The OPC I/O Server supports two different methods of obtaining data from an OPC server: synchronous and asynchronous I/O. Synchronous I/O enables the OPC Client to poll the OPC server at regular intervals, according to the specified poll rate. Synchronous I/O is a reliable means of getting data at a specified time interval.
Asynchronous I/O, by comparison, does not enable the OPC Client to poll the server; it provides data between the driver and the OPC server on an exception basis. Using Asynchronous I/O, the server notifies the OPC Client when data has exceeded the user-supplied percent deadband. This architecture offers low communications overhead because the driver does not repeatedly poll for the same data over and over again.
You can enable polling for asynchronous I/O by enabling the asynchronous watchdog. This feature enables the OPC Client to ask for data from the OPC server and ensures that no data is lost because the OPC server was too busy.
Using Process Database Exception-Based Processing
You can use synchronous and asynchronous I/O with exception-based database blocks by entering a deadband. When incoming data exceeds the specified deadband, SAC updates all the exception-based database blocks.
For synchronous I/O, enter an exception deadband in each OPC item. For asynchronous I/O, enter a percent deadband in the item’s group.
See Also
