Patent Publication Number: US-2016233952-A1

Title: Optical communication module, method for recording log of optical communication module, and optical communication apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an optical communication module, a method for recording a log of an optical communication module, and an optical communication apparatus. More specifically, the present invention relates to an optical communication module configured to store log information. 
     BACKGROUND ART 
     An optical transceiver is a kind of optical communication module. The optical transceiver generally has a capability of converting an electrical signal and an optical signal to and from each other, a capability of receiving an optical signal from an optical communication cable, and a capability of transmitting an optical signal to an optical communication cable. Where the optical transceiver fails, a technical expert of the manufacturer of the transceiver may analyze the optical transceiver, Japanese Patent Laying-Open No. 2004-222297 (PTD 1) or International Publication No. WO2005/107105 (PTD 2) discloses a method according to which information about an optical transceiver is held in the optical transceiver, 
     CITATION LIST 
     Patent Document 
     
         
         PTD 1: Japanese Patent Laying-Open No. 2004-222297 
         PTD 2: International Publication No. WO2005/107105 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the case where an abnormality occurs to optical communication, it is important for a provider of the optical communication system to immediately return the optical communication to a normal state. In many cases, one host substrate is mounted with a plurality of optical communication modules (optical transceivers in many cases). If a certain host substrate is the cause of an abnormality of the optical communication, the provider usually considers replacing the host substrate. Therefore, even if a plurality of optical communication modules mounted on the host substrate are estimated to have caused the abnormality, the host substrate may be replaced. 
     In view of such an operation as described above, a host substrate may be mounted with a memory (nonvolatile memory for example) for storing log information about the state of the whole of the host substrate. A technical expert of the manufacturer of the optical communication modules can test an optical communication module itself to determine whether or not this optical communication module is failing. 
     However, in order to ascertain in what situation the optical communication module enters a failure state, it is necessary to analyze the log information held in the memory of the host substrate. Therefore, when only the optical communication module which has failed is returned to the technical expert of the manufacturer, the technical expert of the manufacturer cannot know the situation in which the optical communication module enters the failure state. 
     In order to solve this problem, a method according to which information is held in the optical transceiver, like the one disclosed in above-referenced Japanese Patent Laying-Open No. 2004-222297 (PTD 1) or International Publication No. WO2005/107105 (PTD 2), can be adopted to configure the optical communication module. However, it is more important for analysis of the cause of the failure of the optical communication module to have the information about the situation in which the optical communication module enters the failure state. Above-referenced PTD 1 and PTD 2 are both silent about the method for leaving, in the optical communication module, information about the situation in which the optical communication module enters a failure state. 
     Furthermore, the failure of the optical communication module may be caused by an abnormality of the host substrate, and thus, the information about the state of the host substrate immediately before the failure of the optical communication module is also considered to be important for analysis of the cause of the failure of the optical communication module. PTD 1 and PTD 2 do not, however, describe in detail the host substrate having the optical communication modules mounted thereon. Therefore, PTD 1 and PTD 2 are both silent about the technique of leaving, in the optical communication module, the information about the state of the host substrate. 
     Japanese Patent Laying-Open No. 2004-222297 (PTD 1) also discloses that any of a volatile storage device and a nonvolatile storage device may be used as a memory for storing information about a failure. In the case where the volatile storage device is used, however, stoppage of supply of a power supply voltage to the optical communication module causes the information stored in the volatile storage device to be lost. Accordingly, there is a possibility that the information stored in the optical communication module is lost, for example, when the power supply voltage becomes unable to be supplied to the optical communication module due to occurrence of an abnormality to the host substrate itself or when the host substrate is removed from an optical communication apparatus. Namely, according to the technique disclosed in PTD 1, adequate consideration is not given to the situation of the failure of the optical communication module or the situation in which only the optical communication module which has failed is returned. 
     For the aforementioned reasons, according to the conventional techniques, when only the optical communication module which has failed is returned to the technical expert of the manufacturer, the technical expert of the manufacturer was not able to know the state of the optical communication module and the state of the host substrate immediately before the optical communication module fails. 
     An object of the present invention is to, when an abnormality occurs in a host substrate, allow not only information about an optical communication module mounted on the host substrate but also information about the host substrate to be left in the optical communication module. 
     Solution to Problem 
     An optical communication module according to an aspect of the present invention is an optical communication module insertable in and removable from a host substrate, including: a control unit; and a nonvolatile memory. The control unit monitors the optical communication module and repeatedly receives log information about a state of the host substrate from the host substrate. When a hazard signal indicating an abnormality of the host substrate is detected, the control unit writes, in the nonvolatile memory, a result of monitoring of the optical communication module and the log information from the host substrate. 
     Owing to the above-described features, when the abnormality occurs in the host substrate, not only the information about the optical communication module mounted on the host substrate but also the information about the host substrate can be left in the optical communication module. The aforementioned information is stored in the nonvolatile memory. Therefore, even when power supply to the optical communication module becomes abnormal due to, for example, the abnormality that has occurred in the host substrate, the information about the state of the host substrate and the state of the optical communication module immediately before the occurrence of the abnormality can be left in the optical communication module. The abnormality of the host substrate indicated by the hazard signal includes, for example, an abnormality relating to power supply from the host substrate to the optical communication module, although it is not limited thereto. “Optical communication module” may have both the transmission and reception capabilities like the optical transceiver, or have only one of the transmission and reception capabilities (like optical receiver or optical transmitter for example). 
     Preferably, the optical communication module further includes a power supply monitoring unit. The power supply monitoring unit outputs the hazard signal when an abnormality relating to power supply from the host substrate to the optical communication module is detected. The abnormality relating to the power supply includes a case in which a power supply voltage supplied from the host substrate during operation of the optical communication module falls outside a prescribed range. 
     Owing to the above-described features, the hazard signal is output in response to detection of the abnormality relating to the power supply. As a result, the optical communication module writes, in the nonvolatile memory, a result of self-monitoring and the log information from the host substrate. Therefore, such a possibility can be increased that the information about the state of the host substrate and the state of the optical communication module is stored in the nonvolatile memory before the optical communication module stops finally due to the abnormality relating to the power supply. The abnormality relating to the power supply includes the case in which the power supply voltage supplied from the host substrate during operation of the optical communication module falls outside the prescribed range, although it is not limited thereto. 
