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60hz interference with CANbus?

Discussion in 'Electronic Design' started by EnigmaPaul, Apr 12, 2007.

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  1. EnigmaPaul

    EnigmaPaul Guest

    Hi All,

    I am working on an electronic engine controller with CANbus at
    250kpbs. It is used on a large 200KW generator set, 480V-3phase.
    Getting some strange CANbus missing data but very intermittent and
    only seems to happen in the testing lab and not in other situations.
    The only difference I can think of is that in the lab the main power
    wires are connected to a load bank to run under load testing. I also
    noticed that the CANbus wire is shield twisted-pair but it running
    directly on top of the AC alternator and current transformer so its
    practically living next to the main power outputs of the generator
    set.

    I'm wonder if its conceivable that 60hz electric or magnetic field
    interference could effect the CANbus at all?

    Thanks,

    Paul
     
  2. CAN is quite robust to the air coupled junk. Most likely you are
    experiencing the strong common mode interference along the CAN line.
    Make sure there is no bouncing between the ground potentials on the both
    ends of the bus, and there is no grounding currents via CAN.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  3. Joerg

    Joerg Guest

    If it is common mode noise Paul could try some clamp-on ferrites on the
    lines. Either order from a place like Amidon or, with lots of luck, a
    Radio Shack here or there might have some. A couple years ago I lucked
    out when I had a similar situation at a client. Their local Radio Shack
    had four of them (down to zero when I left...).

    For hardcore CM noise: Get some large #43 and some large #77 or J
    ferrites. 1" O.D. or more. Then pull the cable through as many times as
    possible without force or cable damage. If that doesn't change a thing
    then most likely it ain't CM.

    Generators produce a lot more than just 60Hz. Besides lots of harmonics
    there may be commutation spikes etc.
     
  4. Joerg wrote:

    The CM input impedance of CAN is at the order of kOhms. Besides, the
    proper termination of CAN on the transceiver should have a decent common
    mode choke. The external ferrite can't really help.
    CAN is very reliable indeed. However there are two typical mistakes when
    it works with glitches:

    1. No common ground connection between the ends of the bus at all.
    2. The huge current along the bus ground.

    I believe it can be either one of those two reasons. An optocoupled CAN
    can help both cases however it looks like an overkill for this application.


    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  5. EnigmaPaul

    EnigmaPaul Guest

    The CAN cable is shielded and is grounded at one end only, which is at
    the engine ECM. Both ends are terminated with a 120ohm resistor and
    the other end of shield is unterminated.
     
  6. try 100nF to ground on the unterminated shield


    martin
     
  7. Joerg

    Joerg Guest

    They can. I have used them successfully on other kinds of buses that
    have inherent CM rejection. Ethernet as well. I would definitely try.

    The 2nd problem can be helped with additional CM chokes if it is noise
    at higher frequencies that rides on that ground. I agree on the
    opto-solution. It's often done in RS232 but that is almost like using a
    Smith&Wesson to kill a fly. I think in "Once upon a Time in the West" a
    guy did that...
     

  8. CAN is *not* differential. It is pseudo differential. CAN requires that
    the both ends of the bus should have the common reference ground. CAN is
    three wires, not two. CAN-H, CAN-L and GND. That makes it very
    different from the TP Ethernet, for example. The most common reason for
    the problems with CAN is the bouncing between the reference grounds at
    the ends.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  9. Joerg

    Joerg Guest

    Supposedly it is to be ground-bounce insensitive. Here is a link:
    http://www.embedded.com/showArticle.jhtml?articleID=13000304

    About half way down that article: "Information is carried on the bus as
    a voltage difference between the two lines. If both lines are at the
    same voltage, the signal is a recessive bit. If the CAN_H line is higher
    than the CAN_L line by 0.9V, the signal line is a dominant bit. There's
    no independent ground reference point for these two lines. The bus is
    therefore immune to any ground noise, which in a vehicle can be
    considerable."

    Now "immune" is maybe a bit of a stretch here but that should leave
    available options for common mode debugging. of course, it's not really
    isolated like LAN but at least differential. And a 0.9V hysteresis is
    pretty good.
     
  10. Joerg wrote:

    The typical CAN transceiver is operable if the CM is in the range of
    +/-12V or so. It is not too much, especially if there is an electric
    power equipment in proximity. The fact is that the CAN is unstable if
    there is no good solid ground, and this is a major source of trouble
    with it.

    VLV
     
  11. Joerg

    Joerg Guest

    Yes, when it hits the rail it'll fall apart. You seem to be quite
    knowledgeable about CAN: Why didn't they select an architecture like
    Ethernet where it is transformer-coupled? From a cost POV those LAN
    transformers are a dime a dozen these days.
     
  12. Joerg wrote:

    I work for the automotive.
    CAN is a common bus architecture. CAN is pretty cheap and simple, and it
    runs via any pair of wires. The neat feature of CAN is the bus
    arbitration and the collision avoidance at the hardware level. This
    implies the DC coupling. With the AC coupled architecture, the star
    configuration with hubs would be required. Also, that would require some
    sort of network management. So it would be rather expensive and
    complicated, especially at the time when CAN was introduced.


    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  13. Joerg wrote:

    P.S.

    The initially proposed CAN standard was also about the fail safe
    operation. The bus was supposed to be fully operable if either CAN-H or
    CAN-L is broken or short to the GND or Vcc. That implies the connection
    to the ground.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  14. Traditional 20 mA current loop, properly terminated RS-422/485 and CAN
    bus systems are very closely related and work without a separate
    ground wire.

