Methods for obtaining data from OPC server

There are several mechanisms for the OPC server to communicate with the OPC client. When installing and configuring the software, the user can select the optimal polling period, as well as the method for obtaining data. There are only two of them: subscription and reading. The description of the characteristics of these methods can be studied on the properties page. Let us consider the main features of the mechanisms and their fundamental differences.

1. Subscription. The client makes a list of necessary variables. A large number of groups can be monitored. The server informs the user about the fact that the values of the monitored data are changed. The parameters of the variable are considered to be changed in case of adjusting its value or quality.
2. Reading. In this case, the OPC server sends the information about variables to the client, even if they are not changed. When working with this method, two types of polling are provided.

• Synchronous. The client sends a request with a list of important parameters and waits for a response from the OPC server. Performing other operations at this time is impossible. This type of polling is used for the software that minimizes the standard.
• Asynchronous. The client sends a request for data and continues to work. When the server sends the information about variables, the user will receive a notification. This is the most common type of polling in most projects.

According to OPC technology developers, the above methods of data request from the server are considered to be the most reasonable ones. The described mechanisms provide efficient maintenance. However, users can receive information in other ways as well, for example, from the internal buffer. The OPC standard only regulates the interface between servers and clients.

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