Assigning and updating dynamically through asynchronous communication non unique second identifiers to a plurality of stations for isochronous data communication

Abstract: A master station 1 1 produces a token packet for isochronous data in which a slave station 1 2 and slave stations 1 3 and 1 4 serving as a sending station and receiving stations are designated using IDs for isochronous data "#2" and "#3" in accordance with contents described in a table 16 1 held in its inner part so as to manage a transmission bandwidth using a token packet sending/analysis executing device 16, and sends out the produced token packet to a transmission path 15 on the downstream side. The slave station 1 2 sends out an isochronous data packet 42 using the token packet for isochronous data 41. According to the token packet for isochronous data 41, the slave stations 1 3 and 1 4 are designated as receiving stations using the ID for isochronous data "#3", so that the slave stations 1 3 and 1 4 perform the above-mentioned operations, to receive the isochronous data packet 42. The sending station can thus transmit the same isochronous data to a plurality of receiving stations belonging to a broadcast group, so that it is possible to establish multicast communication (communication to a plurality of nodes).

Description: CROSS REFERENCE TO RELATED APPLICATION The present application is related to copending U.S. patent application Serial No. 08/843,597 filed on April 16, 1997 published under US 5946679, assigned to the same assignee of the present application. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a communication network, and more particularly, to a communication network capable of establishing asynchronous data communication not requiring real time properties and isochronous data communication requiring the real time properties. Description of the Background Art Examples of a conventional loop communication network include one disclosed in "JP-A-9-289518". Fig. 29 is a block diagram showing the structure of the loop communication network. In Fig. 29, a plurality of stations (four stations 29 1 to 29 4 are illustrated) are connected so as to be capable of communicating with each other by a loop-shaped transmission path 295. Identifiers which are not overlapped with each other (hereinafter referred to as actual IDs) are fixedly assigned to the stations 29 1 to 29 4 , respectively. The station 29 1 of the four stations 29 1 to 29 4 functions as a master station, and the other stations 29 2 to 29 4 function as slave stations (which may, in some cases, be hereinafter referred to as the master station 29 1 and the slave stations 29 2 to 29 4 ). The procedure for communication in the communication network will be described. The master station 29 1 holds a table 296 for managing a transmission bandwidth. The master station 29 1 generates a token packet in which a station to be a source of transmission of data (hereinafter referred to as a sending station) and a station to be a destination of transmission of the data (hereinafter referred to as a receiving station) are designated using actual IDs (hereinafter referred to as a sending station ID and a receiving station ID) in accordance with contents described in the table 296, and sends out the token packet to the transmission path 295. It is assumed that the station 29 4 and the station 29 3 are designated as the sending station and the receiving station. The token packet returns to the master station 29 1 upon circulating around the loop-shaped transmission path 295. The master station 29 1 deletes the returned token packet. When the token packet circulating around the transmission path 295 arrives, each of the slave stations 29 2 to 29 4 branches the token packet into two token packets, to receive one branched token packet, while sending out the other branched token packet to the transmission path 295 on the downstream side. Each of the slave stations 29 2 to 29 4 judges whether or not the sending station ID or the receiving station ID of the accepted token packet identifies with the actual ID of the slave station. The slave station 29 4 judges that data transmission is enable because the sending station ID and the actual ID of the slave station identify with each other. Further, the slave station 29 3 judges that data receiving is enable because the receiving station ID and the actual ID of the slave station identify with each other. The slave station 29 4 sends out, when it holds a data packet to be transmitted immediately after judging that data transmission is enable, the data packet to the transmission path 295. The data packet is transmitted so as to succeed the token packet when it is sent out to the loop-shaped transmission path 295, and returns to the sending station (the slave station 29 4 ) upon circulating around the transmission path 295. The slave station 29 4 deletes the returned data packet. The master station 29 1 controls a switch (not shown) when the token packet and the data packet arrive from the transmission path 295 on the upstream side, to send out only the data packet of the two types of arrived packets to the transmission path 295 on the downstream side. Fig. 29 illustrates the data packet immediately after the master station 29 1 controls the switch. The slave station 29 3 receives, when it judges that data receiving is enable, the data packet arriving within a predetermined time period elapsed since it received the token packet. In the communication network, point to point communication can be established between the sending station and the receiving station which are designated by the token packet. Further, the master station 29 1 sends out the token packet at predetermined time intervals in accordance with contents described in the table 296 for managing the transmission bandwidth in the communication network. When isochronous data is communicated between the sending station and the receiving station, therefore, the real time transfer of the isochronous data are reserved if the transmission bandwidth is ensured on the table 296. In the communication network shown in Fig. 29, a first method in which the master station 29 1 sets a plurality of receiving station IDs in the token packet in order to establish communication between one station and a plurality of stations (which is referred to as multicast communication, and broadcast communication particularly in the case of communication between one station and all stations) is considered. Since the token packet generally has a fixed length, the number of receiving station IDs which can be set has a limit. If the number of stations which are accommodated in the communication network is increased, it is possible to anticipate the problem that multicast communication cannot be ensured. Next, a second method in which special IDs for multicast communication are provided in addition to actual IDs is considered. That is, it is assumed that a special ID (for example, #1) is assigned to a combination of the slave stations 29 2 and 29 3 , for example, in Fig. 29. If the special ID ; #1 is set to the receiving station ID of the token packet, therefore, the master station 29 1 can designate the plurality of stations (the slave stations 29 2 and 29 3 ) as receiving stations. However, an attempt to apply the second method to ensure multicast communication requires the number of special IDs corresponding to combinations of all the stations. If the number of stations accommodated in the communication network is increased, the data length of the IDs is increased. Therefore, it is possible to anticipate a first problem that the special ID cannot, in some cases, be set in the token packet having a fixed length. Further, the master station 29 1 must manage not only the actual ID but also the special ID, whose management becomes difficult. US 4,637,014 describes a method of transmitting isochronous and non-isochronous data in a computer network in which multiple stations have respective input and output parts that are serially coupled together to form a loop. D.E. Comer describes in "Internetworking with TCP/IP: Principles, Protocols, and Architecture", Prentice Hall International, Englewood Cliffs, NJ 07632 1988, on pages 17 to 18, the Ethernet Addressing scheme. In Ethernet, every station is assigned by the manufacturer of the network interface a fixed address called "hardware address" or "physical address". Furthermore, one or more programmable physical addresses and/or "multicast addresses" for one-to-many communication can be assigned to the network interface, which is usually done when the system starts. SUMMARY OF THE INVENTION Therefore, the object of the present invention is to provide a communication network capable of ensuring point to point communication and communication between one station and a plurality of stations without significantly increasing the number of IDs respectively assigned to the stations, as defined in claim 1. In order to attain the above-mentioned object, the present invention has the following aspects. A first aspect is directed to a communication network in which a plurality of stations are connected to each other by a loop-shaped transmission path, any one of the plurality of stations functioning as a master station; and asynchronous data communication relating to asynchronous data not requiring real time properties and isochronous data communication relating to isochronous data requiring the real time properties can be established, wherein first identifiers which are used at the time of the asynchronous data communication and are not overlapped with each other are respectively fixedly assigned to the stations, second identifiers which are used at the time of the isochronous data communication and may be overlapped with each other are assigned to the stations as required, the master station sends out to the transmission path for each predetermined time period a token packet for isochronous data in which a station transmitting the isochronous data (hereinafter referred to as a sending station) and a station receiving the isochronous data (hereinafter referred to as a receiving station) are designated using the second identifiers when the isochronous data communication should be established, so that a plurality of stations receiving the isochronous data can be designated, and the sending station transmits, when the token packet for isochronous data is transmitted through the transmission path, the isochronous data to be transmitted to the receiving stations, so that the isochronous data communication is started. According to the first aspect, the isochronous data communication is started on the basis of the token packet for isochronous data in which the receiving stations are designated using the second identifiers. The second identifiers may be assigned to the stations as required and also be overlapped with each other, so that it is possible to easily ensure multicast communication of the isochronous data. A second aspect is characterized in that in the first aspect, the second identifiers are assigned to the stations establishing the isochronous data communication by the asynchronous data communication. According to the second aspect, the second identifiers are set