Abstract:
Provided is a communication system for logically connecting a source node and one or more destination nodes, and for controlling data communication between the individual nodes by employing a connection ID that is used to identity the logical connection relationship. The communication system may comprise a source node adapted to transmit data packets, a destination node adapted to receive the data packets transmitted from the source node, and a controller adapted to manage a logical connection between the source node and the destination node, the destination node being adapted to abort communication between the source node and the destination node if the destination node received an abort packet transmitted from the controller, and the destination node being adapted to disconnect the logical connection after the communication is aborted by the abort packet. The source node itself, and methods of using the system and that node, are also individual aspects of what is disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a data communication system, a data communication method, a data communication apparatus, and a digital interface. In particular, the present invention pertains to a network for transmitting communication data (including image data) and command data together at a high speed, and a communication protocol that can be applied for the network. 
     2. Related Background Art 
     Conventionally, hard disks and printers are the peripheral devices that are most frequently employed with personal computers (PCs). One of these peripheral devices is connected to a PC via a special input/output interface or via a general-purpose digital interface, such as a SCSI (a small computer system interface). 
     Recently, however, AV (Audio/Visual) devices, such as digital cameras and digital video cameras, have become popular, and taken together they constitute another type of peripheral that can be used with a PC. Such an AV (Audio/Visual) device can be connected to a PC via an interface. 
     FIG. 1 is a diagram illustrating a conventional communication system that comprises a PC and an AV device. 
     In FIG. 1,  101  denotes an AV device (a digital camera),  102  denotes PC and  103  denotes a printer. 
     The digital camera  101  comprises: a memory  104 , in which image data is compressed and recorded; a decoder  105 , for expanding the compressed image data stored in the memory  104  in order to decode them; an image processing unit  106 ; a D/A converter  107 ; a display unit  108  that includes an EVF; and a special digital I/O unit  109 , for connecting the digital camera  101  and the PC  102 . 
     The PC  102  comprises: a special digital I/O unit  110 , for connecting the PC  102  to the digital camera  101 ; an operation unit  111 , including a keyboard and a mouse; a decoder  112 , for expanding the compressed image data in order to decode them; a display unit  113 ; a hard disk  114 ; a memory  115 , such as a RAM; an MPU  116 ; a PCI bus  117 ; and a SCSI interface, for connecting the PC  102  to the printer  103 . 
     The printer  103  comprises: a SCSI interface  119 , for connecting the printer  103  to the PC  102 ; a memory  120 ; a printer head  121 ; a printer controller, for controlling the operation of the printer  103 ; and a driver  123 . 
     In a conventional communication system the special digital interface (digital I/O unit)  109  of the digital camera  101  and the digital interface (SCSI interface)  119  of the printer  103  are not compatible, and one can not be directly connected to the other. Therefore, when, for example, the digital camera  101  is to transmit a still image to the printer  103 , the PC must serve as a relay. 
     The conventional special digital interface  109  and the conventional SCSI interface  119  have many shortcomings: their data transfer rates are low, especially when, for a still image or a moving picture, there is a large amount of data to be transferred from an AV device; thick cables are employed for parallel communication; only a small number and a few types of peripheral devices can be connected; the connection system is limited; and data transfers can not be performed in real time. 
     A fast, high-performance, next generation digital interface that can resolve the above shortcomings is one that conforms to the well known IEEE (the Institute of Electrical and Electronics Engineers, Inc.) 1394-1995 interface standards. 
     A digital interface that conforms to the IEEE 1394-1995 interface standards (hereinafter referred to as a 1394 interface) has the following features. 
     (1) The data transfer speed is high. 
     (2) The real-time data transmission system, i.e., the isochronous transmission system, and the asynchronous transmission system are supported. 
     (3) A connection configuration (topology) having a high degree of freedom can be obtained. 
     (4) The Plug and Play function and the active line detachment function are supported. 
     However, while in the IEEE 1394-1995 standards the physical and electrical connections for a connector and the most fundamental data transmission systems are defined, a data type, a data format and a communication protocol to be employed for the exchange of data are not defined. 
     Since according to the IEEE 1394-1995 standards a response for the receipt of a packet is not defined for the isochronous transmission system, there is no way by which to ensure that an individual isochronous packet has been received. Therefore, the isochronous transmission system can not be employed when a plurality of sets of sequential data are to be transmitted, or when data in a file is to be transmitted by dividing the data into a plurality of data sets. 
     In the isochronous transmission system according to the IEEE 1394-1995 standards, the total number of communications is limited to 64, even though there is an empty space in a transmission band. Therefore, the isochronous transmission system is not adequate for multiple communications carried by a small transmission band. 
     According to the IEEE 1394-1995 standards, the transmission of data must be halted when a bus is reset because the power to a node is turned on or off, or when the connection or disconnection of the node is established. However, according to the IEEE 1394-1995 standards, when data transmission is halted (stopped) due to the resetting of a bus or to an error that occurs during transmission, the contents of the data that are lost can not be identified. Further, very complicated communication processing must be performed to resume the transmission. 
     The bus resetting function is a function for automatically identifying a new topology and for setting an address (node ID) that is allocated to the node. According to this function, the Plug and Play function and the active line detachment function can be provided by applying the IEEE 1394-1995 standards. 
     For a communication system that conforms to the IEEE 1394-1995 standards, real time processing is not required, and no specific communication protocol has been proposed that can be used for dividing a comparatively large amount of object data that must be reliable (e.g., still image data, graphics data, text data, file data or program data) into more than one data segment, and for sequentially transmitting the data segments. 
     In addition, for a communication system that conforms to the IEEE 1394-1995 standards, no specific communication protocol has been proposed that can be used to implement data communications among a plurality of devices by employing a communication method for the asynchronous broadcasting of data. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to solve the above described problems. 
     It is another object of the present invention to provide a technique, for a data communication system, a data communication method, a data communication apparatus and a digital interface, whereby it is ensured that object data for which real time processing is not required can be sequentially transmitted. 
