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J1939 Transport Protocols

J1939 transport protocol breaks up PGs larger than 8 data bytes and up to 1785 bytes, into multiple packets. The transport protocol defines the rules for packaging, transmitting, and reassembling the data.

  • Messages that have multiple packets are transmitted with a dedicated PGN, and have the same message ID and similar functionality.

  • The length of each message in the packet must be 8 bytes or fewer.

  • The first byte in the data field of a message specifies the sequence of the message (one to 255) and the next seven bytes contain the original data.

  • All unused bytes in the data field are set to zero.

  • A different PGN controls the message flow.

The data package is passed to the application layer after it is reassembled in the order specified by the first data-field byte.

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