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Re: control area network (CAN) bus speeds > 1Mbps possible?

Discussion in 'Embedded' started by Paul Keinanen, Aug 8, 2003.

  1. On Fri, 08 Aug 2003 14:31:41 +0100, Dave Hudson
    <-this.net> wrote:

    >Issues of whether you can actually find a CAN controller that can do the
    >higher speeds (it's about 6 or 7 years since I last used CAN), the main
    >question I would have would be whether you can afford to drop you
    >maximum network size? At 5 Mbps the maximum distance between two nodes
    >would reduce to 1/5th of the maximum distance at 1 Mbps.


    In fact the drop would be much more drastic than that due to the fixed
    propagation times in the CAN controller and the CAN transceivers.

    At lower speeds the distance drops to one half when the speed is
    doubled. However the maximum distance for 500 kbit/s is 100 m, for 800
    kbit/s 50 m and for 1 Mbit/s only 25 m (without optocouplers, with
    optocouplers only 9 m).

    The problem is that the total propagation time has to be counted twice
    when processing the ACK bit. Thus, even if the CAN controller is
    overclocked, the internal propagation delays remain the same. With
    typical CAN controller delays of 50 to 62 ns, transceiver delays 120
    to 250 ns and cable propagation delay 5 ns/m, you could not reach very
    far even at 2 Mbit/s.

    With 50 ns controller and 120 ns transceiver delay it would take at
    least 340 ms back-to-back propagation delay even if the devices are
    wired directly to each other and no optocouplers are used. This might
    be just enough for a 500 ns bit time (2 Mbit/s) but I really don't see
    how anything more than that could be used.

    Paul
     
    Paul Keinanen, Aug 8, 2003
    #1
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