     It is an additional object of the present invention to provide a technique, for a data communication system, a data communication method, a data communication apparatus and a digital interface, whereby sequential transmission of data between a source node and one or more destination nodes can be satisfactorily halted through only simple processing, without complicated communication procedures being required. 
     As one preferred embodiment for such objects, according to the present invention, a communication system comprises: 
     a source node adapted to transmit data packets; 
     a destination node adapted to receive the data packets transmitted from the source node; and 
     a controller adapted to manage a logical connection between the source node and the destination node, 
     wherein the destination node is adapted to abort communication between the source node and the destination node if the destination node receives an abort packet transmitted from the controller, and wherein the destination node is adapted to disconnect the logical connection after the communication is aborted by the abort packet. 
     As one more preferred embodiment of the present invention, a communication method for a communication system comprising a source node adapted to transmit data packets, a destination node adapted to receive the data packets transmitted from the source node, and a controller adapted to manage a logical connection between the source node and the destination node, comprises the steps of: 
     aborting communication between the source node and the destination node if the destination node receives an abort packet transmitted from the controller; and 
     disconnecting the logical connection after the communication is aborted by the abort packet. 
     As another preferred embodiment of the present invention, a data communication method for a destination node adapted to receive data packets transmitted from a source node comprises the steps of: 
     aborting communication between the source node and the destination node if the destination node receives an abort packet transmitted from a controller which manages a logical connection between the source node and the destination node; and 
     disconnecting the logical connection after the communication is aborted by the abort packet. 
     As an additional preferred embodiment of the present invention, a destination node adapted to receive data packets transmitted from a source node, comprises: 
     aborting means adapted to abort communication between the source node and the destination node if the destination node receives an abort packet transmitted from a controller which manages a logical connection between the source node and the destination node; and 
     disconnecting means adapted to disconnect the logical connection after the communication is aborted by the abort packet. 
     Still other objects of the present invention and the advantages thereof will become fully apparent during the course of the following detailed description given for the embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating a conventional system; 
     FIG. 2 is a block diagram showing an example arrangement for a communication system according to a first embodiment of the present invention; 
     FIG. 3 is a conceptual diagram for explaining the basic structure of a communication protocol according to the first embodiment of the present invention; 
     FIGS. 4A,  4 B and  4 C are sequence charts for explaining the basic communication procedure covered by the communication protocol according to the first embodiment of the present invention; 
     FIG. 5 is a diagram showing the structure of an asynchronous broadcast packet according to the first embodiment; 
     FIGS. 6A and 6B are diagrams for explaining an address space included in each node; 
     FIG. 7 is a diagram for explaining a transfer model for object data; 
     FIG. 8 is a diagram for explaining the structure of a 1394 interface according to the first embodiment; 
     FIG. 9 is a sequence chart for explaining the communication procedure covered by a communication protocol according to a second embodiment of the present invention; and 
     FIGS. 10A to  10 C are sequence charts for explaining the communication procedure covered by a communication protocol according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described in detail below while referring to the accompanying drawings. 
     FIG. 2 is a diagram illustrating an example arrangement of a data communication system according to a first embodiment of the present invention. As is shown in FIG. 2, the data communication system comprises a computer  10 , a digital video recorder with built-in camera  28 , and a printer  60 . 
     The arrangement of the computer  10  will be described first. An MPU  12  controls the operation of the computer  10 . A 1394 interface  14  includes a function that conforms to the IEEE 1394-1995 standards and a function that is associated with a communication protocol that is specified in this embodiment. An operating unit  16  includes a keyboard and a mouse. A decoder  18  decodes compressed and encoded digital data (moving image data, still image data, audio data, etc.). A display unit  20  includes a display device, such as a CRT display or a liquid crystal panel. A hard disk (HD)  22  is used to store various types of digital data (moving image data, still image data, audio data, graphics data, text data, program data, etc.), and an internal memory  24  is also provided as a storage medium. An internal bus  26  is, for example, a PCI bus that interconnects the individual sections of the computer  10 . 
     The arrangement of the digital video recorder with built-in camera (hereinafter referred to as a DVCR)  28  will now be described. An image pickup unit (opt)  30  converts an optical image of an object into an electrical signal and converts the signal into an analog signal, and an analog/digital (A/D) converter  32  converts the analog signal into a digital signal. An image processing unit  34  changes digital moving image or still image data into digital image data having a predetermined format. A compression/expansion unit  36  includes a function for decoding compressed and encoded digital code (moving image data, still image data, audio data, etc.) and a function for performing the high-efficiency encoding of digital image data (e.g., like the MPEC or DV method, the digital image is vertical converted to provide a variable length predetermined unit image that is then quantized and encoded). A memory  38  is used to temporarily store digital image data for which high-efficiency encoding has been performed, and a memory  40  is used to temporarily store digital image data for which high-efficiency encoding has not been performed. A data selector  42  selects either the memory  38  or the memory  40 . A 1394 interface  44  includes a function that conforms to the IEEE 1394-1995 standards and a function that is associated with the communication protocol that is specified in this embodiment. Memory controllers  46  and  48  control the writing and the reading processes for the memories  38  and  40 . A system controller  50 , which includes a microcomputer, controls the operation of the DVCR  28 . An operating unit  52  includes a remote controller and an operation panel. A electronic viewfinder (EVF)  54  is used to display an analog image signal. A D/A converter  56  converts a digital signal into an analog signal. A recorder/reproducer  58  is a recording medium, such as a magnetic tape, a magnetic disk, or a magneto-optic disk, and is used to record or reproduce various types of digital data (moving image data, still image data or audio data and so on). 
     The arrangement of the printer  60  will now be described. A 1394 interface  62  includes a function for conforming to the IEEE 1394-1995 standards and a function that is associated with a communication protocol that is specified in this embodiment.  64  denotes a data selector. An operating unit  66  includes an operation button and a touch panel and so on. A printer controller  68  controls the operation of the printer  60 .  70  denotes a decoder and  72  denotes an internal memory. An image processing unit  74  processes still image data, text data or graphics data it receives through a 1394 interface.  76  denotes a driver, and a printer head  78  performs printing. 
