Abstract:
An electronic apparatus comprising a detecting unit, a communications unit, and a power control circuit. The power control circuit supplies power to a communication circuit unit based upon a result of detection of the detecting unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic apparatus and, more specifically, to an electronic apparatus having a function of communication using a fiber optic cable or an electric cable, which uses a detachable communication cable. 
     2. Description of the Background Art 
     Among digital video camera (digital VTRs with camera; digital video camera, hereinafter referred to as DVC) recording motion pictures, some have a function of transferring digital data at high speed to other electronic apparatuses, by connecting electronic apparatuses with each other by a cable, utilizing a standard such as IEEE1394. 
     In most cases, such electronic apparatuses are used as portable apparatuses. Therefore, generally, such an apparatus is used for recording pictures or the like while it is not connected to a cable, and communication with other apparatus is established by connecting a cable when communication becomes necessary. 
     An electric cable having a 4-pin or 6-pin terminal has been standardized as a communication medium in accordance with IEEE1394. As to the optical fiber, a standard specifying use of two optical fibers has been proposed. Further, mainly for portable apparatuses, study has been made for transferring signals in accordance with IEEE1394 standard over one optical fiber. 
     According to IEEE1394, when an electronic apparatus has a plurality of ports, it is necessary for the electronic apparatus to function as a repeater to relay a signal from one electronic apparatus to another electronic apparatus. Therefore, power supply to a physical layer circuit is adapted to be supplied constantly. 
     In an electronic apparatus not mainly used for communication, constant power supply to the physical layer circuit when the cable is not connected is unnecessary. Though IEEE1394 suggests possibility of considerable reduction of power consumption of a communication circuit when the communication cable is not connected, specific method thereof is not specified in the standard. 
     Further, a receptacle specified in IEEE1394 does not have any mechanism for detecting whether a plug is inserted to/removed from the receptacle. Therefore, it is necessary to detect voltage levels of signal lines in accordance with IEEE1394, to obtain information of connection between the plug and the receptacle from the result of analysis. This means that power must be supplied constantly to the communication circuit to keep the circuit in operation. 
     Here is the problem that even when the apparatus is fully disconnected from a communication circuit and the communication circuit is not in use, power consumption by the communication circuit portion cannot be made zero. In a portable apparatus such as a DVC, the communication circuit portion operates not only at the time of communication but also at the time of recording, for example, and hence power is consumed at the communication circuit portion which is essentially not in use. In such portable apparatus which are in most cases battery-operated, this leads to shorter discharge life of the battery, and the recordable time in recording operation is undesirably made shorter. 
     Generally, not only in DVCs but also in battery operated portable apparatuses, wasteful power consumption at the communication circuit portion not at the time of communication shortens discharge time of the battery, making it difficult to ensure long operation time of the apparatuses. 
     Therefore, reduction in power consumption of the electronic apparatus is an important problem. 
     As one technique related to lower power consumption of an electronic apparatus, Japanese Patent Laying-Open No. 7-57819 discloses a technique in which power supply of the electronic apparatus as a whole is shut off, when a cable is not connected to the electronic apparatus. 
     Though it is possible to control power supply to the overall electronic apparatus by the technique described in Japanese Patent Laying-Open No. 7-57819, it is not possible to reduce power consumption by the communication circuit portion only of the electronic apparatus, or to control power on port by port basis in accordance with IEEE1394, for example. 
     Japanese Patent Laying-Open No. 10-70508 discloses another related technique providing both spatial optical communication function and optical communication function through fiber optic cable, in which driving power for optical communication is increased to perform spatial optical communication when a fiber optic cable is not connected, and driving power for optical communication is reduced to perform optical communication through an optical fiber when the fiber optic cable is connected. 
     The technique described in Japanese Patent Laying-Open No. 10-70508 is applicable when spatial optical communication and optical communication through fiber optic cable are switched. The technique, however, cannot directly be applied to a DVC, for example, of which communication with other apparatus is established only through a communication cable. 
     In addition to the problem of power consumption, there is another problem of stabilizing operation when electronic apparatuses in operation are to be connected with each other by a communication cable. 
