CAN programming help

balaguru · 11-20-2005, 02:11 PM

This is probably a long shot but I thought I'd ask anyway.

I've been playing around with the Multiplex Engineering T16-014 OBD interface. It supports all the major protocols, CAN included. I wrote a simple Perl script to talk through the device using the ISO 9141 protocol for the standard OBD parameters.

Now I'd like to explore the CAN functions on the adapter. I've been googling around trying to learn about CAN. While I've come across some helpful resources I still have some pretty basic questions left unanswered, so I'll just start with the first:

Anybody understand how CAN masks and filters work? I understand the network is of the broadcast type and I imagine setting the mask and filter for a node on the network determines what messages it pulls from the bus, but how exactly are they used. Are they somehow applied to the SID/EID addresses of each CAN message?

BTW, all the stuff I'm doing is open source (Apache license), so any help I can get will I'm sure be appreciated by others as well.

phc · 11-20-2005, 02:33 PM

You can set the filters to only allow certain messages to be received. Or you can let them all in.

I've not done CAN for vehicles, but devloped my own network. CAN messages can have an 11 bit or 29 bit identifier. I've only done 11 bit.

The filter hardware is such that you can have either
2 x 32 bit filters
4 x 16 bit filters
8 x 8 bit filters

I've done 8 x 8 filters. The 8 bit filters only look at the first 8 bits of the identifier.

Each filter has 2 registers you have to program, a mask register (MR or CANIDMR) and an acceptance register (AR or CANIDAR).

Mask Register = set bit to 1 if you don't care
Acceptance Register = match to bit you

So if you want to receive a message 10101010 (first 8 bits of ID)

You can set the Mask = 00000000
And the AR = 10101010

If you want to receive a group of messages 1010XXXX (x = dont care)

MR = 00001111
AR = 10100000

balaguru · 11-20-2005, 02:40 PM

Awesome! Exactly what I needed. Thanks.