4.10.5 Asynchronous vs. Synchronous Behavior

In general each function call is synchronous. Within FDT there are two special cases of asynchronous behavior.

• After starting the user interface of a DTM, the DTM works asynchronous to the Frame Application. Asynchronous in this case means that the user works with the DTM and the Frame Application is the server for communication and data access. This state ends by a notification to the Frame Application, when the DTM closes the user interface.
  
• While a DTM has opened its user interface, the DTM uses the asynchronous behavior at the communication interface. The time a communication function call needs to return depends on the system topology and the bus protocol and would block an application for seconds. Dividing a communication function call to a request and a response function causes a non-blocking behavior without the pain of multi-threading implementation. The DTM sends its request or several requests without being disturbed by incoming responses. When a response is available, the DTM gets a notification and can receive the response from the communication component. Due to this mechanism, on one hand a DTM should not implement a timeout control and on the other hand the communication has to provide a response for each request. Only a response can contain the timeout information.