[DoCAN] Vehicle Diagnostic Communication Part 40 [UDS 1]

[DoCAN] Vehicle Diagnostic Communication Part 40 [UDS 1]車両診断通信
[DoCAN] Vehicle Diagnostic Communication Part 40 [UDS 1]

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I will start explaining ISO 14229 (UDS) from this time.


Vehicle Diagnostic Communication Hierarchy Review

Previously, I have explained and simulated the transport layer of vehicle diagnostic communication.
From this time onward, I will explain the higher layers.

Now, do you remember those higher layers?
Let’s recap it here.

Application layerISO14229-1ISO15031-5
Presentation layer
Session layerISO14229-2(UDS)
Transport layerISO15765-2(DoCAN)
Network layerISO15765-2(DoCAN)ISO15765-4(OBD)
Data link layerISO11898-1(CAN)ISO15765-4(OBD)
Physical layerISO11898-2(CAN)ISO15765-4(OBD)

Based on this table, the next step is to explain ISO 14229-2 for the session layer.

However, this is also discussed in Wikipedia and Vector’s ” Beginner’s Diagnostics,” so there is no need to explain it again.

Wikipedia(Unified Diagnostic Services)

Vector’s ” Beginner’s Diagnostics”.

You can think of it as the following story from Wikipedia (Unified Diagnostic Services).

  • Diagnostic Session Control
  • Security Access

In addition to the above, “response time” is also explained.


Response message timeout parameter

There is a timeout parameter for vehicle diagnostic communication called “P2 time”.

Some of you may have thought of N_As and N_Bs as timeout parameters.
N_As and N_Bs are network layer timeout parameters.
P2 time is a session layer timeout parameter, so they are different.

In a few words, it is “the timeout time until a response message is received after a request message is sent.

Even this may not be clear to many people.
I am sure you will be able to understand it in due course, so I will end here and go to the next explanation.


Session timeout parameter

There is a timeout parameter called S3 time.

There is a concept of a “session” in vehicle diagnostic communications.
The initial state is the default session.
From there, the session can be “switched” by throwing a message to the session control service.
When the session is switched, “supported services change” and “services that were not available in the default session can be used”.
However, when you switch to a session other than the default session, the system will forcibly return to the default session if no message is received for S3 time.

(Well, it is OK if you don’t understand this part at this point…)


What if you can”t picture it?

So, I have explained the P2 time, S3 time, and session, but many of you probably do not have any image at all.
Therefore, I will try to make a figure that will help you understand the above by the next time.



  • Review of the Diagnostic Troubleshooting Communication Layer.
  • The response message timeout parameter is P2 time.
  • Session timeout parameter is S3 time.

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