There has been entertained a communication system in which Av apparatus, such as a video tape recorder (VTR), a monitor or a tuner, are connected to a bus for exchanging digital video signals or digital audio signals.
FIG. 8 shows an example of such communication system, which is provided with a root node 21, a leaf node 22, a branch node 23, a leaf node 24 and a leaf node 25. Input/output ports between the nodes 21 and 22, also referred to as nodes 21-22, hereinafter the same, the nodes 21-23, the nodes 23-24 and the nodes 23-25, are connected by two sets of twist pair cables. The nodes 21 to 25 are the digital VTRs, tuners or personal computers, as discussed above, each having one or more input/output ports. Each node 21 to 25 has an amplifier and a relay enclosed therein. The communication system shown in FIG. 8 is equivalent to a communication system in which the nodes 21 to 25 are connected to a bus 26.
The structure shown in FIG. 8 is a hierarchical structure in which the nodes 22 and 23 are connected in the lower layer relative to the node 21 and the nodes 24 and 25 are connected in the lower layer relative to the node 23. In other words, the node 21 is a master node for the nodes 22 and 23, while the node 23 is a master node for the nodes 24 and 25. The sequence for determining such hierarchical structure is now explained.
If the nodes 21-22, 21-23, 23-24 and 23-25 are connected by cables, the node only one input/output port of which is connected to an other node notifies the node to which it is connected that the latter node is the master node. In the case of FIG. 8, the nodes 24 and 25 notify the node 23 of the fact that the node 23 is the master node, while the node 22 notifies the node 21 of the fact that the node 21 is the master node.
The node plural input/output nodes of which are connected to other nodes notifies a node other than the node which has notified the firstly-stated node that the firstly-stated node is the master node that such other node is the master node. In the case of FIG. 8, the node 23 notifies the node 21 that the node 21 is the master node, while the node 21 notifies the node 23 that the node 23 is the master node. Since in such case the nodes 21, 23 notify each other that the counterpart node is the master node, the node which has made such notification first becomes the master node. FIG. 8 shows a case in which the node 21 has become the master node.
The sequence of according an address to each node is explained. Basically, the node address is accorded by the master node permitting an address to be accorded to a slave node. If there are plural slave nodes, addresses are accorded in the order of the smaller port numbers to which the slave nodes are connected.
In FIG. 8, in which the node 22 is connected to a port #1 of the node 21 and the node 23 is connected to a port #2 of the node 22, the node 21 permits an address to be accorded to the node 22. The node 22 accords the address (i) to itself and transmits data indicating that the address (i) has been accorded to itself over a bus 26. The node 21 then permits the node 23 to set its own address. The node 23 permits an address to be accorded to the node 24 connected to its port #1. The node 24 accords the address (ii) to itself. The node 23 permits an address to be accorded to a node 25 connected to its port #2. The node 25 accords an address (iii) to itself. After having accorded addresses to its slave node 24 and slave node 25, the node 23 accords an address (iv) to itself. After having accorded addresses to its slave node 22 and slave node 23, the node 21 accords an address (v) to itself.
With the present communication system, it is possible to carry out synchronous communication or continuous communication at a constant data rate and asynchronous communication for transmitting control commands, for example, non-periodically, that is whenever the necessity arises.
With the present communication system, communication is carried out at a communication cycle having a pre-set period, such as 125 .mu.s, as shown in FIG. 10. The communication cycle starts with a cycle start packet csp, followed by a period for transmitting a packet for synchronous communication. By affixing channel numbers 1, 2, 3, . . . N to the respective packets for synchronous communication, plural synchronous communication cycles may be carried out. For example, if the channel 1 is allocated to the communication from the node 22 to the node 23, communication is carried out by the node 22 transmitting the packet for synchronous communication having the channel number 1 directly after the cycle start packet csp and by the node 23 monitoring the bus 26 and fetching the packet for synchronous communication having the channel number 1. Similarly, the communication from the node 24 to the node 21 can be accorded to the channel 2, while the packet of a channel can be received by plural nodes.
If plural synchronous communication cycles are carried out, it is attempted to transmit the packets for synchronous communication over plural channels directly after the cycle start packet cps. In such case, the packet of synchronous communication of a channel is first transmitted by arbitration means (such as CSMA/CD) as determined by the bus 26. The packets of synchronous communication of other channels are then transmitted sequentially.
After termination of transmission of the packets of synchronous communication of all of the channels, the time interval until the next cycle start packet csp is employed for asynchronous communication. To the packets for asynchronous communication (packets A and B in FIG. 10) are affixed addresses of the transmitting node and the receiving node. The respective nodes fetch the packets having the addresses proper to the nodes affixed thereto.
Since the details of the above-described communication system are publicized as "IEEE P1394 Serial Bus Design Statement, they are not explained herein specifically.
In order for the above-described communication system to operate correctly, it is necessary for the respective synchronous communication packets to have different channel numbers, while it is necessary for the sum total of the communication time of the synchronous communication packets of the respective channels not to exceed the period of the synchronous communication. To this end, it is necessary to check before the start of synchronous communication of a node that the communication capacity necessary for the communication is available on the bus and to have an un-used channel allocated for the communication if there is any residual communication capacity in the bus.
For supervising the communication capacity and the channel number employed for synchronous communication, it is commonplace practice that one of the nodes connected to a bus become a bus management node and to effect required management. In such case, other nodes indicate the communication capacity desired to be employed to the bus management node, using the asynchronous communication packet, and require channels to be allocated to them. The bus management node checks if the communication capacity in use added to the communication capacity newly requested does not exceed the maximum communication capacity of the bus. If the sum is not in excess of the maximum communication capacity of the bus, the bus management node notices the channel number and the effect of permission of synchronous communication. If the sum is in excess of the maximum communication capacity of the bus, the bus management node notices that the channel allocation is not permitted. After termination of the synchronous communication, the management node is notified of the channel number and the channel capacity which will not be in use.
Since the bus supervision is in need of complex processing operations, it is commonplace practice with the communication system centered about e.g., a personal computer to use the personal computer as a bus management node and to perform the processing operations using the software possessed by the personal computer. However, if this method is employed for the communication system between the AV apparatus, such as a digital VTR, tuner or a monitor, it becomes necessary to interconnect an apparatus having powerful data processing functions, such as personal computer, to the bus, in addition to the AV apparatus, thus raising the cost of the communication system.
In view of the above problem, it is an object of the present invention to provide a method for realizing facilitated management of a bus in a system for carrying out synchronous communication between plural nodes connected to the bus.