     Generally, when electronic apparatuses in operation are to be connected to each other by a communication cable, it is not guaranteed that the ground (reference potential) of an apparatus is at the same potential as the ground of the other apparatus to be connected, immediately before connection. Accordingly, when signal electrodes of the apparatuses contact with each other before the ground electrodes are connected to each other, it is possible that signal electrodes of the two apparatuses may contact with excessive potential difference therebetween. Therefore, a receiving device or a transmitting device may possibly be damaged. 
     FIG. 10 shows shapes of a plug and a receptacle in compliance with IEEE13344 standard. 
     Referring to FIG. 10, a receptacle  410  includes a power supply related electrode  415 , and a signal electrode  417 . Power supply related electrode  415  is positioned close to a tip end portion of receptacle  410 , whereas signal electrode  417  is placed recessed therefrom. A plug  400  includes plug side electrodes  420  corresponding to respective electrodes of receptacle  410 . 
     Therefore, when plug  400  is connected to receptacle  410 , power supply related electrode  415  comes to be in contact with the corresponding plug side electrode  420  before signal electrode  417  comes into contact with the corresponding plug side electrode  420 . Therefore, between the apparatuses, the ground (reference potential) and the power supply are first connected, and after the apparatuses are ready to have the signal lines connected to each other, the signal related electrode  417  comes to be in contact with the plug side electrode  420 , establishing safe connection of the signal line. Accordingly, problems such as an unexpected application of excessive voltage to the receiving device or the transmitting device or malfunction caused by unexpected data input to a state transition circuit in accordance with IEEE1394 can be solved. 
     Generally, when electronic apparatuses in operation are to be connected with each other by a communication cable, unexpected data may possibly be input as a noise to the receiving side dependent on the order of connection of a plurality of signal lines, causing malfunction of a communication circuit. 
     In order to solve such a problem, a technique for preventing malfunction at the time of connection between the plug and the receptacle is described in Japanese Patent Laying-Open No. 2-53125. 
     FIG. 11 shows configurations of a plug and a receptor disclosed in Japanese Patent Laying-Open No. 2-53125. 
     Referring to FIG. 11, in a plug  500  and a receptacle  510 , electrodes are arranged aligned in the direction vertical to the direction of insertion. At opposing ends of the plug and the receptacle, electrodes  512  and  514  for detecting connection and corresponding plug side electrodes  502  and  504  are provided respectively, separate from the electrodes for transmitting signals. By this configuration, it becomes possible to determine, when voltage levels of plug electrodes  502  and  504  do not match, that connection is now being established, and hence it becomes possible to interrupt operations of the electronic apparatuses and to prevent malfunction. 
     Development has been made to reduce area occupied by a receptacle of an apparatus by applying a miniature concentric plug (hereinafter also referred to as a mini plug) as an electric plug in accordance with IEEE1394 standard, for example, and providing an optical mini jack (OMJ) used commonly for optical and electric applications as a corresponding receptacle. 
     The structure of the OMJ will be described in detail later. In the OMJ, there is provided a terminal corresponding to an electric mini plug, and in addition, a photoreceptor circuit for processing an optical signal and a light emitting circuit (these circuits will be generally referred to as optical front-end circuit in the following) operable when a fiber optic plug having the same shape as the mini plug is inserted. Therefore, one receptacle can be commonly used to receive an optical plug and an electric plug. As a result, it becomes unnecessary to provide two receptacles for an optical signal and an electric signal, enabling reduction in size of the apparatus. 
     Further, study has been made to transfer signals in accordance with IEEE1394 standard over one optical fiber, on the premise that a small receptacle such as the OMJ is applied. 
     In such an electric mini plug and in the OMJ, electrodes are arranged aligned parallel to the direction of insertion, as will be described in detail later. Such arrangement of electrodes prevents application of the prior art shown in FIG. 11, and therefore there is a possibility that signal terminals may be in contact with each other before the ground terminals are connected, during the operation of insertion. Therefore, it is possible that signal electrodes of two apparatuses may be brought into contact with excessive potential difference therebetween. If a transmitting/receiving device having low breakdown voltage is used, such a device may be damaged. 