by the asynchronous data communication. Therefore, it becomes easy to add a new station to the stations receiving the same isochronous data (a broadcast group) by the multicast communication and withdraw the station, which has already belonged to the broadcast group, from the group. A third aspect is characterized in that in the second aspect, when the isochronous data transmitted by the sending station is received by the plurality of receiving stations in the isochronous data communication, the only one receiving station of the plurality of receiving stations sends out to the transmission path a response signal for controlling the transmission of the isochronous data by the sending station and/or reporting the acknowledgment of the isochronous data communication by the receiving station. According to the third aspect, the only one receiving station in the broadcast group sends out the response signal to the transmission path. Therefore, no signal collision occurs on the transmission path. A fourth aspect is characterized in that in the third aspect, the only one receiving station is set by the asynchronous data communication. According to the fourth aspect, the only one receiving station is set by the asynchronous data communication. Therefore, it becomes easy to change the different receiving stations in the broadcast group into the only one receiving station. A fifth aspect is characterized in that in the second aspect, when the isochronous data transmitted by the sending station is received by the plurality of receiving stations in the isochronous data communication, the plurality of receiving stations respectively send out to the transmission path response signals for controlling the transmission of the isochronous data by the sending station and/or reporting the acknowledgment of the isochronous data communication by the receiving stations, and the sending station receives only the response signal sent out from the only one receiving station of the plurality of receiving stations. According to the fifth aspect, only the response signal sent out from the only one receiving station of the response signals sent out from the plurality of receiving stations is transmitted to the sending station. Therefore, no signal collision occurs on the transmission path. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 3 are diagrams for explaining the procedure for point to point communication in a communication network; Fig. 4 illustrates the structure of a communication network according to a first embodiment of the present invention; Fig. 5 is a diagram showing an actual ID and an ID for isochronous data which are set in each of stations 1 1 to 1 4 shown in Fig. 4; Fig. 6 is a timing chart of two types of token packets (for isochronous data and for asynchronous data) used in the communication network shown in Fig. 4; Fig. 7 illustrates the structure of a communication network according to a second embodiment of the present invention; Fig. 8 illustrates the structures of ID holding portions 19 1 to 19 4 in stations 1 1 to 1 4 shown in Fig. 7; Fig. 9 is a timing chart of signals sent out and received by each of stations 1 1 to 1 4 in a communication network according to a third embodiment of the present invention; Fig. 10 illustrates the structure of a communication network according to a fourth embodiment of the present invention; Fig. 11 is a block diagram showing the structures of a token packet sending/analysis executing device 16 and a switch 18 1 in a master station 1 1 shown in Fig. 10; Fig. 12 is a block diagram showing the structures of analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 in slave stations 1 2 to 1 4 shown in Fig. 10; Fig. 13 is a diagram showing on a time base packets transmitted on a transmission path 15 in the communication network shown in Fig. 10; Fig. 14 is a flow chart showing the procedure that the master station 1 1 sends out a token packet shown in Fig. 10; Fig. 15 illustrates the structure of a communication network according to a fifth embodiment of the present invention; Fig. 16 is a flow chart showing the procedure that a master station 1 1 sends out a token packet shown in Fig. 15; Fig. 17 illustrates on a time base an inquiry token packet 131 and identification signals 171 which are transmitted on a transmission path 15 in a communication network according to a sixth embodiment; Fig. 18 is a block diagram showing the other structures of a token packet sending/analysis executing device 16 and a switch 18 1 in a master station 1 1 and analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 in slave stations 1 2 to 1 4 ; Fig. 19 illustrates the structure of a communication network according to an eighth embodiment of the present invention; Figs. 20 to 27 are diagrams for explaining the procedure for automatically setting an actual ID in each station 1 in the communication network shown in Fig. 19; Fig. 28 is a block diagram showing the structures of a token packet sending/analysis executing device 16 and a switch 18 1 in a master station 1 1 shown in Fig. 19 and the structures of analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 in slave stations 1 2 to 1 4 shown in Fig. 19; and Fig. 29 illustrates the structure of a conventional communication network. DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the procedure for point to point communication in a communication network will be first summarized with reference to Figs. 1 to 3, the details of the procedure for communication should be referred to because it is disclosed in copending U.S. patent application Serial No. 08/843,597 filed on April 16, 1997 and assigned to the same assignee of the present application. In Figs. 1 to 3, a plurality of stations (four stations 1 1 to 1 4 are illustrated) are connected so as to be capable of communicating with each other by a loop-shaped transmission path 15. Identifiers (hereinafter referred to as actual IDs) #a to #d which are not overlapped with each other are fixedly assigned to the stations 1 1 to 1 4 . The station 1 1 functions as a master station, and the other stations 1 2 to 1 4 function as slave stations (which may, in some cases, be hereinafter referred to as the master station 1 1 and the slave stations 1 2 to 1 4 ). The master station 1 1 comprises a token packet sending/analysis executing device 16 and a switch 18 1 . The slave stations 1 2 to 1 4 comprise analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 . The procedure for data communication in the communication network constructed as described above will be described below. At the time of the initial state of the communication network, the stations 1 1 to 1 4 connect the switches 18 1 to 18 4 to terminals A, to connect the transmission path 15 (see Fig. 1). The master station 1 1 then produces a token packet 21 in which a station to be a source of transmission of data (hereinafter referred to as a sending station) and a station to be a destination of transmission of the data (hereinafter referred to as a receiving station) are designated using the actual IDs in accordance with contents described in a table 16 1 held so as to manage a transmission bandwidth in the token packet sending/analysis executing device 16. The master station 1 1 connects the switch 18 1 to a terminal B, to disconnect the transmission path 15 immediately after producing the token packet 21, and sends out the token packet 21 to the transmission path 15 on the downstream side through the switch 18 1 from the token packet sending/analysis executing device 16 (see Fig. 2). As shown in Fig. 2, the switches 18 2 to 18 4 in the slave stations 1 2 to 1 4 are connected to the terminals A. Therefore, the slave stations 1 2 to 1 4 branch the token packet 21 arriving from the transmission path 15 on the upstream side into two token packets 21 in portions of the terminals A of the switches 18 2 to 18 4. One branched token packet 21 is received and analyzed by the analysis executing devices 17 2 to 17 4 . On the other hand, the other branched token packet 21 is sent out to the transmission path 15 on the downstream side of the slave stations 1 2 to 1 4 . Accordingly, the token packet 21 sent out from the master station 1 1 circulates around the transmission path 15 while being received by the slave stations 1 2 to 1 4 positioned on the transmission path 15, and returns to the master station 1 1 . The master station 1 1 receives and analyzes the returned token packet 21, and then deletes the token packet 21, and connects the switch 18 1 to the terminal A, to connect the transmission path 15. The token packet sending/analysis executing device 16 and each of the analysis executing devices 17 2 to 17 4 analyzes the received token packet 21, to judge whether or not the actual ID of the sending station or the actual ID of the receiving station which is designated by the token packet identifies with the actual ID of the station. When it is assumed that an actual ID ; #c and an actual ID ; #b are set as the actual ID of the sending station and the actual ID of the receiving station, the slave station 1 3 judges that data transmission is enable because the actual IDs of the sending station and the slave station identify with each other, and the slave station 1 2 judges that data receiving is enable because the actual IDs of the receiving station and the slave station identify with each other. The slave station 1 3 connects, when it holds a data packet 31 to be transmitted immediately after judging that data transmission is enable, the switch 18 3 to the terminal B to disconnect the transmission path 15 as shown in Fig. 3 after an elapse of a predetermined time period since the token packet was received, and sends out the data packet 31 to the transmission path 15 on the downstream side. The data packet 31 circulates around the transmission path 15 when the slave station 1 3 switches the switch 18 3 . The slave station 1 2 receives, when it judges that data receiving is enable, the data packet 31 arriving from the upstream side on the transmission path 15 within a predetermined time period since the token packet 21 was received. Further, the slave station 1 3 deletes, when the data packet 31 sent out by itself returns upon circulating around the transmission path 15, the data packet 31, and connects the switch 18 3 to the terminal A to connect the transmission path 15. Consequently, the communication network enters the same state as that shown in Fig. 1, so that the same procedure for communication as that described above with reference to Figs. 1 to 3 is repeated again. In the communication network, therefore, point to point communication can be established between the sending station and the receiving station which are designated using the actual IDs included in the token packet. Further, the master station 1 1 sends out the token packet 21 at predetermined time intervals in accordance with the contents described in the table 16 1 for managing the transmission bandwidth in the communication network. The master station 1 1 ensures the transmission bandwidth on the table 16 1 when isochronous data is communicated between the sending station and the receiving station. Consequently, the real time properties of the isochronous data (data requiring strict time management such as AV (Audio Video) data) are ensured. As described in the foregoing, in the communication network shown in Figs. 1 to 3, there are some problems. For example, it is difficult to ensure communication between one station and a plurality of stations (multicast communication). The applicant of the present application discloses communication networks according to first to eighth embodiments described below. However, while the first to third embodiments solve the problems outlined above, the fourth to eighth embodiments, as well as Figs. 10 to 28 illustrating the fourth to eighth embodiments, do not form part of the invention and are given as background art which is useful for understanding the invention. (First Embodiment) Fig. 4 illustrates the structure of a communication network according to a first embodiment of the present invention. The communication network shown in Fig. 4 has approximately the same structure as that shown in Figs. 1 to 3. Therefore, in Fig. 4, the same reference numerals are assigned to structures corresponding to the structures shown in Figs. 1 to 3 and hence, the description thereof is not repeated. Stations 1 1 to 1 4 shown in Fig. 4 are the same as the stations shown in Figs. 1 to 3 in that actual IDs ; #a to #d are respectively fixedly assigned thereto, and differ from the stations shown in Figs. 1 to 3 in that IDs for isochronous data are dynamically assigned thereto. The actual IDs are used for not only asynchronous data communication but also isochronous data point to point communication in the communication network shown in Figs. 1 to 3. However, attention must be given to the fact that the actual IDs are used only in the case of asynchronous data communication in the first embodiment. On the other hand, the IDs for isochronous data are used in the case of isochronous data communication because a plurality of IDs for isochronous data can be set with respect to one station and overlapped values can be set in a plurality of stations. Further, the stations 1 1 to 1 4 comprise ID holding portions 19 1 to 19 4 each capable of storing a plurality of IDs for isochronous data. Fig. 5 is a diagram showing an actual ID and an ID for isochronous data which are set in each of the stations 1 1 to 1 4 shown in Fig. 4. As described above, the actual IDs in the respective stations 1 1 , 1 2 , 1 3 and 1 4 shall be "#a", "#b", "#c" and "#d". The actual IDs are thus inherent in the stations, and are not overlapped with each other. In the stations 1 1 , 1 2 , 1 3 and 1 4 , one or more IDs for isochronous data are respectively set in addition to the actual IDs in the ID holding portions 19 1 , 19 2 , 19 3 and 19 4 . It is assumed that the IDs for isochronous data in the stations 1 1 , 1 2 , 1 3 and 1 4 are set to "#1", "#2", "#3" and "#3". The same ID for isochronous data "#3" is thus set in the slave station 1 3 and the slave station 1 4 , so that the slave stations form a broadcast group. Description is now made of the procedure for data communication in the communication network constructed as described above. For example, when isochronous data is transmitted from the master station 1 1 to the slave station 1 2 , the master station 1 1 sends out a token packet for isochronous data 41 in which the master station 1 1 and the slave station 1 2 serving as a sending station and a receiving station (as described above) are designated using the IDs for isochronous data "#1" and "#2". In this case, the above-mentioned normal point to point communication is established and hence, the detailed description thereof is omitted. For example, even when isochronous data is transmitted from the slave station 1 2 to the slave stations 1 3 and 1 4 , the procedure for data communication as shown in Figs. 1 to 3 is basically carried out. That is, the master station 1 1 produces a token packet for isochronous data 41 in which the slave station 1 2 and the slave stations 1 3 and 1 4 serving as a sending station and receiving stations are designated using the IDs for isochronous data "#2" and "#3" in accordance with contents described in the table 16 1 held so as to manage the transmission bandwidth in the token packet sending/analysis executing device 16, and sends out the token packet 41 to the transmission path 15 on the downstream side (operations at this time are the same as those in the case of the master station 1 1 described using Fig. 2). As apparent from the foregoing, the slave station 1 2 sends out an isochronous data packet 42 to the transmission path 15 using the token packet for isochronous data 41. According to the token packet for isochronous data 41, the slave stations 1 3 and 1 4 are designated as receiving stations using the ID for isochronous data "#3", so that the slave stations 1 3 and 1 4 perform the above-mentioned operations, to receive the isochronous data packet 42. A sending station can thus transmit the same isochronous data to a plurality of receiving stations belonging to a broadcast group, thereby multicast communication (communication to a plurality of nodes) can be established. In the stations 1 1 to 1 4 , one or more IDs for isochronous data are respectively held in the ID holding portions 19 1 to 19 4 . IDs for isochronous data having the same value are set in the stations 1 other than the sending station, thereby broadcast communication of the isochronous data can be established. In the first embodiment, each of the stations 1 holds one or more IDs for isochronous data which can be set to arbitrary values in addition to the actual ID. A plurality of stations in which IDs for isochronous data having overlapped values are set form a broadcast group, to receive the same isochronous data. In the first embodiment, the IDs for isochronous data are thus overlapped with each other, so that the plurality of stations can be designated as receiving stations. Consequently, the communication network according to the first embodiment can ensure multicast communication and broadcast communication between one station and a plurality of stations, provided that a token packet includes one ID for isochronous data. Fig. 6 is a timing chart of two types of token packets (for isochronous data and for asynchronous data) used in the communication network shown in Fig. 4. As shown in Fig. 6, the master station 1 1 sends out a token packet for isochronous data 41 or a token packet for asynchronous data 43 for each predetermined time period. In this example, the master station 1 1 continuously sends out m (m is an integer of one or more) token packets for isochronous data 41 onto the transmission path 15, and then sends out one token packet for asynchronous data 43 onto the transmission path 15. The master station 1 1 repeats the series of operations for sending out the token packets. Each of the stations 1 1 to 1 4 can judge the two types of token packets 41 and 43. Specifically, the two types of token packets 41 and 43 include packet type information in addition to actual IDs for designating a sending station and a receiving station (as described above). The packet type information indicates whether the token packet is for isochronous data communication or for asynchronous data communication. Each of the stations 1 1 to 1 4 judges whether the token packet is for isochronous data communication (the token packet 41) or for asynchronous data communication (the token packet 43) on the basis of the packet type information of the received token packet. Each of the stations 1 1 to 1 4 executes, when it receives the token packet for isochronous data 41, analysis as to whether the IDs for isochronous data set in the ID holding portion 19 in the station identify with the IDs for isochronous data of the sending station and the receiving station, to establish the above-mentioned isochronous data communication. Further, each of the stations 1 1 to 1 4 executes, when it receives the token packet for asynchronous data 43, analysis as to whether the actual ID fixedly assigned to the station identifies with the actual IDs of the sending station and the receiving station, to establish the asynchronous data communication. By the asynchronous data communication, the IDs for isochronous data in the ID holding portions 19 1 to 19 4 in the stations 1 1 to 1 4 are rewritten. This rewriting makes it easily feasible to start and terminate the transmission in the case of isochronous data point to point communication, to add the station to the broadcast group and to withdraw the station from the broadcast group in the case of multicast communication. In the communication network according to the first embodiment, the IDs for isochronous data can be thus dynamically rewritten as required, so that multicast communication (and broadcast communication) can be ensured without requiring IDs for isochronous data corresponding to combinations of all the stations. (Second Embodiment) Fig. 7 illustrates the structure of a communication network according to a second embodiment of the present invention. The communication network shown in Fig. 7 has approximately the same structure as the communication network according to the first embodiment. Therefore, in Fig. 7, the same reference numerals are assigned to structures corresponding to the structures shown in Figs. 1 to 3 and Fig. 4 and hence, the description thereof is not repeated. Description is now made of the procedure for data communication in the communication network according to the second embodiment mainly with respect to the difference from the first embodiment. In the initial state of the communication network, IDs for isochronous data are not set in ID holding portions 19 1 to 19 4 in stations 1 1 to 1 4 . The master station 1 1 sends out a token packet for isochronous data 41 or a token packet for asynchronous data 43 onto a transmission path 15 for each predetermined time period, as shown in Fig. 6. However, isochronous data communication is not established, and only asynchronous data communication is established. In the master station 1 1 , the IDs for isochronous data are set in the ID holding portions 19 1 to 19 4 by the asynchronous data communication using the token packet for asynchronous data 43 in order to start point to point communication of isochronous data or multicast communication of isochronous data (see the first embodiment). The master station 1 1 designates the only one station 1 of the plurality of stations 1 in which the same ID for isochronous data is set, that is, the plurality of stations 1 belonging to a broadcast group as a group leader at the time of setting the IDs for isochronous data. The master station 1 1 designates the station 1 in which the ID for isochronous data is set as a group leader even in the case of isochronous data point to point communication. Although isochronous data point to point communication is established at the beginning, the transition to multicast communication may occur by thereafter rewriting the ID for isochronous data (as described above). In the point to point communication of isochronous data and the multicast communication of isochronous data, the station 1 which is a group leader of the stations 1 receiving isochronous data sends out a response signal 71 immediately