CROSS-REFERENCED TO RELATED APPLICATION 
     This is a Divisional application which claims the benefit of U.S. patent application Ser. No. 12/434,415, filed May 1, 2009, which claims priority to Japanese Patent Application 2008-143308, filed on May 30, 2008. The disclosure of the prior applications are hereby incorporated in their entirety by reference. 
    
    
     FIELD 
     The embodiments discussed herein are directed to a packet communication device and a packet communication method. More particularly, the embodiments are directed to a device and method for data communication in packet units through a serial bus compliant with IEEE 1394 standard. 
     BACKGROUND 
     Standards for serial buses for realizing high-speed data transfers include IEEE 1394 standard. Various technologies employing the IEEE 1394 standard have been proposed until present. One such technology is disclosed in the Japanese Patent Applications Laid-Open No. 11-212896 and 2000-253023. 
     The IEEE 1394 standard supports isochronous transfers and asynchronous transfers. Isochronous transfers are transfers which are time-managed in 125 μs cycles (isochronous cycles) and have a constant transfer bandwidth. As the transfer bandwidth is reliably secured for this type of transfers, isochronous transfers are suitable for transfers of video images and the like wherein interruptions are not permitted. Alternatively, asynchronous transfers are transfers wherein data transfers are reliably carried out without any time management. 
       FIG. 21  depicts a format of a cycle start packet which is transmitted in each isochronous cycle. The cycle start packet is made up of 32-bit data sequences (quadrate data). A destination_ID refers to an identifier for a transmission destination, while a source_ID refers to an identifier for a transmission source. A transaction label tl is a label for identifying various transactions. Also, rt indicates a retry code, tcode indicates a transaction code, pri indicates a priority order, destination_offset indicates packet type-inherent information and cycle_time_data indicates packet type-inherent quadrate data, respectively. A header_CRC indicates cyclic redundancy check (hereinafter referred to as CRC) data for checking for errors error in the 4 quadrate data items. 
     SUMMARY 
     According to an aspect of the invention a packet communication device for switching packets to be transferred in packet communication which is time-managed in constant cycles, the packet communication device includes a changing section configured to change a portion of a first packet to be transmitted in each of the constant cycles. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating connections in a first embodiment; 
         FIG. 2  illustrates a specific example of a CRC changing section  3 ; 
         FIG. 3  depicts transfer of packets on the IEEE 1394 bus  200  in the first embodiment; 
         FIG. 4  depicts the display contents to be outputted to a display device  103  in the first embodiment; 
         FIG. 5  is a block diagram of devices in a second embodiment; 
         FIG. 6  is a block diagram illustrating connections in the second embodiment; 
         FIG. 7  depicts transfer of packets on the IEEE 1394 bus  200  in the second embodiment; 
         FIG. 8  depicts the display contents to be outputted to the display device  103  in the second embodiment; 
         FIG. 9  is a block diagram of device in a third embodiment; 
         FIG. 10  is a block diagram of device in the third embodiment; 
         FIG. 11  is a block diagram illustrating connections in the third embodiment; 
         FIG. 12  depicts transfer of packets on the IEEE 1394 bus  200  in the third embodiment; 
         FIG. 13  depicts the display contents to be outputted to the display device  103  in the third embodiment; 
         FIG. 14  is a block diagram of device in a fourth embodiment; 
         FIG. 15  depicts transfer of packets on the IEEE 1394 bus  200  in the fourth embodiment; 
         FIG. 16  is a block diagram illustrating connections in a fifth embodiment; 
         FIG. 17  is a block diagram illustrating connections in a sixth embodiment; 
         FIG. 18  depicts transfer of packets on the IEEE 1394 bus  200  in the sixth embodiment; 
         FIG. 19  depicts the contents to be outputted to the display device  103  and a speaker  113  in the sixth embodiment; 
         FIG. 20  is a block diagram illustrating connections in a seventh embodiment; 
         FIG. 21  depicts a format of a cycle start packet; 
         FIG. 22  illustrates conventional transfer of packets on the IEEE 1394 bus  200 ; 
         FIG. 23  depicts conventional display contents to be outputted to an in-vehicle display device; and 
         FIG. 24  illustrates conventional transfer of packets on the IEEE 1394 bus  200 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In recent years, the IEEE 1394 standard has been used mostly for home networking and in-vehicle networking, rather than as an interface between AV devices, and a computer and peripherals. 
     However, the following problems may occur in the case the IEEE 1394 standard is used for in-vehicle networking, for instance. 
     Video images and the like are transferred in in-vehicle networks in an isochronous transfer suitable for the transfer of such video images as described earlier. Isochronous packets subjected to isochronous transfer as supported by the IEEE 1394 standard are transferred in real time with the transfer bandwidth to be used secured in advance, and without interruptions. Accordingly, in the case of transferring isochronous packets, procedures for reserving the band are preferably carried out in advance. To change isochronous packets which are currently being transferred to transfer of separate isochronous packets, once the operating band is released, a procedure is subsequently preferably carried out to reserve new band. Specifically, these procedures for releasing band and reserving band for switching the transfer data make it impossible to switch transfer data at high speed. 
     For instance, when an attempt is made to switch, in an emergency case, from an environment wherein DVD video images are displayed to video images of a back monitor in an in-vehicle network, packets of DVD video image data and packets of video image data from the back monitor may not be changed instantly, as illustrated in  FIG. 22 . Procedures for releasing band and for reserving band are carried out between a cycle M wherein packets of DVD video image data are transferred and a cycle N wherein transfer of packets of video image data from the back monitor is initiated. As a result, video images to be outputted to an in-vehicle display device are interrupted between DVD video images and video images of the back monitor, as illustrated in  FIG. 23 . 
     To solve this problem, all video image data was transferred on an IEEE 1394 bus in conventional art, as illustrated in  FIG. 24 . Accordingly, an IEEE 1394 bus band is used to transfer video image data which is not to be outputted to the display device, which then restricts the size of the transferable video image data. 
     The embodiments have been worked out in view of the above-described problems. According to the embodiments, a packet communication device and a packet communication method are proposed which enable switching of isochronous packets subjected to isochronous transfer as supported by IEEE 1394 standard, without carrying out procedures such as band releasing and band reserving. 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. 
