Design Of Management System For Large-scale Control Node Cluster

To break the limitation of traditional RS485 serial bus,a networking scheme for large-scale embedded node cluster based on RS485 bus was proposed,which expanded the number of nodes on the RS485 bus to 27000.According to the characteristics of this network,the Modbus protocol was extended to a cluster node communication and control protocol to achieve effective management and reliable control of the cluster nodes.On this basis,a complete information transfer strategy was built and finally the geographic surveying information was correctly mapped from the master controller to each terminal node.Firstly,reasonable physical links were established and a set of corresponding communication protocols were designed.By adding repeaters between the master and leaves,the networking scheme succeeded to increase the number of nodes on the RS485 bus,and constituted a cluster of embedded network nodes in one-to-many control mode.With the method of protocol extension,the repeater layer stipulation was appended to Modbus protocol.Unlike traditional way that simply increased the length of the physical address,the new protocol variant used the way of local-addressing to break the limitation that Modbus address length created for the number of nodes,and introduced the node scanning and error feedback mechanism.As a result,a reliable node control for the master controller was accomplished.Secondly,the transmission strategy of geographical surveying information was designed.Taking advantage of GDAL library,the UTM in combination with positive azimuthal projection transformation method was used on available remote sensing geographic data to extract the coordinates and elevation information.With the origin as the reference,the data matrix was divided into blocks which was the maximum data volume that terminal module could accept.Each block had one to one correspondence with the underlying terminal,so that the elevation information was able to be encapsulated and transmitted successively as provided by the repeater layer stipulation.Since only the elevation information was passed and the coordinates was implicit,the data amount of network was reduced and transmission efficiency was improved.System performance was tested under different topologies experimentally.When the extended protocol was adopted,the turnaround delay increased by 5.6% with one layer of repeaters,while two layers of repeaters caused the delay increased by 36.51%.With two layers of repeaters,the total delay increased by 1.94 times than the original Modbus system.Nevertheless,the average delay in practical two-layer relay system was still controlled at about 100 ms.Transmission accessibility test showed that the successfully sent rate for system frames was as high as 99 percent.The results show that the scheme can attain the goal of clustering management of embedded nodes at the expense of some real-time performance.At the same time,the control protocol and physical links not only keep the simplicity,but also have easily portable and scalable features that can adapt to changes in cluster topology timely and ensure precise mapping and geographic information transmission.