An OPC Client can issue a Cache Read from the OPC Server. The OPC Server will understand that it should reply with the value it already has in its memory, rather than fetch it from the original data source (as would be required by a Device Read), which could take far longer to complete. A Cache Read should take less time to complete than a Device Read.

In OPC, a Cache Read can only be a synchronous operation, and there is no asynchronous Cache Read. This is because a Cache Read retrieves data from memory, so a synchronous call will return as fast as an asynchronous call, but without the programming overhead.

See also: Cache, Device Read