       FIG. 1  is a block diagram illustrating connections in an in-vehicle network using a packet communication device according to a first embodiment. An isochronous packet receiving device  100  and isochronous packet transmitting devices  101  and  102  are connected through an IEEE 1394 bus  200 . The receiving device  100  has a cycle timer  1 , a CRC computing section  2 , a CRC changing section  3 , a cycle start packet generating section  4 , a 1394 block  5 , a FIFO  6  and a video I/F  7 . The cycle timer  1  supplies 125 μs isochronous cycles. The cycle start packet generating section  4  generates a cycle start packet having a header_CRC attached thereto. This header_CRC is computed in the CRC computing section  2  and changed in the CRC changing section  3 . The cycle start packet thus generated is transmitted on the IEEE 1394 bus  200  through the 1394 block  5  and is transferred to the transmitting devices  101  and  102 . The isochronous packets transferred from the transmitting devices  101  and  102  through the IEEE 1394 bus  200  are outputted to the display device  103  through the 1394 block  5 , FIFO  6  and video I/F  7  provided in the receiving device  100 . 
     The transmitting devices  101  and  102  each have a 1394 block  5 , a CRC extracting section  8 , a cycle start format checking section  9 , a CRC checking section  10 , an AND gate  11 , an isochronous packet generating section  12 , a FIFO  6  and a video I/F  7 . The transmitting device  101  further has a CRC changing section  3 . A monitoring camera  104  for the back monitor is connected to the transmitting device  101 . Isochronous packets of video image data for the back monitor are generated in the isochronous packet generating section  12  through the video I/F  7  and FIFO  6 . Alternatively, a DVD player  105  is connected to the transmitting device  102 . Isochronous packets of DVD video image data are generated in the isochronous packet generating section  12  through the video I/F  7  and FIFO  6 . 
     The cycle start packet transmitted from the receiving device  100  is inputted to the CRC extracting section  8  through the 1394 block  5  in the transmitting sections  101  and  102 . The CRC extracting section  8  extracts the header_CRC of the cycle start packet. The cycle start format checking section  9  checks whether or not Data representing data from the cycle start packet excluding the CRC is compliant with the IEEE 1394 standard. In the transmitting device  101 , the CRC data is inputted to the CRC checking section  10  through the CRC changing section  3 . In the transmitting device  102 , the CRC data is inputted to the CRC checking section  10  directly. The CRC checking section  10  then checks whether there are errors in this data. The cycle start format checking section  9  and the CRC checking section  10  input the checking results to an AND gate  11 . An output of the AND gate  11  is connected to an enable terminal EN of the isochronous packet generating section  12 . Accordingly, if the checking results as processed in the cycle start format checking section  9  and the CRC checking section  10  are recognized as normal, enabling of the isochronous packet generating section  12  is asserted. 
       FIG. 2  illustrates a specific example of the CRC changing section  3 . The CRC changing section  3  has an inverter  31 , a selector  32  and a switching register  33 , as illustrated in  FIG. 2 . The selector  32  selects whether to output either a through input without passing through the inverter  31  or an inverted-side input which passes through the inverter  31 , based on a value set in the switching register  33 . In the first embodiment, the CRC changing section  3  provided in the transmitting device  101  is set so as to input an inverted-side input. 
     The operation in the first embodiment having the above-described configuration will now be described. Normally, if the CRC changing section  3  provided in the receiving device  100  is set so as to output the through-side input, the cycle start packets transmitted from the receiving device  100  are normal cycle start packets. Checking results with respect to the cycle start packets to be transmitted from the receiving device  100 , as processed in the cycle start format checking section  9  and the CRC checking section  10  are recognized as normal in the transmitting device  102  which does not have the CRC changing section  3 . Accordingly, enabling of the isochronous packet generating section  12  is asserted, which enables transfer of isochronous packets of video image data from a DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, the CRC changing section  3  provided in the transmitting device  101  is set so as to output an inverted-side input. The checking results with respect to the cycle start packets to be transmitted from the receiving device  100 , as processed in the CRC checking section  10 , are recognized as normal in the transmitting device  101  which has the CRC changing section  3 . Accordingly, enabling of the isochronous packet generating section  12  is not asserted, which disables transfer of isochronous packets of video image data from a monitoring camera  104  of the back monitor to the IEEE 1394 bus  200 . Thus, only isochronous packets of video image data from a DVD player  105  are transferred to the IEEE 1394 bus  200 , and DVD video image data is outputted to the display device  103 . 
     Here, once the display contents displayed on the display device  103  used for backing up the vehicle are switched to video images of the back monitor, setting of the CRC changing section  3  provided in the receiving device  100  is switched so as to output the inverted-side input. Thus, cycle start packets having an inverted header_CRC attached thereto are generated in the cycle start packet generating section  4  provided in the receiving device  100 . Accordingly, the cycle start packet transmitted from the receiving device  100  becomes a cycle start packet with inverted CRC data. As a result, the checking results with respect to the cycle start packets to be transmitted from the receiving device  100 , as processed in the CRC checking section  10  are not recognized as normal in the transmitting device  102  which does not have the CRC changing section  3 . Accordingly, enabling of the isochronous packet generating section  12  is not asserted, which stops transfer of isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, as the CRC changing section  3  provided in the transmitting device  101  is set so as to output an inverted-side input, the checking results with respect to the cycle start packets to be transmitted from the receiving device  100 , as processed in the CRC checking section  10  are recognized as normal in the transmitting device  101  which has the CRC changing section  3 . Accordingly, enabling of the isochronous packet generating section  12  is asserted, which enables transfer of isochronous packets of video image data from the monitoring camera  104  for the back monitor to the IEEE 1394 bus  200 . Thus, only isochronous packets of video image data from a monitoring camera  104  for the back monitor is transferred to the IEEE 1394 bus  200 , in place of video image data from the DVD player  105 , while video image data of the back monitor is outputted to the display device  103 . 
       FIG. 3  depicts transfer of packets on the IEEE 1394 bus  200  in the first embodiment as described earlier.  FIG. 4  depicts the display contents to be outputted to the display device  103 . Starting with  FIG. 3 , gaps such as isochronous gaps and the like in the IEEE 1394 bus  200  of a Phy layer which may occur in packet transmission on the bus are omitted. 
     In the first embodiment described above, the through side input and inverted-side input in the CRC data in the cycle start packets to be transmitted from the receiving device  100  can be switched in the CRC changing section  3  provided in the receiving device  100 . Enabling of the isochronous packet generating section  12  is asserted only at the transmitting device  101  or  102  which received the compliant cycle start packet, which enables transmission of such isochronous packet. Thus, isochronous packets to be transferred on the IEEE 1394 bus  200  as illustrated in  FIG. 3  can be switched without any band release and band reserving procedures being carried out. As a result, the display contents of the display device  103  can be outputted without interruptions between the DVD video images and the video images of the back monitor, as illustrated in  FIG. 4 . 