before the data packet 42, immediately after the data packet 42, both immediately before and immediately after the data packet 42, as shown in Fig. 7. The station 1 which is not a group leader of the stations 1 receiving the isochronous data (a broadcast group) does not send out the response signal 71. The only one station 1 to be a group leader in the broadcast group sends out the response signal 71 immediately before the data packet 42 is sent out, to control the sending of the data packet 42 from a sending station. Consequently, the sending station can wait for the sending of the data packet, so that it can also correspond with data communication at a variable bit rate (hereinafter referred to as VBR). Further, the station 1 sends out the response signal 71 immediately after the data packet, so that the normal acknowledgment of the data communication is confirmed. Since the only one station in the broadcast group sends out the response signal 71, no signal collision occurs on the transmission path 15. The only one station 1 to be a group leader thus exists in the broadcast group, and only the group leader outputs the response signal 71, thereby making it possible to establish reliable data communication corresponding to a VBR and causing no signal collision on the transmission path 15. Fig. 8 illustrates the structures of the ID holding portions 19 1 to 19 4 in the stations 1 1 to 1 4 shown in Fig. 7. In Fig. 8, the ID holding portions 19 1 to 19 4 include fields holding IDs for isochronous data in the stations 1 1 to 1 4 and the set conditions of the group leader. In the initial state of the communication network, the IDs for isochronous data are not also assigned to the stations 1 1 to 1 4 , so that any of the stations 1 1 to 1 4 cannot be a group leader. In order to indicate such situations, "NOTHING" is set as the ID for isochronous data, and "NO" is further set as the set conditions of the group leader in each of the ID holding portions 19 1 to 19 4 . Under such situations, the master station 1 1 sends out the token packet for isochronous data 41 or the token packet for asynchronous data 43 to the transmission path 15 for each predetermined time period, as shown in Fig. 6. However, isochronous data communication is not established, and only asynchronous data communication is established. In the master station 1 1 , IDs for isochronous data are set in the respective ID holding portions 19 1 to 19 4 and a group leader is further set by the asynchronous data communication using the token packet for asynchronous data 43 in order to start point to point communication of isochronous data or multicast communication of isochronous data. By setting the ID for isochronous data and the group leader using the asynchronous data communication, "XX" is set as the ID for isochronous data in the ID holding portions 19 in the plurality of stations 1, and "YES" is further set as the set conditions of the group leader in the ID holding portion 19 in the only one station 1 of the plurality of stations 1 in which the same ID for isochronous data is set, for example. Consequently, only the station 1 set as the group leader can send out the above-mentioned response signal 71 to the transmission path 15. By the asynchronous data communication, the ID for isochronous data and the set conditions of the group leader are thus rewritten in the ID holding portion 19, thereby making it easy to add each of the stations 1 1 to 1 4 to the broadcast group and withdraw each of the stations from the broadcast group as well as to change the group leader in the broadcast group. In the above-mentioned second embodiment, description was made of the situations where the ID for isochronous data and the group leader are simultaneously rewritten in the ID holding portions 19 in the plurality of stations 1. However, the ID for isochronous data and the group leader may be separately rewritten in the ID holding portions 19 in the plurality of stations 1, in which case the same effect as described above is obtained. Further, it is also easily reasible to change the station 1 in which the group leader has been set once into the other station 1. (Third Embodiment) Fig. 9 is a timing chart of signals sent out and received by each of stations 1 1 to 1 4 in a communication network according to a third embodiment of the present invention. In Fig. 9, a square portion indicted by a solid line represents the timing of sending out each signal, and a square portion indicated by a dotted line represents the timing of receiving each signal. Further, the structure of the communication network according to the present embodiment is the same as the structure of the communication network according to the second embodiment. In the present embodiment, therefore, the same reference numerals are respectively assigned to structures corresponding to those in the second embodiment and hence, the description thereof is not repeated. In Fig. 9, a master station 1 1 sends out a token packet for isochronous data 41 to a transmission path 15 at predetermined timing in the same manner as described above. The token packet for isochronous data 41 is received by slave stations 1 2 to 1 4 with delays corresponding to the positional relationships among the slave stations 1 2 to 1 4 on the transmission path 15 on the basis of the time when the master station 1 1 sends out the token packet, and is then received by the master station 1 1 with a delay 91 corresponding to one circulation around the transmission path 15. A response signal 71 sent out by each of the stations 1 1 to 1 4 also circulates around the transmission path 15, and returns to each of the stations 1 1 to 1 4 sending out the response signal 71 when a time period corresponding to the delay 91 has elapsed. More specifically, each of the stations 1 1 to 1 4 disconnects the transmission path 15 by switching control of a switch in the station, to send out the response signal 71 to the transmission path 15 as described above. Each of the stations 1 1 to 1 4 receives the response signal 71 sent out by itself when the time period corresponding to the delay 91 which corresponds to one circulation around the transmission path 15 has elapsed. Immediately after each of the stations 1 1 to 1 4 receives the response signal 71 sent out by itself, a state where the transmission path 15 is disconnected is returned to a state where it is connected by switching control of the switch. It is assumed that the master station 1 1 and the slave stations 1 2 and 1 4 belong the same broadcast group, and the slave station 1 3 transmits isochronous data to the broadcast group. First, a response signal 71 1 sent out by the master station 1 1 is transmitted to the slave station 1 2 through the transmission path 15. At the timing at which the response signal 71 1 sent out from the master station 1 1 arrives at the slave station 1 2 , the slave station 1 2 disconnects the transmission path 15 by switching control of the switch because it is a receiving station, to receive and at the same time send out a response signal 71 2 sent out by itself to the transmission path 15 on the downstream side. Therefore, the response signal 71 1 sent out by the master station 1 1 is transmitted only to the slave station 1 2 , and is deleted by the slave station 1 2 . That is, the response signal 71 1 is not transmitted to the transmission path 15 on the downstream side of the slave station 1 2 . However, the slave station 1 2 sends out the response signal 71 2 sent out by itself, and the response signal 71 2 is transmitted to the slave station 1 3 . The slave station 1 3 which is a sending station receives the response signal 71 2 and at the same time, passes the response signal 71 2 to the transmission path 15 on the downstream side, and then the response signal 71 2 is transmitted to the slave station 1 4 . The slave station 1 4 is operated similarly to the slave station 1 2 because it is also a receiving station, so that the response signal 71 2 which arrived is received without being transmitted to the transmission path 15 on the downstream side and is deleted. Further, the slave station 1 4 sends out a response signal 71 4 sent out by itself to the transmission path 15 on the downstream side. The response signal 71 4 is also received by the master station 1 1 which is a receiving station and is deleted. As a result, even when all the stations 1 belonging to the broadcast group (the master station 1 1 and the slave stations 1 2 and 1 4 in this description) send out the response signals to the transmission path 15, no signal collision occurs on the transmission path 15. To the sending station 1 transmitting the data packet 42 (the slave station 1 3 in this description), only the response signal sent out by the station 1 positioned on the upstream side of the sending station 1 and nearest thereto (the slave station 1 2 in this description) of the stations 1 belonging to the broadcast group is transmitted. The slave station 1 2 which is a sending station receives the response signal 71 2 and at the same time, passes the response signal 71 2 , and then switches the switch 18 2 in the slave station to disconnect the transmission path 15, and sends out the data packet 42 to the transmission path 15 on the downstream side. The data packet 42 is received by all the stations 1 belonging to the broadcast group (the slave station 1 4 , the master station 1 1 , and the slave station 1 2 in this order in this description) as described above. In the third embodiment, all the stations 1 belonging to the broadcast group thus send out the response signals 71, unlike the second embodiment. However, the only one response signal 71 of the plurality of response signals sent out is received by the sending station, so that no signal collision occurs on the transmission path 15. This also makes it possible to establish data communication corresponding to a VBR and high is reliability, as in the second embodiment. (Fourth Embodiment) Fig. 10 illustrates the structure of a communication network according to a fourth embodiment of the present invention. The communication network shown in Fig. 10 has approximately the same structure as that shown in Figs. 1 to 3. Therefore, in Fig. 10, the same reference numerals are assigned to structures corresponding to the structures shown in Figs. 1 to 3 and hence, the description thereof is not repeated. That is, a master station 1 1 shown in Fig. 10 comprises a table 16 1 , whose illustration is omitted, similarly to that shown in Figs. 1 to 3. Fig. 11 is a block diagram showing the structures of a token packet sending/analysis executing device 16 and a switch 18 1 in the master station 1 1 shown in Fig. 10. Fig. 12 is a block diagram showing the structures of analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 in slave stations 1 2 to 1 4 shown in Fig. 10. In Figs. 11 and 12, each of the token packet sending/analysis executing device 16 and the analysis executing devices 17 2 to 17 4 comprises a sending and receiving control portion 111, a transmission buffer 112, a receiving buffer 113, a packet detecting portion 114, and a communication request packet output portion 115. Further, in Fig. 11, the token packet sending/analysis