     Preferably, the optical communication module further includes a volatile memory. When the hazard signal is not detected, the control unit writes, in the volatile memory, the result of monitoring of the optical communication module and the log information from the host substrate, and then, when the hazard signal is detected, the control unit transfers, from the volatile memory to the nonvolatile memory, the result of monitoring of the optical communication module and the log information from the host substrate. 
     Owing to the above-described features, when the hazard signal is not detected, i.e., when the host substrate is normal, the information is written in the volatile memory. Therefore, the information can be left in the volatile memory. Only after the hazard signal is detected, the result of monitoring of the optical communication module and the log information from the host substrate are written in the nonvolatile memory. As a result, frequent writing in the nonvolatile memory can be prevented. Therefore, a considerable reduction of the number of times the nonvolatile memory is permitted to be written can be prevented. The lifetime of the nonvolatile memory is prevented from being considerably shortened, and accordingly shortening of the lifetime of the optical communication module depending on the lifetime of the nonvolatile memory can be prevented. 
     Preferably, when receiving the log information from the host substrate, the control unit writes the result of monitoring of the optical communication module in the volatile memory, together with the received log information. 
     Owing to the above-described features, a time lag between the log information from the host substrate and the result of monitoring of the optical communication module can be reduced. Therefore, for example, when the optical communication module which has failed is returned to a technical expert of the manufacturer, the technical expert can analyze the cause of the failure of the optical communication module in detail by checking the state of the host substrate indicated by the log information from the host substrate and the state of the optical communication module indicated by the result of monitoring of the optical communication module. 
     A method for recording a log of an optical communication module according to another aspect of the present invention includes the steps of: causing an optical communication module insertable in and removable from a host substrate to perform self-monitoring; causing the optical communication module to receive log information about a state of the host substrate from the host substrate; and writing, in a nonvolatile memory mounted on the Optical communication module, a result of the self-monitoring by the optical communication module and the log information received by the optical communication module, when an abnormality relating to power supply from the host substrate to the optical communication module is detected. 
     Owing to the above-described features, when the abnormality relating to the power supply from the host substrate to the optical communication module occurs, not only the information about the optical communication module mounted on the host substrate but also the information about the host substrate can be left in the optical communication module. The abnormality relating to the power supply includes, for example, the case in which the power supply voltage supplied from the host substrate to the optical communication module falls outside the prescribed range. For example, when the power supply voltage during operation of the optical communication module falls outside the prescribed range, it is detected as the abnormality relating to the power supply from the host substrate. The case in which the power supply voltage falls outside the prescribed range includes, for example, a case in which the power supply voltage falls below a determination level when the power supply voltage is positive. 
     An optical communication apparatus according to still another aspect of the present invention includes: a host substrate; and a plurality of optical communication modules. Each of the plurality of optical communication modules is insertable in and removable from the host substrate and includes a nonvolatile memory. Each of the plurality of optical communication modules performs self-monitoring and repeatedly receives log information about a state of the host substrate from the host substrate. When an abnormality relating to power supply from the host substrate to the optical communication modules occurs, each of the plurality of optical communication modules writes, in the nonvolatile memory, a result of the self-monitoring and the log information from the host substrate. The host substrate transmits the log information to the plurality of optical communication modules at different timings. 
     Owing to the above-described features, when the abnormality relating to the power supply from the host substrate to the optical communication modules occurs, not only the information about the optical communication module mounted on the host substrate but also the information about the host substrate can be left in the optical communication module. Furthermore, the log information is sent to the plurality of optical communication modules at different timings. Therefore, the log information generated at different times is held in the plurality of optical communication modules. When a plurality of optical communication modules fail, a temporal change in the state of the host substrate (conversely, it may be no change in the state of the host substrate) can be known by analyzing the log information stored in these optical communication modules which have failed. Therefore, the cause of the failure of the optical communication modules can be analyzed in more detail. “Abnormality relating to power supply” is defined similarly to the aforementioned definition. 
     Preferably, the optical communication apparatus further includes a power supply switching unit in the optical communication apparatus. When the abnormality relating to the power supply from the host substrate is detected, the power supply switching unit supplies power to the optical communication modules in place of the host substrate, at least until writing in the nonvolatile memory is completed. 
     Owing to the above-described features, the power supply switching unit can ensure the operation of each of the plurality of optical communication modules to write, in the nonvolatile memory, the result of self-monitoring and the log information from the host substrate. Therefore, the result of self-monitoring by the optical communication modules and the log information from the host substrate can be left in the optical communication modules more reliably. The power supply switching unit may at least supply, to the optical communication modules, the power supply voltage required for the operation of writing in the nonvolatile memory. Therefore, the power supply voltage supplied to the optical communication modules by the power supply switching unit is not limited to be the same as the power supply voltage supplied from the host substrate before the host substrate becomes abnormal. 
     Advantageous Effects of Invention 
     According to the present invention, when the abnormality occurs in the host substrate, not only the information about the optical communication module mounted on the host substrate but also the information about the host substrate can be left in the optical communication module. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an optical communication apparatus in an embodiment of the present invention. 
         FIG. 2  is a block diagram showing an example configuration of an optical transceiver I shown in  FIG. 1 . 
         FIG. 3  is a functional block diagram of a power supply switching circuit  6  shown in  FIGS. 1 and 2 . 
         FIG. 4  is a diagram showing one specific example configuration of power supply switching circuit  6  shown in  FIG. 3 . 
         FIG. 5  is a block diagram showing a configuration of a controller  20  shown in  FIG. 2 . 
         FIG. 6  is a diagram illustrating an example configuration of log information stored in a nonvolatile memory  22  shown in  FIG. 5 . 
         FIG. 7  is a flowchart showing one process at the time of normal operation of the optical transceiver in the embodiment of the present invention. 