     As is shown in FIG. 2, the individual communication devices (hereinafter referred to nodes) of the computer  10 , the DVCR  28  and the printer  60  are interconnected via 1394 interfaces  14 ,  44  and  62 . Hereinafter a network constituted by the 1394 interfaces is referred to as a 1394 serial bus. Since a predetermined communication protocol is defined, the nodes can exchange various object data (e.g., moving image data, still image data, audio data, graphics data, text data, program data, etc.), and command data can be used to remotely control the nodes. In this embodiment, the communication protocol for the employment of the asynchronous transmission system is defined. 
     An explanation will now be given, while referring to FIG. 2, of the operations performed by the individual nodes constituting the communication system in this embodiment. 
     First, the functions of and the operations performed by the individual units of the computer  10  will be described. 
     In this embodiment, the computer  10  is operated, for example, as a controller for controlling the exchange of image data between the DVCR  28  and the printer  60 , or as a controller for remotely controlling the DVCR  28  and the printer  60 . 
     The MPU  12  executes software recorded on the hard disk  22 , and moves various data to the internal memory  24 . The MPU  12  also provides an arbitration function for the individual units that are connected by the internal bus  26 . 
     The 1394 interface  14  can receive image data from the 1394 serial bus, and can transfer to the 1394 serial bus image data received from the hard disk  22  or from the internal memory  24 . The 1394 interface  14  can also relay command data for exercising remote control of the other nodes along the 1394 serial bus. Further, the 1394 interface  14  has a function for transmitting to a different node a signal received via the 1394 serial bus. 
     A user selects desired software by using the operating unit  16  to instruct the MPU  12  to execute software recorded on the hard disk  22 . Information concerning the software is provided the user by the display unit  20 . In accordance with the software, the decoder  18  decodes image data received via the 1394 serial bus. The decoded image data are provided the user by the display unit  20 . 
     The functions and operations of the individual units of the DVCR  28  will now be described. 
     In this embodiment, DVCR  28  is operated, for example, as an image transmitter (source node) for asynchronously transmitting image data based on the communication protocol for this embodiment. 
     The image pickup unit  30  converts the optical image of an object into an electrical signal consisting of a luminance signal (Y) and a color signal (C), and supplies the electrical signal to the A/D converter  32 . The A/D converter  32  then converts the electrical signal into a digital signal. 
     The image processing unit  34  performs predetermined image processing for the digital luminance signal and the digital color signal, and mulitplexes the resultant digital signals. And thereafter the compression/expansion unit  36  compresses the digital luminance signal and the digital color signal. The compression/expansion unit  36  may employ a separate compression-circuit and process the luminance signal and the color signal in parallel, or it may employ time sharing and process the two signals by using a compression circuit that is employed in common. 
     The compression/expansion unit  36  shuffles the compressed image data in order to provide a means for countering transmission path errors. Therefore, sequential code errors, i.e., consecutive errors, can be changed to dispersed errors, i.e., random errors, that can be easily corrected or interpolated. When a data volume that varies due to the density of the images projected onto a screen is to be made uniform, this process should be performed before compression, so that it will be convenient to employ variable length encoding, such as run length. 
     The compression/expansion unit  36  adds, to the compressed image data, data identification information (ID) for recovering from the shuffling. In addition, the-compression/expansion unit  36  adds an error correction code (ECC) to the compressed image data in order to reduce the number of errors that occur during recording and reproduction. 
     The image data that are compressed by the compression/expansion unit  36  are transmitted to the memory  38  and the recorder/reproducer  58 . The recorder/reproducer  58  adds the ID and the ECC to the compressed image data and records it on a recording medium, such as a magnetic tape. The compressed image data are stored in a different recording area from that used for audio data. 
     The D/A converter  56  converts the image data received from the image processing unit  34  into an analog image signal, and the EVF  54  displays the analog image signal it receives from the D/A converter  56 . The image data processed by the image processing unit  34  are also transmitted to the memory  40 . In this case, uncompressed image data are transmitted to the memory  40 . 
     The data selector  42  selects the memory  38  or the memory  40  in accordance with an instruction issued by a user, and transmits either the compressed image data or the uncompressed image data to the 1394 interface  44 . The data selector  42  transmits, to either the memory  38  or the memory  40 , the image data received from the 1394 interface  44 . 
     Based on the communication protocol that will be described later, the 1394 interface  44  asynchronously transmits the compressed image data or the uncompressed image data. Further, the 1394 interface  44  receives, via the 1394 serial bus, a control command for exercising control of the DVCR  28 . The received control command is transmitted via the data selector  42  to the controller  50 . The 1394 interface  44  issues a response acknowledging receipt of the control command. 
     The functions and operations of the individual units of the printer  60  will now be described. 
     The printer  60  in this embodiment is operated, for example, as an image receiver (destination node) for receiving image data that is asynchronously transmitted, based on the communication protocol for this embodiment, and for printing the received image data. 
     The 1394 interface  62  receives image data and a control command that are asynchronously transmitted via the 1394 serial bus. Thereafter, the 1394 interface  62  issues a response acknowledging receipt of the control command. 
     The received image data are transmitted via the data selector  64  to the decoder  70 . The decoder  70  decodes the image data, and outputs the results to the image processing unit  74 . The image processing unit  74  temporarily stores the decoded image data in the memory  72 . 
     The image processing unit  74  converts the image data temporarily stored in the memory  72  into print data, and transmits the print data to the printer head  78 . The printer head  78  executes a printing process under the control of the printer controller  68 . 
     The received control command is transmitted via the data selector  64  to the printer controller  68 . The printer controller  68  employs the control data to control various printing related procedures. For example, the printer controller  68  controls the driver  76  that feeds paper, and adjusts the position of the printer head  78 . 