executing device 16 further comprises a token packet output portion 116. As apparent from comparison between Figs. 11 and 12, the difference between the master station 1 1 and the slave stations 1 2 to 1 4 is the presence or absence of the token packet output portion 116 and the table 16 1 . Fig. 13 is a diagram showing on a time base packets transmitted on a transmission path 15 in the communication network shown in Fig. 10. In Fig. 13, an inquiry token packet 131, a token packet for asynchronous data 132, a token packet for isochronous data 133, a packet for asynchronous data 134, a packet for isochronous data 135, and a communication request packet (not shown) are transmitted to the transmission path 15 shown in Fig. 10. The token packet for asynchronous data 132 and the packet for asynchronous data 134 as well as the token packet for isochronous data 133 and the packet for isochronous data 135 are used in the case of asynchronous data communication and isochronous data communication described in the first embodiment and hence, the description thereof is simplified. The inquiry token packet 131 is a token packet for collecting communication requests of the respective stations 1, and a transmission bandwidth immediately after the inquiry token packet 131 is previously divided into a plurality of slots 136. Each of the stations 1 shown in Fig. 10 is previously assigned any one of the plurality of slots 136. Each of the stations 1 sends out the communication request packet to the slot assigned to itself. The procedure for data communication in the communication network constructed as described above will be described. The switch 18 1 in the master station 1 1 (see Fig. 11) and the switches 18 2 to 18 4 in the slave stations 1 2 to 1 4 (see Fig. 12) are in a state where the transmission path 15 is connected (on the side of terminals A) when the stations send out no packets. The switch 18 1 in the master station 1 1 and the switches 18 2 to 18 4 in the slave stations 1 2 to 1 4 disconnect the transmission path 15 (on the side of terminals B) only when the stations send out the packets (the packets other than the communication request packet), and connect the transmission path 15 again (on the side of the terminals A) immediately after packets which sent out by the station returns upon circulating around the transmission path 15. The basic operations are the same as those described in the first embodiment. As to the communication request packet, the stations 1 1 to 1 4 immediately return the switches 18 1 to 18 4 , after sending out the communication request packet. The token packet output portion 116 in the master station 1 1 sends out to the transmission path 15 the token packet for asynchronous data 132 for establishing asynchronous data communication, the token packet for isochronous data 135 for establishing isochronous data communication, or the inquiry token packet 131 for each predetermined time period. Examples of a sequence of sending of tokens from the master station 1 1 include a sequence for isochronous data communication and a sequence for asynchronous data communication. In the sequence for isochronous data communication, the token packet for isochronous data 135 is sent out. On the other hand, in the sequence for asynchronous data communication, the token packet for asynchronous data 132 or the inquiry token packet 131 is sent out. In the case of the sequence for asynchronous data communication, the sending and receiving control portion 111 in the master station 1 1 (see Fig. 11) connects the switch 18 1 to the terminal B to disconnect the transmission path 15, and then sends out the produced inquiry token packet 131 (see Fig. 13) to the transmission path 15 from the token packet output portion 116. In each of the stations 1 1 to 1 4 , the inquiry token packet 131 transmitted from the transmission path 15 on the upstream side is detected by the packet detecting portion 114. The sending and receiving control portion 111 in each of the stations 1 1 to 1 4 sends out, when the inquiry token packet 131 is detected, and when a communication request of asynchronous data to be transmitted is stored in the transmission buffer 112 in the station, a communication request packet representing the communication request of the asynchronous data from the communication request packet output portion 115 to the transmission path 15 on the downstream side using the slot for a communication request 136 previously assigned to the station. On the other hand, the sending and receiving control portion 111 in each of the stations 1 1 to 1 4 performs nothing or sends out, even when the inquiry token packet 131 is detected, and when a communication request of asynchronous data to be transmitted is not stored in the transmission buffer 112 in the station, a packet not representing the communication request of the asynchronous data to the transmission path 15 on the downstream side. The packet detecting portion 114 in the master station 1 1 detects that the inquiry token packet 131 returns upon circulating around the transmission path 15, and detects and analyzes the communication request packet transmitted immediately after the inquiry token packet 131. Consequently, the master station 1 1 can receive the communication request packets in the stations 1 which are sent out to a plurality of slots for a communication request 136, and successively sends out, when there exits a station 1 which has sent a communication request, the token packet for asynchronous data 132 in which the station 1 which has sent a communication request is designated as a sending station to the transmission path 15 on the downstream side in the subsequent sequence for asynchronous data communication. The station 1 which has sent a communication request can send out asynchronous data stored in the transmission buffer 112 in the station using the token packet for asynchronous data 132. The master station 1 1 sends out, after finishing outputting the token packets for asynchronous data 132 corresponding to all the stations 1 which has sent the communication request, the inquiry token packet 131 again. By repeating the foregoing operations, asynchronous data communication between the stations is established. Fig. 14 is a flow chart showing the procedure for sending of the token packets from the master station 1 1 shown in Fig. 10. The master station 1 1 judges whether or not a sequence to be currently established is a sequence for asynchronous data communication (step S141). In the case of the sequence for asynchronous data communication, there exists no station 1 which has sent the communication request in the initial state (step S143), so that the master station 1 1 sends out the inquiry token packet 131 to the transmission path 15 on the downstream side in the above-mentioned procedure (step S145). Consequently, the master station 1 1 can receive communication request packets in the stations 1 which are sent out to the slots for a communication request 136 in the above-mentioned procedure. The master station 1 1 sends out, when there exits a station 1 sending out the communication request packet, the token packet for asynchronous data 132 in which the station 1 is designated as a sending station to the transmission path 15 in the subsequent sequence for asynchronous data communication (step S144). The master station 1 1 sends out, when it finishes sending out the token packet for asynchronous data 132 in each of the stations 1 sending out the communication request packets, the inquiry token packet 131 again in the subsequent sequence for asynchronous data communication (step S145). The master station 1 1 sends out the token packet for isochronous data 133 when a sequence to be currently established is not the sequence for asynchronous data communication in the step S141 (step S142). The master station 1 1 repeats the procedure in the steps S141 to S145 described above. The master station 1 1 sends out the inquiry token packet 131, the token packet for asynchronous data 132, or the token packet for isochronous data 133 for each predetermined time period, as described above. Each of the stations 1 thus outputs the communication request packet to the assigned slot for a communication request 136, provided that it holds asynchronous data to be transmitted in response to the inquiry token packet 131 sent out by the master station 1 1 . The master station 1 1 sends out the token packet for asynchronous data 132 in which the station 1 sending out the communication request packet is designated as a sending station in the subsequent sequence for asynchronous data communication. Consequently, the master station 1 1 need not send out an unnecessary token packet for asynchronous data 132, and can reliably send out a token packet for asynchronous data in which the station 1 holding asynchronous data to be transmitted is designated as a sending station. This makes it possible to establish fair and efficient asynchronous data communication between the stations 1. Each of the stations 1 1 to 1 4 designates an actual ID of the station 1 which should receive asynchronous data using the communication request packet sent out, making it possible for the master station 1 1 to send out the token packet for asynchronous data 132 in which the station 1 designated by the actual ID is designated as a receiving station. Simultaneously, each of the stations 1 1 to 1 4 sends out an actual ID of the station when it sends out the communication request packet, also making it possible for the master station 1 1 to easily send out the token packet for asynchronous data 132. Furthermore, if information representing the amount of asynchronous data to be sent out and/or the priority of the asynchronous data to be sent out is added to the communication request packet sent out by each of the stations 1 1 to 1 4 , the master station 1 1 can send out the token packet for asynchronous data 132 corresponding to the amount and/or the priority held by each of the stations 1 1 to 1 4 . (Fifth Embodiment) Fig. 15 illustrates the structure of a communication network according to a fifth embodiment of the present invention. The communication network according to the present embodiment has approximately the same structure as the communication network according to the fourth embodiment. Therefore, in the structure shown in Fig. 15, the same reference numerals are assigned to structures corresponding to the structures shown in Fig. 10 and hence, the description thereof is not repeated. That is, a master station 1 1 shown in Fig. 15 comprises a table 16 1 , whose illustration is omitted, similarly to that shown in each of Figs. 1 to 3, and a token packet sending/analysis executing device 161 in the master station has the same structure as that shown in Fig. 11. Slave stations 1 2 to 1 4 shown in Fig. 15 have the same structures as those shown in Fig. 12. Packets transmitted on a transmission path 15 in the communication network according to the fifth embodiment is the same as those shown in Fig. 13 if they are illustrated on a time base and hence, Fig. 13 will be incorporated in the following description. The significant difference between the fifth embodiment and the fourth embodiment is that