Assembling a Wire harness for a CAN Bus with 3 or more devices

Discussion in 'Automotive Electronics' started by joewiki, Jul 7, 2017.

  1. joewiki

    Thread Starter New Member

    Jul 7, 2017
    I'm working on a car project where I need to connect multiple devices ( more than 3) to the same CAN bus. One of these devices is supposed to go in the engine compartment and the rest of them inside the car (possibly as far as the trunk). Each device is an electronic board, like the CAN-BUS-Shield-V1.2 for Arduino, with screw terminals to connect to the CAN-H and CAN-L signal wires of the bus.

    My questions...

    1) What type of connectors, if any, should I use? Specially to tap a device (node) into the bus? . I read that DB-9 is the industry standard connector for CAN bus, but can't a female-female-male Tee-connector . I also read that M12 and 5 pin mini C connectors are used in some CAN bus applications.
    Links to online parts are greatly appreciated.

    2) Assuming no connectors are necessary, whats the best way to make a 3 way connection for each wire in a twisted pair?

    3) Should I look for a specific cable . I'm specifically wondering about,
    (1) Wire gauge (considering that the wires have to be run several feet go from trunk to engine compartment)
    (2) Shielded cable vs unshielded twisted pair of wires.
    (3)Characteristic impedance.
    A link to a suitable cable/wires sold online is greatly appreciated.

    4) Assuming a shielded cable is used, is it ok if the shield conductor doesn't carry the reference ground?

    5) All of the boards already have the CAN signals connected with 120 ohms resistors, but those resistors can be switched out. I understand that two and only two terminations should exist between the two CAN signals . Should I consider switching out the resistors in the boards, and instead use external 120 ohm terminators?

    6) Lastly. I never done any hands-on work. Anybody has an idea how to run wires through the firewall?. The car we are working on is a Kia Soul.

    Please note that this CAN bus is separate from any CAN bus already in the vehicle and has nothing to do with the OBD connector that is found in the vehicle's cabin.

    Thanks so much
  2. cork_ie

    Active Member

    Oct 8, 2011
    A lot depends on the speed of your CAN Network, slow speeds are a lot more tolerant.
    There is absolutely no need for any kind of special connector in a short three node network like you have,unless you are running at high speed like 500Kbps or so.Two pin plug/sockets or screw terminals, as commonly found on scrap electronics or ebay will do fine. Most important is that CAN needs a network impedance of 60 Ohm,this is achieved by fitting 120 Ohm terminating resistors at either end of your network.
  3. cork_ie

    Active Member

    Oct 8, 2011
    My favourite tool is one I made myself, old very long 14" (350mm) screwdriver , about 1/8" (3mm) redden the tip beat it flat and drill a 2mm hole in it , round the edges until they are nice and smooth. You now have a needle with a handle in it. Push it through the centre of the wires in the existing harness firewall bung. I will guarantee you, that you will do no damage. Then simply thread your wire around the eye from the other end , a small bit of soap and pull through.
  4. mwahlgreen

    New Member

    Oct 6, 2017
    I’ll answer some of this, since you have not got any valid answers or answered them yourself.
    I can recommend TE Superseal
    Also Delphi makes some as well (E.G. CONN ASSY 150 SERIES )
    For your wire harness you need to have a two-wire system with ~40 turns pr meter.
    You should connect any third unit on the bus in a “Y” connection with as short a wire as possible, making sure the 60Ohm impedance of the bus does not degrade too much. When connecting a third wire set make a mechanical connection rather than soldering it. And make sure to make it water/splash proof with hot melt glue in heat shrink.
    Make sure your wire have 40 turns/m and that the wire have sufficient properties, like water and heat resistance for the environment (like Alpha wire)
    If you choose a shielded wire, the shield of any wire will need to be connected only ONCE for every bus wire. E.G. One wire with D-SUB 9 in both ends will need only to have shield connected at one of the D-SUB 9s, never both.
    Shielded should not be needed if you have the 40turns/m (possible with CAN GND on a third wire)
    Again your bus should have 60Ohm impedance with two terminating 120Ohm resistors at each end.
    If you look at Google (D-SUB 9 CAN-BUS) images, you will find that the standard have CAN GND, CAN Shield, and optional POWER GND.
    What do you need REF GND for? Two or more CAN-BUS modules should never have shield connected anyway. If you connect something of the nature of the connection shield, it should be CAN GND or POWER GND.
    It is very rare to have two or more CAN units on the same GND in a car, since GND tends to float due to corrosion issues.
    Right, only two 120Ohm resistors should do the trick, however, if you have Bus-heavy or Bus-busy error, maybe you need only one or three. However look for this elsewhere (Google)
    See the other answer in the forum, I have no other better solution

    Hobe this will help you or anyone else reading this post.
  5. bwilliams60

    Senior Member

    Nov 18, 2012
    Mwahlgreen. Welcome to the forum but I'd like to point out that although your answers may be good and true, berating members of the forum doesn't score you any points, especially on your first post. It is okay to be smart, much better to be wise. Your answers are solid for the most partbut not the only answers. There are some very knowledgeable people on here and although I know my fair share about vehile CANBUS, there are people better suited to answer these questions, and I will wait for them to chime in. By the way, Deutch connectors work really well on CANBUS systems, good enough for aerospace so....good enough for cars, trucks etc.
  6. mwahlgreen

    New Member

    Oct 6, 2017
    Hello bwilliam

    I have just put down the answers in a jiffy at work. I have no intention to berate anyone, just in a hurry. I was sad no one had answered the questions by now. I would have been more than glad to see someone else with valid and good answer. The poor guy seems to be in a hurry and had very few answers since July. And I just put down whatever I have learned over the years working in the automotive industry in development in Denmark, to make sure others did not come here to an "empty" search for answers.
    Many of my answers builds on troubleshooting CANBUS more than once. And more than once having to Google something over and over again, loosing my hair in the process to find E.G. that I forgot a third terminator not suppose to be there (jumper setting), wrong clock in IC/prescale or EMC being reflected too much due to lack of twisting leads/wires.