     The structures of the 1394 interfaces  14 ,  44  and  62  in this embodiment will now be described, while referring to FIG.  8 . 
     The 1394 interface is functionally constituted by a plurality of layers. In FIG. 8, the 1394 interface is connected to the 1394 interface of another node via a communication cable  801  that conforms to the IEEE 1394-1995 standards. The 1394 interface has one or more communication ports  802 , each of which is connected to a physical layer  803  that is included in the hardware portion. 
     In FIG. 8, the hardware portion includes the physical layer  803  and a link layer  804 . The physical layer  803  serves as a physical and electrical interface with another node, detects the resetting of a bus and preforms associated processes, encodes/decodes an input/output signal, and provides an arbitration function to settle conflicts concerning the right of use of a bus. The link layer  804  generates a communication packet, exchanges various types of communication packets, and controls a cycle timer. In addition, the link layer  804  has a function for generating asynchronous broadcast packets and a function for exchanging such packets, which will be described later. 
     In FIG. 8, the firmware portion includes a transaction layer  805 , and a serial bus management portion  806 . The transaction layer  805  manages the asynchronous transmission system and provides various types of transactions (reading, writing and locking). The transaction layer  805  also provides an asynchronous broadcast transaction function, which will be described later. The serial bus management portion  806  provides a function for, based on the IEEE1212 CSR standards that will be described later, controlling the node that it belongs to, managing the connection state of the node, managing the ID information of the node, and managing the resources of the serial bus network. 
     The hardware portion and the firmware portion in FIG. 8 substantially constitute the 1394 interface, and its basic structure is as specified in the IEEE 1394-1995 standards. 
     The functioning of an application layer  807 , which is included in a software portion and which designates object data and the method to be used for its transmission, varies in accordance with the application software that is to be used. 
     The communication protocol in this embodiment expands the functions of the hardware portion and the firmware portion of the 1394 interface, and provides innovative transmission processing for the software portion. 
     The basic structure of the communication protocol defined in this embodiment will now be explained while referring to FIG.  3 . 
     In FIG. 3, the basic structure comprises: a controller  300 ; a source node  302 ; n (n≧1) destination nodes  304 ; a sub-unit  306  included in the source node  302 ; and object data  308 , such as still image data, graphics data, text data, file data or program data. 
     A first memory space  310  is defined in the destination node  304  by employing a predetermined destination offset (destination_offset #0). A first connection  312  represents a logical connection relationship established between the source node  302  and the destination node  304 . It should be noted that the destination offset is an address by which to designate in common memory spaces in n destination nodes  304 . 
     An n-th memory space  314  is defined in the destination node  304  by a predetermined destination offset (destination_offset #n). An n-th connection  316  represents the logical connection relationship established between the source node  302  and the destination node  304 . 
     In this embodiment, the individual nodes manage the first to the n-th memory spaces  310  to  314  by using 64-bit address spaces that conform to the IEEE1212 CSR (Control and Status Register Architecture) standards (or the ISO/IEC 13213: 1994 standards). The IEEE 1212 CSR standards are those for specifying the control, the management and the address allocation for the serial bus. 
     FIGS. 6A and 6B are diagrams for explaining the address space included in each node. In FIG. 6A is shown a logical memory space that is represented by a 64 bit address. In FIG. 6B is shown one part of the address space shown in FIG. 6A, where the upper 16 bits represent FFFF 16 . The first memory space  310  to the n-th memory space  314  in FIG. 3 employ a part of the memory space in FIG. 6B, and a destination offset address for each of them is included in the lower 48 bits of an address. 
     In FIG. 6B, for example, 000000000000 16  to 0000000003FF 16  define a reserved area, while actually the object data  308  are written in an area for which the starting address in the lower 48 bits is FFFFF0000400 16 . 
     In FIG. 3, the source node  302  is a node that includes a function for transmitting the object data  308  in accordance with the communication protocol that will be described later. The destination node  304  is a node that includes a function for receiving the object data  308  from the source node  302 . The controller  300  is a node for establishing a logical connection relationship between the source node  302  and one or more destination nodes  304  in accordance with the communication protocol that will be described later, and for managing the logical connection relationship. 
     Separate nodes may be provided as the controller  300 , the source node  302  and the destination node  304 . A single node may be provided as the controller  300  and the source node  302 , and a single node may be provided as the controller  300  and the destination node  304 . In this case, no transaction is required to be effected between the controller  300  and the source node  302 , or the destination node  304 , and the processing is simplified. 
     In this embodiment, the separate nodes are provided as the controller  300 , the source node  302  and the destination node  304 . The computer  10 , including the 1394 interface  14 , serves as the controller  300 , the DVCR  28 , including the 1394 interface  44 , serves as the source node  302 , and the printer  60 , including the 1394 interface  62 , serves as the destination node  304 . 
     As is shown in FIG. 3, one or more connections can be established between the source node  302  and one or more destination nodes  304 . When a request for the transmission of specific object data is issued, one or more controllers  300  establish these connections in accordance with the communication protocol that will be described later. 
     In this embodiment, one or more destination offsets can be set that can be used for one connection. The value of the destination offset may be either a value that is set in advance, or a variable value that the controller  300  or the source node  302  sets. It should be noted that the relationship between the connection and the destination offset is set in accordance with the communication protocol that will be described later. 
     When a plurality of destination offsets are to be set for one connection, data communication having a plurality of forms can be provided with a single connection. For example, when different offset addresses are allocated to different forms of data communication, one-to-one communication, one-to-N communication, and N-to-N communication can be implemented at the same time by a single connection. 
     In this embodiment, the computer  10  that serves as the controller  300  may act as the destination node  304 . In this case, a connection is established between the source node  302  and two destination nodes  304 , and the object data  308  are transmitted. 
     In this embodiment, the computer  10  serves as the controller  300 , but it may not necessarily be designated the controller  300 . The DVCR  28  or the printer  60  may also act as the controller  300 . 
     An explanation will now be given for the basic transmission processing according to the communication protocol defined in this embodiment. 
