric communication cable  809  to electric connectors  803 ,  804  mounted on electronic devices  801 ,  802 , using electric connectors  805 ,  806 , 
         [0006]    or a method for connecting an optical active cable that performs communication via an optical communication cable  810 , using photoelectric conversion circuits incorporated in electric connectors  807 ,  808  provided at both ends thereof as illustrated in  FIG. 8 , or. 
         [0007]    (2) a method for connecting an electric cable  923  to electric communication modules  915 ,  916  which can be attached to, and detached from, electric connectors  913 ,  914  mounted on electronic devices  911 ,  912 , using electric connectors  917 ,  918 , 
         [0008]    or a method for connecting an optical communication cable  924  to photoelectric conversion modules  919 ,  920 , using optical connectors  921 ,  922 , as illustrated in  FIG. 9 . 
         [0009]    In the method (2) above, a small form-factor pluggable (SFP) module is widely known. 
         [0010]    In general, optical communication enables higher-speed and longer-distance communication than electric communication. Furthermore, waveform correction techniques for achieving higher-speed and longer-distance communication with electric communication have been developed although less effective than optical communication, wherein pre-emphasis and de-emphasis, and an equalizer are widely used. If a cable that is connected is known at the side of an electronic device, it is possible to select a communication speed and to determine whether or not to use a waveform correction method, whereby communication optimally utilizing the performance of each cable can be performed. 
         [0011]    As a method for informing an electronic device of a selected communication method, electric communication or optical communication, a user may set the selected communication method in an electronic device, or an electronic device may automatically discriminate the selected communication method. Examples of such automatic discrimination by an electronic device include a configuration in which a terminal for discriminating the communication method selected is provided, as disclosed in Japanese Patent Application Laid-Open No. 2008-097040, and a configuration in which a transceiver (electric communication module or photoelectric conversion module) has therein information on corresponding communication standard including information on which of optical communication or electric communication is used and is configured to acquire information by using a dedicated communication path between an electronic device and the transceiver, as in an SFP transceiver. 
         [0012]    In case where a system is established in which communication between electronic devices is performed by an inexpensive electric communication in a short-distance communication and a low-speed communication, whereas communication between electronic devices is performed by optical communication in a long-distance communication and high-speed communication, if both of the electronic devices that perform communication know the communication method before the start of communication, it is possible to determine whether or not a technique for correcting a waveform is to be used at the time of electric communication and to select a communication speed. However, adding a specific terminal for discriminating a communication method or a dedicated communication path increases costs. Because it is sufficient that the communication method is known before the start of communication, a terminal for communication used after the start of communication can be temporarily used. Furthermore, for electric communication, connection with AC coupling may be required to eliminate influence of a signal potential difference between electronic devices that perform communication. 
       SUMMARY OF THE INVENTION 
       [0013]    The object of the present invention is to provide a communication system capable of determining, at a reception side of a communication circuit, whether a communication method connected is of optical communication or electric communication without adding a signal terminal for discrimination. 
         [0014]    A communication system according to the present invention is a communication system enabling communication between electronic devices with either of an optical communication path including an optical communication cable or an electric communication path including an electric communication cable connected thereto. Each of the electronic devices includes a communication circuit transmission unit that outputs serial data to be transmitted in the form of a differential signal and a communication circuit reception unit that receives a differential signal. 
         [0015]    In a communication system according to a first embodiment of the present invention, the optical communication path includes a transmission-side connector and a reception-side connector. The transmission-side connector includes a transmission-side photoelectric conversion circuit that converts the differential signal into an optical signal. The reception-side connector includes a reception-side photoelectric conversion circuit with a signal detection circuit incorporated therein. The reception-side photoelectric conversion circuit converts the received optical signal into an electric signal when the received optical signal includes an AC signal and sets an electric output signal to a high fixed signal or a low fixed signal when the received optical signal does not include an AC signal. The communication circuit transmission unit outputs a high fixed signal or a low fixed signal of differential for determination of communication path when determining a communication path connected thereto. On the other hand, the communication circuit reception unit receives a non-inverted signal of a signal output by the communication circuit transmission unit when the electric communication path is connected, receives an inverted signal of a signal output by the communication circuit transmission unit with the signal detection circuit when the optical communication path is connected, and determines whether the connected communication path is an electric communication path or an optical communication path based on the received signal. 
