Skip to content

Latest commit

 

History

History
98 lines (66 loc) · 2.29 KB

can-j1939.md

File metadata and controls

98 lines (66 loc) · 2.29 KB
Raw

CAN-J1939 on linux

The Kickstart guide is here

CAN on linux

See Wikipedia:socketcan

J1939 networking in short

  • Add addressing on top of CAN (destination address & broadcast)

  • Any (max 1780) length packets. Packets of 9 or more use Transport Protocol (fragmentation) Such packets use different CANid for the same PGN.

  • only 29bit, non-RTR CAN frames

  • CAN id is composed of

    • 0..8: SA (source address)
    • 9..26:
      • PDU1: PGN+DA (destination address)
      • PDU2: PGN
    • 27..29: PRIO

  • SA / DA may be dynamically assigned via j1939-81 Fixed rules of precedence in Specification, no master necessary

J1939 on SocketCAN

J1939 is just another protocol that fits in the Berkely sockets.

socket(AF_CAN, SOCK_DGRAM, CAN_J1939)

differences from CAN_RAW

addressing

SA, DA & PGN are used, not CAN id.

Berkeley socket API is used to communicate these to userspace:

PRIO is a datalink property, and irrelevant for interpretation Therefore, PRIO is not in sockname or peername.

The data that is [recv][recvfrom] or [send][sendto] is the real payload. Unlike CAN_RAW, where addressing info is data.

Packet size

J1939 handles packets of 8+ bytes with Transport Protocol fragmentation transparently. No fixed data size is necessary.

send(sock, data, 8, 0);

will emit a single CAN frame.

send(sock, data, 9, 0);

will use fragmentation, emitting 1+ CAN frames.

Using J1939

BSD socket implementation

  • socket
  • bind / connect
  • recvfrom / sendto
  • getsockname / getpeername

Modified struct sockaddr_can

struct sockaddr_can {
	sa_family_t can_family;
	int         can_ifindex;
	union {
		struct {
			__u64 name;
			__u32 pgn;
			__u8 addr;
		} j1939;
	} can_addr;
}

  • can_addr.j1939.pgn is PGN

  • can_addr.j1939.addr & can_addr.j1939.name determine the ECU

    • receiving address information, addr is always set, name is set when available.

    • When providing address information, name != 0 indicates dynamic addressing