         FIG. 8  is a flowchart showing a process of the optical transceiver when the host substrate is powered down. 
         FIG. 9  is a diagram showing candidate examples of monitor values for the optical transceiver and types of abnormalities that can be known from the monitor values. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding components are denoted by the same reference characters, and a description thereof will not be repeated. 
       FIG. 1  is a schematic configuration diagram of an optical communication apparatus in an embodiment of the present invention. Referring to  FIG. 1 , optical communication apparatus  101  includes a plurality of optical transceivers  1 , a host substrate  2 , a casing  5 , and a fan  10 . Optical transceivers  1  are shown in  FIG. 1  as one specific form of the optical communication module of the present invention. 
     A plurality of optical transceivers  1  are mounted on host substrate  2 . A plurality of optical transceivers  1  are pluggable optical transceivers. Namely, optical transceiver  1  is configured to be insertable in and removable from host substrate  2 . 
     Optical transceiver  1  converts an electrical signal sent from host substrate  2  into an optical signal and outputs the optical signal to an optical network. Optical transceiver  1  also converts an optical signal sent through the optical network into an electrical signal and sends the electrical signal to host substrate  2 . A front face  1   a  of optical transceiver  1  is configured so that a connector (not shown) provided at an end of an optical communication cable is attachable to and detachable from front face  1   a  of optical transceiver  1 , which, however, is not shown in detail in  FIG. 1 . 
     Host substrate  2  is installed in casing  5 . Casing  5  may for example be a rack. The direction in which host substrate  2  is oriented is not particularly limited.  FIG. 1  shows, for the sake of ease of recognition of a plurality of optical transceivers  1  and host substrate  2 , an arrangement where the surface of host substrate  2  is parallel to the horizontal direction. Host substrate  2  may be arranged in the manner shown in  FIG. 1 . Alternatively, host substrate  2  maybe arranged upright (host substrate  2  is placed to stand in the vertical direction). In addition, one host substrate  2  or a plurality of host substrates  2  may be mounted on optical communication apparatus  101 . 
     Host substrate  2  is mounted with a host CPU (Central Processing Unit)  3  and a nonvolatile memory  4 . Host CPU  3  and nonvolatile memory  4  are shown as typical devices mounted on host substrate  2 . 
     Host CPU  3  communicates with each of a plurality of optical transceivers  1 . Host CPU  3  further generates log information concerning monitoring of the situation of host substrate  2  by host CPU  3 . The log information is stored in nonvolatile memory  4 . The log information stored in nonvolatile memory  4  includes, for example, information about a time when host CPU  3  monitored the situation of host substrate  2 , and information about the situation of host substrate  2  at that time. 
     Nonvolatile memory  4  is a memory in which information can be written and the information can be stored in a nonvolatile manner. Nonvolatile memory  4  is implemented for example by an EEPROM. Host CPU  3  and nonvolatile memory  4  may be integrated into one unit. 
     To each of the plurality of optical transceivers  1 , a power supply switching circuit  6  supplies a power supply voltage supplied from host substrate  2 . The power supply voltage is supplied to host substrate  2  from outside host substrate  2 . An example configuration of power supply switching circuit  6  will be described below. 
     When host substrate  2  is powered down, power supply switching circuit  6  supplies prestored electric power to each of the plurality of optical transceivers  1 . Namely, power supply switching circuit  6  has a capability as a backup power source for the plurality of optical transceivers  1 . Power supply switching circuit  6  outputs a hazard signal indicating an abnormality relating to power supply from host substrate  2 . 
     In the present embodiment, as one example of the abnormality relating to power supply from host substrate  2 , an abnormality that the power supply voltage supplied from host substrate  2  to optical transceivers  1  during operation of optical transceivers  1  decreases is described. “Abnormality that the power supply voltage decreases” specifically means that the power supply voltage supplied from host substrate  2  to each of optical transceivers  1  falls below a determination level. The determination level may be appropriately set depending on specifications of host substrate  2  and optical transceivers  1 . For example, when the normal power supply voltage of optical transceiver  1  is set within the range of 3.3 V±5%, the determination level is set to be, for example, 3 V (this value is only an example provided for understanding of the present embodiment and the determination level is not limited to this value). It is to be noted that “abnormality that the power supply voltage decreases” may be expressed hereinafter as “host substrate  2  is powered down”. 
     In addition, when an abnormality occurs in host substrate  2 , the power supply voltage supplied from host substrate  2  to each of the plurality of optical transceivers  1  is considered to instantaneously decrease from the normal voltage (e.g., 3.3 V described above) to 0 V in many cases. However, the aforementioned abnormality that “host substrate  2  is powered down” includes a situation in which the power supply voltage falls below the determination level, and does not limit the speed of decrease in the power supply voltage. In addition, the final value of the power supply voltage when the power supply voltage falls below the determination level is not limited to 0 V. 
     Specific examples of occurrence of the abnormality that “host substrate  2  is powered down” include a case in which supply of the power supply voltage to host substrate  2  becomes impossible-due to failure of optical communication apparatus  101  as a whole. The specific examples also include a case in which supply of the power supply voltage to host substrate  2  is interrupted due to, for example, abnormal increase in temperature of host substrate  2 . The specific examples are not, however, limited to these examples and other examples can also he assumed. 
     When host substrate  2  is powered down, each optical transceiver  1  receives the hazard signal indicating this abnormality. As a result, each optical transceiver  1  records log information about the state of optical transceiver  1  itself and log information about host substrate  2  in optical transceiver  1  in a nonvolatile manner. This process will be described in detail below. 
     Fan  10  releases heat generated at host substrate  2  to outside optical communication apparatus  101 . According to the configuration shown in  FIG. 1 , fan  10  is provided on a rear face of casing  5 . Fan  10  operates, and thereby, external air is introduced from a front face of optical communication apparatus  101  into optical communication apparatus  101 , and heat generated at host substrate  2  is released from the rear face of optical communication apparatus  101  to the outside. Fan  10  is not limited to being provided on the rear face of casing  5 . Fan  10  may be provided on an arbitrary surface (such as an upper surface, a lower surface, the front face, and a side surface) of casing  5 , or may be provided on host substrate  2 . 
