Abstract:
A communication apparatus for relaying a host device connected thereto via a transmission line and a plurality of terminals connected thereto via a network is provided which includes relaying means for relaying information received from the host device to the plurality of terminals. The information causes the plurality of terminals to perform predetermined processes. The information contains logic name of a destination terminal. The communication apparatus further includes destination information converting means for replacing the logic name of the destination terminal with a real address of the destination address so that that information containing the real address is transmitted to the destination terminal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a communication apparatus which is connected to a host apparatus via a transmission line and is connected to a plurality of terminal via a network and particularly relates to a communication apparatus and a communication method for relaying the host apparatus and the terminals.  
           [0003]    2. Description of the Related Art  
           [0004]    10BASE5 cables are commonly used for constructing a LAN (Local Area Network) of a considerably large scale. 10BASE5 is a standardized LAN cable conforming to IEEE 802.3. A 50Ω Thick cable having a bit-rate of 10 Mbps, a diameter of about 12 mm and a maximum segment length of 500 meters is used for 10BASE5. In addition to 10BASE5, LAN cables such as 10BASE2, 10BASE-T and 100BASE-T are known in the art. 10BASE2 is a LAN cable that is a 50Ω Thin cable having a diameter of about 5 mm and has a bit-rate of 10 Mbps and a maximum segment length of 185 meters. 10BASE-T also has a bit-rate of 10 Mbps and uses an unshielded twisted pair cable (UTP) and has a maximum segment length of 100 meters.  
           [0005]    Terminals to be connected via LAN have addresses that can be uniquely identified in a LAN environment. Such addresses may be IP (Internet Protocol) addresses and MAC (Media Access Control) addresses.  
           [0006]    A plurality of terminals may be connected by a communication cable without using destination addresses. Such a connection method can be achieved using coaxial cables. An example of a coaxial cable is a cable having an impedance of 75Ω and a maximum segment length of 1500 meters. The coaxial cable has a longer maximum segment length and therefore can be connected to a remote terminal by only one coaxial cable. Further, according to the connection method using the coaxial cable, a counterpart that is physically or electrically connected is recognized as a destination terminal. Therefore, the connection method using the coaxial cable does not require an address management function and, therefore, is highly reliable.  
           [0007]    An advantageous network environment can be constructed using LAN cables such as 10BASE-T together with a coaxial cable. In order to provide an interface between such two different types of cables or to extend communication distance, it is known to provide a communication apparatus for connecting the cables.  
           [0008]    [0008]FIG. 1 is a diagram showing how a communication apparatus may be connected. In FIG. 1, a communication apparatus  10  is connected to a host computer  12  in a one-to-one relationship by means of a coaxial cable. On the other hand, the communication apparatus  10  is connected to a terminal  14  by means of a cable such as 10BASE-T. In this example, the terminal  14  is intended to be a printer or a monitor.  
           [0009]    The communication apparatus  10 , the host computer  12  and the terminal  14  each have an address used for communications between each other. As has been described above, the addresses may be IP addresses or MAC addresses in case where LAN cables are used (e.g., 10BASE-T and 10BASE5). In case of a one-to-one connection using a coaxial cable, addresses are not required. Therefore, in a structure shown in FIG. 1, those elements having addresses are only the communication apparatus  10  and the terminal  14 .  
           [0010]    A plurality of logic terminals are defined on the terminal  14  such that the terminal  14  virtually serves as a plurality of terminals. In the example shown in FIG. 1, four logic terminals DspA-DspD are defined in the terminal  14 . FIGS. 2 and 3 are data format used for exchanging data between the logic terminals Dspa-DspD and the host computer  12 . FIG. 2 shows a data format  16  used for exchanging data between the host computer  12  and the communication apparatus  10 . The data format  16  has a sub-address which is a logic terminal number and a real data which contains commands and data of the terminal. FIG. 3 is a data format  16  used for exchanging data between the communication terminal  10  and the terminal  14 . The data format  18  has areas for storing a sender address, a destination address, a sub-address and real data. The sender address and the destination address are the addresses described above. When relaying the host computer  12  and the terminal  14 , the communication apparatus  10  attaches and removes the addresses of those data formats on the basis of the logic terminal numbers and the destinations.  
