1. Field of the Invention
This invention relates to a multiplex transmission system for carrying out a data transmission among a plurality of multiplex nodes connected to a common multiplex transmission line, and more particularly, to a multiplex transmission system using CSMA/CD (carrier sense multiple access/collision detection).
2.Description of the Related Art
In conventional multiplex transmission systems of this type, a plurality of terminals (multiplex nodes) are connected to a common multiplex transmission line (multiplex bus) formed of, e.g., a paired cable. One of the multiplex nodes sends a predetermined data frame to the multiplex bus in accordance with the CSMA/CD method, to thereby transmit data simultaneously to the other multiplex nodes. The data frame includes an acknowledge signal (ACK signal) area at an end thereof.
In such systems, it is known that each of the multiplex nodes that received the data frame returns an ACK signal to a bit position of the ACK signal area preassigned thereto.
The multiplex nodes each include a computer for communication, and these computers operate independently of one another and are able to send a data frame to the multiplex bus at desired timings. Therefore, a collision of transmitted messages can occur on the multiplex bus, and conventionally this is prevented by setting a priority sequence for the transmission of individual messages.
In accordance with the priority sequence thus set, each computer carries out a priority control to send a message without destroying same. Specifically, each computer carries out a data transmission in accordance with a nondestructive arbitration type CSMA/CD access method in which the transmission of a message with a lower priority is automatically interrupted and only the higher-priority message is continuously transmitted. When the data is properly received, each multiplex node returns an ACK signal to a location of address uniquely assigned thereto (respective bit area in the ACK signal area).
If a local error occurs in the network and the data received by a multiplex node is erroneous, then no ACK signal is returned from this multiplex node, and therefore, the sending multiplex node determines that an abnormal situation occurred, and retransmits the data. The sending multiplex node repeats the transmission of the data, e.g., three times at the maximum, until all of the registered multiplex nodes return the ACK signals. In prior art systems, all nodes have the function of determining whether or not ACK signals have been returned from the nodes and retransmitting data.
A sending multiplex node uses an ACK management function, i.e., if a node fails to return the ACK signal even though the data transmission has been repeated three times, such a node is regarded as being in trouble and is excluded from the registered nodes, and when an extra ACK signal is received, the node corresponding to this ACK signal is registered.
In this multiplex transmission system, since all of the nodes connected to the network return ACK signals, the ACK signal area occupies a large portion of the data frame if the number of nodes connected to the network is increased, and thus a problem arises in that the transmission efficiency is lowered during a data frame transmission. In other words, to ensure a certain transmission efficiency, the number of nodes connectable to the network is limited.