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60hz interference with CANbus?

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

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  1. Tim Wescott

    Tim Wescott Guest

    One of the salient features of CAN is they do collision detection
    bit-by-bit. Basically, during the packet header a transmitter backs off
    if it sees a dominant bit (a 0) being transmitted when it is
    transmitting a recessive bit. This is cool because the transmitter with
    the highest priority message automatically wins, and gets to transmit
    immediately. Compare this with the inherently peer-peer nature of
    Ethernet, which detects a collision at the packet level then backs off
    for a random interval.

    This collision detection puts a number of restrictions on the bus as far
    as propagation delays and noise (and a requirement that any physical
    layer be wired-or in one way or another), but the no-delay priority
    arbitration makes it easy to build a soft real-time bus.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  2. Tim Wescott

    Tim Wescott Guest

    P.S. Yes, I know they have real-time, or at least pseudo real-time
    Ethernet these days. But I don't have the details, and I do know that
    micros with embedded CAN are a lot less expensive than micros with
    embedded Ethernet.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  3. Joerg

    Joerg Guest

    Right, I haven't come across a uC in the lower cost class that
    inherently supports Ethernet. But a lot of times I have to do this stuff
    for medical products and a DC path tyically isn't in the cards there.
    Unless you do a DC restore via clamping but that kind of gets old ;-)
     
  4. I didn't think a high end CAN controller would be in your price range, I
    just wanted to provide an existance proof ;) I figured you'd build your
    own transciever to implement the technique for whatever controller you
    were using if it seemed applicable. Mind you if anyone does know of
    this being implemented I'd be curious.
    Don't ask me to defend NXP's Web site. Every 'improvement' appears to
    take another step backwards. The latest being popups that hide the
    actual datasheet links behind some sort of annoying collective download
    thingy.

    Robert
     
  5. Joerg

    Joerg Guest

    So would I. An AC coupled CAN bus could be quite useful in
    safety-critical applications.
    That's when their ridiculous zip file habit seemed to have started.
    Instead of just starting the PDF download like everyone else they show a
    popup box. Oh man. It's quite sad to see so much self-destruction at work.
     
  6. jasen

    jasen Guest

    If you don't find one, have a look at the (National Semiconductor) DP8352C
    data sheet it appears the implementors of this card used the circuit
    basically as shown there

    Bye.
    Jasen
     
  7. Tim Wescott

    Tim Wescott Guest

    The CAN standard itself doesn't require that you have a DC path, or even
    that you use electrons to carry your data. It just requires that you
    have a media that will support recessive and dominant bits, and that the
    delay between the farthest points in the network be short enough that a
    bit collision can be detected during the bit time.

    Practically this means that your network can't span more than 1/2 bit
    time divided by your local propagation speed. This rules out high-speed
    CAN in a building, but doesn't restrict you much in a car or something
    smaller.

    The commonly used CAN transceivers _do_ specify just how to push
    electrons around, but they are specified with a separate SAE spec -- not
    CAN. There's no reason you couldn't come up with your own CAN physical
    layer that could be AC coupled (transmitting or refraining from
    transmitting a tone comes to mind), but then you'd have to come up with
    your own hardware to make it work.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  8. CAN is a lightweight network with send and forget philosophy. The CPU
    load in CAN is determined by the need to parse the inbound. Although all
    of CAN controllers provide some filtering of addresses, it is not enough
    of hardware filters as a rule. The typical requirement is somewhat about
    2K RAM and 10MHz CPU.

    Ethernet is much heavier then CAN both in the processing, memory and
    hardware requirements. By design, it is intended for the different class
    of applications: point - to - point transfer of the large chunks of
    data. I would stay away from Ethernet + TCP/IP for anything smaller then
    256k/100MHz. That would be an incomplete realization with very limited
    throughput.


    VLV
     
  9. Tim Wescott wrote:

    Yes. CAN 2.0 spec is not concerned with the physical layer. However why
    reinventing the phy?
    As pointed by Robert Adsett, the SJA1000 from Phillips has a mode of
    operation for AC coupled CAN. Quite simple indeed: they invert the
    polarity after every dominant bit. In recessive state, the transmitter
    is in high impedance. Although I have some doubts about the error
    immunity of this method, it should generally work.


    (transmitting or refraining from
    Not a very good idea. On collision, there will be an interference. It
    would be difficult to extract a meaningful data from it.


    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  10. Nico Coesel

    Nico Coesel Guest

    It is used mainly in cars. I once shorted one of the CAN wires in a VW
    Passat. It stopped the engine immediately after getting started, but
    the diagnostics still worked.
     
  11. Joerg wrote:

    [%X]
    I have found some, probably quite informative, websites that leave me
    staring at nothing more than a blank page after they have loaded what
    looked like good information. Some weirdly crude Javascript is usually to
    blame for this facet. I am not sure they read the Postmaster or Webmaster
    in-boxes either but I make the complaint about it anyway, sometimes
    including a W3C report as an attachment.

    If ever you get hold of one of their reps let them know that you have
    problems with their web-site. It just might get them fixed.

    --
    ********************************************************************
    Paul E. Bennett ....................<email://>
    Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
    Mob: +44 (0)7811-639972
    Tel: +44 (0)1235-811095
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
  12. Which is all very well until a low priority device begins to be very verbose
    and hogs the bus continuously (it is one of the documented foreseen failure
    modes for CANBus).

    --
    ********************************************************************
    Paul E. Bennett ....................<email://>
    Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
    Mob: +44 (0)7811-639972
    Tel: +44 (0)1235-811095
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
  13. You could even implement a CAN network using a traditional 20 mA
    current loop with a few nodes. Put all node transmitters and receivers
    in series and drive the loop with a single floating power supply. When
    a node wants to send a dominant state, it just cuts the loop current.

    Creating an isolated current loop system is easy, when the loop
    current passes directly through the optocouplers.

    Paul
     
  14. Nico Coesel wrote:

    No wonder. Your VW has at least three independent CAN buses: critical,
    non-critical and diagnostic. (There could be more then three in our
    days). The failure of either of the buses forces the shutdown.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant

    http://www.abvolt.com
     
  15. Do you mean high priority Chris?

    Robert
     
  16. Joerg

    Joerg Guest

    Their site doesn't find it anymore. But then again they seemed to have
    butchered their web site lately. The ones I found were CAT-5 only and
    that's AC coupled.
     
  17. Joerg

    Joerg Guest

    Been there, done that, even wrote to a few CEOs. No answers. So I don't
    write anymore because it would be a waste of time :-(

    I just skadaddle over to a mfg that allows easy access to their product
    info.
     
  18. Oops, soory about that Paul.

    Robert
     
  19. No. I was describing a failure mode and really did mean a low priority
    device running amok on the bus continuously streaming (probably) rubbish
    data. This sort of thing would lock out the high priority and any other
    devices on the bus. Hopefully the watchdogs catch this problem.


    --
    ********************************************************************
    Paul E. Bennett ....................<email://>
    Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
    Mob: +44 (0)7811-639972
    Tel: +44 (0)1235-811095
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
  20. jasen

    jasen Guest

    Sorry to send you on a fool's errand, the correct number is DP8392.

    Using google I found the datasheet on "digchip.com", (that site requires
    free regitration).

    Bye.
    Jasen
     
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