         [0016]    A communication system according to a second embodiment of the present invention includes an electric communication module, a transmission-side photoelectric conversion module, and a reception-side photoelectric conversion module. The electric communication module connects an electric connector provided at both ends of the electric communication path and an electric connector of the electronic device when the electric communication path is connected. The transmission-side photoelectric conversion module includes a transmission-side photoelectric conversion circuit that is connected to a transmission-side optical connector of the optical communication path and converts the differential signal into an optical signal when the optical communication path is connected. The reception-side photoelectric conversion module includes a reception-side photoelectric conversion circuit with a signal detection circuit incorporated therein. The reception-side photoelectric conversion circuit converts the received optical signal into an electric signal when the received optical signal includes an AC signal and sets an electric output signal to a high fixed signal or a low fixed signal when the received optical signal does not include an AC signal. The communication circuit transmission unit outputs a high fixed signal or a low fixed signal of differential for determination of communication path when determining a communication path connected thereto. On the other hand, the communication circuit reception unit receives a non-inverted signal of a signal output by the communication circuit transmission unit when the electric communication path is connected, receives an inverted signal of a signal output by the communication circuit transmission unit with the signal detection circuit when the optical communication path is connected, and determines whether the connected communication path is an electric communication path or an optical communication path based on the received signal. 
         [0017]    The communication system may further include a terminal resistor that makes an inverted signal and a non-inverted signal of differential transmission after AC coupling the same potential when no AC signal is present before an input to the communication circuit reception unit and a signal detection circuit that is provided in a rear stage of the terminal resistor and that instructs a circuit in a rear stage, when an inverted signal and a non-inverted signal are in the same potential, to transmit a non-inverted signal of a signal output by the communication circuit transmission unit, indicating that no signal is present, at the time of communication path determination, wherein when an electric communication path is connected, an AC coupling element is incorporated between the communication circuit transmission unit and the communication circuit reception unit. 
         [0018]    A communication system according to the present invention is capable of determining, at a reception side of a communication circuit, whether a communication method connected is of optical communication or electric communication, without adding any signal terminal for determination. With the communication system bidirectionally implemented, both of the electronic devices that perform communication can discriminate the communication method. With respect to electric communication, both connection methods of DC coupling and AC coupling can be available. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The forgoing and other objects and feature of the invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a diagram illustrating an example of configuration of electric communication connection at DC coupling in a communication system according to an embodiment of the present invention; 
           [0021]      FIG. 2  is a diagram illustrating an example of configuration of optical communication connection at DC coupling in a communication system according to an embodiment of the present invention; 
           [0022]      FIG. 3  is a diagram illustrating an example of configuration of electric communication connection at AC coupling in a communication system according to an embodiment of the present invention; 
           [0023]      FIG. 4  is a diagram illustrating an example of configuration of optical communication connection at AC coupling in a communication system according to the embodiment of the present invention; 
           [0024]      FIG. 5  is a diagram illustrating an example of a positive emitter coupled logic (PECL) terminal circuit; 
           [0025]      FIG. 6  is a diagram illustrating an example of a low voltage differential signaling (LVDS) terminal circuit; 
           [0026]      FIG. 7  is a diagram illustrating an example of a current mode logic (CML) terminal circuit; 
           [0027]      FIG. 8  is a diagram explaining a first example of a communication system according to prior art technique, which is capable of selecting between optical signal communication and electric signal communication; and 
           [0028]      FIG. 9  is a diagram explaining a second example of a communication system according to prior art technique, which is capable of selecting between optical signal communication and electric signal communication. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    According to the present invention, when a system is activated, a differential high fixed output is transmitted from a communication circuit transmission unit and if the differential high fixed output is received by a communication circuit reception unit, then it is determined that an electric communication is used, whereas if a differential low fixed output is received by the communication circuit reception unit, on the other hand, then it is determined that an optical communication is used. It should be noted here that a signal to be transmitted and determination logic of a received signal can be reversed. 
         [0030]    A. When electronic devices are DC-coupled in electric communication, connection in electric communication is illustrated in  FIG. 1  and connection in optical communication is illustrated in  FIG. 2 . 
         [0031]    A-1: In electric communication, as illustrated in  FIG. 1 , a differential high fixed signal output from a communication circuit transmission unit  102  included in an electronic device transmission part  101  reaches a communication circuit reception unit  107  included in an electronic device reception part  106  via an electric connector or a module transmission side  103 , an electric communication cable  104 , and an electric connector or a module reception side  105 , still in the form of differential high fixed signal. 
