[DoCAN] Vehicle Diagnostic Communication Part 55 [UDS 16]

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Introduction.
Request message for TesterPresent service
suppressPosRspMsgIndicationBit
Response message for TesterPresent service
Conclusion

Introduction.

Explanation of ISO 14229, UDS.
In this article, the messages of the TesterPresent service will be explained.

Request message for TesterPresent service

Let’s take a look at the TesterPresent service message.

Position	Parameter Name	Value
1	TesterPresent Request SID	0x3E
2	sub-function = [ zeroSubFunction ]	0x00,0x80

You may have noticed that there are only two sub-functions, 0x00 and 0x80.
As for 0x80, this number is not important, but the most significant bit is important.

The most significant bit of this sub-function is called the suppressPosRspMsgIndicationBit.

suppressPosRspMsgIndicationBit

The suppressPosRspMsgIndicationBit is not a specification limited to TesterPresent service.
It is applicable to all services that have sub-function parameters.
The effect is to suppress PositiveResponse responses.
It is located at the most significant bit of sub-function, so it is ORed with the normal sub-function and 0x80.

Why does PositiveResponse response suppression exist?
With the TesterPresent service, for example, it is rather straightforward and is used to notify that “ECUs that receive this do not have to respond”.

The TesterPresent service itself does not require any particular processing, and is often used only to maintain S3 time.

For example, if there are 8 ECUs in a CAN network and you send a TesterPresent with a functional address, what do you think will happen?
All ECUs would respond.

In contrast, what happens if the suppressPosRspMsgIndicationBit is standing?
All ECUs will not respond.

Considering only the extension of S3 time, it is more efficient to have a functional address and a suppressPosRspMsgIndicationBit.

Incidentally, even if the suppressPosRspMsgIndicationBit is set, NRC$78 is returned.
If NRC$78 is returned, PositiveResponse is also returned even if suppressPosRspMsgIndicationBit is set.
This is the specification.

If NRC$78 is returned once, it will be handled as a timeout if PositiveResponse is not returned, so it is necessary to return PositiveResponse in this case.

Response message for TesterPresent service

The response message is as follows.

Position	Parameter Name	Value
1	TesterPresent Response SID	0x7E
2	sub-function = [ zeroSubFunction ]	0x00

This could have been expected, couldn’t it?

There is no possible response in sub-function 0x80.
The sub-function of 0x80 is the case of “suppressPosRspMsgIndicationBit”, and in the case of TesterPresent, it is assumed that there is no NegativeResponse.

This explanation should be enough for TesterPresent service.

Conclusion

Explanation of the messages of the TesterPresent service.
Explanation of suppressPosRspMsgIndicationBit.
- PositiveResponse suppression.
- NegativeResponse is returned.
- PositiveResponse after NRC$78 is also returned.

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