     FIGS. 4A,  4 B and  4 C are sequence charts showing the processing performed for the transmission of one set of object data. FIG. 4B is a sequence chart showing the processing performed when a bus is reset or a transmission error occurs during the transmission of one set of object data. 
     According to the communication protocol in this embodiment, when the controller  300  has established the previously described connection, it transmits one set of object data by performing one or more asynchronous broadcast transactions. The detailed asynchronous broadcast transaction processing will be described while referring to FIGS. 4A to  4 C. A packet used for an asynchronous broadcast transaction (hereinafter referred to as an asynchronous broadcast packet) will be explained while referring to FIG.  5 . 
     The asynchronous broadcast transaction and the asynchronous broadcast packet are an innovative process and an innovative packet format that are specified by the communication protocol in this embodiment. 
     The basic transmission processing in accordance with the communication protocol in this embodiment will now be described while referring to FIGS. 4A and 4C. FIG. 4A is a sequence chart for explaining how data communication is to be performed when a connection is established with only one destination node  304 . FIG. 4C is a sequence chart for explaining how data communication is performed when a single connection is employed for three destination nodes  304 . 
     The controller  300  establishes a connection ID for identifying the logical connection relationship existing between the source node  302  and one or more destination nodes  304 . The controller  300  then notifies the individual nodes of the connection ID that is to be used, and establishes a single connection ( 401  and  402  in FIGS.  4 A and  4 C). 
     After relaying the connection ID notification, the controller  300  instructs the source node  302  to initiate the transmission of the object data  308  ( 403  in FIGS.  4 A and  4 C). 
     Upon receiving the instruction, the source node  302  begins negotiations with one or more destination nodes  304 , and performs the initial setup for the asynchronous broadcast transaction ( 404  and  405  in FIGS.  4 A and  4 C). 
     After performing the initial setup, the source node  302  executes the asynchronous broadcast transaction, and sequentially broadcasts the object data  308 , which consists of one or more data segments ( 406  to  409  in FIGS.  4 A and  4 C). 
     A transfer model for the object data  308  in this embodiment will now be described while referring to FIG.  7 . The object data  308  in FIG. 7 are still image data of, for example, 128 Kbytes. 
     The source node  302  divides the object data  308  into, for example, 500 data segments (one data segment is 256 bytes) in accordance with the reception capabilities of the individual destination nodes  304  that are identified during the initial setup process. The size of one data segment is variably determined by the source node  30  by referring to the size of the internal buffer at each of the destination nodes  304 . In FIG. 7 is shown a case where internal buffers having the same data size as that of the object data  308  are available. 
     The source node  302  transmits one or more data segments by performing at least one asynchronous broadcast transaction. In FIG. 7, one data segment is transmitted by performing one asynchronous broadcast transaction. 
     When all the data segments have been transmitted, the source node  302  terminates the data communication connection with one or more destination nodes  304  ( 410  and  411  in FIGS.  4 A and  4 C). 
     The operation of the controller  300  will now be explained in detail while referring to FIGS. 4A and 4C. 
     The controller  300  asynchronously transmits a packet for establishing a connection (hereinafter referred to as a connection request packet) to the source node  302  that was selected by the user and to one or more destination nodes  304  ( 401  and  402  in FIGS.  4 A and  4 C). A connection ID is stored in the payload of the packet to identify the connection established between the source node  302  and the destination node  304 . 
     The connection between the source node  302  and one or more destination nodes  304  is established by the controller  300  in accordance with the connection ID previously allocated for the source node  302 , and the connection,ID previously allocated for each of the destination nodes  304 . 
     The controller  300  asynchronously transmits a transaction command packet to the source node  302  ( 403  in FIGS.  4 A and  4 C). 
     Upon receiving the transaction command packet, the source node  302  performs the initial setup in accordance with the connection ID received from the controller  300 , and executes an asynchronous broadcast transaction ( 404  to  409  in FIGS.  4 A and  4 C). By executing the asynchronous broadcast transaction, the source node  302  can sequentially transmit the object data  308  that consists of one or more data segments. 
     In the communication protocol in this embodiment, the controller  300  provides a function for managing the connection and the disconnection of nodes. Therefore, after the connection has been established, the transmission of the object data  308  is initiated by negotiations performed between the source node  302  and the destination nodes  304 . 
     When a series of asynchronous broadcast transactions has been completed, the source node  302  outputs an asynchronous broadcast packet indicating the end of the segment (hereinafter referred to as a segment end packet) ( 410  in FIGS.  4 A and  4 C). 
     Upon receiving the segment end packet from the source node  302 , the controller  300  disconnects the nodes and terminates the data transmission process ( 411  in FIGS.  4 A and  4 C). 
     Since the segment end packet is broadcast, the contents of the packet can also be detected by a destination node  304 . Therefore, the destination node  304  instead of the controller  300  may disconnect the source node  302 . 
     The operation of the source node  302  will now be described in detail while referring to FIGS. 4A and 4C. 
     When the source node  302  receives the connection request packet and the transaction command packet from the controller  300 , the source node  302  transmits, to a destination node  304 , an asynchronous broadcast packet requesting the transmission of a data transmission request (hereinafter referred to as a send request packet) ( 404  in FIGS.  4 A and  4 C). 
     The send request packet is a packet used to obtain the initial information that is required for an asynchronous broadcast transaction for the object data  308 . A connection ID designated by the controller  300  is written in the packet. 
     The destination node  304  broadcasts an asynchronous broadcast packet (hereinafter referred to as an ack response packet) that constitutes a response to the send request packet ( 405  in FIGS.  4 A and  4 B). The same connection ID as that used for a send request packet is written in the ack response packet. Therefore, the source node  302  can examine the connection ID in the ack response packet that is received, and can identify the connection through which that packet has been transmitted. 
     In the ack response packet are stored the size of the internal buffer available at the destination node  304  and the offset address for a specific memory space. Upon receiving the ack response packet, the source node  302  sets the destination offset for it that designates in common the memory spaces in the destination nodes  304 , and begins the asynchronous broadcast transaction. The destination offset is designated by using the offset address included in the ack response packet received from each destination node  304 . 
