OPC I/O Servers supports two different methods of obtaining data: Synchronous and Asynchronous I/O.
In typical OPC client, this selection updates items at regular intervals.
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.
Information that is not synchronized to a continuously running clock. With asynchronous transmission, each character begins with a 'start' bit and ends with a 'stop' bit and the clock is only synchronized for the duration of the character.