关键词: RS485总线, 嵌入式结点集群, 中继层规约, 可靠控制, 集群化管理

Abstract:

Since the current network design of RS485 has limited the communication node number and scalability, a networking scheme for embedded control node cluster and a corresponding control protocol based on RS485 bus were proposed. By adding repeaters between the master and leaves, the networking scheme expanded the number of nodes on the RS485 bus to 27000, and constituted a cluster of embedded network nodes in one-to-many control mode. For this characteristic, the Modbus protocol was extended and the repeater layer stipulation was appended. Unlike traditional way that simply increases the length of the physical address, the new protocol used the method of local-addressing to break the restriction on the number of nodes which created by Modbus address length, and introduced the node scanning and error feedback mechanism. As a result, a reliable node control for the master controller was achieved. The control protocol not only keep the original simplicity of Modbus protocol, but also owns the features of portability and scalability, and it is easy to implement on a micro control unit. When the extended protocol was adopted, the turnaround delay increased by 10.36% with one layer of repeaters, while two layers of repeaters caused the delay increased by 69.9%. With two layers of repeaters, the total delay increased by 2.4 times than that of the original Modbus system. Nevertheless, the average delay in practical two-layer relay system was still controlled below 70 ms. The results show that the scheme can attain the goal of clustering management of embedded nodes at the expense of some real-time performance.

Key words: RS485 bus, embedded control node cluster, repeater layer stipulation, reliable control, clustering management