CAN source/destination address query

We utilise a control system (by IFM) in our vehicles and as part of this there is a J1939 connection to a third-party accident event recorder. Messages are sent to the recorder from the IFM system with PGN set to 65521 and destination address (DA) set to 249

The event recorder was not receiving any data so I decided to un-plug it and connect dewesoft in its place (using a Dewe43).

By scanning all channels I found that the data is there, on PGN 65520 Source address #3

My CAN knowledge is expanding but not too advanced so I'll use an analogy to explain what I think is happening, please confirm if this is correct........

We're in a street with 253 houses in it, each house represents a CAN node. We'll call it "J1939 Street"

Several people want to live on J1939 street where Mr Recorder has been told to purchase #249 whilst others are happy to take the next available house. Mr IFM moves into the area and is allocated the lowest number next available house which #3 (owners of number #1 and number #2 are unknown, but we assume Mr Isuzu lives there).

Mr IFM at #3 is sending messages to house #249 but they're not receiving them. Meanwhile Mr Dewesoft is parked in a car on the street and can see all the information that #3 are sending, regardless of whether #249 is home or not. Mr DeweSoft can snoop around and knows everything that's going on in the street.

Mr Recorder at #249 hasn't setup his front door correctly so no matter what Mr IFM does, there's no way those messages can get through. Meanwhile Mr DeweSoft continues to see everything.

To get the messages to pass correctly from #3 to #249 there are two options:

1) Mr Recorder sets up #249 correctly to allow receipt of the messages. They probably just need to correctly allocate the number of their node

2) Mr Recorder starts looking at #3 for the messages. This way it doesn't matter what his own node is called, he'll always receive the information from IFM

Of these two options, it would appear that the first is preferable as it allows Mr IFM to be allocated any house on the street and he'll always be able to send messages to #249. There is a chance that Mr Isuzu will one day have some friends who want to live close to him so Mr IFM could end up allocated any number house and Mr recorder will never be able to find him.

I should add an additional query - I want Dewesoft to temporarily take the place of the event recorder at #249 - How to I allocate node #249 in Dewesoft so that it accurately represents the event recorder that is normally there?

Hello Phil,

I hope that we correctly understand your problem.

Structure of general CAN J1939 arbitration ID:

|| P | R | DP | PDU Format | PDU Specific | Source Address ||

The PDU Specific part of the arbitration ID has a different meaning depending on the value in PDU Format. If this id below 240 the PDU specific field represents a destination address:

|| P | R | DP | PDU Format | Destination Address | Source Address ||

If PDU format is over 239 the PDU Specific represents a group extension:

|| P | R | DP | PDU Format | Group Extension | Source Address ||

When DEWESoft CAN port is in J1939 mode the message ID that you can see in DEWESoft is read out from the PDU Format or PDU Format + Group Extension (PDU Format > 239). This fields identify the type of data that is contained in the message. Inside of the message definition there is also an option to filter the messages depending on the Source address.

There is no option inside DEWESoft to filter the messages by their destination address. Therefore you won't be able to exactly know if a message on your Recorder is ignored only because of the incorrect Destination Address that is written in the message that is sent by the IFM.

All of the nodes on the same CAN network can see all of the messages that are passing through the network. The settings of the node determine if the node uses that message or not. Therefore if the IFM is sending a message for the recorder with a specific Destination address (#249 Recorder ID) and Source address (#3 IFM number) your Recorder has to be set up to receive the messages with that Destination and Source address.