           [0011]    [0011]FIGS. 4 and 5 are diagrams showing how data is actually stored in such data formats. FIG. 4 shows a transmission data  20  transmitted from the host computer  12  to the communication apparatus  10  and a transmission data  22  from the communication apparatus  10  to the terminal  14 . The transmission data  20  and  22  are both data to be sent to the logic terminal DspA. Therefore, the address DspA is stored in the sub-address storing area shown in FIGS. 2 and 3. In the transmission data  22 , an address AdrZ of the communication device  10 , which is sending data, is stored in the sending address storage area and an address AdrA of the terminal  14 , which is a destination of the data, is stored in the destination address storage area.  
           [0012]    [0012]FIG. 5 shows a transmission data  24  from the terminal  14  to the communication apparatus  10  and a transmission data  26  from the communication apparatus  10  to the host computer  12 . These data are data sent when the terminal  14  regarded as the logic terminal DspD. Therefore, the address DspD of the logic terminal DspD is stored in the sub-address storage area of both of the transmission data  24  and  26 . In the transmission data  24 , an address AdrA of the terminal  14 , which is sending data, is stored in the sending address storage area and an address AdrZ of the communication apparatus  10 , which is a destination of the data, is stored in the destination address storage area.  
           [0013]    Using the data formats described above, the communication apparatus  10  relays between the host computer  12  and the terminal  14 .  
           [0014]    However, with such an environment including a host computer and a single terminal that are connected via a coaxial cable, even though a single terminal is logically handled as a plurality of terminals, it is difficult to improve a processing efficiency since there is only one physical terminal. Also, with such an environment with only one terminal, no process can be performed in case of a failure of the terminal.  
         SUMMARY OF THE INVENTION  
         [0015]    Accordingly, it is a general object of the present invention to provide a communication apparatus and a communication method that can obviate the problems described above.  
           [0016]    It is another and more specific object of the present invention to provide a communication apparatus and a communication method for relaying a host device connected via a communication line and a plurality of terminals physically connected via a network which can efficiently perform process of the plurality of terminals by management of status of the terminals and failures of the terminals.  
           [0017]    In order to achieve the above objects, the communication apparatus for relaying a host device and a plurality of terminals includes:  
           [0018]    first connection means which can be connected to the host device via a transmission line;  
           [0019]    second connection means which can be connected to the plurality of terminals via a network; and  
           [0020]    relaying means for relaying information received from the host device to the plurality of terminals, the information causing the plurality of terminals to perform predetermined processes.  
           [0021]    With the communication apparatus described above, the host device can transmit information for causing the plurality of terminals to perform predetermined processes. Therefore, predetermined processes can be performed in parallel by the plurality of terminals by an instruction from the host device.  
           [0022]    According to another embodiment of the present invention, the information contains first destination information designating a destination terminal of the information and the communication apparatus further includes destination information converting means for replacing first destination information with second destination information so that that information containing the second destination information is transmitted to the destination terminal.  
           [0023]    Accordingly, since the communication apparatus is provided with the destination information converting means, it is not necessary for the host device to store destination information of the destination terminals.  
           [0024]    A communication method for relaying a host device and a plurality of terminals is also provided which includes the steps of:  
           [0025]    a) establishing a connection with the host device via a communication line;  
           [0026]    b) establishing a connection with the plurality of terminals via a network; and  
           [0027]    c) relaying information received from the host device to the plurality of terminals, the information causing the plurality of terminals to perform predetermined processes.  
           [0028]    With the communication method described above, the host device can transmit information for causing the plurality of terminals to perform predetermined processes. Therefore, predetermined processes can be performed in parallel by the plurality of terminals by an instruction from the host device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a diagram showing how a communication apparatus is connected in the related art.  
         [0030]    [0030]FIG. 2 is a diagram showing a data format which is communicated between a host computer and a communication apparatus.  
         [0031]    [0031]FIG. 3 is a diagram showing a data format which is communicated between a communication apparatus and a terminal.  
         [0032]    [0032]FIG. 4 is a diagram showing data transmitted from a host computer to a communication apparatus and data transmitted from a communication apparatus to a terminal.  