     In this embodiment, the destination offset used for the asynchronous broadcast transaction is set using the offset address included in the ack response packet. However, this destination offset may be set in a different manner. For example, the controller  300  may have a function for managing the destination offsets used for individual connections, and may set destination offsets that correspond to the connection IDs. In this case, destination offsets corresponding to the connections are transmitted by the controller  300  to the source node  302 . 
     The source node  302  writes the first asynchronous broadcast packet in the memory space indicated by the destination offset ( 406  in FIGS.  4 A and  4 C). The connection ID and the sequence number of a data segment are stored in the packet. 
     After transmitting the first asynchronous broadcast packet, the source node  302  waits for a response packet from the destination node  304 . The destination packet  304  transmits, as a response packet, an asynchronous broadcast packet in which its connection ID and the sequence number are stored. Upon receiving the response packet, the source node  302  increments the sequence number, and transmits another asynchronous broadcast packet that includes the sequence number of the next data segment ( 407  in FIGS.  4 A and  4 C). 
     By repeating the above process, the source node  302  sequentially performs the asynchronous broadcast transactions ( 408  and  409  in FIGS.  4 A and  4 C). The maximum waiting time for a response from a destination node  304  is determined in advance. When no response is transmitted before the maximum waiting time has expired, the same sequence number is employed to re-transmit the same data segment. 
     When a response packet requesting re-transmission is issued by a destination node  304 , the source node  302  can broadcast the data that correspond to the designated sequence number. 
     When all of the object data  308  has been transmitted by means of the asynchronous broadcast transactions, the source node  302  broadcasts the segment end packet and terminates the data transmission ( 410  and  411  in FIGS.  4 A and  4 C). 
     As is described above, the source node  302  divides the object data  308  into one or more segments as needed. Therefore, the transmission of above response packet will occur in association with the asynchronous broadcast transmission of the data segments. One data segment is transmitted for each asynchronous broadcast transaction that is performed. The destination node  304  includes a buffer having the above described capacity. 
     In this embodiment, it is so designed that a response packet is transmitted in association with the asynchronous broadcast transaction of one data segment. However, a destination node  304  may transmit a response packet after the data buffer at the destination node  304  has been filled with a plurality of sequential data segments. 
     The operation of destination node  304  will now be described in detail while referring to FIGS. 4A and 4C. 
     When a connection request packet is received from the controller  300 , the destination node  304  waits for the send request packet from the source node  302  ( 404  in FIGS.  4 A and  4 C). 
     Upon receiving the send request packet, the destination node  304  compares the connection ID written in the packet with the connection ID received from the controller  300 , and determines whether the received packet originated at the source node  302 . 
     When the send request packet that is received is from the source node  302 , the destination node  304  broadcasts the ack response packet in which are written the connection ID, the size of the available internal buffer, and the offset address that for the a specific memory space ( 405  in FIGS.  4 A and  4 C). 
     When an asynchronous broadcast packet received from the source node  302  is written in the memory space, the destination node  304  inspects the connection ID contained in the packet. When the connection ID stored in the packet matches the connection ID of the destination node  304 , the destination node  304  broadcasts a response packet in which are stored the connection ID and the sequence number included in the received packet ( 406  and  409  in FIGS.  4 A and  4 C). In this case, the data segment included in the received asynchronous broadcast packet is stored in the internal buffer. When the connection ID included in the received packet differs from the connection ID of the destination node  304 , the destination node  304  abandons the received packet. 
     When the destination node  304  ascertains that the sequence number of the received packet does not match, it can transmit a response packet to request a re-transmission. In this case, the destination node  304  notifies the source node  302  of the sequence number for which the re-transmission is requested. 
     When all the asynchronous broadcast transactions above been completed, the source node  302  broadcasts the segment end packet. Upon receiving this packet, the destination node  304  terminates the data transmission processing ( 410  in FIGS.  4 A and  4 C). 
     After receiving the segment end packet, the destination node  304  broadcasts a response packet indicating that it has received the segment end packet ( 411  in FIGS.  4 A and  4 C). 
     As is described above, the communication system in this embodiment can resolve the inconveniences encountered with a conventional communication system. In addition, the communication system in this embodiment can easily and quickly perform the transmission of data even when real-time processing is not required. 
     Since, when the controller establishes the connection, the object data are exchanged between the source node and the destination nodes, the controller need not be employed for the transmission, and the data transmission can be performed easily, with no complicated processing being required. 
     Since a destination node always transmits a response packet for each broadcast transaction, a satisfactory communication protocol can be provided. 
     In order to implement a more satisfactory data transmission, data transmission must be resumed rapidly without any data being lost, even when the data transmission is halted due to resetting of a bus or the occurrence of a transmission error. While referring to FIG. 4B, an explanation will now be given for the resumption processing that is specified in accordance with the communication protocol in this embodiment. 
     Assume that a bus reset occurs after an asynchronous broadcast packet having sequence number of i is received. Each of the nodes halts the transmission and initializes the bus, identifies the connection configuration, and sets the node ID in accordance with the procedures defined in the IEEE1394-1995 standards ( 420  and  421  in FIG.  4 B). 
     When the bus has been rebuilt, the destination node  304  broadcasts a resumption request packet (resend request packet) in which the connection ID and the sequence number i are stored ( 422  in FIG.  4 B). 
     When the asynchronous broadcast transaction can be resumed, the source node  302  identifies the connection ID contained in a received resend request packet, and broadcasts an ack response packet in which that connection ID is stored ( 423  in FIG.  4 B). 
     Then, starting with the sequence number that was requested by the resend request packet, the source node  302  begins to sequentially broadcast data segments, i.e., data segments beginning with sequence number (i+1) ( 424  in FIG.  4 B). 