     There is still another problem. When an electric mini plug and an OMJ receptacle, for example, to be fitted in the mini plug are applied to the communication standard such as IEEE1394, it is the case that electrodes of the plug are brought into contact with electrodes other than the corresponding electrodes of the receptacle in the process of insertion, before the plug is fully inserted and connected to the corresponding electrodes of the receptacle, if the mini plug having such an electrode arrangement is inserted to the receptacle. 
     In the IEEE1394 standard, such an event in that plug electrodes are brought into contact with electrodes other than the corresponding electrodes of the receptacle when the plug is inserted is not considered. Therefore, when a mini plug represented by the OMJ is used, unexpected data may possibly be input, causing malfunction, as electrodes not corresponding to each other are brought into contact when the plug is inserted, affecting the communication circuit. 
     More specifically, though the plug and the receptacle having such shapes as shown in FIG. 11 solve the general problem of malfunction, the mini plug and the OMJ having such shapes in that electrodes are arranged parallel to the direction of insertion cannot solve these problems. 
     Further, in the plug and the receptacle defined by IEEE1394 shown in FIG. 10, positional relation between the power supply terminal and the signal terminal are adjusted to prevent the above described problems of malfunction or electric breakdown. The receptacle, however, occupies a large area on the surface of the apparatus. A receptacle considering common use with an electric cable, when an optical fiber is used, is not defined. Therefore, it is necessary to provide separate receptacles for optical signals and the electric signals. From these points, the plug and the receptacle defined by IEEE1394 shown in FIG. 10 are not suitable for application to a small portable apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce power consumption of an electronic apparatus which communicates with other apparatus using a communication cable and capable of limited communication with the other apparatuses. 
     Another object of the present invention is to realize stable operation at the time of connection, in an electronic apparatus using a mini plug, which allows reduction in size and common use for optical and electric applications, and a fitting receptacle. 
     Briefly stated, the present invention provides an electronic apparatus to and from which a communication cable is attached and detached, including a detecting unit, a communication unit and a power control unit. 
     The detection unit detects state of connection of the communication cable. The communication unit communicates signals with the communication cable. The power control unit controls power supply to the communication unit, based on the result of detection by the detecting unit. 
     According to another aspect, the present invention provides an electronic apparatus to and from which a communication cable can be attached and detached, including a receptacle, a detecting unit, a communication unit and a switch unit. 
     The receptacle is provided to receive a plug of the communication cable inserted thereto. The detecting unit detects the state of insertion of the plug to the receptacle. The communication unit communicates signals with the communication cable. The switch unit is provided between the receptacle and the communication unit, and operates based on the result of detection by the detecting unit. 
     Therefore, an advantage of the present invention is that, in an electronic apparatus having the function of attaching/detaching a communication cable, power supply to the communication unit is stopped and power consumption is reduced except in a communication period in which the communication cable is connected. Therefore, the time of battery discharge of a portable apparatus can be made longer, and the operation time of the apparatus can be made longer. 
     Further, connection between the receptacle and the communication unit is established after insertion of the plug into the receptacle is detected. Therefore, electric breakdown of the receiving device or the transmitting device at the time of inserting the plug can be prevented. Further, malfunction at the communication unit caused by undesirable contact with other terminal in the course of inserting the plug can be prevented. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram representing a configuration of an electronic apparatus  100  in accordance with a first embodiment of the present invention. 
     FIG. 2 shows an appearance of an OMJ. 
     FIGS. 3A and 3B show appearances of an electric cable plug and a fiber optical plug used corresponding to the OMJ. 
     FIGS. 4A and 4B represent configurations of electric contact unit of the OMJ. 
     FIG. 5 is a circuit diagram showing an example of a configuration of a power control circuit. 
     FIG. 6 is a block diagram representing a configuration of an electronic apparatus  200  in accordance with a second embodiment of the present invention. 
     FIG. 7 is a circuit diagram showing another configuration of the power control circuit. 
     FIG. 8 is a block diagram representing a configuration of an electronic apparatus  300  in accordance with a third embodiment of the present invention. 
     FIGS. 9A and 9B represent configurations of an electric contact unit of the OMJ. 