     As per the conventional art example depicted in  FIG. 24 , as all video image data is not transferred on the IEEE 1394 bus  200 , bus band can be efficiently used. Further, as the transmitting device  101  or  102  from which no output is made to the display device  103  transmits no isochronous packets, power consumption can be suppressed. 
       FIG. 5  is a block diagram of an isochronous packet receiving device  100  and isochronous packet transmitting devices  101  and  102  provided in a packet communication device according to a second embodiment.  FIG. 6  is a block diagram illustrating connections in an in-vehicle network using the packet transmitting devices depicted in  FIG. 5 . The receiving device  100  and transmitting devices  101  and  102  are omitted in  FIG. 6 . The second embodiment will now be described based on  FIG. 5  and  FIG. 6 . 
     In the second embodiment, the receiving device  100  has a switching register  13  and a selector  14  (refer to  FIG. 5 ). The transmitting devices  101  and  102  each have a CRC and tl extracting section  8  and tl checking section  15 . As the rest of the constituting elements are the same as in the first embodiment, further description thereof is hereby omitted. In an in-vehicle network using the packet communication device according to the second embodiment, the isochronous packet receiving device  100 , the two isochronous packet transmitting devices  101  and the transmitting device  102  are connected through an IEEE 1394 bus  200  (refer to  FIG. 6 ). A display device  103  is connected to the receiving device  100 . Monitoring cameras  104  for a back monitor and a side monitor are respectively connected to the two transmitting devices  101 . A DVD player  105  is connected to the transmitting device  102 . 
     As was described earlier, a transaction label tl is included in the components of the cycle start packet (refer to  FIG. 21 ). The transaction label tl represents a label for identifying each transaction. A fixed value 00h is included in conventional cycle start packets. In the second embodiment, a selector  14  can select either one of the fixed values 01h and 02h, other than the conventional fixed value 00h, with respect to transaction labels tl, based on a value set in the switching register  13  provided in the receiving device  100 . Accordingly, the transaction labels tl of the cycle start packets generated in the cycle start packet generating section  4  can be set to any of the fixed vales 00h, 01h and 02h. 
     In the transmitting devices  101  and  102 , the cycle start packets to be transmitted from the receiving device  100  are inputted to the CRC and the ti extracting section  8  through the 1394 block  5 . The CRC and the ti extracting section  8  extract a header_CRC and a transaction label tl of the cycle start packet. In the second embodiment, the cycle start format checking section  9  checks the configuration of DATA representing data in the cycle start packet with the CRC and tl removed therefrom. Also, the CRC checking section  10  checks for errors in the CRC data, and in addition, the tl checking section  15  checks the transaction labels tl. 
     In conventional art, the transaction labels tl are checked in the tl checking section  15  by comparison with the fixed value 00h. However, in the second embodiment, the tl checking section  15  provided in the transmitting device  101  receives a changeable comparison value. Accordingly, the transaction labels tl are checked in the tl checking section  15  provided in the transmitting device  101  by comparison with the changeable comparison value. As illustrated in  FIG. 6 , the transaction labels tl are compared with the comparison value 01h in the transmitting device  101  connected to the monitoring camera  104  of the back monitor, and with the comparison value 02h in the transmitting device  101  connected to the monitoring camera  104  of the side monitor. 
     In the transmitting devices  101  and  102 , the respective checking results of the cycle start format checking section  9 , the CRC checking section  10  and the tl checking section  15  are inputted to an AND gate  11 . An output of the AND gate  11  is connected to an enable terminal EN of the isochronous packet generating section  12 . Accordingly, if the respective checking results as processed in the cycle start format checking section  9 , the CRC checking section  10  and the tl checking section  15  are recognized as normal, enabling of the isochronous packet generating section  12  is asserted. 
     Operation in the second embodiment having the above-described configuration will now be described. If the CRC changing section  3  provided in the receiving section  100  normally outputs a through-side input, and the changing register  13  provided in the receiving device  100  is set so that selector  14  always selects the conventional fixed value 00h, the cycle start packets transmitted from the receiving device  100  are normal cycle start packets. Thus, the checking results with respect to the cycle start packets transmitted from the receiving device  100 , as processed in the cycle start format checking section  9 , the CRC checking section  10  and the tl checking section  15  are recognized as normal in the transmitting device  102  which does not have the CRC changing section  3  and compares the transaction labels tl with the fixed value 00h. Accordingly, enabling of the isochronous packet generating section  12  is asserted, enabling transfer of isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, the CRC changing section  3  provided in the two transmitting devices  101  are set so as to output an inverted-side input, as is the case with the first embodiment. The tl checking sections  15  provided in the two transmitting devices  101  each compares the transaction labels tl with the comparison values 01h and 02h. Thus, the checking results with respect to the cycle start packets transmitted from the receiving device  100 , as processed in the CRC checking section  10  and the tl checking section  15  are not recognized as normal in the two transmitting devices  101 . Accordingly, enabling of the isochronous packet generating section  12  is not asserted, which disables transfer of isochronous packets of video image data in the respective monitoring cameras  103  of the back monitor and the side monitor to the IEEE 1394 bus  200 . Only isochronous packets of video image data from the DVD player  105  are transferred to the IEEE 1394 bus  200 , while the DVD video image data is outputted to the display device  103 . 
     Here, once the display contents of the display device  103  for backing up a vehicle are switched to video images of the back monitor, settings of the CRC changing section  3  provided in the receiving device  100  are switched so as to output an inverted-side input. Also, settings of the switching register  13  provided in the receiving device  100  are switched so that the selector  14  selects the comparison value 01h. The cycle start packet generating section  4  provided in the receiving section  100  generates cycle start packets having an inverted header_CRC and value 01h included in the transaction labels tl. Accordingly, the cycle start packets transmitted from the receiving device  100  become cycle start packets having inverted CRC data and the transaction labels tl set to 01h. As a result, the checking results with respect to the cycle start packets transmitted from the receiving device  100  as processed in the CRC checking section  10  and the ti checking section  15  are not recognized as normal in the transmitting device  102  which does not have the CRC changing section  3  and compares the transaction labels tl with the fixed value 00h. Accordingly, enabling of the isochronous packet generating section is not asserted, which stops transfer of the isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, as the CRC changing sections  3  provided in the two transmitting devices  101  are set so as to output inverted-side input, checking results with respect to cycle start packets transmitted from the receiving device  100  as processed in the CRC checking section  10  are recognized as normal in the two transmitting devices  101 . The tl checking sections  15  provided in the two transmitting sections  101  respectively compare the transaction labels tl with the comparison values 01h and 02h. In the cycle start packets transmitted from the receiving section  100 , the transaction labels tl are set to 01h. Thus, the checking results as processed in the tl checking section  15  are recognized as normal only in the transmitting device  101 , from the two transmitting devices  101 , which makes comparison with the comparison value 01h, specifically, only in the transmitting device  101  which is connected with the monitoring camera  104  of the back monitor. Accordingly, enabling of the isochronous packet generating section  12  is asserted only in the transmitting device  101  connected with the monitoring camera  104  of the back monitor. This enables transfer of isochronous packets of video image data of the monitoring camera  104  of the back monitor to the IEEE 1394 bus  200 . Only isochronous packets of video image data of the monitoring camera  104  of the back monitor are transferred to the IEEE 1394 bus  200  in place of video image data of the DVD player  105 , while video image data of the back monitor is outputted to the display device  103 . 
