OPC interface is developed by the OPC Foundation, based on the DCOM technology, for the control system software for data communication between the interface specification. Because of its openness and high efficiency, it has been widely used in the field of automation control and production information management. Zijin Bridge software to comply with the OPC interface specification to achieve the corresponding interface, greatly increasing the openness of Zijin bridge software. In the use of OPC interface driver, due to configuration parameters or improper operation environment settings, may achieve unexpected results, and some even affect the stability of the control system. Here are some examples of OPC interface usage tips based on actual work experience: Read the OPC server user manual carefully
When using the OPC interface for data acquisition, be sure to read the user manual of the OPC server or the online Help, because the OPC server name (ProgID) used in communication and the component name used for DCOM configuration are usually defined by the OPC server software provider. I usually get relevant information from the user manual or online Help. Some OPC server is not support online browsing, communication when using data items (connection items) are defined by a certain specification, we also need to read the user manual or online Help to obtain relevant information. For example, when collecting data from an old OPTO22 system, it is not supported for online browsing. By online help We know that its data item definition is as follows: Controllername[refreshgroupname]:itemtype.itemname[start-end]. Field.bitn and the Access path ("Opto_mds_0") is to be filled in when the data item is literal. Controller name, brush write group name, item class type and other information we need to control the system completion data or OPTO22 configuration works to find. Correctly configure the acquisition cycle
Since the fastest acquisition cycles that can be supported by different OPC servers are different, we have to configure them according to the best acquisition speed that OPC server can provide when implementing data acquisition. Most of the OPC server support acquisition period is not continuous, if we set the acquisition period is not supported by the server acquisition cycle, OPC server will use a similar set period of time to communicate with the natural cycle of scheduling. Traffic control
In general, it is recommended to use asynchronous communication, because in this way the client program obtains the data by the OPC server according to the data Change situation the initiative sends, has the high communication efficiency. In the practical application we may encounter the problem: the control system that communicates with the real-time database is running full load, if do not control the traffic volume can affect the normal operation of the control system (in the OPC server through the app node read the TDC3000 system data, it is foolish to have a similar problem). In this case, it is recommended to use synchronous or single-group synchronous mode of data acquisition, in which we can adjust the number of data items collected per group and the acquisition period to control the traffic. Note Server Special restrictions
In the use of OPC interface for data communication, there are some special limitations of OPC server, some can be connected to the number of clients are limited, some can access the maximum number of data items are limited, and some of each group contains a limit on the number of data items. For these servers with special restrictions, if the communication parameters are not properly configured or the purchase is authorized, there will be a communication error or only some data can be collected. For OPC servers with a limited number of access customers, if the maximum number of customers is reached, we will have to re-purchase the authorized increase in the number of clients allowed to access. The best way to have an OPC server that has access to the number of data items is to re-purchase the number of allowed access data items, but you can also add the data points that can be collected by the "one-group synchronization" method available in the interface. The "one-group synchronization" Communication method is at the expense of communication speed, and in this way data writeback (updating the server's data from the client) is not possible. For each set of data item limit of the OPC server is better processing, as long as the interface device definition, the limit number to fill in the number of items per group. Distributed communication via DCOM configuration
In the practical application sometimes because of the safety or the operation environment consideration cannot the data acquisition program and the OPC server installs on the same computer, then we need to use the OPC's DCOM attribute realizes the data acquisition program and the OPC server network-based distributed communication. DCOM technology is a kind of communication technology among the components which is supported by operation from the NT4.0 system. When implementing a component, as long as the interface is defined in accordance with the COM specification, when the server and client programs are distributed on different computers, remote communication can be made over the network as long as the DCOM properties of the server and client are configured accordingly. The DCOM configuration is configured differently on different operating systems, except in the Windows XP SP2 system, where other systems can communicate with the specified user or anonymously. About the DCOM configuration process is not described in detail here, you can refer to the "Zijin Bridge OPC Server and interface" document. |