     FIG. 10 represent appearances of a plug and a receptacle standardized in accordance with IEEE1394. 
     FIG. 11 represents configurations of a plug and a receptacle in accordance with the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, an embodiment of the present invention will be described in detail with reference to the figures. 
     First Embodiment 
     In the first embodiment, a configuration of an electronic apparatus allowing reduction in power consumption will be described. 
     Referring to FIG. 1, an electronic apparatus  100  in accordance with the first embodiment communicates signals with other apparatuses through an optical fiber or an electric cable, by means of a communication cable plug  111  inserted to a receptacle  101 , to which the OMJ is applied. The appearance of the OMJ used as the receptacle is as shown in FIG.  2 . In the following, the receptacle may also be simply referred to as OMJ. 
     Receptacle  101  includes a contact unit  108  which is brought into contact with an electric plug inserted thereto, and an optical front-end circuit  107  including a light emitting device, a driving circuit therefor, a photoreceptor device and an amplifying circuit therefor, corresponding to a fiber optic cable. 
     Therefore, the electronic apparatus in accordance with the present invention allows a user to selectively use either an electric cable plug and a fiber optic plug having the same shape, as shown in FIGS. 3A and 3B. 
     An OMJ provided with either a photoreceptor unit or a light emitting unit in the receptacle has already been adopted in a portable MD recorder, for example, and already implemented in products and on the market, for digital audio communication. In the present invention, it is assumed that a plug having such a shape as shown in FIG. 3A is used to cope with the OMJ, when an electric signal in accordance with IEEE1394 standard is to be handled. 
     Referring to FIG. 3A, at a tip end portion of the electric cable plug, conductor portions and insulating portions are arranged alternately, and the plurality of conductor portions arranged along the direction of insertion constitute electrodes. 
     FIG. 3B shows the appearance of the fiber optic plug having the same shape as the electric cable plug shown in FIG.  3 A. 
     Again referring to FIG. 1, electronic apparatus  100  includes a receptacle  101  receiving a plug inserted thereto, an IEEE1394 communication circuit  106  for communication in accordance with the IEEE1394 standard through receptacle  101 , and a main circuit  105 . 
     IEEE1394 communication circuit  106  is a circuit or an LSI processing a protocol for transferring signals in accordance with IEEE1394 over one optical fiber. Main circuit  105  is the main circuit portion of the electronic apparatus and, when the electronic apparatus is a DVC, it includes a recording and reproducing circuit. Details of main circuit  105  will not be given here. 
     Electric contact unit  108  in the OMJ will be described. 
     FIG. 4A shows the plug not inserted to the OMJ, and FIG. 4B shows the plug inserted to the OMJ. 
     Referring to FIG. 4A, the contact unit includes a movable contact  302  and a fixed contact  303 . The fixed contact  303  is connected to the fourth terminal  301 . Terminal  301  is pulled up to a power supply node  305  through an internal resistance of the OMJ, and therefore, when the plug is not inserted, terminal  301  provides the same level as the pulled-up power supply potential, that is, H level. 
     Next, referring to FIG. 4B, when the plug is inserted to the OMJ, movable contact  302  is moved by the plug, and movable contact  302  is electrically connected to fixed contact  303 . Thus, terminal  301  outputs a signal at the same potential level as ground node  306 , that is, L level. 
     In FIGS. 4A and 4B, the signal output from terminal  301  corresponds to the connection detecting signal CNS of FIG.  1 . 
     More specifically, the connection detecting signal CNS attains to the L level when the plug is inserted to contact unit  108 , and it attains to the H level when the plug is removed, that is, when the plug is not inserted. 
     In this embodiment, an example in which the OMJ is used with an optical fiber will be described. As transmission of signals over an electric cable and an optical fiber using plugs of the same shape is handled here, the operation at the contact unit  108  is the same no matter which is used. Therefore, the embodiment of the present invention does not differ when the plug for an electric cable is used or when a plug for an optical fiber is used. 
     IEEE1394 communication circuit  106  and receptacle  101  are connected by signal lines  109  and  110 . When the plug for an optical fiber is connected, communication is established through signal line  109  between IEEE1394 communication circuit  106  and the optical front-end circuit. When a plug for an electric cable is connected, communication is established through signal line  110  between IEEE1394 communication circuit  106  and contact unit  108 . 
