CAN Bus Interpretation

[gpeterson]

Originally Posted by lkv0315

They believe the bus traffic may be encrypted

It is absolutely not encrypted. If if the message payload were you would still see CAN messages if your hardware/software were functioning properly.

[80stech]

RE the isolation: If isolated the CAN module will need power and ground (depending on the level of isolation) on the data input side SEPERATE from the Pi supply. Yes it could be coming from the same place but the makers of the module don't know how you are going to use it. I also noticed that the CAN module has it's own crystal so it will be limited to only certain data rates that can be derived from the crystal which might not match up with the BMW data rate. Just putting that out there as a possibility.

[Overboost]

BMW and almost every street car CAN bus baud rate is 500k. Work with that as default.

[80stech]

500k is pretty fast! I tested my RS485 setup up to 512k and was error free (I was totally blown away that it could go that fast!) I didn't even try faster because I thought that was already ridiculous and about 100x faster than I would really need. Looking at the CAN bus protocol though there is a huge amount of bits around each packet so will need at least 3 times the speed to move the same data as what I'm doing with the RS485 bus. For sure good to know though and you would think if 500k is pretty universal for automotive the CAN bus module and software would be set up for that.

[80stech]

Haha! I had just had a look at my CAN module and it has a 8,000 mhz crystal so a divide by 16 counter would give exactly 500k so that all makes sense as does the reason for the crystal in the first place since 500k is not normally dividable from most processor clocks I'm thinking.

[80stech]

@ikv0315 maybe try setting the address filter for a specific piece of data. Polling for anything probably won't return anything if the filter is exclusive to everything it's not set to. Makes sense in a way because there is no way you can display scrolling data fast enough. If you can get into the source code and set a "data capture" to a buffer for set time or number of frames and then display the buffer it might help to see what's going on.

There also seems to 2 different bit frame lengths, 11 and 29, so that might be something to look into.

Keep the wiring short and twisted pair. BTW How are you tapping in to the wires?

Also maybe make sure you have the right identifiers for the CAN bus you are working with. AFAIK the other diagnostic and communication lines are Can buses(use CAN protocol) as well and probably use different identifiers.