    In all these systems, one end of the transmitter sends a current into
    the loop, which flows through some kind of load or termination
    resistance and return to the other end of the transmitter.

    At the other end the voltage drop across the load/termination
    resistance is sensed (either directly or as the light intensity in the
    optoisolator in an isolated 20 mA current loop).

    RS-422/485 are bipolar current loops, while the "20 mA" and CAN are
    unipolar with nominally 20 mA in the Mark state or 10-70 mA Dominant
    state and practically 0 mA in the Space or Recessive state and hence
    no voltage drop across the load/termination resistance.

    The only thing that might justify the separate ground wire is the
    receiver common mode voltage problem and the receiver transistor
    biasing. By using galvanic isolation with a floating transceiver power
    supply, the high resistance receiver transistor biasing resistors will
    force the floating transceiver power supply around the actual line
    potential.

    Some clamping diodes may be needed between the line and the floating
    transceiver Vcc and Gnd, if there is a large stray capacitance between
    the floating electronics and some kind of physical ground, since if
    there would be a very fast common mode interference on the line, the
    receiver biasing transistors would not be able to charge that stray
    capacitance fast enough.

    Some RS-232/422/485 converter manufacturers have even removed the
    signal ground terminal from the RS-422/485 side of their never
    converter versions.
    While the Ethernet hardware these days is dirt cheap, the situation
    was quite different in the 1980s. The original Ethernet vampire tap
    transceivers were very expensive, the first Ethernet I built used AUI
    cabling to avoid the transceivers. Even the first external Thin Wire
    Ethernet transceivers were quite expensive (but at least comparable to
    the actual Ethernet AUI card in the computer).

    The CANbus is from the same era.

    The 10baseT Ethernet became popular much later.

    Paul
     
  15. Paul Keinanen wrote:


    Try to take two CAN nodes and make them work together without connecting
    the GND. That may even work at the bench top conditions and for the
    moderate speed and the short wire length. Practically, the GND has to be
    connected for good, and the GND problems is the most common reason for
    the CAN not working properly.

    CAN transmitter is pulling in one direction only. If there is a CM,
    there will be a huge CM glitch with ringing when the dominant level is
    changed to the recessive.
    CAN is bidirectional bus with the arbitration at the bit level. It is
    fairly sensitive to the fast CM glitches. The OP question was about
    operating CAN in the proximity of the powerful alternator.

    VLV
     
  16. Joerg

    Joerg Guest

    I was almost going to ask whether you could build cars with less
    electronics in there but now I better not ... ;-)

    Why does it require DC coupling to allow a hub-less design? NRZ would
    mean more signal analysis to detect a contention but that's cheap these
    days. It's been a bit long ago but at my first job (80's) we had a coax
    LAN. There was no hub, just a really long line of RG58 coax which
    everyone tapped into. No DC-coupling. The only time this LAN ever
    collapsed was when someone needed a 50ohm barrel terminator and "found"
    one under the table and the end of "some" piece of coax. He was
    sentenced to bringing popcorn for all the next day.
     
  17. Joerg

    Joerg Guest

    Ok, this was in the 80's: We needed ECG trigger inputs for our
    ultrasound machines so we figured we might as well design a simple ECG
    unit and put it in there. Must be isolated to IEC/EN (and the old UL544
    back then). As usual, this extra feature was supposed to cost next to
    nothing. So, someone had the idea to use what was then called "Star-LAN"
    transformers. I had a hard time talking the group out of that because
    they were only available up to 1.5kV breakdown which isn't enough for
    medical. But I remember those were really cheap, a few dimes at the most
    and that was in low quantities.

    Custom-made could be much cheaper, provided you need some large 6-7
    digit quantity. Around 1990 I designed a switcher where I needed one.
    Not happy with the 50c price tag we asked a Taiwanese company. They made
    a custom part for us at a fraction of that.
     
  18. If you would know how all of that software looks like, you would never
    drive a car of fly an airplane :)

    Detecting of the collisions of the AC signals at the bit level wouldn't
    be very trivial.

    It would be difficult to distinguish the dominant and recessive levels
    with the transformer coupled signal.
    In the coax Ethernet, the tranceivers are DC coupled at the bus side. It
    detects the collisions by the DC level. There were bus-type LANs with
    the AC coupling, however they detected the collisions by the packets
    destroyed, and/or used the centralized management to eliminate the
    contentions. You can afford that if you have a lot of bandwidth and
    computing power, however this is not the case with CAN.
    I can remember that too. The coax ethernet was a hassle: every time a
    cable was jammed somewhere, the whole network was down. I had to check
    the cabling section by section. An enlightened installer used some
    pieces of 75 Ohm cable intermixed with 50 Ohm, which also contributed to
    the reliability...



    VLV
     
  19. Joerg

    Joerg Guest

    After an engine-out over the Atlantic (in a two-engine aircraft) I
    became a bit hardened in that domain ;-)

    But I do prefer cars with the least amount of electronics. The others
    tend to be in the shop too often.

    Ideally that would have to be at the signal signature level. I guess
    that could be called "sub-bit level".
    Yes, that is certainly true.

    I have yet to see a client where I don't find a mix of 50/75 coaxes in
    the lab that look deceptively similar.
     
  20. Jim Thompson

    Jim Thompson Guest

    [snip]

    That seem to be peculiar to European vehicles... Mercedes, in
    particular, has big problems here in AZ.

    ...Jim Thompson
     
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