         [0032]    A-2: In optical communication, as illustrated in  FIG. 2 , a differential high fixed signal output from a communication circuit transmission unit  202  included in an electronic device transmission part  201  is input to an electric connector incorporated in a photoelectric conversion circuit or a module reception side  205  via an electric connector incorporated in a photoelectric conversion circuit or a module transmission side  203  and an optical communication cable  204 . 
         [0033]    In the electric connector incorporated in a photoelectric conversion circuit or a module reception side  205  in  FIG. 2 , an optical signal received by a light receiving element  210  is converted into an electric signal and transmitted to differential amplifiers  212 ,  213  in rear stages via an amplifier  211  as well as branched to be transmitted to AC coupling elements  214 ,  215 . As DC components are cut by the AC coupling elements  214 ,  215 , a signal detection circuit  216  determines that no signal is detected, and an output stop is instructed to the differential amplifier  213  via a signal line  217 . An output of the differential amplifier  213  when the output stop is instructed is set to differential low fixed, whereby a differential low fixed signal reaches a communication circuit reception unit  207  included in an electronic device reception part  206 . 
         [0034]    B: When electronic devices are AC-coupled in electric communication, connection in electric communication is illustrated in  FIG. 3  and connection in optical communication is illustrated in  FIG. 4 . 
         [0035]    B-1: In electric communication, as illustrated in  FIG. 3 , a differential high fixed signal output from a communication circuit transmission unit  302  included in an electronic device transmission part  301  is transmitted via an electric connector or a module transmission side  303  and an electric communication cable  304  to an electric connector or a module reception side  305 , in which AC coupling elements  308 ,  309  cut DC components of the differential high fixed signal. An inverted side and a non-inverted side become the same potential by a terminal circuit  310 , at a certain potential. 
         [0036]    The inverted side and the non-inverted side being at the same potential causes a signal detection circuit  313  to determine that no signal is present and instruct an output stop to a differential amplifier  311 . The operation of the differential amplifier  311  at the time of output stop is set to differential high fixed, whereby a differential high fixed signal reaches a communication circuit reception unit  307 . It should be noted here that the AC coupling elements  308 ,  309  may be arranged in any positions between the electric connector or the module transmission side  303  and the electric connector or the module reception side  305 . 
         [0037]    B-2: In optical communication, as illustrated in  FIG. 4 , a differential high fixed signal output from a communication circuit transmission unit  402  included in an electronic device transmission part  401  is transmitted to an electric connector incorporated in a photoelectric conversion circuit or a module reception side  405  via an electric connector incorporated in a photoelectric conversion circuit or a module transmission side  403  and an optical communication cable  404 . 
         [0038]    In the electric connector incorporated in a photoelectric conversion circuit or the module reception side  405  in  FIG. 4 , an optical signal received by a light receiving element  410  is converted into an electric signal and transmitted to differential amplifiers  412 ,  413  in rear stages via an amplifier  411  as well as branched to be transmitted to AC coupling elements  414 ,  415 . As DC components are cut by the AC coupling elements  414 ,  415 , a signal detection circuit  416  determines that no signal is detected, and an output stop is instructed to the differential amplifier  413  via a signal line  417 . An output of the differential amplifier  413  when the output stop is instructed is set to differential low fixed, whereby a differential low fixed signal reaches an electronic device reception part  406 . In the electronic device reception part  406 , the differential low fixed signal is branched to reach a signal detection circuit  421  via a terminal circuit  418 . The differential low fixed signal causes the signal detection circuit  421  to determine that a signal is present, and is thus transmitted to a communication circuit reception unit  407  via a differential amplifier  419 , still in the form of the differential low fixed signal. 
         [0039]    Functions corresponding to the signal detection circuits  216 ,  313 ,  416 ,  421  are generally implemented in an SFP module or the like in the names of Signal Detect (SD) and Loss of Signal (LOS), for example, and used for detecting that no optical signal reaches a photoelectric conversion circuit. Signal detection circuits having a function to stop a reception output in accordance with a signal detection result are disclosed in Japanese Patent Application Laid-Open No. 2009-044228 and Japanese Patent Application Laid-Open No. 2007-005968, for example. 
         [0040]    In addition, as the terminal circuits  310 ,  418 , PECL ( FIG. 5 ), LVDS ( FIG. 6 ), CML ( FIG. 7 ) circuits and the like are available, in each of which when no AC component is present in an input signal, a positive side and a negative side of differential after AC coupling become the same potential. 
         [0041]    The embodiments of the present invention has been described above, however, the present invention is not limited to the examples described in the embodiments above and may be implemented in various forms by adding modification as appropriate.