     Electronic apparatus  100  further includes a power control circuit  104 . Power control circuit  104  operates in response to the connection detecting signal CNS, and supplies power to IEEE1394 communication circuit  106  and optical front-end circuit  107  when the connection detecting signal CNS is at the L level. When the plug  111  is removed from OMJ 101  and the connection detecting signal CNS is at the H level, communication to other apparatus is unnecessary, and therefore, power supply to IEEE1394 communication circuit  106  and optical front-end circuit  107  is stopped. 
     Power control circuit  104  may be implemented by a circuit using a transistor and a resistor. 
     Referring to FIG. 5, power control circuit  104  includes a transistor QP connected between a power supply node  105  and a node Np. Node Np is connected to IEEE1394 communication circuit  106  and optical front-end circuit  107 . Transistor QP receives at its base the connection detecting signal CNS. Because of this structure, when the connection detecting signal is at the L level, that is, when the cable is connected to the receptacle, base current flows in transistor QP, and the power supply current is supplied, as corrector current of transistor QP, to IEEE1394 communication circuit  106  and optical front-end circuit  107  through node Np. 
     When the connection detecting signal CNS is at the H level, that is, when the cable is not connected to the receptacle, transistor QP is off, and therefore, the power supply current is not supplied to node Np. Therefore, it is possible to detect whether the communication cable is connected to the electronic apparatus or not by the connection detecting signal CNS, and it is possible to stop power supply to the communication circuit portion when the electronic apparatus is separated from a communication network, whereby power consumption can be reduced. 
     Further, as receptacle  101  has the function of detecting whether the communication cable is connected or not in itself, the connection detecting signal CNS for performing power control can be obtained directly, without the necessity of providing a new detecting unit. This enables reduction in the number of components on the circuit board and area of occupation. 
     Second Embodiment 
     In the second embodiment, a configuration where one electronic apparatus has a plurality of communication ports will be described. 
     FIG. 6 shows, as an example, a configuration of an electronic apparatus  200  having two communication ports. 
     Referring to FIG. 6, electronic apparatus  200  includes receptacles  101   a  and  101   b  corresponding to two communication ports, respectively. As in the first embodiment, receptacle  101   a  includes a contact unit  108   a  and an optical front-end circuit  107   a . Similarly, receptacle  101   b  includes a contact unit  108   b  and an optical front-end circuit  107   b . Power control circuits  104   a  and  104   b  are provided for respective receptacles. Configuration and operation of contact units  108   a ,  108   b , optical front-end circuits  107   a ,  107   b  and power control circuits  104   a  and  104   b  are the same as those of the first embodiment. Therefore, description is not repeated. 
     Thus, connection detecting signals CNSa and CNSb are generated for respective receptacles, and power control is performed accordingly. 
     Electronic apparatus  200  further includes a power control circuit  103  controlling power supply to IEEE1394 communication circuit  106 , and a logic gate  112  generating a signal dependent on the state of connection at each receptacle. It is possible for IEEE1394 communication circuit  106  to communicate each of receptacles  101   a  and  101   b , by signal lines  109  and  110 . 
     Logic gate  112  performs an AND operation of connection detecting signals CNSa and CNSb output from respective receptacles, and applies the result of operation to power control circuit  103 . In response to the output signal from logic gate  112 , power control circuit  103  stops supply of power to IEEE1394 communication circuit  106 , when none of the communication ports, that is, none of the receptacles, is in the state of connection. 
     Control of power supply to optical front-end circuits  107   a  and  107   b  of respective receptacles is performed by power control circuits  104   a  and  104   b  provided for respective receptacles. Therefore, power consumption can efficiently saved, as the power supply to the port to which the cable is not connected is shut out. 
     FIG. 6 shows an example in which there are two communication ports. Even when there are three or more ports, similar effects can be obtained by arranging a power control circuit corresponding to each receptacle and IEEE1394 communication circuit  106 , with the connection detecting signals output from respective receptacles supplied as inputs to logic gate  112 . 
