Microsoft Windows uses a callback mechanism to implement asynchronous communication. When an OPC Client application issues an asynchronous call, the OPC Server acknowledges receipt of the call itself. The OPC Server then “calls” the OPC Client application back with the result using a callback. The OPC Client application does not know when it will receive the callback, and therefore the call is said to be asynchronous, or without time.

Using asynchronous communication, an OPC Server relies on callbacks to deliver data back to an OPC Client application. Integrators must configure the PC with the OPC Client application to accept the callbacks. Failure to do so will cause callbacks to fail and consequently any asynchronous communication will fail. This is a common cause for OPC Item Subscriptions to fail.

See also: Asynchronous, Subscription