         [0033]    [0033]FIG. 5 is a diagram showing data transmitted from a terminal to a communication apparatus and data transmitted from a communication apparatus to a host computer  
         [0034]    [0034]FIG. 6 is a system configuration diagram including a communication apparatus of an embodiment of the present invention.  
         [0035]    [0035]FIG. 7 is a system configuration diagram with addresses being indicated.  
         [0036]    [0036]FIG. 8 is a hardware block diagram of the host computer.  
         [0037]    [0037]FIG. 9 is a hardware block diagram of the communication apparatus.  
         [0038]    [0038]FIG. 10 is a software block diagram of the communication apparatus.  
         [0039]    [0039]FIG. 11 is a hardware block diagram of the terminal.  
         [0040]    [0040]FIG. 12 shows a destination conversion table of the present invention.  
         [0041]    [0041]FIG. 13 is a system configuration diagram including a communication apparatus of an embodiment of the present invention provided with an alternative terminal.  
         [0042]    [0042]FIG. 14 shows a destination conversion table including alternative terminal address.  
         [0043]    [0043]FIG. 15 is a diagram showing a chart of settings.  
         [0044]    [0044]FIG. 16 is a flowchart of a process performed on settings.  
         [0045]    [0045]FIG. 17 is a flowchart of a process performed by the communication apparatus when data from a host device is relayed to a terminal.  
         [0046]    [0046]FIG. 18 is a flowchart of a process performed by the communication apparatus when data from a terminal is relayed to a host device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]    In the following, principles and embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0048]    [0048]FIG. 6 is a diagram showing a system configuration including a communication apparatus  10  of an embodiment of the present invention whereto a host computer  12  and a plurality of terminals  28 ,  30 ,  32  and  34  are connected. It is to be noted that four terminals are illustrated in the figure, but any number of terminals may be connected to the system. In FIG. 6, the host computer  12  and the communication apparatus  10  are connected via a coaxial cable and the communication apparatus  10  and the terminals  28 ,  30 ,  32  and  34  are connected via LAN. The communication apparatus  10  relays communications between the host computer  12  and the terminals  28 ,  30 ,  32  and  34 . The terminal may be a monitor serving as a display device or a printer.  
         [0049]    [0049]FIG. 7 is a diagram of the system configuration as shown in FIG. 6 accompanied by addresses for the communication apparatus  10 , the host computer  12  and the terminals  28 ,  30 ,  32  and  34 . As shown in FIG. 7, the communication apparatus  10  has an address AdrZ that is used for establishing a connection with LAN. The terminals  28 ,  30 ,  32  and  34  have IP addresses or MAC addresses, AdrA, AdrB, AdrC and AdrD, respectively. In this embodiment, no address is required for a connection between the host computer  12  and the communication apparatus  10 , since the connection is a one-to-one connection via a coaxial cable. If a connection between the host computer  12  and the communication apparatus  10  is not via a coaxial cable, the host computer  12  is allotted an address AdrX and the communication apparatus  10  is allotted an address AdrY for communicating with the host computer  12 .  
         [0050]    [0050]FIG. 8 is a hardware block diagram of the host computer  12 . As shown in FIG. 8, the host computer  12  includes a CPU (Central Processing Unit)  36 , a memory  38 , a display controller  40 , a display  42 , a communication control circuit  44 , an input unit  46  and a HDD (Hard Disk Drive)  48 , which are connected via a bus. The CPU  36  follows instructions of a program to execute processes. The memory  38  includes a RAM (Random Access Memory) and a ROM (Read Only Memory). The memory  38  may store BIOS (Basic Input/Output System) and may be used as a work storage area for a program. The HDD  48  stores program and data. The display controller  40  controls a displaying operation of the display  42 . The display  42  may be a CRT (Cathode-Ray Tube).  
         [0051]    [0051]FIG. 9 is a hardware block diagram of the communication apparatus  10 . As shown in FIG. 9, the communication apparatus  10  includes an MPU (MicroProcessing Unit) module  52 , a LAN connection circuit  54 ,  56 , a line connection circuit  58 , an HDD  50 , an input part  59  and a display part  60 , which are connected via a bus. The MPU module  52  includes a CPU and a memory and executes processes for a relaying operation in accordance with a program. Such a program is stored in the HDD  50 . The display part  60 , for example, includes LEDs and displays operational statuses such as “error”, “connection” and “READY”. The LAN connection circuits  54  and  56  are circuits for communicating data on LAN and the line connection circuit  58  is a circuit for communicating data between the communication circuit  10  and the host computer  12 .  
