From: Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx> Use table markup to show the structure of the CAN identifier, PGN, PDU1, and PDU2 formats. Also add introductory sentence. Signed-off-by: Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx> --- Documentation/networking/j1939.rst | 46 +++++++++++++++++++++++++++--- 1 file changed, 42 insertions(+), 4 deletions(-) diff --git a/Documentation/networking/j1939.rst b/Documentation/networking/j1939.rst index 0a4b73b03b99..19d9878d7194 100644 --- a/Documentation/networking/j1939.rst +++ b/Documentation/networking/j1939.rst @@ -69,18 +69,56 @@ J1939 concepts PGN --- +The J1939 protocol uses the 29-bit CAN identifier with the following structure: + + ============ ============== ==================== + 29 bit CAN-ID + -------------------------------------------------- + Bit positions within the CAN-ID + -------------------------------------------------- + 28 ... 26 25 ... 8 7 ... 0 + ============ ============== ==================== + Priority PGN SA (Source Address) + ============ ============== ==================== + The PGN (Parameter Group Number) is a number to identify a packet. The PGN is composed as follows: -1 bit : Reserved Bit -1 bit : Data Page -8 bits : PF (PDU Format) -8 bits : PS (PDU Specific) + + ============ ============== ================= ================= + PGN + ------------------------------------------------------------------ + Bit positions within the CAN-ID + ------------------------------------------------------------------ + 25 24 23 ... 16 15 ... 8 + ============ ============== ================= ================= + R (Reserved) DP (Data Page) PF (PDU Format) PS (PDU Specific) + ============ ============== ================= ================= In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU2 format (where PF >= 240). Furthermore, when using the PDU2 format, the PS-field contains a so-called Group Extension, which is part of the PGN. When using PDU2 format, the Group Extension is set in the PS-field. + ============== ======================== + PDU1 Format (specific) (peer to peer) + ---------------------------------------- + Bit positions within the CAN-ID + ---------------------------------------- + 23 ... 16 15 ... 8 + ============== ======================== + 00h ... EFh DA (Destination address) + ============== ======================== + + ============== ======================== + PDU2 Format (global) (broadcast) + ---------------------------------------- + Bit positions within the CAN-ID + ---------------------------------------- + 23 ... 16 15 ... 8 + ============== ======================== + F0h ... FFh GE (Group Extenstion) + ============== ======================== + On the other hand, when using PDU1 format, the PS-field contains a so-called Destination Address, which is _not_ part of the PGN. When communicating a PGN from user space to kernel (or vice versa) and PDU2 format is used, the PS-field -- 2.17.0
