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Kickstart guide to can-j1939 on linux

Prepare using VCAN

You may skip this step entirely if you have a functional can0 bus on your system.

Load module, when vcan is not in-kernel

modprobe vcan

Create a virtual can0 device and start the device

ip link add can0 type vcan
ip link set can0 up

First steps with j1939

Use testj1939

When can-j1939 is compiled as module, opening a socket will load it, or you can load it manually

modprobe can-j1939

Most of the subsequent examples will use 2 sockets programs (in 2 terminals). One will use CAN_J1939 sockets using testj1939, and the other will use CAN_RAW sockets using cansend+candump.

testj1939 can be told to print the used API calls by adding -v program argument.

receive without source address

Do in terminal 1

./testj1939 -r can0:

Send raw CAN in terminal 2

cansend can0 1823ff40#0123

You should have this output in terminal 1

40 02300: 01 23

This means, from NAME 0, SA 40, PGN 02300 was received, with 2 databytes, 01 & 02.

now emit this CAN message:

cansend can0 18234140#0123

In J1939, this means that ECU 0x40 sends directly to ECU 0x41 Since we did not bind to address 0x41, this traffic is not meant for us and testj1939 does not receive it.

Use source address

Binding a can-j1939 socket to a source address will register allow you to send packets.

./testj1939 can0:0x80

Your system had, for a small moment, source address 0x80 assigned.

receive with source address

Terminal 1:

./testj1939 -r can0:0x80

Terminal 2:

cansend can0 18238040#0123

Will emit this output

40 02300: 01 23

This is because the traffic had destination address 0x80 .

send

Open in terminal 1:

candump -L can0

And to these test in another terminal

./testj1939 -s can0:0x80,0x3ffff

This produces 1BFFFF80#0123456789ABCDEF on CAN.

Multiple source addresses on 1 CAN device

./testj1939 -s can0:0x90,0x3ffff

produces 1BFFFF90#0123456789ABCDEF ,

Use PDU1 PGN

./testj1939 -s can0:0x80,0x12345

emits 1923FF80#0123456789ABCDEF .

Note that the real PGN is 0x12300, and destination address is 0xff.

Use destination address info

The destination field may be set during sendto(). testj1939 implements that like this

./testj1939 -s can0:0x80,0x12345 can0:0x40

emits 19234080#0123456789ABCDEF .

The destination CAN iface must always match the source CAN iface. Specifing one during bind is therefore sufficient.

./testj1939 -s can0:,0x12300 :0x40

emits the very same.

Emit different PGNs using the same socket

The PGN is provided in both bind( sockname ) and sendto( peername ) , and only one is used. peername PGN has highest precedence.

For broadcasted transmissions

./testj1939 -s can0:0x80,0x12300 :,0x32100

emits 1B21FF80#0123456789ABCDEF rather than 1923FF80#012345678ABCDEF

Desitination specific transmissions

./testj1939 -s can0:0x80,0x12300 :0x40,0x32100

emits 1B214080#0123456789ABCDEF .

It makes sometimes sense to omit the PGN in bind( sockname ) .

Larger packets

J1939 transparently switches to Transport Protocol when packets do not fit into single CAN packets.

./testj1939 -s20 can0:0x80 :,0x12300

emits:

18ECFF80#20140003FF002301

This is the first fragment for broadcasted Transport Protocol. testj1939 returns before the subsequent packets can leave, and as the last socket on the system closes, can-j1939 effectively cleans up all resources. Real-world applications will run like forever, and will not encounter this side-effect.

Try again, and instruct testj1939 to keep the socket open for 1 second.

./testj1939 -w1.0 -s20 can0:0x80 :,0x12300

emits:

18ECFF80#20140003FF002301
18EBFF80#010123456789ABCD
18EBFF80#02EF0123456789AB
18EBFF80#03CDEF01234567

The fragments for broadcasted Transport Protocol are seperated 50ms from each other.
Destination specific Transport Protocol applies flow control and may emit CAN packets much faster.

First assign 0x90 to the local system.
This becomes important because the kernel must interact in the transport protocol sessions before the complete packet is delivered.

./testj1939 can0:0x90 -r &

Now test:

./testj1939 -s20 can0:0x80 :0x90,0x12300

emits:

18EC9080#1014000303002301
18EC8090#110301FFFF002301
18EB9080#010123456789ABCD
18EB9080#02EF0123456789AB
18EB9080#03CDEF01234567
18EC8090#13140003FF002301

The flow control causes a bit overhead. This overhead scales very good for larger J1939 packets.

Advanced topics with j1939

Change priority of J1939 packets

./testj1939 -s can0:0x80,0x0100
./testj1939 -s -p3 can0:0x80,0x0200

emits

1801FF80#0123456789ABCDEF	
0C02FF80#0123456789ABCDEF

using connect

advanced filtering

dynamic addressing