The OPC client driver in ClearSCADA allows a user to define whether synchronous or asynchronous reads and writes are performed (these are configured independently). This article describes the difference between synchronous and asynchronous reads and writes.
In a synchronous read, ClearSCADA will send a 'Refresh' to retrieve an update from every single point in the third party OPC server.
In an asynchronous read, ClearSCADA waits for a list of updated OPC items from the OPC server and then ClearSCADA sends a refresh request for only the points from the list that it receives (from the OPC server).
Asynchronous is generally a preferred method of reading and writing . When an OPC client (i.e. ClearSCADA) connects to an OPC server, it subscribes to 'x' number of points and a poll cycle is created. Sending a write command from the client interrupts the read cycle such that the next transmission contains the write command. The current read transmission, however, must complete before the write transmission can occur. If the current set of points to read is quite large then there could be a delay before the write occurs.
Asynchronous writes are typically seen as more efficient than synchronous writes.