       FIG. 2  is a block diagram showing an example configuration of optical transceiver  1  shown in  FIG. 1 . Referring to  FIG. 2 , optical transceiver  1  includes an optical device  11 , a transmission circuit  14 , a reception circuit  17 , and a controller  20 . 
     Optical device  11  includes a laser diode (LD)  12  and a photodiode (PD)  13 . Laser diode  12  receives a power supply voltage and a control voltage that are fed from transmission circuit  14 . Laser diode  12  converts an electrical signal (transmission signal) which is sent from transmission circuit  14  into an optical signal and outputs the optical signal through an optical cable (not shown) to the optical network. 
     Photodiode  13  receives a power supply voltage and a control voltage that are fed from reception circuit  17 . Photodiode  13  receives an optical signal through an optical cable (not shown) from the optical network and converts the optical signal into an electrical signal. Photodiode  13  outputs the electrical signal as a reception signal to reception Circuit  17 . 
     Transmission circuit  14  includes a driver  15  for feeding the power supply voltage and the control voltage to laser diode  12 . Transmission circuit  14  further includes a D/A converter (DAC)  16 . D/A converter  16  converts a digital transmission signal which is sent from host CPU  3  into an analog signal. Driver  15  applies the analog signal to laser diode  12 . Further, transmission circuit  14  outputs to controller  20  a monitor voltage indicating a state of transmission circuit  14  or laser diode  12 . This monitor voltage is for example a voltage representing the intensity of light which is output by laser diode  12 . 
     Reception circuit  17  feeds the power supply voltage and the control voltage to photodiode  13 . Reception circuit  17  includes an amplifier  18  and an A/D converter (ADC)  19 . Amplifier  18  amplifies the reception signal (analog signal) which is sent from photodiode  13 . A/D converter  19  converts the amplified analog signal into a digital signal. Reception circuit  17  outputs this digital signal to host CPU  3 . Further, reception circuit  17  outputs to controller  20  a monitor voltage indicating a state of reception circuit  17  or photodiode  13 . This monitor voltage is for example a voltage representing the intensity of light which is received by photodiode  13 . 
     Controller  20  performs centralized control of optical transceiver  1 . For this sake, controller  20  supplies a control signal and a control voltage to each of transmission circuit  14  and reception circuit  17 . Further, based on the monitor voltage from each of transmission circuit  14  and reception circuit  17 , controller  20  monitors the state of optical transceiver  1 . Furthermore, in response to a request from host CPU  3 , controller  20  transmits to host CPU  3  information about the state of optical transceiver  1 . 
     Furthermore, controller  20  repeatedly receives log information sent from host CPU  3 . Controller  20  temporarily holds the log information. Controller  20  may receive the log information on a regular basis (e.g., at intervals of one second), or may receive the log information on an irregular basis. 
     Controller  20  receives the power supply voltage from power supply switching circuit  6 . Furthermore, when host substrate  2  is powered down, controller  20  receives the hazard signal. In response to the hazard signal, controller  20  stores the log information about host substrate  2  and the log information about the state of optical transceiver  1  in a nonvolatile manner. 
       FIG. 3  is a functional block diagram of power supply switching circuit  6  shown in  FIGS. 1 and 2 . Referring to  FIG. 3 , power supply switching circuit  6  includes a power storage device  7 , a comparing unit  8  and a switching circuit  9 . 
     In the present embodiment, power storage device  7  is implemented by a device configured to be charged and discharged, such as, for example, a secondary battery and a capacitor. Power storage device  7  is charged with the power supply voltage supplied from host substrate  2 . When host substrate  2  is powered down, power storage device  7  emits the stored electric power. The capacity of power storage device  7  is set such that power supply to the plurality of optical transceivers  1  is possible at least until the end of an operation of each of the plurality of optical transceivers  1  to write a result of self-monitoring and the log information from the host substrate in a nonvolatile memory included in optical transceiver  1 . 
     Comparing unit  8  is a circuit for detecting decrease in the power supply voltage from host substrate  2 . When the power supply voltage from host substrate  2  falls below the determination level, comparing unit  8  outputs the hazard signal. The hazard signal is sent to switching circuit  9  and controller  20  of optical transceiver  1  . Namely, comparing unit  8  functions as a power supply monitoring unit for monitoring the power supply voltage from host substrate  2 . 
     When receiving the hazard signal, switching circuit  9  supplies the voltage of power storage device  7  to optical transceiver  1 . When not receiving the hazard signal, switching circuit  9  supplies the power supply voltage from host substrate  2  to optical transceiver  1 . 
       FIG. 4  is a diagram showing one specific example configuration of power supply switching circuit  6  shown in  FIG. 3 . Referring to  FIG. 4 , resistance elements RI and R 2  divide the voltage of power storage device  7  and generate a reference voltage corresponding to the determination level. 
     Comparing unit  8  compares the reference voltage generated as described above and the power supply voltage from the host substrate. In the normal case, i.e., when the host substrate is not powered down, the voltage of power storage device  7  is equal to the power supply voltage from host substrate  2 . Therefore, the power supply voltage from host substrate  2  is higher than the reference voltage. In this case, comparing unit  8  does not output the hazard signal. On the other hand, when the power supply voltage from the host substrate is lower than the reference voltage, comparing unit  8  outputs the hazard signal. The hazard signal is input to an input port of controller  20 . 
     The power supply voltage for operating comparing unit  8  is, for example, supplied from the host substrate. For example, “output the hazard signal” refers to a state in which the hazard signal is in the L (low) level, and not output the hazard signal” refers to a state in which the hazard signal is in the H (high) level. According to this configuration, since the power supply voltage from the host substrate is normally higher than the reference voltage, the hazard signal is in the H level. Namely, the hazard signal is not output from comparing unit  8 . On the other hand, when the power supply voltage from the host substrate becomes lower than the normal level, the hazard signal enters the L level. Namely, the hazard signal is output from comparing unit  8 . When host substrate  2  is powered down, supply of the power supply voltage from host substrate  2  to comparing unit  8  is considered to be impossible. In this case as well, the hazard signal enters the L level. Namely, the hazard signal is output from comparing unit  8 . With the aforementioned configuration, the hazard signal can be output from comparing unit  8  when host substrate  2  is powered down. 
