Tapping the CAN bus

[Thom]

Wondering where and how those of you who have added upgrades that talk to the CAN bus have made the connection. Since it's twisted pair and is capable of data speeds up to 1 Mb/s, I'm assuming a copule wire taps and some 22 gauge wire isn't the way to go?

http://www.can-cia.de/can has the following info:

A connector used to plug devices to the bus line shall have nominal impedance of 120 Ohm and a nominal transmission resistance of 70 mOhm. The cables chosen for the CAN bus lines shall have a nominal impedance of 120 Ohm, a length-related resistance of 70 mOhm/m, and a specific line delay of nominal 5 ns/m. At bus line length greater than 40 m, the specific resistance of the bus cable should be lower. The aim of these specifications is to standardize the electrical characteristics and not to specify mechanical and material parameters of the connectors respectively cables. The bus wires may be routed parallel, twisted and/or shielded, depending on EMC requirements.

 

[Thom]

No takers on this one, huh?

 

[Rob_from_Elkmont]

Thom,

What exactly do you have in mind? The diagnostic connection that the VAGCOM utilizes at the J1962 connector is ISO-9141. If VW has a CAN bus on the vehicle, it is probably only High Speed CAN running at 500 kbps. It would be utilized for critical component communication (i.e. Engine Controller; Transmission Controller). A medium speed or low speed CAN bus (single or dual wire) would be utilized for such mundane things as body control or audio control communications.

Best Regards,

Rob

 

[MOGolf]

Those who have added the rear parking aid have tapped into the CAN bus. IIRC, they did just use wire taps. Search a thread on the rear parking aid upgrade. Michael Moore, in Toronto, put one on.

 

[Thom]

Thanks Rob & MOGolf. I'll take a look at that thread.

I've installed the Multi-function steering wheel and controller, which required tapping the can-bus. I did this using taps, however I believe the connection is not functioning properly as communication with the control unit (via vag-com) is intermittant, and the leds on the wheel also only light intermittantly. If the can-bus taps are disconnected, the leds don't light at all and I can't talk to the control unit with vag-com at all (which seems odd to me as there is a k-wire connection as well). All the other power leads check out as per the bentley diagrams. The intermittant connection issue reminds me of what happens to cat5 ethernet cabling when it comes untwisted, hence my suspicion that there may be an issue with the way I tapped the can-bus.

 

[PTC]

There is a whole procedure for tapping the CAN-bus (or "repair" in the manual since they don't count on you adding things). Like, IIRC the length of the untwisted wire mustn't be more than 2 cm (or maybe 1 cm), and the wire MUST be twisted pair, VW sells special twisted pair wires (2x0,35 square mm) for repairs on the CAN-bus.
000 979 987 is the part number for it, it's 10 meters and costs ~$8 in Europe.

 

[Thom]

Thanks, PTC. That makes sense.

I can't find the procedure for tapping (repairing) the CAN bus in either my paper or CD Bentley, however. ELSA, perhaps?