The Design And Implementation Of Multi-master Mechanism Of Half-duplex Fieldbus Based On Linux

With the process of industrial automation rocketing, the number of meters used in industrial field is greatly increased. Apparently, the manual operation for data acquisition cannot meet the need of data monitoring in modern industrial field. Intelligent instruments with digital output and communication port begin to take place of traditional mechanical meters. Generally, serial bus is used as data transition medium in distributed control system. RS-485, as communication bus, which performance well, and use less signal lines, and has a simple networking and long transmission distance, is widely used in industrial field. Modbus protocol is widely used in data transmission. This protocol defines a PDU which is independent of hardware. Thus, Modbus can be applied to different types of equipment.In default, data is sent and received by RTU mode when Modbus is applied to serial link. Communication can only started by a master node in a Modbus network, limited by the half duplex nature of RS-485, and master must get access to slaves by polling. Now consider such a controlling network used in industrial field, if abnormal data is detected by a slave, master won’t do anything until it get access to this very slave.The more the slaves, the longer it takes to make a polling. Which means abnormal data processing requires a long time waiting. We proposed a new communication mechanism by which a slave node can start communication, compatible with Modbus protocol at the same time. Abnormal data processing in this mechanism has good real-time response. In this design, we use ARM9processor and Linux OS to control master node, while STM32processor and μC-OS for slave nodes.Firstly, we introduce the principle of communication in serial link. By doing research in the existing communication protocols, we proposed the multi-master sending mechanism on serial link. Secondly, we designed and described the details of this mechanism, especially its principles and how it works. At last, we built some experimental to test the mechanism, and drew a conclusion.The results showed that this mechanism is of high reliability, and its characteristic of multi-master sending meets the need of real-time response for abnormal data in control network.