The communications mode (Asynchronous or Synchronous) is determined by the OPC Server. Most new OPC Servers use Both Asynch and Synchronous modes, however many early OPC Servers use Synchronous polling only.
Synchronous Mode requires that the Client continuously poll the OPC Server for data.
Asynchronous Mode allows report by exception where the Client initially polls the Server when it starts, then the Server reports only when a value changes.