On 24/02/06, Mauricio Jancic <info@janso.com.ar> wrote:
> A customer already wired an installation like it's shown in the diagram. The
> communications are made using CAN at 250 kb/s.
>
> I would like that someone guides me on how to connect the nodes so I can
> have a reliable network.
>
> The CAN standard specifies a max length of 250m for 250kb/s, and a max stub
> length of 10m.
>
> First of all, I do not understand what it is exactly Tpropseg, used on the
> calculation of the line lengths.
>
> Now, all the cables are CAT5 wires, the same used con Ethernet networks. One
> solution was to simulate a line going back and forth using different pairs,
> like Dwayne suggested, but the bus length will be around 700m, which is a
> lot more that 250m :)
>
> I first thought of using a hub, but now I saw on the standard that the like
> can be evenly balanced, in my case it will have termination resistors on
> every node that will be of 390 ohm. That will give me the ~60 ohm impedance
> I need...
>
> Is that correct? Does someone has any other suggestion?
>
> Mauricio Jancic
> Janso Desarrollos
> Microchip Consultant Program Member
> info@janso.com.ar
> www.janso.com.ar
> +54 11 4542 3519

Mauricio,
The termination impedance (60ohms?) needs to be put in place to
prevent reflections down the lines. Otherwise signals will refelct
from the ends and propogate back slightly delayed, interfering with
following data packets.
Therfore each line needs to be terminated with the correct impedance.
(^0 ohms seems very low - shouldn't it be 120 ohms? - or even higher
for 250kb/sec)

This then raises a problem with stubs, as the impedance at the
junction will be 1/2 that of the line and will therefore cause a
reflection also. If the stub is short. it can be left unterminated as
the delay will be insignificant. If the stub is long you will probably
need to fit some sort of repeater

If you drop the bit rate then the delay permitted before reflections
cause problems increases, and once the permitted delay is long enough,
then terminations and stub lengths become less of an issue.

at 250kb/sec, the "fundamental" frequency is 125kHz with a wavelength
in cable of about 1608m.  The 250m limit may come from  cable
attenuation as well as possible reflection problems. The stub length
issue is directly related to reflections - works out to a delay of
about 1/8 wavelength so that seems about right.

Richard P

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