     Similarly, once the display contents of the display device  103  are switched to video of a side monitor for pulling the vehicle over to the kerb, settings of the CRC changing section  3  provided in the receiving device  100  are switched so as to output inverted-side input. Settings of the switching register  13  provided in the receiving device  100  are switched so that the selector  14  will select the comparison value 02h. Accordingly, in this case, enabling of the isochronous packet generating section  12  is asserted only in the transmitting device  101  connected with the monitoring camera  104  of the side monitor. This enables transfer of isochronous packets of video image data in the monitoring camera  104  of the side monitor to the IEEE 1394 bus  200 . Only isochronous packets of video image data of the monitoring camera  104  for the side monitor are transferred to the IEEE 1394 bus  200 , while video image data of the side monitor is outputted to the display device  103 . 
       FIG. 7  illustrates transfer of packets on the IEEE 1394 according to the second embodiment as described herein.  FIG. 8  illustrates the display contents to be outputted to the display device  103 . In the second embodiment as described above, the CRC changing section  3  provided in the receiving device  100  switches between the through side and the inverted side with respect to the CRC data of the cycle start packets to be transmitted from the receiving device  100 . Also, settings can be made so that the selector  14  selects either of values 01h and 02h, other than the conventional fixed value 00h, in the switching register  13  provided in the receiving device  100 , with respect to the transaction labels tl of the cycle start packets transmitted from the receiving device  100 . Enabling of the isochronous packet generating section  12  is asserted only at the transmitting device, of the two transmitting devices  101  and  102 , which received the compliant cycle start packets, which enables transmission of isochronous packets. As a result, isochronous packets to be transferred on the IEEE 1394 bus  200  as illustrated in  FIG. 7  can be switched without carrying out procedures such as band releasing and band reservation as is the case with the first embodiment. As a result, display contents of the display device  103  are outputted without interruptions between the respective video images of the DVD, the back monitor and the side monitor. Thus, as is the case with the first embodiment, bus band can be efficiently used, making it possible to suppress power consumption. 
     Further, in the second embodiment, the receiving device  100  has a switching register  13  and a selector  14  in addition to the CRC changing section  3 . Here, three types of isochronous packets can be switched by changing the transaction labels tl of the cycle start packets. 
       FIG. 9  illustrates a block diagram of an isochronous packet receiving device  100  in a packet communication device according to a third embodiment.  FIG. 10  is a block diagram of an isochronous packet transmitting device  101  provided in a packet communication device according to the third embodiment.  FIG. 11  is a block diagram illustrating connections in an in-vehicle network using the packet communication device depicted in  FIG. 9  and  FIG. 10 . In  FIG. 11 , the receiving device  100  and the transmitting devices  101  and  102  are omitted. The third embodiment will now be described while referring to  FIG. 9  through  FIG. 11 . 
     In the third embodiment, the receiving device  100  has an isochronous packet selecting section  16 , two status storing registers  17  and an MPU_I/F  18  (refer to  FIG. 9 ). The transmitting device  101  has two CRC checking sections  10 , two tl checking sections  15 , three AND gates  11 , an OR gate  20 , two selectors  14 , an MPU_I/F  18 , a status storing register  17 , and two data length setting registers  19 , one register being employed for video data and the other one being employed for status (refer to  FIG. 10 ). The rest of the constituent elements are the same as those in the second embodiment, and therefore, further description thereof is hereby omitted. In an in-vehicle network using the packet communication device according to the third embodiment, the isochronous packet receiving device  100 , the two transmitting devices  101  and the transmitting device  102  are connected through the IEEE 1394 bus  200  (refer to  FIG. 11 ). A display device  103  and an MPU  106  are connected to the receiving device  100 . A monitoring camera  104  of an obstacle monitor or a tuner  107  for a broadcast receiver and an MPU  106  are respectively connected to the two transmitting devices  101 . A DVD player  105  is connected to the transmitting device  102 . 
     The isochronous packets which are transferred from the respective transmitting devices through the IEEE 1394 bus  200  are inputted to the isochronous packet selecting section  16  through an 1394 block  5  provided in the receiving device  100  (refer to  FIG. 9 ). The isochronous packet selecting section  16  outputs the isochronous packets thus received to the FIFO  6  or the status storing register  17 , in accordance with the contents of such packets. The isochronous packets inputted to the FIFO  6  are outputted to the display device  103  through the video I/F  7 . Alternatively, isochronous packets inputted to the status storing register  17  are inputted to the MPU  106  through the MPU_I/F  18 . The MPU  106  controls the CRC changing section  3  and the switching register  13  in accordance with the isochronous packets thus received. 
     The CRC data of the cycle start packets extracted in the CRC and the tl extracting section  8  provided in the transmitting device  101  is inputted to one of the two CRC checking sections  10  directly, and to the other CRC checking section  10  through the CRC changing section  3  (refer to  FIG. 10 ). The transaction labels tl of the cycle start packets as extracted in the CRC and tl extracting section  8  are compared with a fixed value 00h at one of the two tl checking sections  15 , and, with a changeable comparison value at the other tl checking section  15 . The checking results as processed at the CRC checking section  10  to which CRC data is inputted directly, and the checking results as processed at tl checking section  15  which makes comparison with the fixed value 00h are inputted to a first AND gate  11 . Then, output of this AND gate  11  is inputted to the OR gate  20 . Checking results of the CRC checking section  10  to which the CRC data is inputted through the CRC changing section  3  and checking results of the ti checking section  15  which makes comparison with the comparison value are inputted to a second AND gate  11 . Then, output of this AND gate  11  is inputted to the OR gate  20  and to the two selectors  14 . Output of the OR gate  20  is inputted to a third AND gate  11 , together with output of the cycle start format checking section  9 . Output of the third AND gate  11  is inputted to an enable terminal EN of the isochronous packet generating section  12 . 