     Switching circuit  9  is formed of diodes D 1  to D 3 . Diode D 1  is arranged to allow the current to flow from power storage device  7  to a power supply input terminal of controller  20 . Diode D 2  is arranged to allow the current to flow from host substrate  2  to power storage device  7 . Diode D 3  is arranged to allow the current to flow from host substrate  2  to the power supply input terminal of controller  20 . 
     Normally, diode D 3  allows the current to flow from host substrate  2  to the power supply input terminal of controller  20 . Furthermore, diode D 2  allows the current to flow from host substrate  2  to the power supply input terminal of controller  20 , and power storage device  7  is charged. On the other hand, when the power supply voltage from host substrate  2  decreases, supply of the power supply voltage from host substrate  2  to controller  20  is interrupted by diode D 3 . Furthermore, by diode D 1 , the power supply voltage is supplied from power storage device  7  to controller  20 . 
     According to the configuration shown in  FIG. 4 , the power supply voltage supplied from power storage device  7  is substantially equal to the power supply voltage supplied from host substrate  2  to optical transceiver  1  before host substrate  2  is powered down. However, the power supply voltage supplied from power storage device  7  does not necessarily need to be the same as the power supply voltage supplied from host substrate  2  to optical transceiver  1 . The power supply voltage supplied from power storage device  7  may at least fall within a range predetermined as the power supply voltage of optical transceiver  1 . 
     In addition, any power storage device may be used as power storage device  7  as long as it can supply the power to optical transceiver  1  when power supply from host substrate  2  is abnormal (particularly when the power supply voltage from host substrate  2  falls below the determination level during operation of optical transceiver  1 ). Therefore, power storage device  7  is not limited to a power storage device such as a secondary battery that can be both charged and discharged, and a primary battery may be used. The voltage of the primary battery may at least fall within a range predetermined as the power supply voltage of optical transceiver  1 , and does not necessarily need to be the same as the power supply voltage supplied from host substrate  2  to optical transceiver  1 . 
     In addition, according to the structure shown in  FIGS. 3 and 4 , power supply switching circuit  6  is mounted on host substrate  2  separately from optical transceiver  1 . However, a part or all of the components of power supply switching circuit  6  may be built into optical transceiver  1 . According to one embodiment, comparing unit  8  serving as the power supply monitoring unit may be built into optical transceiver  1 . According to this configuration, optical transceiver  1  has a capability of monitoring the power supply voltage, and thus, optical transceiver  1  can be highly functionalized. Power supply switching circuit  6  is not limited to being provided within optical transceiver  1  or on host substrate  2 , and may at least be provided somewhere in optical communication apparatus  101 . 
       FIG. 5  is a block diagram showing a configuration of controller  20  shown in  FIG. 2 . The configuration shown in  FIG. 5  can be implemented by either a plurality of semiconductor integrated circuits or a single semiconductor integrated circuit. 
     Referring to  FIG. 5 , controller  20  includes a control unit  21 , a nonvolatile memory  22 , a volatile memory  23 , a bus  24 , an A/D converter  25 , a D/A converter  26 , a data bus interface  27 , a logic port  28 , a data bus interface  29 , and a temperature sensor  30 . 
     Control unit  21  controls the operation of the whole of controller  20 . Nonvolatile memory  22  is a memory in which information can be written and from which information can be read and further the information written therein can be stored in a nonvolatile manner. “Store in a nonvolatile manner” means that nonvolatile memory  22  can still hold the information even while no power supply voltage is fed thereto. Nonvolatile memory  22  is implemented for example by an EEPROM. 
     Regarding volatile memory  23 , information can be written and read in and from the volatile memory. When the power supply voltage is stopped from being fed to volatile memory  23 , the information stored in volatile memory  23  is lost. Volatile memory  23  is implemented for example by a DRAM (Dynamic Random Access Memory) or SRAM (Static Random Access Memory) or the like. 
     Bus  24  is provided for transmitting information for example between control unit  21  and nonvolatile memory  22  or between control unit  21  and volatile memory  23 . 
     A/D converter  25  converts a monitor voltage which is sent from transmission circuit  14  or reception circuit  17  shown in  FIG. 2  for example into a digital signal. A/D converter  25  outputs this digital signal to control unit  21 . D/A converter  26  converts a digital control signal which is sent from control unit  21  for example into an analog control signal. D/A converter  26  outputs this analog control signal to transmission circuit  14  or reception circuit  17  shown in  FIG. 2 . 
     Data bus interface  27  is a circuit for transmitting and receiving data for example between transmission circuit  14  or reception circuit  17  shown in  FIG. 2  and control unit  21 . Logic port  28  is a circuit for control unit  21  for example to transmit a digital control signal to transmission circuit  14  or reception circuit  17 . Data bus interface  27  is a circuit for transmission and reception of data between transmission circuit  14  or reception circuit  17  shown in  FIG. 2  and control unit  21 , for example. Data bus interface  29  is a circuit for control unit  21  for example to transmit and receive data to and from host CPU  3  or another device (another optical transceiver for example) mounted on host substrate  2 . 
     Control unit  21  receives the log information from the host substrate via data bus interface  29 . Alternatively, in response to a request from host substrate  2  (host CPU  3 ), control unit  21  outputs the log information stored in volatile memory  23  to data bus interface  29 . 
     Temperature sensor  30  detects the temperature of optical transceiver  1  and outputs a signal representing the temperature to control unit  21 . Since temperature sensor  30  may at least be disposed inside optical transceiver  1 , temperature sensor  30  may be provided separately from controller  20 . 