all the stations 1 1 to 1 4 are classified into a plurality of groups. More specifically, each of the stations 1 1 to 1 4 belongs to only one group, and is assigned a group ID for identifying the group to which the station itself belongs. In Fig. 15, it is assumed that the stations 1 1 and 1 2 belong to a group A (a group ID ; #A), and the stations 1 3 and 1 4 belong to a group B (a group ID ; #B). The procedure of data communication in the communication network constructed as described above will be described. The procedure for data communication in the communication network according to the fifth embodiment is partially the same as the above-mentioned procedure for data communication in the communication network according to the fourth embodiment. Description is now made mainly with respect to the difference between them. In the case of a sequence for asynchronous data communication, the master station 1 1 sends out a produced inquiry token packet 131 (see Fig. 13) to the transmission path 15. A group ID ; #A is set in the inquiry token packet 131 currently sent out. The stations 1 1 to 1 4 detect the inquiry token packet 131 transmitted from the transmission path 15. However, only the stations 1 1 and 1 2 receive, because the group ID designated by the currently detected inquiry token packet and the group ID of the stations identify with each other, the token packet. Each of the stations 1 1 and 1 2 sends out, provided that it receives the inquiry token packet 131, and it holds a communication request of asynchronous data to be transmitted, a communication request packet including the communication request of the asynchronous data and an actual ID of a station 1 which is a destination of transmission of the asynchronous data to the transmission path 15 using a slot 136 for a communication request packet assigned to the station (see Fig. 13). The master station 1 1 detects that the inquiry token packet 131 returns upon circulating around the transmission path 15, and receives a communication request packet transmitted immediately after the inquiry token packet 131. The master station 1 1 successively sends out, provided that there exists a station 1 which has sent out a communication request of asynchronous data, a token packet for asynchronous data 132 in which a sending station and a receiving station are designated using actual IDs of the station 1 which has sent out the communication request of the asynchronous data and the station 1 which is a destination of transmission of the asynchronous data to the transmission path 15 in the subsequent sequence for asynchronous data communication. The station 1 which has sent the communication request can send out the asynchronous data held therein in response to the token packet for asynchronous data 132. The master station 1 1 sends out token packets for asynchronous data 132 corresponding to all the stations 1 which has sent out communication requests. In the subsequent sequence for asynchronous data, the master station 1 1 sends out a newly produced inquiry token packet 131 (see Fig. 13) to the transmission path 15. A group ID ; #B is set in the inquiry token packet 131 currently sent out. The stations 1 1 to 1 4 detect the inquiry token packet 131 transmitted from the transmission path 15. However, only the stations 1 3 and 1 4 receive, because the group ID designated by the currently detected inquiry token packet and the group ID of the stations identify with each other, the token packet. Each of the stations 1 3 and 1 4 sends out, provided that it receives the inquiry token packet 131, and it holds a communication request of asynchronous data to be transmitted, a communication request packet including the communication request of the asynchronous data and an actual ID of a station 1 which is a destination of transmission of the asynchronous data to the transmission path 15 using a slot for a communication request 136 assigned to the station (see Fig. 13). The subsequent procedure for communication is the same as that described above and hence, the description thereof is not repeated. The master station 1 1 repeats the foregoing operations, whereby asynchronous data communication is performed. Fig. 16 is a flow chart showing the procedure that the master station 1 1 sends out the token packets shown in Fig. 15. The master station 1 1 judges whether or not a sequence to be currently established is a sequence for asynchronous data communication (step S161). In the initial state of the sequence for asynchronous data communication, there exists no station 1 which has sent out a communication request (step S163), so that the master station 1 1 sends out the inquiry token packet 131 in which the group ID ; #A is designated to the transmission path 15 (step S165), and then changes the group ID to be designated by an inquiry token packet 131 to be subsequently sent out into #B (step S166). Consequently, the master station 1 1 can receive communication request packets transmitted from the stations belonging to the group A (the stations 1 1 and 1 2 ) in the above-mentioned procedure. The master station 1 1 sends out, when there exits a station 1 which has sent out the communication request packet, the token packet for asynchronous data 132 in which a sending station and a receiving station are designated to the transmission path 15 in the subsequent sequence for asynchronous data communication (step S164). The master station 1 1 belongs to the group A, and sends out, when it finishes sending out the token packet for asynchronous data 132 in each of the stations 1 which have sent out communication request packets (step S163), the inquiry token packet 131 in which the group ID ; #B is designated to the transmission path 15 (step S165). The designated group ID is changed every time when the inquiry token packet is sent out. Consequently, asynchronous data communication is established fairly for each group. The master station 1 1 sends out the token packet for isochronous data 133 when a sequence to be currently established is not the sequence for asynchronous data communication in the step S161 (step S162). The master station 1 1 repeats the procedure in the steps S161 to S166 as described above. The master station 1 1 sends out the inquiry token packet 131, the token packet for asynchronous data 132, or the token packet for isochronous data 133 for each predetermined time period, as described above. Each of the stations 1 1 to 1 4 can easily send out, in sending out the communication request packet, the token packet for asynchronous data 132 if it sends out an actual ID of the station together with the communication request packet because the master station 1 1 can easily designate a sending station. If information representing the amount of asynchronous data to be sent out and/or the priority of the asynchronous data to be sent out is added to the communication request packet sent out by each of the stations 1 1 to 1 4 , the master station 1 1 can send out the token packet for asynchronous data 132 corresponding to the amount and/or the priority held by each of the stations 1 1 to 1 4 . (Sixth Embodiment) A communication network according to a sixth embodiment has approximately the same structure as the communication network according to the fourth or fifth embodiment (see Fig. 10 or 15). In the above-mentioned fourth or fifth embodiment, the master station 1 1 merely sends out an inquiry token packet 131, as shown in Fig. 13. In the sixth embodiment, however, a master station 1 1 sends out an identification signal in order that each of stations 1 can easily identify the above-mentioned slot 136 after an inquiry token packet 131. Fig. 17 illustrates on a time base the inquiry token packet 131 and identification signals 171 which are transmitted on a transmission path 15 in the communication network according to the sixth embodiment. In Fig. 17, each of the identification signals 171 is sent out from the master station 1 1 so as to represent the head of each of slots 136 subsequently to the inquiry token packet 131. Consequently, each of the stations 1 detects, when it detects the inquiry token packet 131, the identification signals 171 succeeding the inquiry token packet 131, so that a communication request packet (as described above) can be easily inserted into the slot 136 assigned to the station. Further, the master station 1 1 also detects the identification signals 171 succeeding the inquiry token packet circulating around the transmission path 15, to recognize the communication request packet. In the foregoing description, each of the identification signals 171 shall be sent out to the transmission path 15 so as to represent the head of each of the slots 136. However, it may represent the end of the slot for a communication request 136, in which case the same effect is obtained. Although in the above-mentioned sixth embodiment, description was made of a communication network which is constituted by one loop, the sixth embodiment is also applicable in a communication network which is constituted by a plurality of loops. (Seventh Embodiment) In the fourth embodiment, description was made of the structures of the token packet sending/analysis executing device 16 and the switch 18, in the master station 1 1 and the analysis executing devices 17 2 to 17 4 and the switches 18 2 to 18 4 in the slave stations 1 2 to 1 4 with reference to Figs. 11 and 12. Fig. 18 is a block diagram showing the other structures of a token packet sending/analysis executing device 16 and a switch 18 1 in a master station 1 1 and analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 in slave stations 1 2 to 1 4 . In Fig. 18, the token packet sending/analysis executing device 16 comprises a sending and receiving control portion 181, a transmission buffer 182, a receiving buffer 183, a packet detecting portion 184, a communication request packet output portion 185, a token packet output portion 186, a register for a destination of transmission 187, and a register for a source of transmission 188. Each of the analysis executing devices 17 2 to 17 4 in the slave stations 1 2 to 1 4 differ from the token packet sending/analysis executing device 16 in the master station 1 1 in that it does not comprise the token packet output portion 186 and a table 16 1 . In Fig. 18, structures corresponding to the structures shown in Figs. 11 and 12 are assigned difference reference numerals and the same names and hence, the description thereof is not repeated. In transmitting asynchronous data, an equipments 1820 connected to the outside of the token packet sending/analysis executing device 16 or the analysis executing devices 17 2 to 17 4 write asynchronous data to be transmitted to the transmission buffer 182 and write an actual ID of a station to be a destination of transmission of the asynchronous data in the register for a destination of transmission 187. The sending and receiving control portion 181 in each of the stations sends out, after the asynchronous data to be transmitted and the actual ID of the station to be the destination of transmission start to be stored in the transmission buffer 182 and the register for a destination of transmission 187, a communication request packet to