     An MPU  106  is connected to the status storing register  17 , and the two data length setting registers  19 , one being employed for video data and the other one being employed for status, through the MPU_I/F  18  (refer to  FIG. 10  and  FIG. 11 ). One of the two selectors  14  receives video data from the FIFO  6  and status information from the status storing register  17 . The other one of the two selectors  14  receives data length information from two data length setting registers  19 , one being employed for video data and the other one being employed for status. The two selectors select which input to output, based on the output from the second AND gate  11 . Outputs of the two selectors  14  are inputted to the isochronous packet generating section  12 , where isochronous packets are generated. 
     As illustrated in  FIG. 11 , of the two transmitting devices  101 , the transmitting device  101  connected to the monitoring camera  104  employed for the obstacle monitor has a comparison value 01h, and the transmitting device  101  connected to the tuner  107  for the broadcast receiver has a comparison value 02h as comparison values for the transaction labels tl. The MPU  106  to which the two transmitting devices  101  are respectively connected is connected with a monitoring camera  104  of the obstacle monitor and a tuner  107  for the broadcast receiver, respectively. The MPU  106  monitors the status of the respective input devices. 
     The operation of the third embodiment having the above-described configuration will now be described. The CRC changing section  3  provided in the receiving device  100  normally receives a through-side input and the switching register  13  provided in the receiving device  100  is set so that the selector  14  will select the conventional fixed value 00h. In this case, cycle start packets to be transmitted from the receiving device  100  are normal cycle start packets. As a result, the checking results with respect to the cycle start packets to be transmitted from the receiving device  100 , as processed in the cycle start format checking section  9 , the CRC checking section  10  and the tl checking section  15  are recognized as normal in the transmitting device  102  which does not have the CRC changing section  3  and compares the transaction labels tl with the fixed value 00h. Accordingly, enabling of the isochronous packet generating section is asserted, which enables transfer of the isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, in the two transmitting devices  101 , checking results as processed in the CRC checking section  10  to which the CRC data is inputted directly and checking results of the tl checking section  15  which makes comparison with the fixed value 00h are recognized as normal (refer to  FIG. 10  and  FIG. 11 ). However, the CRC changing section  3  is set so as to receive inverted-side input, as is the case with the second embodiment. Thus, the checking results as processed in the CRC checking section  10  to which CRC data is inputted through the CRC changing section  3  and the checking results of the tl checking section  15  which makes comparison with the comparison value are not recognized as normal. Accordingly, a first AND gate  11  and a second AND gate  11  output 1 and 0, respectively. Upon receiving checking results from the a cycle start format checking section  9  and the output (=1) from the OR gate  20 , a third AND gate  11  outputs 1, and enabling of the isochronous packet generating section  12  is asserted. 
     As the output of the second AND gate  11  is 0, two selectors  14  select and then output status information from the respective status storing register  17  and data length information from the data length setting register  19  for status. Accordingly, isochronous packets of status information are generated in the isochronous packet generating section  12 . 
     As a result, video image data of the DVD player  105 , status information from the monitoring camera  104  for the obstacle monitor and status information from the tuner  107  for the broadcast receiver are transferred in isochronous packets to the IEEE 1394 bus  200 . In the receiving device  100 , DVD video image data is outputted to the display device  103 , while status information from the monitoring camera  104  of the obstacle monitor and the tuner  107  of the broadcast receiver is separately stored in the two status storing registers  17 . 
     Here, once the MPU  106  connected to the monitoring camera  104  of the obstacle monitor detects obstacle proximity, the status storing register  17  inside the transmitting device  101  connected to the monitoring camera  104  of the obstacle monitor is updated. Contents of the register are transferred to the IEEE 1394 bus  200  as isochronous packets and are then received at the receiving device  100 . These contents are then notified to the MPU  106  connected to the receiving device  100 . The MPU  106  at which the notification is received switches settings of the CRC changing section  3  provided in the receiving device  100  so as to output an inverted-side input. Settings of the switching register  13  provided in the receiving device  100  are switched by the MPU  106  so that the selector  14  will select the comparison value 01h. Thus, the cycle start packet generating section  4  provided in the receiving device  100  generates cycle start packets having inverted header_CRC attached thereto and having transaction labels tl including value 01h. Accordingly, as is the case with the second embodiment, the checking results as processed in the CRC checking section  10  and the tl checking section  15  are not recognized as normal in the transmitting device  102 . Accordingly, enabling of the isochronous packet generating section  12  is not asserted in the transmitting device  102 , which stops transfer of isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, checking results of the CRC checking section  10  to which CRC data is inputted directly and checking results of the tl checking section  15  making comparison with the fixed value 00h or the comparison value 02h are not recognized as normal in the two transmitting devices  101 . However, checking results of the CRC checking section  10  to which the CRC data is inputted through the CRC changing section  3  and checking results of the tl checking section  15  which makes comparison with the comparison value 01h are recognized as normal (refer to  FIG. 10  and  FIG. 11 ). Accordingly, enabling of the isochronous packet generating section  12  is asserted only at the transmitting device  101 , of the two transmitting devices  101 , which makes comparison with the comparison value 01h, specifically, only at the transmitting device  101  which is connected with the monitoring camera  104  of the obstacle monitor. 
     At this time, the output of the second AND gate  11  provided in the transmitting device  101  connected with the monitoring camera  104  of the obstacle monitor is 1. Thus, the two selectors  14  select and output video data from the respective FIFO  6  and data length information from the data length setting register  19  for video data. Accordingly, the isochronous packet generating section  12  generates isochronous packets of video data of the obstacle monitor, in place of stats information. 
     As a result, only isochronous packets of video data of the obstacle monitor are transferred to the IEEE 1394 bus  200 . In the receiving device  100 , the video data of the obstacle monitor is outputted to the display device  103 . Thereafter, if the obstacle is avoided and the display contents of the display device  103  are switched again to the DVD video image, cycle start packets to be transmitted from the receiving device  100  will be switched back to normal cycle start packets. As a result, this makes it possible to enjoy watching the DVD again. 
     Similarly, if the MPU  106  connected to the tuner  107  of the broadcast receiver detects emergency information such as occurrence of an earthquake and the like, the status storing register  17  provided in the transmitting device  101  connected to the tuner  107  of the broadcast receiver is updated. The contents of the register are transferred to the IEEE 1394 bus  200  as isochronous packets and are received at the receiving device  100 . Then, a notification is transmitted to the MPU  106  connected to the receiving device  100 . The MPU  106  which received such notification switches the settings of the CRC changing section  3  provided in the receiving device  100  so as to output an inverted-side input. Thus, the MPU  106  switches the settings of the switching register  13  provided in the receiving device  100  so that the selector  14  selects the comparison value 02h. Accordingly, in this case, enabling of the isochronous packet generating section  12  is asserted only at the transmitting device  101  connected with the tuner  107  of the broadcast receiver  107 , enabling generation of isochronous packets of video data for the broadcast receiver. 