     Control unit  21  repeatedly monitors the state of optical transceiver  1 . Namely, control unit  21  achieves a self-monitoring capability of optical transceiver  1 . Furthermore, when the log information is sent from host substrate  2  to optical transceiver  1 , control unit  21  adds information about monitoring of the state of optical transceiver  1  to the log information from host substrate  2  and writes these log information in volatile memory  23 . 
     The hazard signal is input to control unit  21  via, for example, logic port  28 . When control unit  21  receives the hazard signal, control unit  21  stops the normal routine process and transfers the log information stored in volatile memory  23  to nonvolatile memory  22 . Namely, control unit  21  reads the log information from volatile memory  23  and writes the log information in nonvolatile memory  22 . As described above, the power supply voltage for the operation of control unit  21 , volatile memory  23  and nonvolatile memory  22  at this time is supplied by power storage device  7  (see  FIGS. 3 and 4 ) included in power supply switching circuit  6 . 
     As a result of the aforementioned operation, nonvolatile memory  22  stores, in a nonvolatile manner, the log information about the state of optical transceiver  1  and the state of host substrate  2  immediately before host substrate  2  is powered down. Log information  42  shown in  FIG. 5  represents the log information stored in nonvolatile memory  22 . 
       FIG. 6  is a diagram illustrating an example configuration of the log information stored in nonvolatile memory  22  shown in  FIG. 5 . Referring to  FIGS. 5 and 6 , log information  42  includes an optical transceiver status  42   a  (hereinafter simply referred to as “status  42   a ”), alarm information  42   b,  temperature monitor information  42   c,  time information  42   d,  and a host substrate log  42   e.    
     An address A 1  is allocated to status  42   a.  An address A 2  is allocated to alarm information  42   b.  An address A 3  is allocated to temperature monitor information  42   e,  An address A 4  is allocated to time information  42   d.  An address A 5  is allocated to host substrate log  42   e.  Addresses A 1  to A 5  are determined depending on sizes of the respective items of log information  42 . 
     Status  42   a  is a code representing a state of optical transceiver  1  when control unit  21  monitors the optical transceiver. Alarm information  42   b  is information indicating the fact that an abnormality of optical transceiver  1  has occurred. For example, in the case where the temperature of optical transceiver  1  has exceeded a reference value, a flag (“1” for example) representing this fact is generated as alarm information  42   b.  Temperature monitor information  42   c  is information indicating the temperature of optical transceiver  1  when the temperature thereof exceeds the reference value. Based on the output of temperature sensor  30 , control unit  21  generates a value of the measured temperature, and includes, in the log information, the value of the measured temperature as temperature monitor information  42   c.    
     Time information  42   d  is time information provided from the host substrate. This time may be, for example, a time when host substrate  2  (host CPU  3 ) generates the log information, or may be a time when host CPU  3  monitors host substrate  2 . 
     Host substrate log  42   e  is log information sent from host substrate  2 . Host substrate log  42   e  includes, for example, information about the temperature of host substrate  2 . In addition to or instead of the information about the temperature of host substrate  2 , another information may be included in host substrate log  42   e.  Examples of such “another information” include, for example, information indicating whether fan  10  (see  FIG. 1 ) is normal or not. The information included in host substrate log  42   e  is not limited thereto and may include other information about host substrate  2 . 
     Log information  42  shown in  FIG. 6  is temporarily stored in volatile memory  23 . Control unit  21  may temporarily store time information  42   d  and host substrate log  42   e  sent from host substrate  2  in volatile memory  23 , and may generate and write status  42   a,  alarm information  42   b  and temperature monitor information  42   c  in nonvolatile memory  22  when control unit  21  writes time information  42   d  and host substrate log  42   e  in nonvolatile memory  22 . 
     Furthermore, a plurality of pieces of log information  42  may be stored in volatile memory  23 . Each of the pieces of log information  42  has the configuration shown in  FIG. 6 . In response to reception of the hazard signal, control unit  21  collectively transfers the plurality of pieces of log information  42  to nonvolatile memory  22 . 
       FIG. 7  is a flowchart showing one process at the time of normal operation of the optical transceiver in the embodiment of the present invention. Referring to  FIG. 7 , the process of the main routine is started. In step S 1 , control unit  21  receives a measurement value of temperature sensor  30  to thereby monitor the temperature of optical transceiver  1 . Further, in step S 1 , control unit  21  updates a status (corresponding to status  42   a ). For example, control unit  21  holds the status in control unit  21  and updates the status. Step S 1  corresponds to a step for optical transceiver  1  to perform self-monitoring. 
     In step S 2 , control unit  21  determines whether there is communication from host substrate  2  or not. For example, when a transmission request or a reception request is sent from host substrate  2  to control unit  21 , control unit  21  determines that there is communication from the host substrate: If there is communication from host substrate  2  (YES in step S 2 ), the process proceeds to step S 3 . If there is no communication from host substrate  2  (NO in step S 2 ), the process is returned to step S 1 . That is, if there is no communication from host substrate  2 , the process in steps S 1  and S 2  is repeated. 
     In step S 3 , control unit  21  determines whether there is log information from host substrate  2  or not. When host substrate  2  sends the log information to control unit  21 , control unit  21  determines that there is log information from host substrate  2 . In this case (YES in step S 3 ), the process proceeds to step S 4 . On the other hand, if the log information is not sent from host substrate  2  to control unit  21  although there is communication between control unit  21  and host substrate  2  (NO in step S 3 ), the process is returned to step S 1 . 
     In step S 4 , control unit  21  obtains the log information from host substrate  2 . In step S 5 , control unit  21  updates the log information (host substrate log) on volatile memory  23 . Specifically, control unit  21  adds the information about monitoring of the state of optical transceiver  1  to the log information from host substrate  2 , and writes this log information in volatile memory  23 . The log information has the same configuration as the configuration shown in  FIG. 6 . After completion of the process in step S 5 , the process is returned to step S 1 . 