     Even when there is only one port, power control circuits corresponding to IEEE1394 communication circuit  106  and optical front-end circuit  107  can be provided independently. 
     Other configuration of power control circuit  103  provided corresponding to IEEE1394 communication circuit  106  will be described. 
     Power control circuit  103  may have the similar structure as power control circuit  104  shown in FIG.  5 . It is possible, however, that the circuit is configured as shown in FIG. 7 in which clock supply to a digital circuit included in IEEE1394 communication circuit  106  is stopped. Though not shown in detail, in this example, the power is constantly supplied to IEEE1394 communication circuit  106 . 
     Referring to FIG. 7, power control circuit  103  outputs a clock signal for IEEE1394 communication circuit  106  at an output node  502 , in response to an output signal from logic gate  112  provided at control node  501 . 
     Because of this structure, when a communication cable is not connected to any of the communication ports, the signal level at control node  501  attains to the H level, and the output of the NAND gate attains to the L level regardless of the output of an oscillating element. Therefore, oscillation of power control circuit  103  is stopped and signal level at output node  502  becomes constant. Thus, the clock signal is not supplied to IEEE1394 communication circuit  106 . 
     Power control circuit  103  shown in FIG. 7 is a general purpose oscillation circuit with a control terminal (corresponding to control node  501 ), including a quartz oscillator, a capacitor, an NAND logic gate and an inverter, and therefore the principal of operation thereof is not described here. In a CMOS circuit, the higher the clock frequency, the larger the is connected to the communication ports, power consumption of the overall electronic apparatus can efficiently be reduced. The configuration of power control circuit  103  based on the clock control shown in FIG. 7 may be applicable to electronic apparatus  100  in accordance with the first embodiment, as a power control circuit provided corresponding to IEEE1394 communication circuit  106 , if power control circuits corresponding to IEEE1394 communication circuit  106  and optical front-end circuit  107  are provided independent from each other. 
     In this manner, when power control is performed by controlling supply of digital clock signal, power consumption can efficiently be reduced without the necessity of a large current control element in the power control circuit, while the communication circuit is kept in a standby state. 
     Further, power control can be performed independently for the optical front-end circuit included in each of the plurality of ports. Therefore, efficient power control of the optical front-end circuit, which consumes considerable power in the communication circuit unit, becomes possible. 
     Third Embodiment 
     FIG. 8 is a block diagram representing a configuration of an electronic apparatus  300  in accordance with the third embodiment of the present invention. 
     Referring to FIG. 8, electronic apparatus  300  differs from electronic apparatus  100  shown in FIG. 1 in that in place of signal line  110  between contact unit  108  and IEEE1394 communication circuit  106 , signal lines  325  and  326  and a switch circuit  328  arranged therebetween and a standby circuit  324  controlling switching circuit  328  are provided. In order to show the details of connection control between the OMJ and IEEE1394 communication circuit  106 , main circuit  105  is not shown in FIG.  8 . 
     FIG. 9A shows the state of the contact unit when the plug is not inserted, and FIG. 9B shows the state of the contact unit when the plug is inserted. The configuration of contact unit  108  is the same as that shown with reference to FIGS. 3A and 3B, and therefore, detailed description thereof is not repeated. Of the terminals  314  and  315  shown in FIGS. 9A and 9B,  314  denotes a signal terminal corresponding to four signal lines in accordance with IEEE1394, and  315  denotes a power supply terminal including the ground. The terminals  314  and  315  are connected to IEEE1394 communication circuit  106  through signal line  325 , switch  328  and signal line  326 . 
     As shown in FIGS. 9A and 9B, the connection detecting signal CNS is generated at terminal  301 , in the similar manner as described with reference to FIGS. 3A and 3B. The connection detecting signal CNS is the signal for detecting full insertion of the plug into the receptacle. Therefore, when the plug is fully inserted to the receptacle, the connection detecting signal CNS attains to the L level, and otherwise, H level. 