         [0052]    [0052]FIG. 10 is a software block diagram of the communication apparatus  10 . The communication apparatus  10  includes an upper interface (I/F)  72 , a lower interface (I/F)  100 , send/receive buffers  74 ,  98 , a terminal status monitoring part  76 , a command analyzing part  78 , a command storing part  80 , a sub-address storing part  82 , a command generating part  84 , an address analyzing part  86 , an address converting part  88 , a destination conversion table  90 , an address switch setting part  92 , an address switch informing part  94  and a setting part  96 .  
         [0053]    Each block will be described in detail. The upper I/F  72  is an interface to the host computer  12  and the lower I/F  100  is an interface to LAN. The send/receive buffers  74  and  98  buffer data received at the upper I/F  72  and the lower I/F  100 , respectively. The terminal status monitoring part  76  monitors communication statuses of the terminals  28 ,  30 ,  32  and  34 . The command analyzing part  78  analyzes a content of a command to determine, for example, whether the received command is normal. The command storing part  80  stores the received command and the sub-address storing part  82  stores a sub-address. The command generating part  84  converts the received commands into commands for the destination and generates commands to be transmitted by the communication apparatus  10 . The address analyzing part  86  checks and analyzes addresses of senders and destinations. The address converting part  88  converts addresses in accordance with the destination conversion table  90  described below. The destination conversion table  90  converts the address of a logic terminal designated by the host computer  12  to a real address of the terminal. The address switch setting part  92  determines whether to switch an address of a particular terminal to an alternative terminal described below. The address switch informing part  94  generates data to be sent to a manager terminal described below to inform that the address has been switched. The destination conversion table  90 , the address switching part  92  and the address switch informing part  96  are set by operating a terminal connected to an operation panel or an external interface described below.  
         [0054]    [0054]FIG. 11 is a hardware block diagram of the terminals  28 ,  30 ,  32  and  34 . In the present embodiment, the terminals  28 ,  30 ,  32  and  34  are considered as printers. Therefore, as shown in FIG. 11, the terminals  28 ,  30 ,  32  and  34  includes a MPU  64 , a communication circuit  66 , a memory  68  and a driving part  62 , which are connected via a bus. The MPU  64  operates in accordance with a program stored in the memory  68  and controls the driving part  62  in accordance with data received by the communication circuit  66 . The communication circuit  66  is a circuit for communicating data over LAN.  
         [0055]    Based on the system configuration described above, data communicated between the communication apparatus  10 , the host computer  12  and the terminals  28 ,  30 ,  32  and  34  will be described in detail. Data communicated between the communication apparatus  10 , the host computer  12  and the terminals  28 ,  30 ,  32  and  34  have the formats and contents as illustrated in FIGS. 2 through 5. As shown in FIGS. 2 through 5, the data format of the data communicated between the host computer  12  and the communication apparatus  10  and the data format of the data communicated between the communication apparatus  10  and the terminals  28 ,  30 ,  32 ,  34  are different. A method of converting data format used when the communication apparatus  10  transmits data received from the host computer  12  to the terminals  28 ,  30 ,  32 ,  34  and a method of converting data format used when the communication apparatus  10  transmits data received from the terminals  28 ,  30 ,  32 ,  34  to the host computer  12  will be described below.  
         [0056]    [0056]FIG. 12 shows a destination conversion table. The communication apparatus  10  refers to the destination conversion table upon data format conversion. The destination conversion table includes five items: Logic Terminal, Terminal Address, Apparatus Address (Lower), Apparatus Address (Upper) and Host Address. Logic Terminal represents the terminals  28 ,  30 ,  32 ,  34 . For example, “DpsA” represents the terminal  28  (see FIG. 7). Terminal Address represents addresses of the terminals  28 ,  30 ,  32 ,  34 . Apparatus Address (Lower) represents an address of the communication apparatus  10  that is used to connect to LAN and Apparatus Address (Upper) represents an address of the communication apparatus  10  that is used to connect to the host computer  12 . Host Address represents an address of the host computer  12 . It is to be noted that Apparatus Address (Upper) and Host Address is not required in case where a coaxial cable is used.  
