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

Paul Keinanen
Posts: n/a
      08-08-2003, 04:10 PM
On Fri, 08 Aug 2003 14:31:41 +0100, Dave Hudson
<(E-Mail Removed)> 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.


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