     As a result, only isochronous packets of video data for the broadcast receiver will be transferred to the IEEE 1394 bus  200 . In the receiving device  100 , video data of the broadcast receiver is outputted to the display device  103 . Thereafter, once emergency information is terminated and the display contents on the display device  103  are switched again to DVD video image, cycle start packets which are transmitted from the receiving device  100  are switched back to normal cycle start packets, making it possible to enjoy watching the DVD again. 
       FIG. 12  illustrates transfer of packets on the IEEE 1394 bus  200  in the third embodiment as described hereinbefore.  FIG. 13  illustrates display contents to be outputted to the display device  103 . In the third embodiment as described above, the receiving device  100  and the transmitting device  101  have a status storing register  17  which stores status information of the input device and an MPU  106  which is connected to such status storing register  17 . As a result, the transmitting device  101  can normally transfer only isochronous packets of status information of the input device. However, when proximity to an obstacle is detected, the transmitting device  101  can transfer isochronous packets of video data of the input device (refer to  FIG. 12 ). As the size of the status information is smaller as compared to video data, the operating band of the bus can be normally suppressed for efficient usage of the bus band. At the same time, the size of the packets to be transferred can be reduced, which reduces power consumption. Further, switching of the cycle start packets can be controlled based on the notification from the transmitting devices  101  connected to the respective input devices. Accordingly, isochronous packets to be transferred can thus be switched, making it possible to output display contents of the display device  103  without interruptions (refer to  FIG. 13 ). 
       FIG. 14  illustrates a block diagram of an isochronous packet transmitting device  101  in a packet communication device according to a fourth embodiment. The fourth embodiment will be described while referring to  FIG. 14 . 
     The fourth embodiment differs from the third embodiment in that the transmitting device  101  does not have two selectors  14 , but has two isochronous packet generating devices  12  and four AND gates  11 . The rest of the constituent elements are the same as those in the third embodiment, and therefore, further description thereof is hereby omitted. Description in the fourth embodiment focuses on the points which differ from the third embodiment. In the third embodiment, the output of a second AND gate  11  is inputted to an OR gate  20  and to the two selectors  14  (refer to  FIG. 10 ). In the fourth embodiment, the output of the second AND gate  11  is inputted to the OR gate  20  and to a fourth AND gate  11  thus added, without being inputted to the two selectors  14  (refer to  FIG. 14 ). The fourth AND gate  11  further receives the output of the cycle start format checking section  9 . Of the two isochronous packet generating sections  12 , the output of the third AND gate  11  is inputted to the enable terminal EN of one of the isochronous packet generating sections  12 , while the output of the fourth AND gate  11  is inputted to the enable terminal EN of the other isochronous packet generating section  12 . 
     The isochronous packet generating section  12  connected to the third AND gate  11  receives status information from a status storing register  17  and data length information from a data length setting register  19  for status. The isochronous packet generating section  12  connected to the fourth AND gate  11  receives video data from the FIFO  6  and data length information from a data length setting register  19  for video data. The two isochronous packet generating sections  12  respectively generate isochronous packets of status information and video data. 
     The operation in the fourth embodiment as described earlier will now be described. The receiving device  100  and the transmitting device  102  are the same as the third embodiment. Normally, normal cycle start packets are transmitted from the receiving device  100 . Enabling of the isochronous packet generating section  12  is asserted in the transmitting device  102 , enabling transfer of isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, in the two transmitting devices  101 , the first AND gate  11  outputs 1, the second AND gate  11  outputs 0 and the OR gate  20  outputs 1, as is the case with the third embodiment (refer to  FIG. 14 ). Thus, the third AND gate  11  outputs 1 and the fourth AND gate  11  outputs 0. Accordingly, enabling of the isochronous packet generating section  12  to which the third AND gate  11  is connected, specifically, the isochronous packet generating section  12  which receives status information from the status storing register  17  and data length information from the data length setting register  19  for status is asserted. 
     As a result, isochronous packets of video image data of the DVD player  105 , status information from the monitoring camera  104  of the obstacle monitor and status information from the tuner  107  of the broadcast receiver is normally transferred to the IEEE 1394 bus  200 , as is the case with the third embodiment. In the receiving device  100 , the video image data from the DVD is outputted to the display device  103  and status information from the monitoring camera  104  of the obstacle monitor and the tuner  107  of the broadcast receiver is separately stored in two status storing registers  17 . 
     Here, once the MPU  106  connected to the monitoring camera  104  for the obstacle monitor detects proximity to an obstacle, the status storing register  17  provided inside the transmitting device  101  connected to the monitoring camera  104  of the obstacle monitor is updated. Thus, similarly with the third embodiment, cycle start packets having inverted header_CRC attached thereto and transaction labels tl including value 01h are outputted from the receiving device  100 . Accordingly, enabling of the isochronous packet generating section  12  is not asserted in the transmitting device  102 , which stops transfer of the isochronous packets of video image data from the DVD player  105  to the IEEE 1394 bus  200 . 
     Alternatively, enabling of the isochronous packet generating section  12  is asserted only at the transmitting device  101 , from the two transmitting devices  101 , which makes comparison with the comparison value 01h, specifically, the transmitting device  101  which is connected with the monitoring camera  104  of the obstacle monitor, as is the case with the third embodiment. In the transmitting device  101  connected with the monitoring camera  104  of the obstacle monitor, the output of the first AND gate  11  is 0, while the output of the second AND gate  11  is 1. Thus, the output of the OR gate  20  is 1, the output of the third AND gate  11  and the output of the fourth AND gate  11  are both 1. Accordingly, enabling of both the isochronous packet generating section  12  connected to the third AND gate  11  and the isochronous packet generating section  12  connected to the fourth AND gate  11  is asserted. 
     Thus, isochronous packets of status information are transferred from the monitoring camera  104  of the obstacle monitor, in addition to the isochronous packets of video data of the obstacle monitor, to the IEEE 1394 bus  200 . In the receiving device  100 , video data of the obstacle monitor is outputted to the display device  103  and status information from the monitoring camera  104  of the obstacle monitor is stored in the status storing register  17 . Status information from the monitoring camera  104  of the obstacle monitor as stored in the status storing register  17  is monitored by the MPU  106  connected to the receiving device  100 . Thus, once it is detected that the obstacle was avoided and the normal state is restored, the cycle start packets are switched back to the normal cycle start packets. Specifically, the display contents of the display device  103  can be automatically switched back to the original DVD video image data. For instance, this is the same for the case that the MPU  106  connected to the tuner  107  of the broadcast receiver detects emergency information such as occurrence of earthquakes or the like. 
