Richard,
	As you correctly pointed out the impedance is 120 ohm. I was
"paralleling" both termination resistor and that gaves me the 60 ohms. There
is a part on a CiA document
http://www.diakom.ru:8001/el/communication/can/canphy.pdf 

(page 34)  that gives an example where one can use a 3 way star with 180
termination resistors. 

Regards,

Mauricio Jancic
Janso Desarrollos
Microchip Consultant Program Member
info@janso.com.ar
www.janso.com.ar
+54 11 4542 3519

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Richard Prosser
> Sent: Thursday, February 23, 2006 5:00 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE]CAN Protocol wiring
> 
> 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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