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60hz interference with CANbus?

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

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  1. Joerg

    Joerg Guest

    Out here it runs the gamut. The exception being some Japanese cars. But
    even there the number of electric/electronic gizmos is inversely
    proportional to reliability. When we came from church last Sunday I
    wanted to roll down the rear right window in my wife's car (Toyota).
    It's electric. Click - nada. Zilch. Again. Arrgh. Well, at least this
    time it didn't quit in the rolled-down position. That'll be $200+ for
    the part again I guess (last time it was under warranty). Mine has
    cranks and should that ever break I am sure I can fix it with some scrap
    metal parts I have in the garage.
     
  2. Jim Thompson

    Jim Thompson Guest

    Maybe it's frozen shut ?:)

    ...Jim Thompson
     
  3. Lanarcam

    Lanarcam Guest

    Joerg :
    I seem to remember some formula...
    When I make that sort of remarks at my workplace, I get blank stares.
    I don't say it much anymore, what is the point? ;)

    It is annoying when non critical parts break down, but at the very
    least they should ensure that safety parts are immune.
     
  4. Joerg

    Joerg Guest


    They do a good job in that domain. But it was a hassle the first time
    when the window wouldn't roll up anymore. My wife could not go anywhere
    much because you don't want to park a car in a public lot with a window
    open.
     
  5. I have been using CAN at 50-250 kbit/s up to several hundred meters in
    industrial environments, with dozens of nodes on the bus with or
    without a signal ground. I haven't seen big differences one way or the
    other. Of course the CAN transceiver on each node is isolated from the
    rest of the node.
    The signal ground may help in some situations or it may worsen the
    situation, especially when a separate wire is running parallel to the
    twisted pair carrying the data (i.e. not cancelling any external
    field).
    When isolated transceivers are used, they usually tolerate 0.5-2.5 kV
    common mode voltages. In order to cause data integrity problems, the
    common mode voltage would have to be translated to a differential
    voltage. When the transmitter goes to recessive state, the current
    stops flowing and no voltage drop is generated across the termination
    resistance.

    Only if the stray capacitance from CAN-H to external ground is much
    different from the stray capacitance from CAN-L to external ground, a
    voltage difference could be generated across the termination
    resistance, when current from the wire with lower stray capacitance
    flows through the termination resistance to the other side with a
    larger stray capacitance.
    Such alternators generate very strong slowly varying magnetic fields,
    but it will generate high frequency interference only if the slip
    rings are sparking (assuming synchronous generator).

    If a separate signal ground is used, use a wire from the same quad
    pair in order to cancel out any voltage induced between the signal
    pair and the ground wire. However, when using a quad pair, the balance
    in the actual CAN-H and CAN-L lines may be worse than when using an
    ordinary twisted pair.

    Paul
     
  6. Guest

    According to the J1939 standard, there are two configurations for
    cable: shielded twisted pair and un-shielded twisted pair. We are
    using a shielded configuration with two wires and a shield. The
    standard further states that the shield should only be grounded at a
    single point on the bus, at the point of the best ground. In our
    case, the shield is grounded at the engine ECM and the other end (at
    our box) is unterminated. So it seems this should be correct as per
    the standard.

    The question is: is it reasonable to expect interference from the
    power alternator on the CANbus cable if it is run in very close
    proximity, or would this only happen when there is high frequency
    emissions from the sparking rings? If there is a reasonable
    expectation of a problem, what is the best way to avoid it and be
    consistent with the J1939 standard?
     
  7. Joerg

    Joerg Guest

    Please post below quoted text. Makes it easier for most of us.

    Maybe you could assess the situation at the receiving end where you get
    errors. Hang a fast dual-channel scope from CAN-H to board ground and
    CAN-L to board ground. These are the signals the chip actually sees. But
    be careful, if the noise spikes are extremely bad you could blow the
    scope inputs. So you might have to initially do that with protection
    diodes against VCC and board ground. If you see noise with full swings
    against the rails that means trouble.

    I found that in situations like this common mode filtering was required.
    This wasn't CAN bus but similar buses in the presence of large switched
    loads.
     
  8. jasen

    jasen Guest

    for a bus topology you need a high-impedance input that can switch to a
    low impedance output, the former is hard to do with a transformer in the
    input. 10base2 used expensive isolated powersupply modules to get power
    to the high impedance input
    rg58: sounds like 10base2 (aka 'thin' ethernet), see above.

    the 10base2 card I have in front of me reads 0 ohms from the coax shield to
    pins 9 and 10 of the DP8392CN chip, again and from the centre conductor to
    pin 14
    I see it here

    Bye.
    Jasen
     
  9. Have you looked at the signal lines with a battery powered (floating)
    oscilloscope in differential (A-B) mode in order to determine, if this
    is a 50/60 Hz issue due to the magnetic field or some high frequency
    issue due to the slip rings ?