         [0057]    Accordingly, using a table that links the logic names and real addresses of the terminals, the communication  10  can send data from the host computer  12  to the destination terminals. It is also possible for the communication apparatus  10  to receive data from the terminals and add logic terminal number to the data to be transmitted so that the host computer  12  can distinguish from which terminal the data has been transmitted.  
         [0058]    An alternative terminal will be described in detail. An alternative terminal is such a terminal that takes over the processes to be executed by, for example, the terminal  28  in case of a failure of the terminal  28  in FIG. 7. The alternative terminal may be provided as a dedicated alternative terminal that is prepared in advance or the neighboring terminal  30 .  
         [0059]    [0059]FIG. 13 is a diagram showing a system configuration with a dedicated alternative terminal being provided in advance. The system configuration of FIG. 13 is similar to that shown in FIG. 7 except that the alternative terminal  70  having an address AdrE is added. Further, an alternative terminal for the terminal  30  is also provided which is the neighboring terminal  32  in the illustrated embodiment. Accordingly, in a system shown in FIG. 13, the alternative terminal  70  serves as an alternative terminal of the terminal  28  and the terminal  30  serves as an alternative terminal of the terminal  32 .  
         [0060]    [0060]FIG. 14 shows a destination conversion table of the communication apparatus  10  in which an item related to an alternative terminal address is added. In FIG. 14, Alternative Terminal Address of the logic terminal DspA is selected as address AdrE that indicates the alternative terminal  70 . Similarly, Alternative Terminal Address of the logic terminal DspB representing the terminal  30  is selected as an address AdrC of the terminal  32 . In case of a failure of the terminals, the communication apparatus  10  switched the destination of information such as process request from the host computer  12  to the alternative terminals using the destination conversion table.  
         [0061]    Settings of the alternative terminal and other settings for the communication apparatus  10  are set in the following manner. The settings may be set via an operation panel provided on the communication apparatus  10  or via a console terminal connected to an external interface such as a serial port of the communication apparatus  10 . The settings may also be set by means of a terminal connected to the communication apparatus  10  via LAN.  
         [0062]    The details of the settings will be described with reference to FIG. 15 showing a chart of the settings. The chart includes: items, details and settings. The following description is made for each item. “ADDRESS SWITCHING” relates to the above-mentioned alternative terminal and determines, in case of a failure to a terminal, whether or not to switch the address to another terminal. Therefore, the item “ADDRESS SWITCHING” determines whether “to switch” or “not to switch”. The item “ALTERNATIVE TERMINAL ADDRESS” specifies the address of the alternative terminal to be switched to. The item “TIMING OF ADDRESS SWITCHING” specifies at which timing the switching operation to the alternative terminal is to be performed and therefore specifies the trigger for performing the switching operation. The switching operation is performed when the alternative terminal is in an abnormal state, that is to say, “NO RESPONSE FROM TERMINAL” and “RESPONSE FROM TERMINAL IS ABNORMAL”. Accordingly, the item “TIMING OF ADDRESS SWITCHING” may be selected as “NO RESPONSE FROM TERMINAL”, “RESPONSE FROM TERMINAL IS ABNORMAL” and “NO RESPONSE FROM TERMINAL OR RESPONSE FROM TERMINAL IS ABNORMAL”.  
         [0063]    Further, a selection can be made between an automatic switching operation or a manual switching operation, and this is specified in the item “MANUAL/AUTOMATIC OPERATION OF SWITCHING OPERATION”. For manual switching, if any failure of the alternative terminal is detected, a selection dialog screen “SWITCH TO ALTERNATIVE TERMINAL? (YES/NO)” is displayed on the operation panel so that the user can manually select “YES/NO”. If the terminal displaying such a selection dialog screen is connected to LAN, the communication apparatus  10  should know addresses of the terminals connected via LAN. This is specified in “SENDING INFORMATION UPON ADDRESS SWITCHING”. When an address is specified in this item, the above-mentioned selection dialog or an information screen for informing that the terminal has been automatically switched is displayed on the terminal having the specified address.  