       FIG. 15  illustrates transfer of packets on the IEEE 1394 bus  200  in the fourth embodiment as described hereinbefore. In the fourth embodiment as described hereinbefore, the transmitting device  101  has two isochronous packet generating sections  12  and four AND gates  11 , in place of the two selectors  14 . As a result, the transmitting device  101  normally transfers isochronous packets of status information of the input device. Alternatively, when proximity of an obstacle is detected, the transmitting device  101  can transfer isochronous packets of status information of the input device, in addition to isochronous packets of video data of the input device (refer to  FIG. 15 ). Switching of the cycle start packets can be controlled based on the notification from the transmitting device  101  connected to each input device. Thus, it can be detected that the normal state is restored. Accordingly, the display contents of the display device  103  can be automatically restored to the original state. 
       FIG. 16  is a block diagram illustrating connections in an in-vehicle network using a packet communication device according to a fifth embodiment. The fifth embodiment is similar with the first embodiment as described above, other than the fact that an output of an AND gate  11  provided in a transmitting device  101  is connected to a monitoring camera  104  of a back monitor. Thus, the characteristics of the fifth embodiment will now be described. 
     In the fifth embodiment, the output of the AND gate  11  provided in the transmitting device  101  is inputted to the monitoring camera  104  of the back monitor. Accordingly, the monitoring camera  104  can be set to be turned on only when isochronous packets of video image data for the monitoring camera  104  of the back monitor are transmitted to the IEEE 1394 bus  200 . If DVD video image data is outputted to the display device  103  and the video image data of the back monitor is not outputted to the display device  103 , the monitoring camera  104  of the back monitor is set to a low-power-consumption mode, further suppressing power consumption. 
     Insofar, examples were described in the first through fifth embodiments wherein cycle start packets are transmitted from an isochronous packet transmitting device. In a sixth embodiment, an example is described wherein cycle start packets are transmitted from an isochronous packet transmitting device.  FIG. 17  is a block diagram depicting connections in an in-vehicle network using a packet communication device according to a sixth embodiment. An isochronous packet transmitting device  108  and isochronous packet receiving devices  109  and  110  are connected through an IEEE 1394 bus  200 . 
     The transmitting device  108  has a cycle timer  1 , a CRC computing section  2 , a CRC changing section  3 , a cycle start packet generating section  4 , a 1394 block  5 , a video I/F  7 , an audio I/F  21 , two FIFO  6 , two isochronous packet generating sections  12  and a selector  14 . The cycle timer  1 , the CRC computing section  2 , the CRC changing section  3  and generation of the cycle start packets in the cycle start packet generating section  3  are the same as described in the above text. The cycle start packets thus generated are transmitted on the IEEE 1394 bus  200  through the 1394 block  5 , for transfer to the receiving devices  109  and  110 . 
     A DVD player  105  is connected to the transmitting device  108  and isochronous packets of DVD video image data are generated in the isochronous packet generating section  12  through the video I/F  7  and the FIFO  6 . A tuner  111  of an audio broadcasting receiver is connected to the transmitting device  108 , and isochronous packets of audio data of the audio broadcasting receiver are generated in the isochronous packet generating section  12  through the audio I/F  21  and the FIFO  6 . Isochronous packets thus generated in the respective isochronous packet generating sections  12  are inputted to the selector  14 . 
     A switch  112  is provided. The switch  112  is switched so that the CRC changing section  3  provided in the transmitting device  108  can switch between outputting a through side output and an inverted side output. Also, by switching the switch  112 , the selector  14  can switch between selecting and outputting isochronous packets from either one of the two isochronous packet generating sections  12 . 
     The receiving devices  109  and  110  each have a 1394 block  5 , a packet selecting section  22 , a CRC extracting section  8 , a cycle start format checking section  9 , a CRC checking section  10 , and an AND gate  11 , an isochronous packet receiving section  23 , a FIFO  6 , an audio I/F  21  or a video I/F  7 . The receiving device  109  further has a CRC changing section  3  which is set so as to output an inverted-side input. A speaker  113  is connected to the receiving device  109 . Alternatively, a display device  103  is connected to the receiving device  110 . Cycle start packets and isochronous packets transferred from the transmitting device  108  through the IEEE 1394 bus  200  are inputted to the packet selecting section  22  through the 1394 block  5  in the receiving devices  109  and  110 , respectively. Cycle start packets are outputted to the CRC extracting section  8 , while isochronous packets are outputted to the isochronous packet receiving section  23 . 
     Cycle start packets to be inputted to the CRC extracting section  8  are checked by a cycle start format checking section  9  and a CRC checking section  10 , as described above. The checking results are inputted to the AND gate  11  and an output of the AND gate  11  is connected to an enable terminal EN of the isochronous packet receiving section  23 . Thus, if the checking results of the cycle start format checking section  9  and the CRC checking section  10  are recognized as normal, enabling of the isochronous packet receiving section  23  is asserted. Once enabling of the isochronous packet receiving section  23  is asserted, isochronous packets transmitted from the transmitting device  108  are outputted to the speaker  113  or the display device  103  through the FIFO  6 , and the audio I/F  21  or video I/F  7 . 
     Operation of the sixth embodiment as described above will now be described. The example described herein refers to the case that the switch  112  is set so that the CRC changing section  3  provided in the transmitting device  108  outputs a through-side input and, so that, of the two isochronous packet generating sections  12 , the selector  14  outputs isochronous packets of the DVD player  105 . In this case, normal cycle start packets and isochronous packets of DVD video image data are transferred to the IEEE 1394 bus  200 . Thus, checking results with respect to the cycle start packets which are transmitted from the transmitting device  108 , as processed in the cycle start format checking section  9  and the CRC checking section  10  are recognized as normal in the receiving device  110  which does not have the CRC changing section  3 . Accordingly, enabling of the isochronous packet receiving section  23  is asserted in the receiving device  110 , enabling output of isochronous packets of video image data from the DVD player  105  to the display device  103 . 
     Alternatively, the CRC changing section  3  provided in the receiving device  109  is set so as to receive an inverted-side input. Thus, checking results with respect to the cycle start packets which are transmitted from the transmitting device  108 , as processed in the CRC checking section  10  are not recognized as normal in the receiving device  109  which has the CRC changing section  3 . Accordingly, enabling of the isochronous packet receiving section  23  is not asserted in the receiving device  109 , disabling reception of isochronous packets of video image data from the DVD player  105  and output thereof to the speaker  113 . 