       FIG. 8  is a flowchart showing a process of the optical transceiver when the host substrate is powered down. Referring to  FIG. 8 , in step S 11 , control unit  21  determines whether interruption of power supply from host substrate  2  has been detected or not. Specifically, by detecting the hazard signal, control unit  21  detects interruption of power supply from host substrate  2  (i.e., power-down of host substrate  2 ). Detection of interruption of power supply from host substrate  2  may be executed by an interrupt process. Alternatively, by detecting a state change caused by polling, interruption of power supply from host substrate  2  may be detected. As a result of the process in step S 11 , the abnormality relating to power supply from host substrate  2  to optical transceiver  1  is detected. 
     In step S 12 , control unit  21  determines whether a shutdown instruction has been received from host substrate  2  or not. In order to achieve electric power saving in optical communication apparatus  101 , for example, supply of the power supply voltage to optical transceiver  1  may be intermittently stopped in some cases. In such a case, optical transceiver  1  stops the operation thereof in response to the shutdown instruction from host substrate  2 , for example. 
     If the shutdown instruction has been received from host substrate  2  (YES in step S 12 ), the process shown in  FIG. 8  is completed. On the other hand, if there is no shutdown instruction from host substrate  2  (NO in step S 12 ), the process proceeds to step S 13 . 
     The fact that interruption of power supply from host substrate  2  has been detected although there is no shutdown instruction means that there is a high possibility that an abnormality has occurred in host substrate  2 . Therefore, in step S 13 , control unit  21  determines that a condition for recording the log information has occurred. In step S 14 , control unit  21  transfers the log information stored in volatile memory  23  to nonvolatile memory  22 , and writes the log information in nonvolatile memory  22 . As a result, log information  42  (see  FIG. 6 ) is stored in nonvolatile memory  22 . After completion of the process in step S 14 , the overall process is completed. 
     Host substrate  2  may transfer the same log information to each of the plurality of optical transceivers  1  shown in  FIG. 1 . Normally, it cannot be predicted which of the plurality of optical transceivers  1  will fail. By storing the same log information in the plurality of optical transceivers  1 , the probability of leaving the log information about host substrate  2  in optical transceiver  1  which has failed can be increased. 
     As an alternative method, host substrate  2  may transfer the log information to the plurality of optical transceivers  1  at shifted timings. For example, host substrate  2  may cyclically transfer the log information among the plurality of optical transceivers  1 . Namely, when the number of optical transceivers  1  is N (N is an integer not smaller than 2), host substrate  2  transfers the log information to the first optical transceiver, the second optical transceiver, . . . the Nth optical transceiver, the first optical transceiver, the second optical transceiver, . . . in this order. In this case, the log information generated at different times is held in N optical transceivers  1 . When a plurality of optical transceivers  1  fail, a temporal change in the state of host substrate  2  (conversely, it may be no change in the state of host substrate  2 ) can be known by analyzing the log information stored in these optical transceivers  1  which have failed. Therefore, the cause of the failure of the optical transceivers can be analyzed in more detail. 
     According to the present embodiment, the log information about the host substrate is repeatedly sent to optical transceiver  1  that is insertable in and removable from host substrate  2 . Furthermore, optical transceiver  1  performs self-monitoring. Optical transceiver  1  (control unit  21  of controller  20 ) stores the log information about the host substrate and the result of self-monitoring by optical transceiver  1  in the volatile memory. When the hazard signal indicating the abnormality relating to power supply from host substrate  2  (that the power supply voltage supplied to optical transceiver  1  during operation of optical transceiver  1  becomes lower than the normal level) is detected, the log information and the result of self-monitoring by the optical transceiver stored in the volatile memory are written in nonvolatile memory  22 . 
     In the case where a plurality of optical transceivers I are connected to host substrate  2  as shown in  FIG. 1  and one of the plurality of optical transceivers  1  has failed, it is unnecessary to remove the whole host substrate  2  from optical communication apparatus  101  and thereby return the substrate, and only the optical transceiver  1  which has failed may be returned. Therefore, the burden on the provider of the optical communication system that is required for returning the optical transceiver  1  which has failed can be reduced. 
     Further, in the case where an abnormality occurs to optical communication, it can easily be determined whether the cause of the abnormality is an optical transceiver or a host apparatus (host substrate). For example, the provider of the optical communication replaces the optical transceiver which has failed with a new (normal) optical transceiver. If the optical communication accordingly recovers from the abnormality, it is easily determined that the cause of the abnormality is the optical transceiver. 
     Further, not only the information about the optical transceiver but also the information about the host substrate is stored in optical transceiver  1  in a nonvolatile manner. If the log information remains stored in volatile memory  23 , the log information stored in volatile memory  23  is lost when host substrate  2  is powered down. According to the present embodiment, when detecting the hazard signal, control unit  21  transfers the log information stored in volatile memory  23  to nonvolatile memory  22 . Therefore, when the optical transceiver fails due to the abnormality like power-down of host substrate  2 , not only the information about the state of optical transceiver  1  immediately before the failure but also the log information about the state of the host substrate immediately before the abnormality can be taken out from optical transceiver  1  which has failed. As a result, the cause of the failure of optical transceiver  1  can be analyzed in detail. 
     It is assumed, for example, that the temperature of host substrate  2  increases excessively, which causes not only power-down of host substrate  2  but also the failure of the optical transceiver. According to the temperature monitor information included in the log information, a temperature value higher than normal is shown. For example, when the log information (host information log) includes information indicating an abnormality of fan  10  in addition to the information about the temperature of the host substrate, worsened heat release at host substrate  2  can be estimated to be the cause of the failure of optical transceiver  1 . 
     Further, according to the present embodiment, power supply switching circuit  6  is provided in case the power supply voltage from host substrate  2  is powered down. Power supply switching circuit  6  can supply the power supply voltage to optical transceiver  1  until writing of the log information in nonvolatile memory  22  included in optical transceiver  1  is completed. Therefore, the log information about the state of the optical transceiver and the state of the host substrate can be stored in optical transceiver  1  in a nonvolatile manner. 
     In addition, the nonvolatile memory such as an EEPROM generally has a limitation in the write count. If the log information is frequently written in nonvolatile memory  22 , the lifetime of nonvolatile memory  22  may be shortened. According to the present embodiment, the log information is temporarily stored in volatile memory  23 . The log information is written in nonvolatile memory  22  only when the host substrate is powered down. As a result, the number of times the log information is written in nonvolatile memory  22  can be reduced. Therefore, shortening of the lifetime of optical transceiver  1  due to the lifetime of nonvolatile memory  22  can be prevented. 