         [0064]    [0064]FIG. 16 is a flowchart showing how an address is switched from a terminal with a failure to an alternative terminal. Step S 101  branches in three branches according to the settings of the above-described “TIMING OF ADDRESS SWITCHING”. When the setting is “NO RESPONSE”, the process proceeds to step S 102 . In step S 102 , the type of failure of the terminal is detected. If the detected failure is a no response state, the process proceeds to step S 106  where an address switching operation is performed. If the detected failure is a response indicating an abnormal operation, an error handling operation is performed in step S 103 . When the setting is “ABNORMAL”, the process proceeds to step S 104 . In step S 104 , the type of failure of the terminal is detected. If the detected failure is a response indicating an abnormal operation, the process proceeds to step S 106  where an address switching operation is performed. If the detected failure is a no response state, an error handling operation is performed in step S 103 . When the setting is “NO RESPONSE OR ABNORMAL”, the process proceeds to step S 105 . In step S 105 , the type of failure of the terminal is detected. If the detected failure is a no response state or a response indicating an abnormal operation, the process proceeds to step S 106  where an address switching operation is performed.  
         [0065]    Step S 106  branches according to the settings in “ADDRESS SWITCHING”. If the setting is “SWITCH”, the process proceeds to step S 108 . If the setting is “DO NOT SWITCH”, the process proceeds to step S 107  where an error handling operation is performed.  
         [0066]    In step S 108 , an address of an alternative terminal is obtained from the table of alternative address that is set in the “ALTERNATIVE TERMINAL ADDRESS” settings. Step S 109  branches according to the settings of “MANUAL/AUTOMATIC OPERATION OF ADDRESS SWITCHING”. If the setting is “AUTOMATIC SWITCHING”, the process proceeds to steps S 111  to obtain an address of a manager terminal. In step S 112 , a message “Destination terminal has been switched due to a failure in logic terminal XX” is displayed on the manager terminal. In step S 115 , destination terminal is switched to an alternative terminal so that the use can continue his work using the alternative terminal. Referring again to Step S 109 , when the setting is “MANUAL SWITCHING”, an address a manager terminal is obtained in step S 110 . In step S 113 , a message “Failure in the terminal. Switch destination terminal?” is displayed on the manager terminal. If the choice for this message is “NO”, the process proceeds to step S 114  where an error handling operation is performed. If the choice for this message is “YES”, the process proceeds to step S 115  where destination terminal is switched to an alternative terminal so that the use can continue his work using the alternative terminal.  
         [0067]    Referring now to FIGS. 17 and 18, processes performed on the communication apparatus  10  will be described. In the following description, elements of the system are described using the reference numerals indicated in FIG. 10.  
         [0068]    [0068]FIG. 17 is a flowchart showing a process for transmitting data from the host computer  12  to a destination terminal. Initially, in step S 201 , data from the host computer  12  is received via the upper I/F  72 . In step S 202 , the received data is stored in a send/receive buffer  74 . In step S 203 , the terminal status monitoring part  76  detects a response from the terminal. Step S 204  branches the process in accordance with the result of detection in step S 203 . If the result of detection shows that there is a response from the terminal, the process proceeds to step S 205  where the command analyzing part  78  determines whether a command received from the host computer  12  is an end command. In step S 206 , if it is determined as an end command, the command stored in step S 208  is reset and if it is not the end command, the received command is stored in step S 207 . In step S 209 , the address analyzing part  86  analyzes address of the sender and destination of the received data. In step S 210 , the address converting part  88  converts the destination address by referring to an address table stored in the sub-address storage part. In step S 211 , the command generating part  84  generates commands to be sent to the destination address that has been converted to an appropriate address. In step S 212 , the lower send buffer  98  sends the generated command to the lower I/F  100  and the command is output on LAN via the lower I/F  100 .  
         [0069]    If the result step shows that there is no response from the terminal, the following steps are performed. When there is no response from the terminal, the address switch setting part  92  detects in step S 214  whether the setting is set such that the destination is converted when there is no response from the terminal. In step S 215 , the result of the detection is determined. If the setting indicates that the destination terminal is not to be converted, in step S 221 , the command generating part  84  generates commands for informing to an upper side that there is no response from the terminal. In step S 222 , the upper send/receive buffer  74  sends the command to the upper I/F  72 . In step S 224 , the upper I/F sends the command to the host computer  12 . In step S 223 , the command storage part  80  resets the command stored therein.  
