Transport Protocol

The J1939 standard supports more than 8 bytes of data for a single message. However, the CAN specification allows a maximum of 8 bytes of data per single CAN message frame. Therefore, a single large message must be sent in multiple packets. The J1939 transport protocol defines how to disassemble, transfer, and reassemble CAN messages larger than 8 bytes.

Supported Transfer Mode

CANtrace fully supports monitoring and transmitting J1939 transport protocol message:

BAM (Broadcast Announce Message): Used for broadcasting large messages to the entire network (all nodes).

CANtrace parses the multiple packets for BAM transfers, assembles them into a single large message, and displays it in the Trace tab. You can also send large messages from the Send tab as long as the messages are found in the associated J1939 database and added to the tab.

Sending Large Messages

The example below shows how to send a DM1 message from the Send tab in CANtrace. You can configure the raw value of the signal and then click on the Send button.

Figure: Sending J1939 transport protocol frame in the Send Tab

Receiving Large Messages

As shown in the Trace tab below, the first five 8-byte messages make up the complete 22-byte DM1 message. The final entry displays this fully assembled long frame generated by CANtrace.

Figure: Displaying J1939 transport protocol frame on Trace Window