Study And Development Of OPC Data Access Server Based On Modbus Protocol

OPC (OLE for Process Control) is to solve the problem that process control equipment and commerce application soft from different factory can't communicate freely. OPC technique based on OLE/COM/DCOM technique, develops very quickly and applies more and more widely, it is a new pop technology of automation. Many companies increase OPC speciality for their product, including increase OPC client function for their control application software and providing OPC server for their field equipments etc. OPC DA Specification is OPC Specification about data accessing.Aiming at present situation above, on Windows OS,OPC DA server software of the circuit breaker based on Modbus protocol is designed and developed through the study of OPC specification and COM technology.(1) This paper analyzes in detail the problems of the traditional data access methods and the strongpoint of OPC technology, study OPC technology and its foundation technology—COM/DCOM technology and Modbus field bus.(2) This paper discusses important key and difficult technology in developing OPC Server software: advisory connection mechanism and connection object mechanism in OPC data access method, develops three ways to data access are analyzed in detail, asynchronous access , synchronous access and subscription.(3) The paper studies all important keys in developing OPC Server software, designs the unitary structure of the OPC Server software, develops Server object, Group object, Item object and equipment class, tests the OPC DA server software, discusses and analyzes the results of the tests.(4) OPC server running results indicates that OPC server can communicate with OPC client software through DCOM mode, monitor and control the Modbus network of the communicated circuit breaker.OPC DA server software is designed and developed for the circuit breaker based on Modbus protocol, but used for any equipments based on Modbus protocol. If its I/O DLL(equipment class) is changed, the OPC DA server software is used for equipments based on another protocol;If its I/O DLL(equipment class) is developed to handle all protocols, it is used for equipments based on all protocol.