Commit b7d3c0e5 authored by Yegor Yefremov's avatar Yegor Yefremov Committed by Marc Kleine-Budde
Browse files

can: j1939: add tables for the CAN identifier and its fields 



Use table markup to show the structure of the CAN identifier, PGN, PDU1, and
PDU2 formats. Also add introductory sentence.

Signed-off-by: default avatarYegor Yefremov <yegorslists@googlemail.com>
Link: https://lore.kernel.org/r/20201104155730.25196-1-yegorslists@googlemail.com


[mkl: removed trailing whitespace]
Signed-off-by: default avatarMarc Kleine-Budde <mkl@pengutronix.de>
parent 4082c502

Documentation/networking/j1939.rst

+42 −4
Original line number Diff line number Diff line
@@ -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

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