     Standby circuit  324  activates a control signal CSG after a prescribed standby time after the transition of the connection detecting signal CNS from the H level to the L level, and turns on switch  328 . In the period while the connection detecting signal CNS is maintained at the L level, standby circuit  324  generates the control signal CSG so as to maintain the on state of switch  328 . Thereafter, simultaneously with the transition of connection detecting signal to the H level, standby circuit  324  inactivates the control signal CSG and opens the switch  328 . Standby circuit  324  may be implemented by using a digital timer or an analog element having a time constant. Generally, standby time of about 300 ms to about 400 ms is desirable. 
     Switch circuit  328  may be implemented by a mechanical relay circuit or an electrical semiconductor circuit. 
     IEEE1394 communication circuit  106  is as described above. 
     IEEE1394 communication circuit  106  is connected to optical front-end circuit  107  through signal line  109 , and connected to contact unit  108  through signal lines  325  and  326  as well as switch  328 , as described above. 
     Therefore, when a plug for an optical fiber is inserted, communication is performed with the optical front-end circuit  107  through signal line  109 , and when a plug used for an electric cable is inserted, communication is performed between contact unit  108  and IEEE1394 communication circuit  106  through switch  328  and signal lines  325  and  326 . As the fiber optic plug does not have any electric terminal, the problem as discussed in the prior art is not generated. Therefore, description for the use of fiber optic plug is not given here. 
     When an electric plug is inserted to OMJ 101 , before the plug is fully inserted, movable contact  302  and fixed contact  303  shown in FIG. 9A are separated, and the connection detecting signal CNS is at the H level. Thereafter, when plug  111  is almost fully inserted, movable contact  302  and fixed contact  303  are brought into contact and the connection detecting signal CNS changes to the L level. Standby circuit  324  operates and after a prescribed time period, switch circuit  328  is turned on, whereby power and the signal of plug  111  are connected to IEEE1394 communication circuit  106  through OMJ 1  and switch circuit  328 . 
     From the start to the end of insertion of plug  111 , electrodes of plug  111  may be in contact with electrodes other than the corresponding electrodes of OMJ 101 . Switch circuit  328 , however, is opened, and therefore the power or the signal of plug  111  is not supplied to IEEE1394 communication circuit  106 . When plug  111  is almost fully inserted and fixed contact  303  and movable contact  302  of FIG. 9A are brought into contact, signal terminal  314  or power supply terminal  315  is almost reaching the corresponding prescribed electrode of plug  111 . In the short period until the plug is fully inserted, however, each electrode on the side of the OMJ is slid over the corresponding electrode on the side of the plug, causing unstable state of the electrode (bouncing, chattering). The period of this unstable contact state is short, and therefore, IEEE1394 communication circuit  106  and plug  111  can be connected electrically after the end of this period of unstable state if standby circuit  324  maintains switch  328  open for the standby time period. 
     More specifically, when electrodes of plug  111  and OMJ  101  are in contact with electrodes not corresponding to each other or in the unstable period at the end of insertion, signals from plug  111  and IEEE1394 communication circuit  106  are not electrically connected, and all the electrodes are connected simultaneously by means of switch  328  after the insertion is completed and the signals are stabilized. Therefore, the signal terminals will never be connected before the ground is connected. Thus, there is no possibility of damaging the transmitting device or the receiving device in IEEE1394 communication circuit  106 . Further, unexpected data will never be input in the course of inserting the plug to IEEE1394 communication circuit  106 . Therefore, the state of operation of IEEE1394 communication circuit  106  is not affected. 
     Application of a small single head plug in this manner reduces area occupied by the receptacle of the surface area of the apparatus, which contributes to reduction in size of the electronic apparatus. 
     Though an example of a plug having a power supply terminal has been described in the third embodiment, a plug and a receptor with a signal line only and not including the power supply terminal may be used in accordance with IEEE1394 standard. The configuration of the present invention is similarly applicable to such an example. 
     Though a configuration in accordance with IEEE1394 standard has been described in the first to third embodiments, the present invention is also applicable to electronic apparatuses using other communication standards, such as USB (Universal Serial Bus). 
     In an apparatus of which specification does not involve use of an optical fiber, use of the OMJ is unnecessary. Therefore, the invention in accordance with the first to third embodiments may be applied to an ordinary mini jack. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.