    The twisted pair relies on cancelling induced interference during each
    turn, so make sure that a good quality twisted pair cable is used with
    a constant turns/m ratio. Also avoid sharp bends (created by an
    enthusiastic installer making a "pretty" wiring with sharp bends),
    which would destroy the symmetry.

    If everything else fails, there are various "CAN extenders" using
    fiber optics, but these create extra propagation delay, thus reducing
    the maximum distance. However, at 250 kbit/s (J1939) this is usually
    not a problem.

    Paul
     
  10. When trying to measure differential voltages riding on a large (>10x)
    common mode voltage, it is important that the test equipment is truly
    floating (e.g. battery powered). Keep the A and B channel test leads
    close to each other and away from any grounded objects and put the
    test equipment away from any grounded objects (e.g. on a wooden chair)
    and do not touch the instrument or the test leads during the
    measurements.

    Touching or even being close to one signal conductor only may increase
    the stray capacitance by more than 100 pF. This is a serious problem
    with high impedance systems, but with properly terminated
    RS-422/485/CAN systems with 50-120 ohm termination resistance, this is
    only an issue for common mode interference in the MHz range. The
    unbalanced stray capacitance will cause a current to flow in the load
    resistor and convert common mode interference to differential
    interference, which should be avoided.

    Paul
     
  11. Joerg

    Joerg Guest

    For the bus one would have to define what constitutes a bus access. It
    can be done without DC coupling. But I guess once a protocol is in place
    it'll be too late. WRT power transfer we do isolated power transfers in
    medical electronics all the time. It isn't at all expensive. Except a
    one-time cost for agency certification but that's something med gear has
    to undergo anyhow.

    The parts of the system that have patient contact typically need to be
    100% isolated from everything that is mains connected. Including the bus.
    I'll have to see whether I still have an old card somewhere and measure it.
     
  12. I thought the bi-phase mode was supposed to be transformer coupled?
    OTOH I've never heard of anyone actually using that mode and not all
    controllers support it. I do seem to remember it neing psrt of the
    original chips though.

    Robert
     
  13. Nico Coesel

    Nico Coesel Guest

    Yes and no.. CAN is specified to keep working with one signal wire
    shorted to ground or 12V. If it where a pure differential bus, that
    wouldn't be possible.
     
  14. Joerg

    Joerg Guest

    Hey, that would be interesting. If CAN does offer a somewhat "official"
    AC-coupled mode I might be able to dig into it. Have to google for it...

    If the controllers don't support it, oh well. Most of the time we do
    such buses sans controller anyhow. But it would be nice not to stray too
    far from the officially blessed modes of operation.
     
  15. See http://www.nxp.com/pip/SJA1000_N1.html for NXP's CAN controller that
    supports the mode. Philips previous CAN controller also supported it
    IIRC.

    I've never heard of the mode being used but the fact that Philips
    retained the mode in the new chip may indicate someone uses it, or maybe
    it was just cheaper to keep it.

    Robert
     
  16. Joerg

    Joerg Guest

    But it seems to be over $2 in higher quantities. Ouch. And the datasheet
    link leads to a zip file. What were they thinking?

    If I'd place a serial bus handler above a Dollar my clients would have
    me flogged.
     
  17. To be precise: that's the specification for the low-speed, a.k.a.
    fault-tolerant CAN PHY layer. Which only works up to a certain speed
    (250 kBits/s if memory serves).
    High-speed CAN is pure differential, low-speed isn't.
     
  18. CAN is almost perfect protocol for the plastic fiber. If the ideal
    isolation is a goal, you can consider going optical. It is standardized.
    I don't know of this mode or any controllers supporting for it.

    Most of the time we do
    Why would you need CAN for your applications?

    VLV
     
  19. Joerg

    Joerg Guest

    Robert mentioned one from NXP in his post.

    We don't need it. But we always try to stick closely to some kind of
    established protocol. A nice feature of an established protocol is that
    many uC contain a multi-purpose comms block that supports 2, 3 or
    sometimes even 4 methods.
     
  20. Fault tolerant CAN works up to 125k. Actually, this is how the
    originally proposed CAN standard looked like.
    Fault tolerant CAN is is rather queer mode of operation, still it is
    used sometimes. It's advantage is the very low EMI.

    VLV
     
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