[DoCAN] Vehicle Diagnostic Communication Part 50 [UDS 11]

[DoCAN] Vehicle Diagnostic Communication Part 50 [UDS 11] 車両診断通信
[DoCAN] Vehicle Diagnostic Communication Part 50 [UDS 11]

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Explanation of ISO 14229, UDS.
In this article, P2 time and P2* time will be explained.

P2 time

P2 time has been explained previously.
It was explained in the following issue.

You may recognize the following figure.

P2 time,Request side,Response side,SF,FF,FC,CF

In other words, P2 time is the time between the request message and the response message.

P2* time

And I have yet to explain P2* time.
The relation to P2 time is to be expected given the name.

It is not unrelated, of course.
P2* time is also known as P2 extend time.
And you will want to know which time is indicated.

Last time I talked about NegativeResponse, there was ResponsePending.
The description was, “Response pending; wait P2* time.”
The problem would be that we don’t know the specifics of this.

Simply put, when ResponsePending, in other words NRC$78, is received, the response message wait switches from P2 time to P2* time.
After that, if a response message comes in within P2* time, it is OK.

As for the actual time between P2 time and P2* time, ISO 1429-2 recommends 1 second for P2 time and 5 seconds for P2* time.

There is a slight discrepancy if it is simply assumed that the P2 time can be extended to 5 seconds.
If NRC$78 is received again within the P2* time, the specification includes the possibility of extending the P2* time by 5 seconds again.
In other words, if NRC$78 continues to be received, it can be extended indefinitely.

In the standard, this is true, but in reality, the maximum waiting time is defined by the manufacturer, so it must be terminated at some point.
As one might expect, we don’t want a specification that allows infinite waiting.

Is it possible to wait more than 5 seconds?
Basically, no, but if the entire FlashRom is erased in an ECU, the worst-case value may not be readable.
It is safe to assume that it is used in such a situation.

So, we will return to the topic of services in the next issue.


  • Review of P2 time.
    • P2 time is 1 second
  • Explanation of P2* time
    • P2* time is 5 seconds.
    • When the off-board tester receives NRC$78, it switches from P2 time to P2* time.
    • When the off-board tester receives NRC$78 again, it extends P2* time from there.

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