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Diagnostic Session : Function 1 - Maintain Connection-Oriented Communication

Maintain Connection-Oriented Communication
Whenever ECU is awake and ready to process any message on CAN , it can also process a diagnostic message. 

So we do not need any special connection establishment for starting the Diagnostic Communication. But we need to maintain a Diagnostic Communication Link between client and server when we need it for longer time till our purpose is not served. Going ahead , we will see what does it mean. 

There are two use cases of Session Management:

Diagnostic Connection:
When Client is connected to server, the server has to notify the client that it is connected to the server and diagnostic communication is not yet finished.

The need of this feature is in the power management of the Server ECU . Server ECU will be in  wake up state during the diagnostic communication and shall not go to sleep while communication is going on. Also it shall not remain in wake up  state unnecessarily if no more diagnostic communication is going to happen. Otherwise it will drain the ECU battery.

To achieve this, the Client needs to notify the server about opening, maintaining and closing the Diagnostic Communication.

Session Extension and Closure

ECU supports 3 basic sessions and only non-default sessions are time bound, means need to be extended explicitly otherwise will  expire. Hence Server needs to notify the client about this.

This is done through a Service $3E Tester present and $10 Diagnostic Session Control.

It necessary to know that; when ECU is in non-default session, a Service $ 3E Tester Present needs to be sent to server by client to server, so as to extend the current diagnostic session.

 Let us see how these two things are done.

Server and client both are have a Timer called “S3 timer” for session management.

When a client changes a session a timer is started. The timer name is S3 timer.
Server maintains S3 Server and Client maintains S3 Client timer.

Below sequence follows:

1.  S3 timer : Session Extension


2. S3 Timer Expiry

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