a transmission path 15 through the switch 18 in the station from the communication request packet output portion 186 in response to an inquiry token packet sent out by the master station 1 1 . Thereafter, the station 1 sends out the asynchronous data stored in the transmission buffer 182 to the transmission path 15 through the switch 18 in the station using a token packet for asynchronous data which is sent out by the master station 1 1 and in which the station is designated as a sending station. Furthermore, the asynchronous data is received by a token packet for asynchronous data which is sent out by the master station 1 1 and in which the station is designated as a receiving station. The asynchronous data received by each of the stations is held once in the receiving buffer 183. When asynchronous data corresponding to one packet is held in the receiving buffer 183, an actual ID of a station transmitting the asynchronous data (a sending station) is stored in the register for a source of transmission 188. The equipment 1820 connected to the receiving station can read out, after the asynchronous data corresponding to one packet and the actual ID of the sending station are stored in the receiving buffer 183 and the register for a source of transmission 188, the received asynchronous data and the actual ID of the sending station. In transmitting asynchronous data, the asynchronous data is thus stored in the transmission buffer 182, and an actual ID of a station which should receive the data (a receiving station) is stored in the register for a destination of transmission 187, after which each of the sending stations responds to the inquiry token packet sent out by the master station 1 1 . On the other hand, in receiving asynchronous data, the equipment 1820 can read out, when the received asynchronous data corresponding to one packet is stored in the receiving buffer 183, and the actual ID of the station which is a source of transmission of the data (a sending station) is stored in the register for a source of transmission 188, the asynchronous data and the actual ID. Consequently, it is possible to transmit the asynchronous data to each of the stations 1 and receive the asynchronous data from the station 1. (Eighth Embodiment) Fig. 19 illustrates the structure of a communication network according to an eighth embodiment of the present invention. In Fig. 19, a plurality of stations (four stations 1 1 to 1 4 are illustrated) are connected so as to be capable of communicating with each other by a loop-shaped transmission path 15. Further, the station 1 1 functions as a master station, and the other stations 1 2 to 1 4 function as slave stations (which are hereinafter referred to as the master station 1 1 and the slave stations 1 2 to 1 4 ). The master station 1 1 comprises a token packet sending/analysis executing device 16 and a switch 18 1 . The slave stations 1 2 to 1 4 comprise analysis executing devices 17 2 to 17 4 and switches 18 2 to 18 4 . The procedure for automatically setting an actual ID in each of the stations 1 accommodated in the network in the communication network constructed as above will be described with reference to Figs. 20 to 28. First, Fig. 20 illustrates the initial state such as the time of turning on the power supply of the communication network according to the present embodiment. In the initial state, "0x3E" is set as initial IDs in the stations 1 1 to 1 4 , and the switches 18 1 to 18 4 are further connected to terminals A, respectively. The initial IDs are identifiers which are common among the stations 1 in the communication network. The common initial ID need not be necessarily set in the master station 1 1 , and an actual ID (for example, "0x00") may be set from the beginning, unlike the other slave stations 1 2 to 1 4 . The master station 1 1 first produces a token packet for switching using the token packet sending/analysis executing device 16, and sends out the token packet to a transmission path 15 upon connecting the switch 18 1 to a terminal B. In the token packet for switching, a station to be a source of transmission of data (a sending station) and a station to be a destination of transmission of the data (a receiving station) are designated using the initial ID ("0x3E"). Therefore, the station 1 having the initial ID on the transmission path 15 connects the switch 18 (switches 18 1 to 18 4 in all the stations 1 1 to 1 4 in this description) to the terminal B in an attempt to transmit a data packet, as shown in Fig. 21. The process is hereinafter referred to as a switching process. The master station 1 1 must connect, when it is assigned the actual ID from the beginning, the switch 18 1 in the station to the terminal B after sending the token packet for switching and receiving the sent token packet which circulated. By the switching process, the master station 1 1 sends out an actual ID setting command 221 to the transmission path 15 as one type of data packet as shown in Fig. 22 while the switches 18 1 to 18 4 in all the stations 1 1 to 1 4 are connected to the terminals B. The actual ID setting command 221 is sent out in order to set the actual ID of a receiving station to "0x01". Since all the stations 1 1 to 1 4 on the transmission path 15 are designated as receiving stations, and all the switches 18 1 to 18 4 are connected to the terminals B, the actual ID setting command currently sent out is received only by the slave station 1 2 which is a station subsequent to the master station 1 1 , not to be transmitted to the stations 1 on the downstream side of the slave station 1 2 . Consequently, only the slave station 1 2 performs processing conforming to the received actual ID setting command 221, so that "0x01"is fixedly set therein as an actual ID (see Fig. 22). A process as shown in Fig. 22 is hereinafter referred to as an actual ID setting command sending process. The station 1 in which the switch 18 is connected to the terminal B using the current token packet for switching (all the stations 1 1 to 1 4 in this description) connects the switch 18 to the terminal A after an elapse of a predetermined time period. The master station 1 1 sends out the actual ID setting command 221, and then judges whether or not the actual ID setting command 221 returns again upon circulating around the transmission path 15. Since the actual ID setting command is not transmitted to the stations 1 on the downstream side of the slave station 1 2 , it is not returned to the master station 1 1 . The master station 1 1 considers, when the actual ID setting command 221 sent out by itself does not return within a predetermined time period, that the actual ID setting command 221 is received by any one of the slave stations 1 2 to 1 4 , and waits for an actual ID setting acknowledgment transmitted from the slave station 1. In the case, the master station 1 1 designates a sending station using the actual ID ("0x01") and sends out a token packet for asynchronous data in which the master station 1 1 is designated as a receiving station to the transmission path 15 in order that the slave station 1 in which the actual ID is set can transmit the actual ID setting acknowledgment. In the token packet for asynchronous data, when the master station 1 1 has an initial ID ("0x3E") which is common to those of the slave stations 1 2 to 1 4 , the master station 1 1 may be designated as a receiving station using the common initial ID. On the other hand, when the master station 1 1 has an actual ID from the beginning, the master station 1 1 may be designated as a receiving station using the actual ID ("0x00"). The slave station 1 2 in which the actual ID ("0x01") is set sends out, when data transmission is enable by a token packet for asynchronous data, an actual ID setting acknowledgment indicating that the actual ID is correctly set as a data packet. The actual ID setting acknowledgment is received by the station 1 having an initial ID "0x3E" or an actual ID "0x00". Although the actual ID setting acknowledgment is also received, when the master station 1 1 has the common initial ID, by the other slave station 1 having the initial ID (the slave stations 1 3 and 1 4 at the present time point), the contents thereof exert no adverse effect because they are interpreted only by the master station 1 1 . When the master station 1 1 receives the actual ID setting acknowledgment from the slave station 1 2 as described above, it is recognized that the actual ID "0x01"is correctly set in any one of the slave stations 1. This process is hereinafter referred to as an actual ID confirmation process. The master station 1 1 recognizes, when it does not receive the actual ID setting acknowledgment, that the actual ID is not correctly set in any of the slave stations 1. In this case, the above-mentioned switching process to the actual ID confirmation process are repeated in the communication network. When the master station 1 1 receives the above-mentioned actual ID setting acknowledgment, the subsequent switching process is carried out in the communication network. The master station 1 1 first determines an actual ID to be currently set. The actual ID to be currently set is so selected as not to be overlapped with the actual ID previously set ("0x01"). For example, the master station 1 1 increments the actual ID previously set, to select the actual ID to be currently set. By following this selecting method, the actual ID to be subsequently set is "0x02". The master station 1 1 produces the same token packet for switching as described above, and sends out the produced token packet to the transmission path 15 after selecting the actual ID. Also in the current token packet for switching, both a sending station and a receiving station are designated using the initial ID ("0x3E"). Therefore, the switch 18 in the station 1 having the initial ID (the switches 18 1 , 18 3 and 18 4 in the stations 1 1 , 1 3 and 1 4 in this description) on the transmission path 15 is connected to the terminal B, as shown in Fig. 23. The switch 18 2 in the station 1 2 remains connected to the terminal A because the actual ID is set therein. While the switches 18 1 , 18 3 and 18 4 in the stations 1 1 , 1 3 and 1 4 are connected to the terminals B by the switching process, the master station 1 1 sends out the actual ID setting command 221 to the transmission path 15, as shown in Fig. 24. The current actual ID setting command 221 is sent out in order to set an actual ID of a predetermined receiving station to "0x02". Since the stations 1 1 ,1 3 and 1 4 on the transmission path 15 are designated as receiving stations, and the switch 18 2 is connected to the terminal A, the actual ID setting command 221 currently sent out is received only by the slave station 1 3 through the slave station 1 2 , not to be transmitted to the stations 1 on the downstream side of the slave station 1 3 . Consequently, only the slave station 1 3 performs processing conforming to the received actual ID setting command 221, so that "0x02" is fixedly set therein as an actual ID (see Fig. 24). The station 1 in which the internal switch 18 is connected to the terminal B using the current token packet for switching (the stations 1 1 , 1 3 and 1 4 in this description) connects