     In the above described processing, even when data transmission has been halted, the controller  300 , the source node  302  and the destination nodes  304  can easily and satisfactorily resume the transmission of data, without taking their node IDs into account. 
     As is described above, in this embodiment, the control process performed by the controller  300  can be simplified even when the data transmission has been is halted. 
     The structure of the asynchronous broadcast packet specified in this embodiment will now be described, while referring to FIG.  5 . The asynchronous broadcast packet is a data packet having one quadlet (four bytes=32 bits) as one unit. 
     The structure of a packet header  521  will be described first. 
     In FIG. 5, a field  501  (16 bits) represents destination_ID, which is a node ID of a recipient, i.e., a destination node  304 . Since an asynchronous broadcast transaction of the object data  308  is implemented in accordance with the communication protocol of this embodiment, the value of the field  501  is employed as a broadcast ID, i.e., FFFF 16 . 
     A field  502  (6 bits) represents a transaction level (t 1 ) and is a tag inherent to each transaction. 
     A field  503  represents a retry (rt) code to designate a retry of the packet. 
     A field  504  (4 bits) represents a transaction code (tcode). The transaction code tcode designates a packet format and the type of transaction that must be performed. In this embodiment, the value of this field is set, for example, to 0001 2 , and requests a process (i.e., write transaction) for writing a data block  522  of this packet in the memory space defined by a destination_offset field  507 . 
     A field  505  (4 bits) represents a priority (pri), and designates the priority order. In this embodiment, the value of this field is set to 0000 2 . 
     A field  506  (16 bits) represents a variable source_ID that is the node ID of the transmission side, i.e., the source node  302 . 
     The field  507  (48 bits) represents a variable destination_offset, and designates in common the lower 48 bits of the address spaces included in the individual destination nodes  304 . The same destination_offset value may be set for all the connections, or a different destination_offset value may be set for each connection. However, it is efficient for a different destination_offset value to be set because the asynchronous broadcast packets from a plurality of connections can be processed in parallel. 
     A field  508  (16 bits) represents a variable data_length, and employs bytes to indicate the length of a data field that will be described later. 
     A field  509  (16 bits) represents a variable extended_tcode. In this embodiment, the value of this field is set to 0000 16 . 
     A field  510  (32 bits) represents a variable header_CRC, in which error detection code corresponding to the fields  501  to  509  are stored. 
     A data block  522  will now be described. In this embodiment, the data block  522  is composed of header information  523  and a data field  524 . 
     A connection ID for identifying the logical connection relationship between the nodes is included in the header information  523 . The structure of the header information  513  is varied in accordance with the purpose of its use. 
     The data field  524  is a field having a variable length, and the data segments are stored therein. When the data segment stored in the data field  524  is not a multiple of the quadlet, a 0 is entered in a portion that does not reach the quadlet. 
     A field  511  (16 bits) represents a variable connection_ID, and the connection ID in this embodiment is stored therein. The 1394 interface of this embodiment employs the connection ID stored in this field  511  to identify a connection that is established between the source node  302  and one or more destination nodes  304 . In this embodiment, 2 16 × (the number of nodes) connections can be established. Therefore, a plurality of connections can be established before the total communication bands used by the connections reach the capacity limit for the transmission path. 
     A field  512  (8 bits) represents a variable protocol_type, and indicates the communication processing (i.e., the communication protocol type) that is based on the header information  5213 . When the communication protocol in this embodiment is indicated, the field value is, for example, 01 16 . 
     A field  513  (8 bits) represents a variable control_flags, and predetermined control data are set therein to control the communication order according to the communication protocol in this embodiment. The most significant bit in this field  513  is employed, for example, as a re-transmission request (resend_request) flag. When the value of the most significant bit in the field is 1, it is assumed that a re-transmission has been requested according to the communication protocol of this embodiment. 
     A field  514  (16 bits) represents a variable sequence_number. A sequential value, i.e., a sequence number, is set for a packet that is transmitted in accordance with a specific connection ID (the connection ID designated in the field  511 ). With the sequence number, the destination node  304  can monitor the continuity of data segments that are sequentially transmitted by the asynchronous broadcast transactions. If the sequence number and the data segment do not match, the destination node  304  can request a re-transmission based on the sequence number. 
     A field  515  (16 bits) represents a variable reconfirmation_number. In this embodiment, this field is meaningful only when the re-transmission request flag is set to a value of 1. In this case, the sequence number of the packet for which re-transmission is requested is set in the field  515 . 
     A field  516  (16 bits) represents a variable buffer_size. The buffer size for the destination node  304  is set in this field  516 . 
     A field  517  (48 bits) represents a variable offset_address. The lower 48 bits in the address space included in the destination node  304  are stored in this field  517 . With this field, one of the first memory spaces  310  in the n-th memory space  314  shown in FIG. 3 is designated. 
     A field  518  (32 bits) represents a variable data_CRC. An error detection code for the fields  511  to  517  (including the header information  523  and the data field  524 ) is stored in the variable data_CRC, as well as in the variable header_CRC described above. 
     While referring to FIGS. 8 and 9, a detailed explanation will now be given for the communication processing specified by the communication protocol for this embodiment. 
     In this embodiment, an explanation will be especially given for the processing for, during transmission period, terminating a series of asynchronous broadcast transactions performed between the source node  302  and the destination node  304 . 
     FIG. 9 is a sequence chart showing an example where the transmission is easily halted by the source node  302  and the destination node  304 . In FIG. 9, the asynchronous broadcast transaction between the source node  302  and one destination node  304  is employed for simplifying the explanation. However, the same processing can be preformed for the transaction with N destination nodes  304 . 
     In FIG. 9, the destination node  304  can halt the transmission of data to the source node  302  by no transmission of a response packet. In the example in FIG. 9, the destination node  304  does not transmit a response packet relative to the n-th asynchronous broadcast transaction ( 901  in FIG.  9 ). 
     In this case, when a response packet is not received from the destination node  304  within a period of time (response timeout  901 ) that is designated in advance, the source node  302  automatically re-transmits the data segment having the same sequence number as that of the preceding asynchronous broadcast packet ( 903  in FIG.  9 ). 
