Decoding CANbus messages

[GrandAlf]

I have been asked to look into extracting the following info from a vehicle CANbus and convert to relay output.

Footbrake ON or OFF
Handbrake ON or OFF
Engine running (Rpm related >500 say?)

Now hardware is not a problem, just a pic with a bit of interfacing. Am thinking of using CCS for the built in CAN library.

Problem is how to get the appropriate messages. I understand that you have to request most messages rather than they are just sent periodically. Now I don't even know if they are on the high or low speed bus or even what commands to send. Cannot find any lists online and I am not even sure if they differ between vehicles, will be mostly VAG I think. Now I am sure I will be able to find all the info given time, but if anyone has done anything similar I would be most grateful for any advice. Seems to be a better option than re-inventing the wheel.

 

[horace1]

have you had a look at the J1939 documentation, e.g.
https://store.sae.org/j1939/contents/

also sources such as ISO 11992
https://www.iso.org/iso/catalogue_detail.htm?csnumber=55046

 

[Borisw37]

I think its the opposite. CAN does not have a "master" slaves send data periodically. This of course all depends on the vehicle and their CAN implementation.
1st thing would be to google the sh*)& out of that vehicle and see if somebody has decoded the messages.
You can certainly get a CAN analyzer (I've used this one ) but there is a A LOT of data being sent over CAN. Just pressing the footbrake on/off it may be hard to see the related can messages among 100s of other ones being sent.
Basically every device on the network has a unique CAN ID, perhaps if you know / find the ID of the footbrake swtich, handbrake switch, etc.. then you can filter out the messages you need.
Take a look at this: http://opengarages.org/handbook/2014_car_hackers_handbook_compressed.pdf
http://bobodyne.com/web-docs/robots/MINI/CAN/MINI_CAN.pdf

 

[GrandAlf]

Thanks guys. I thought it may be a little difficult to get specific info. There are manufacturers that make specific units to say just extract engine running and give an signal output, but nothing that is specific to my needs. They do however claim compatibility for most vehicles which suggests there may be some commonality between manufacturers with common signals. As you can probably tell I have no prior experience with CAN, but always nice to learn something new. Useful links, appreciated.