the switch 18 to the terminal A after an elapse of a predetermined time period. The master station 1 1 judges whether or not the actual ID setting command sent out by itself returns upon circulating around the transmission path 15. Since the current actual ID setting command does not return, however, the master station 1 1 waits for the actual ID setting acknowledgment transmitted from the slave station 1 receiving the actual ID setting command. In the case, the master station 1 1 designates a sending station using the actual ID ("0x02") in order that the slave station 1 in which the actual ID is currently set can transmit the actual ID setting acknowledgment, and sends out to the transmission path 15 the token packet for asynchronous data in which the master station 1 1 is designated as a receiving station. The slave station 1 3 in which the actual ID ("0x02") is set sends out, when data transmission is enable by the token packet for asynchronous data, the actual ID setting acknowledgment as a data packet. The actual ID setting acknowledgment is received and interpreted only by the master station 1 1 as described above, so that it is recognized that the actual ID "0x02" is correctly set in any one of the slave stations 1 (an actual ID confirmation process). When the master station 1 1 does not receive the actual ID setting acknowledgment as described above, the above-mentioned switching process to the actual ID confirmation process are repeated in the communication network. When the master station 1 1 receives the above-mentioned actual ID setting acknowledgment, the subsequent switching process to the actual ID confirmation process are carried out in the communication network. The master station 1 1 first determines an actual ID to be currently set. The actual ID to be currently set is so selected as not to be overlapped with the actual IDs ("0x01" and "0x02") previously set. The master station 1 1 increments the actual ID "0x02" previously set, for example, to select an actual ID "0x03" to be currently set. As apparent from the foregoing, the actual ID "0x03" is fixedly set in the slave station 1 4 . When the master station 1 1 receives the above-mentioned actual ID setting acknowledgment, the subsequent switching process to the actual ID confirmation process are carried out in the communication network. The master station 1 1 first determines an actual ID to be currently set. The actual ID to be currently set is so selected as not to be overlapped with the actual IDs ("0x01", "0x02" and "0x03") previously set. For example, the master station 1 1 increments the actual ID "0x03" previously set, to select an actual ID "0x04" to be currently set. The master station 1 1 produces the same token packet for switching as described above, and sends out the produced token packet to the transmission path 15 after selecting the actual ID to be currently set. Also in the current token packet for switching, both a sending station and a receiving station are designated using an initial ID ("0x3E"). Therefore, the switch 18 in the station 1 having the initial ID (the switch 18 1 in the station 1 1 in this description) on the transmission path 15 is connected to the terminal B, as shown in Fig. 25. The switches 18 2 to 18 4 in the stations 1 2 to 1 4 remain connected to the terminals A because the actual IDs are set therein. While the switch 18 1 in the station 1 1 is connected to the terminal B by the switching process, the master station 1 1 sends out the actual ID setting command 221 to the transmission path 15, as shown in Fig. 26. The actual ID setting command 221 is sent out in order to set an actual ID of a predetermined receiving station to "0x04". Since the station 1 1 on the transmission path 15 is designated as a receiving station, and the switches 18 2 to 18 4 are connected to the terminals A, the actual ID setting command 221 currently sent out is received only by the master station 1 1 through the slave stations 1 2 to 1 4 . Consequently, only the master station 1 1 performs processing conforming to the received actual ID setting command 221, so that "0x04" is fixedly set therein as an actual ID (see Fig. 26). The station 1 in which the switch 18 is connected to the terminal B using the current token packet for switching (the station 1 1 in this description) connects the switch 18 to the terminal A after an elapse of a predetermined time period. The master station 1 1 judges whether or not the actual ID setting command 221 sent out by itself is returned upon circulating around the transmission path 15. The master station 1 1 recognizes that the actual IDs are fixedly set in all the stations 1 accommodated in the communication network because the current actual ID setting command 221 returns, to complete the setting of the actual IDs (see Fig. 27). In the communication network, isochronous data communication or the like described in the first embodiment, for example, will be thereafter established. As described in the foregoing, if the token packet for switching and the actual ID setting command are repeatedly transmitted, it is possible to automatically set the actual IDs in the stations from the station subsequent to the master station 1 1 to the station on the downstream side (the slave stations 1 2 , 1 3 and 1 4 and the master station 1 1 in this order). Fig. 28 is a block diagram showing the structures of the token packet sending/analysis executing device 16 and the switch 18 1 in the master station 1 1 shown in Fig. 19 and the structures of the analysis executing devices 17 2 to 17 4 and the switches 18 2 to 18 4 in the slave stations 1 2 to 1 4 shown in Fig. 19. The token packet sending/analysis executing device 16 differs from the analysis executing devices 17 2 to 17 4 in that it has a function of sending out token packets. It will be previously noted that the function of sending out token packets is not illustrated in Fig. 28. In Fig. 28, each of the token packet sending/analysis executing device 16 and the analysis executing devices 17 2 to 17 4 comprises a storage device 281, a sending and receiving control portion 282, and a packet detecting portion 283. Although the storage device 281 initially stores an initial ID ("0x3E"), it stores an actual ID fed by the master station 1 1 through the above-mentioned setting of actual IDs. The packet detecting portion 283 detects a special pattern included in a token packet (a token packet for switching, for example) sent out for each predetermined time period from the master station 1 1 , and detects an actual ID setting command. The sending and receiving control portion 282 in the station receives an actual ID setting command and transmits an actual ID setting acknowledgment while a clock of the station is synchronized with a clock of the master station 1 1 on the basis of the special pattern detected by the packet detecting portion 283. This establishes the synchronization between clocks of all the stations 1 1 to 1 4 and a clock of the communication network. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: A communication network in which a plurality of stations (1 1 , 1 2 , 1 3 , 1 4 ) are connected to each other by a loop-shaped transmission path (15), where any one of the plurality of stations functions as a master station (1 1 ), and asynchronous data communication relating to asynchronous data not requiring real time properties and isochronous data communication relating to isochronous data requiring the real time properties can be established, wherein first identifiers are uniquely and fixedly assigned to each of said plurality of stations connected to said network, and are used to identify stations for sending and receiving of asynchronous data, characterized in that second identifiers are assigned and updated dynamically to said plurality of stations to identify stations for sending and receiving of isochronous data, said second identifiers being non-unique, said second identifiers being dynamically assigned or updated to said plurality of stations through asynchronous communication informing each station of their second identifier, said master station (1 1 ) sends out to said transmission path (15) for each predetermined time period a token packet for isochronous data (41) in which a second identifier identifying a sending station (1 1 , 1 2 , 1 3 , 1 4 ). transmitting said isochronous data and a second identifier identifying at least one receiving station (1 1 , 1 2 , 1 3 , 1 4 ) receiving the isochronous data are designated when said isochronous data communication should be established, so that one or more stations (1 1 , 1 2 , 1 3 , 1 4 ) can be designated to receive the isochronous data, and each of said stations analyzes the second identifiers in said token packet for isochronous data communication received through said transmission path, sends out an isochronous data packet held therein to said transmission path if said second identifier identifying a sending (station 1 1 , 1 2 , 1 3 , 1 4 ) matches with said second identifier assigned to said station, and receives the isochronous data packet sent out to said transmission path if said second identifier identifying a receiving station (1 1 , 1 2 , 1 3 , 1 4 ) matches with said second identifier assigned to said station. The communication network according to claim 1, wherein when said isochronous data transmitted by said sending station (1 1 , 1 2 , 1 3 , 1 4 ) is received by said plurality of receiving stations (1 1 , 1 2 , 1 3 , 1 4 ) in said isochronous data communication, only one receiving station (1 1 , 1 2 , 1 3 , 1 4 ) of said plurality of receiving stations (1 1 , 1 2 , 1 3 , 1 4 ) sends out to said transmission path (15) a response signal (71) for controlling the transmission of said isochronous data by said sending station (1 1 , 1 2 , 1 3 , 1 4 ) and/or reporting the acknowledgement of said isochronous data communication by the receiving station (1 1 , 1 2 , 1 3 , 1 4 ). The communication network according to claim 2, wherein said only one receiving station (1 1 , 1 2 , 1 3 , 1 4 ) is set by said asynchronous data communication. The communication network according to claim 1, wherein when said isochronous data transmitted by said sending station (1 1 , 1 2 , 1 3 , 1 4 ) is received by said plurality of receiving stations (1 1 , 1 2 , 1 3 , 1 4 ) in said isochronous data communication, said plurality of receiving stations (1 1 , 1 2 , 1 3 , 1 4 ) respectively send out to said transmission path (15) response signals (71) for controlling the transmission of said isochronous data by said sending station (1 1 , 1 2 , 1 3 , 1 4 ) and/or reporting the acknowledgement of said isochronous data communication by the receiving stations (1 1 , 1 2 , 1 3 , 1 4 ), and said sending station (1 1 , 1 2 , 1 3 , 1 4 ) receives only the response signal (71) sent out from the only one receiving station (1 1 , 1 2 , 1 3 , 1 4 ) of said plurality of receiving stations (1 1 , 1 2 , 1 3 , 1 4 ).

Inventors: IKEDA TOSHIHISA, KUROSAKI TOSHIHIKO, MIZUGUCHI YUJI, MORIGUCHI KENICHI, OGA TOSHIO, SAKAI TAKAHISA, IKEDA, TOSHIHISA, KUROSAKI, TOSHIHIKO, MIZUGUCHI, YUJI, MORIGUCHI, KENICHI, OGA, TOSHIO, SAKAI, TAKAHISA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H04L12
IPC codes: H04L12