     The example described herein refers to the case that, in order to listen to music on the radio, the switch  112  is switched so that the CRC changing section  3  provided in the transmitting device  108  outputs an inverted-side input, and the selector  14  outputs isochronous packets for the tuner  111  of the audio broadcasting receiver, of the two isochronous packet generating sections  12 . In this case, cycle start packets having inverted CRC data and isochronous packets of audio data from the audio broadcasting receiver are transferred to the IEEE 1394 bus  200 . Thus, the checking results with respect to the cycle start packets transmitted from the transmitting device  108 , as processed in the CRC checking section  10 , are not recognized as normal in the receiving device  110  which does not have the CRC changing section  3 . Accordingly, enabling of the isochronous packet receiving section  23  is not asserted in the receiving device  110 , disabling reception of isochronous packets of audio data from the tuner  111  of the audio broadcasting receiver and stopping output thereof to the display device  103 . 
     Alternatively, as the CRC changing section  3  provided in the receiving device  109  is set so as to output an inverted-side input, the checking results with respect to the cycle start packets transmitted from the transmitting device  108 , as processed in the CRC checking section  10 , are recognized as normal in the receiving device  109  which does have the CRC changing section  3 . Accordingly, enabling of the isochronous packet receiving section  23  is asserted in the receiving device  109 , enabling reception of isochronous packets of audio data from the tuner  111  of the audio broadcasting receiver and output thereof to the speaker  113 . 
       FIG. 18  illustrates transfer of packets on the IEEE 1394 bus  200  according to the sixth embodiment as described hereinbefore.  FIG. 19  depicts the contents to be outputted to the display device  103  and the speaker  113 . In the sixth embodiment as described hereinbefore, in the CRC changing section  3  provided in the transmitting device  108 , switching can be made between the through side and the inverted side with respect to the CRC data of the cycle start packets transmitted from the transmitting device  108 . Enabling of the isochronous packet receiving section  23  is asserted only at the receiving device, of the receiving devices  109  and  110 , which received the compliant cycle start packets, enabling reception of isochronous packets. As a result, isochronous packets to be transferred on the IEEE 1394 bus  200  can be switched as illustrated in  FIG. 18 , without carrying out procedures such as band releasing and band reservation. Thus, DVD video images and audio of the audio broadcasting receiver are outputted to the display device  103  and the speaker  113 , as illustrated in  FIG. 19 . 
     Even in an example wherein the cycle start packets are transmitted from the transmitting device-side, instead of the isochronous packet receiving device-side, isochronous packets can be switched without carrying out procedures such as band releasing and band reservation. Of the receiving devices  109  and  110 , the device at which isochronous packets are not received has no output, which makes it possible to suppress power consumption. 
     Here, correspondences with claims are as follows. 
     An isochronous cycle is one example of a constant cycle. 
     An isochronous transfer is one example of packet communication time-managed in constant cycles. 
     A cycle start packet is one example of a first packet. 
     A CRC and a transaction label tl are one example of a portion of the first packet. 
     A CRC is one example of a redundant bit. 
     A CRC changing section  3  is one example of a redundant bit changing section. 
     An isochronous packet is one example of a second packet. 
     As was described in detail earlier, according to the first through 6 embodiments, the CRC changing section  3  is provided in the isochronous packet receiving device  100  or  109  and in the isochronous packet transmitting device  101  or  108 . Thus, isochronous packets to be transferred on the IEEE 1394 bus  200  can be switched without carrying out procedures such as band releasing and band reservation. As a result, bus band can be efficiently used, making it possible to suppress power consumption. 
     According to the second through fourth embodiments, the receiving device  100  has a switching register  13  and a selector  14 , in addition to the CRC changing section  3 . The selector  14  selects transaction labels tl of the cycle start packets in accordance with the output of the switching register  13 . As a result, isochronous packets can be switched using the transaction labels tl. 
     According to the third and fourth embodiments, the receiving device  100  and the transmitting device  101  have a status storing register  17  which stores status information of the input device and an MPU  106  which is connected with the status storing register  17 . Thus, the MPU  106  monitors the status information of the input device, enabling control of the CRC changing section  3  and the switching register  13 . Accordingly, isochronous packets to be transferred can be switched based on the status information of the input device. 
     Although the embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 
     For instance, in the above-described embodiments, a description was given of an example wherein the header_CRC of the cycle start packets is changed to switch the isochronous packets to be transferred. However, this is not limited to this example. The purpose of the embodiment can be realized even in case only the transaction labels tl are switched. An example of switching the isochronous packets by switching the transaction labels tl only is illustrated in a seventh embodiment in  FIG. 20 . Unlike conventional isochronous packet transmitting devices which have one tl checking section  15 , the isochronous packet transmitting device  101  illustrated in  FIG. 20  has three tl checking sections  15 . As a result, the transaction labels tl of the cycle start packets are checked, making it possible to automatically switch the three types of isochronous packets in response to reception of cycle start packets. Accordingly, the isochronous packet transmitting device  101  does not transmit isochronous packets which are not to be outputted to the display device  103  connected to the receiving device  100 . As a result, bus band can be efficiently used, making it possible to suppress power consumption. 
     Thus, a portion of the cycle start packets which are changed in the embodiment are not limited to the CRC. Components other than the CRC of the cycle start packets may be changed, as long as changing such components does not result in damages. The CRC is changed by inversion in the CRC changing section  3 . It should be noted that the changing method is not limited to this example. The changing method may also include attaching some kind of distinction to the isochronous cycles. 
     In the above-described embodiments, an example of switching 2 to 3 types of isochronous packets is described. It should be noticed, however, that three or more types of isochronous packets may be switched. Further, the embodiment is also applicable to standards other than the IEEE 1394 standard. 
     According to the packet communication device and the packet communication method in the embodiment, isochronous packets subjected to isochronous transfer as supported based on the IEEE 1394 standard can be switched without carrying out procedures such as band releasing and band reserving. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
A packet communication device for communicating a packet to be transferred in constant cycle, comprising one of a logic inversion section configured to invert a logical value with respect to at least one bit included in a first string of bits included in a first packet; and a register section configured to store another string of bits having a logical value different from a given logical value of the first string of bits; and a selector section configured to select one of the first string of bits and a second string of bits that is output from one of the logic inversion section and the register section to designate any one of a plurality of devices, wherein the packet communication is performed when a selected string of bits selected by the selector section conforms to a setting value of a receiving side.