         [0070]    If it is determined in step S 215  that an address converting operation is required, the process proceeds to step S 216 . In step S 216 , the address analyzing part  86  checks the sender address and the destination address to be converted to. In step S 217 , the address converting part  88  converts the destination address by referring to the address table stored in the sub-address storage part  82 . In step S 218 , the command generating part  84  generates commands stored in the command storage part  80 . In step S 219 , the address switching informing part  94  generates address convert informing data which informs the manager terminal that the destination address has been converted. In step S 220 , the send/receive buffer  98  sends the generated command and the address convert informing data to the lower I/F  100 . In step S 213 , these data are transmitted on LAN from the lower I/F  100 .  
         [0071]    As has been described above, data from the host computer  12  is transmitted to the terminal with the address being converted by the communication apparatus  10 . In case of a failure of the terminal the communication apparatus  10  further performs processes such as transmitting data to an alternative terminal and informing the host computer  12  that the terminal has failed.  
         [0072]    Referring to FIG. 18, a process of transmitting data received from the terminal to the host computer will be described. In step S 301 , data is received at the lower I/F  100  which is provided for receiving data from the terminal. In step S 302 , the received data is stored in the send/receive buffer  98 . In step S 303 , the command analyzing part  84  determines whether the data is a normal data. Based on the result of step S 303 , the process is branched in step S 304 . If the data is a normal data, the process proceeds to step S 305  where the command analyzing part  78  determines whether the data is an end command. If the data is not an end command, in step S 307 , the command storage part  80  stores the command. If the data is an end command, in step S 308 , the stored command is reset. In step S 309 , the address analyzing part  88  analyzes the sender address and the destination address of the received data. In step S 310 , the address converting part  88  converts the destination address by referring to the address table stored in the sub-address storage part  82 . In step S 311 , the command generating part  84  generates a command for the destination address that has been converted to an appropriate address. In step S 312 , the send/receive buffer  74  sends the generated command to the upper I/F  72 . In step S 313 , the command is transmitted from the upper I/F to the host computer  12 .  
         [0073]    Referring again to step S 304 , if the command from the terminal is abnormal, the following processes are performed. If the command is abnormal, in step S 314 , the address switch setting part  92  verifies whether the setting indicates that the terminal is converted to the destination terminal in case of a failure of the terminal. In step S 315 , the result of step S 314  is determined whether the setting indicates that the terminal is to be converted or not. If the terminal is not to be switched, the process proceeds to step S 322  where the command generating part  84  generates a command for informing of a failure of the terminal to the host computer  12 . in step S 323 , the upper send buffer  74  sends the generated command to the upper I/F  72 . In step S 324 , the command storage part resets the command stored therein. In step S 313 , the upper I/F  72  transmits the generated command to the host computer  12 .  
         [0074]    Referring again to step S 315 , if the terminal is to be switched, the process proceeds to steps S 316 . In step S 316 , the address analyzing part  86  checks the sender address and the destination address to be converted to. In step S 317 , the address converting part  88  converts the destination address by referring to the address table stored in the sub-address storage part  72 . In step S 318 , the command generating part  84  generates a command stored in the command storage part  80  for the converted destination terminal. In step S 318 , the address switch informing part  94  generates destination address convert message to be sent to the manager terminal for informing that the destination address has been converted. In step S 320 , the send/receive buffer  98  sends the address convert message and the generated command to the lower I/F  100 . In step S 321 , the lower I/F  100  sends the command and the message to LAN.  
         [0075]    As has been described above, the data from the terminal is transmitted to the host computer with the address being converted by the communication apparatus  10 . Further, In case of a failure of the terminal the communication apparatus  10  further performs processes such as transmitting data to an alternative terminal and informing the host computer  12  that the terminal has failed.  
         [0076]    Further, the present invention is not limited to these embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
         [0077]    The present application is based on Japanese priority application No. 2001-394727 filed on Dec. 26, 2001, the entire contents of which are hereby incorporated by reference.