     If the response packet is not received though the above process is repeated at a predetermined number of times ( 904  in FIG.  9 ), the source node  302  assumes that the destination node  304  halted the data transmission, and broadcasts an abort packet ( 905  in FIG.  9 ). The abort packet is a packet to halting a series of asynchronous broadcast transactions performed between the source node  302  and the destination node  304 . 
     With the abort packet, the controller  300  and the destination node  304  are notified of the end of the transmission, and the source node  302  terminates the data transmission. The controller  300  disconnects the node corresponding to the abort packet. 
     According to the communication protocol of this embodiment, through the above processing, the destination node  304  can easily halt the data transmission, without performing special processing, and the controller  300  can perform disconnection. 
     As is shown in FIGS. 10A to  10 C, a series of asynchronous broadcast transactions can also be halted when one of the controller  300 , the source node  302  or the destination node  304  broadcasts an abort packet that requests the halt of the transmission. 
     FIG. 10A is a diagram showing an example where the source node  302  issues a halt request. FIG. 10B is a diagram showing an example where the destination node  304  issues a halt request. FIG. 10C is a diagram showing an example where the controller  300  issues a halt request. In FIGS. 10A to  10 C, the asynchronous broadcast transaction between one source node  302  and one destination node  304  is employed for simplifying the explanation. However, the same process can be performed for N destination nodes  304 . 
     A node that desires to halt the asynchronous broadcast transaction broadcasts an abort packet during the data transmission period. Upon receipt of the abort packet, the source node  302  or the destination node  304  halts the data transmission ion accordance with the predetermined procedures, and the controller disconnects the node. 
     In FIG. 10A, the source node  302  broadcasts the abort packet after the n-th asynchronous broadcast transaction is completed ( 1001  in FIGS. 10A to  10 C). In FIG. 10B, the destination node  304  broadcasts the abort packet after the n-th asynchronous broadcast transaction is completed ( 1002  in FIGS. 10A to  10 C). In FIG. 10C, the controller  300  broadcasts the abort packet after the n-th asynchronous broadcast transaction is completed ( 1003  in FIGS. 10A to  10 C). 
     Through the above processing, the controller  300 , the source node  302  and the destination node  304  ensure to halt the data transmission by performing simple procedures, and can disconnect the other nodes. 
     As is described above, according to the individual embodiments, a logical connection relationship that does not depend on the physical connection form can be built in a bus network that conforms to the IEEE1394-1995 standards. 
     In these embodiments, for the communication system that conforms to the IEEE1394-1995 standards, an innovative communication protocol can be provided according to which a comparatively large amount of object data (e.g., still image data, graphics data, text data, file data, program data, etc.), for which reliability is requested even though real-time processing is not required, can be divided into one or more data segments and the data segments can be sequentially transmitted. 
     In addition, according to the above embodiments, for a communication system that conforms to the IEEE1394-1995 standards, an innovative communication protocol can be provided with which data communication between a plurality of devices can be implemented by using a communication method for the asynchronous broadcasting of data. 
     Furthermore, according to the above embodiments, a plurality of sets of continuous data can be satisfactorily transmitted, without requiring the isochronous transmission method that conforms to the IEEE1394-1995 standards. One set of object data can be divided into a plurality of data segments that can be individually transmitted. 
     Further, according to the above embodiments, since communication among a plurality of devices is managed at one connection, multiple communications that do not require a very large communication band can be performed at the same time. 
     Multiple communications can be performed in a transmission band wherein only a few nodes are employed. 
     In the above embodiments, even when the data transmission is halted due to a bus reset or a transmission error, information can be transmitted concerning the contents of data that have been lost, and the transmission can be resumed without very complicated processing being required. 
     (Other Embodiment) 
     The communication protocols in the above embodiments and the various operations required to implement them can be achieved by software. 
     For example, a storage medium on which program code is stored to implement the functions in the first to the fifth embodiments is supplied to the controllers (the MPU  12 , the system controller  50  and the printer controller  68  in FIG. 2) of apparatuses that constitute the communication system in the individual embodiments. The controllers permit the communication system or the apparatuses to read the program code from the storage medium, and to implement the functions of the embodiments in accordance with the program code, so that the above embodiments can be implemented. 
     Further, a storage medium on which program code is stored to implement the functions in the first to the fifth embodiments is supplied to the 1394 interfaces  14 ,  44  and  62  of the apparatuses. The controller (e.g., the serial bus management unit  806  in FIG. 8) permits the 1394 interfaces  14 ,  44  and  62  to implement the functions of the embodiments in accordance with the program code stored in the storage medium, so that the above embodiments can be implemented. 
     In this case, the program code read from the storage medium is used to implement the functions of the above described embodiments. The program code or the means (e.g., the storage medium) on which the program code is stored constitutes the present invention. 
     A storage medium for supplying such program code can be, for example, a floppy disk, a hard disk, an optical disk, a magneto optical disk, a CD-ROM, a magnetic tape, a nonvolatile memory card, or a ROM. 
     In addition, the scope of the present invention includes a case wherein the functions of the first to the fifth embodiments can be implemented when the program code is read from the storage medium and stored in a memory included in a function expansion unit that is connected to the above controller, and wherein the controller in the function expansion unit performs one part, or all of the actual processing, in accordance with the program code stored in the memory. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. 
     For example, in the above embodiments, the communication protocol that can be applied to the network that conforms to the IEEE1394-1995 has been explained. However, the communication protocol in these embodiments can be applied for a bus network that conforms to the IEEE1394-1995 standards, and a network that can virtually constitute a bus network. 
     Therefore, the above mentioned embodiments are merely examples in all respects, and must not be construed as limiting the invention. 
     The scope of the present invention is defined by the scope of the appended claims, and is not limited at all by the specific descriptions given in this specification. Furthermore, all the modifications and changes belonging to the equivalents of the claims are considered as falling within the scope of the present invention.