     According to the aforementioned embodiment, in the optical transceiver, the temperature of the optical transceiver is monitored. However, various causes of the abnormality of optical transceiver  1  are conceivable. Therefore, in optical transceiver  1 , other monitor values may be monitored in addition to or instead of the temperature monitor value. In this case, control unit  21  stores the monitored monitor value in volatile memory  23 , and transfers the monitor value to nonvolatile memory  22  when host substrate  2  is powered down. 
       FIG. 9  is a diagram showing candidate examples of the monitor values for the optical transceiver and types of abnormalities that can be known from the monitor values. Referring to  FIG. 9 , the temperature of optical transceiver  1 , the intensity of light output by laser diode  12 , the intensity of light received by photodiode  13 , and the power supply voltage supplied to optical transceiver  1  are considered as the monitor values. A method for monitoring the temperature of optical transceiver  1  is as described above, and thus, a detailed description will not be repeated below. 
     Monitoring of the intensity of light output by laser diode  12  and the intensity of light received by photodiode  13  is performed for example in the following manner. Transmission circuit  14  outputs to controller  20  a monitor voltage indicating the intensity of light output by laser diode  12 . Reception circuit  17  outputs to controller  20  a monitor voltage indicating the intensity of light received by photodiode  13 . Controller  20  performs, by means of A/D converter  25 , an analog to digital conversion of the monitor voltage which is output from transmission circuit  14  and the monitor voltage which is output from reception circuit  17 . A digital signal which is output from A/D converter  25  is a monitor value indicating the intensity of the output light or a monitor value indicating the intensity of the received light. Control unit  21  receives these monitor values. Thus, control unit  21  monitors the intensity of light output by laser diode  12  and the intensity of light received by photodiode  13 . 
     When the temperature is high, the components (e.g., laser diode  12 ) of the optical transceiver may be damaged. In addition, the temperature of laser diode  12  is normally managed by, for example, a Peltier device and the like such that the intensity of the output light is constant. However, when a difference between the temperature of laser diode  12  and the temperature of its surroundings becomes too large, it becomes difficult to manage the temperature of laser diode  12  to be constant. Therefore, it becomes difficult to keep constant the intensity of light output by laser diode  12 . Accordingly, the temperature may be monitored as described above. 
     In addition, excessively high intensity of the output light is not preferable from the viewpoint of, for example, safety (e.g., safety against human eyes). Conversely, when the intensity of the output light is lower than a lower limit value, there is a possibility that laser diode  12  has reached the end of its lifetime. Therefore, the intensity of the output light may be monitored. 
     A highly-sensitive photodiode is used in optical communication. When the intensity of an optical signal input to the photodiode for optical communication is too high, the photodiode may be damaged. Therefore, the intensity of the received light may be monitored. 
     The monitor values are not limited to the examples shown in  FIG. 9 , and the optical transceiver may monitor other items about the optical transceiver. 
     In the present embodiment, the abnormality relating to power supply from host substrate  2  has been described as the abnormality of host substrate  2 . However, the types of abnormalities of the host substrate to be detected are not particularly limited. The present invention may at least be configured such that the hazard signal indicating the abnormality is transmitted to control unit  21 . 
     In addition, in the present embodiment, the case in which the power supply voltage supplied from host substrate  2  to optical transceiver  1  during operation of the optical transceiver falls below the determination level has been described as one example of the abnormality relating to power supply from host substrate  2 . However, the power supply voltage used for detection of the abnormality is not limited to the power supply voltage during operation of the optical transceiver. 
     In addition, not only the case in which the power supply voltage supplied from host substrate  2  to optical transceiver I falls below the determination level but also a case in which the power supply voltage exceeds an upper limit value in the predetermined range may be detected as the abnormality relating to power supply from host substrate  2 . Namely, the abnormality detected as the abnormality relating to power supply from host substrate  2  may be an abnormality that the power supply voltage supplied to optical transceiver  1  falls outside the predetermined range. 
     Further, in the present embodiment, the case in which the power supply voltage to optical transceiver  1  is a positive voltage has been described. However, the abnormality that the power supply voltage supplied to optical transceiver  1  falls outside the predetermined range may only be detected, and thus, the power supply voltage to optical transceiver  1  may be a negative voltage. 
     Further, in the aforementioned embodiment, all of the plurality of optical transceivers shown in  FIG. 1  have a capability of storing the log information in a nonvolatile manner. However, a part of the plurality of optical transceivers may have the log information storing capability described in the present embodiment and the remaining optical transceivers do not need to have the log information storing capability. 
     The optical transceiver has been illustrated herein as one specific form of the optical communication module according to the present invention. The optical communication module of the present invention, however, is not limited to the one like the optical transceiver having both the transmission capability and the reception capability. The optical communication module of the present invention may have only one of the transmission capability and the reception capability. Therefore, the optical communication module of the present invention may be an optical receiver or an optical transmitter. 
     It should be construed that the embodiments disclosed herein are by way of illustration in all respects, not by way of limitation. It is intended that the scope of the present invention is defined by claims and encompasses all modifications and variations equivalent in meaning and scope to the claims. 
     REFERENCE SIGNS LIST 
       1  optical transceiver;  1   a  front face (optical transceiver);  2  host substrate;  3  host CPU;  4 ,  22  nonvolatile memory;  5  casing;  6  power supply switching circuit;  7  power storage device;  8  comparing unit;  9  switching circuit;  11  optical device;  12  laser diode;  13  photodiode;  14  transmission circuit;  15  driver;  16 ,  26  D/A converter;  17  reception circuit;  18  amplifier;  19 ,  25  A/D converter;  20  controller;  21  control unit;  23  volatile memory;  24  bus;  27 ,  29  data bus interface;  28  logic port;  30  temperature sensor;  42  log information;  101  optical communication apparatus.