Multi packet J1939 message changing priority

Jag 2 years ago in IQANdesign updated by Gustav Widén (System support) 2 years ago 9


I am sending a multi packet J1939 message as shown below with a priority 6. I am using MC41 as master. For some reason TP packets are braodbcast the priority is being changed to 7 from 6. See snapshot of the data log showing identifier 1CECFF7F instead 18ECFF7F. As a result my request from MC41 is being ignored by the Vehicle controller.

I have manually broadcast 18ECFF7F and 18EBFF7F using Busmaster and it works OK. however when I manually transmit

1CECFF7Fand 1CEBFF7F vehicle controller ignores TP packet.

I am not sure if this is a bug or I am doing something wrong appreciate if you can point me in the right direction.

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SAE J1939-21 specifies prio 7 on both messages that are used for multipacket transport, TP.CM and TP.DT

IQAN will use prio 7 on these regardless of what you specify on the PGN being transported. 

I think the mistake may be in the node that listens to the multipacket message. 

Hi Gustav,

Thanks for your reply.

I read through SAE J1939-21 and only thing I could find in the standard regarding TP message priority was that default priority for  TP.CM and TP.DT is 7 but it doesnt say TP message priority has to be 7. I believe the same applies for all other SAE J1939 messages.

I have worked with other controllers where I haven't experienced this problem of controller overriding the message priority of the requestor.

Can you please review this information as I believe SAE J1939-21 doesnt specify that priority of TP messages has to be 7?

If I am missing something in the standard then please share this information so that I take this up with OEM.



For TP.DT, the standard states "Priority used regardless of the PGN being transported"

But I see that TP.CM only says Default priority 7, without the additional text. 

At the IQAN team, we have made the interpretation that prio 7 is to be used on both TP.CM and TP.DT.

Thank you again Gustav. Sorry I was referring to older version of SAE J1939-21 which did not have this requirement on TP.DT priority. Looks like this requirement was added in 2018 revision of this standard.

Is there any chance to consider that priority for TP.DT and TP.CM be left to up to the user? As you can understand there will be controllers which will be following earlier revision of the standard which did not have requirement of mandatory priority of TP.DT and TP.CM.

This will be great help as I am struggling to find a solution on our customer truck.


Hi Gustav,

Sorry to bother you with this. Appreciate if you have any update on the issue mentioned here.

Thanks in advance.


I think you will have to ask the manufacturer of the device that reads DM1 to also listen to prio 7. 

Hi Gustav,

Sorry I am not sure what you mean. As you will see my first screen shots I am trying to send or listen DM1 message.

I am trying to sending 3 packets with 21 bytes of information using multi packet option in J1939 Frame.


sorry I meant I am not sending or listening DM1.

Sorry, I forgot about the original post and somehow thought it was DM1 multipacket. The transport protocol is the same. 

Anyways, the solution will have to be to ask the manufacturer of the device that listens to the multi-packet message to accept priority 7 on the frames TP.CM and TP.DT.