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'[EE]CAN Protocol wiring'
2006\02\20@131807 by Mauricio Jancic

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Hi,

           I have an already installed wiring in a factory with CAT 5 UTP
cable. The wire is not used at the moment and its installed in a star
topology. At the center of the wiring I would like to put a CAN device and
there would be another can device on each end of the cables.



           I understand that this will not work for CAN since it would need
a BUS with the shortest possible stubs. Now, how can I make (or where can I
buy) a hub for this type of connection? Anyone sell it? Oh, I'm using
micrchip's MCP2551 as the physical interface.



Regards,



Mauricio Jancic

Janso Desarrollos

Microchip Consultant Program Member

spam_OUTinfoTakeThisOuTspamjanso.com.ar

http://www.janso.com.ar

+54 11 4542 3519



2006\02\22@135811 by Dwayne Reid

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At 11:18 AM 2/20/2006, Mauricio Jancic wrote:

>             I have an already installed wiring in a factory with CAT 5 UTP
>cable. The wire is not used at the moment and its installed in a star
>topology. At the center of the wiring I would like to put a CAN device and
>there would be another can device on each end of the cables.
>
>             I understand that this will not work for CAN since it would need
>a BUS with the shortest possible stubs. Now, how can I make (or where can I
>buy) a hub for this type of connection? Anyone sell it? Oh, I'm using
>micrchip's MCP2551 as the physical interface.

What is the length limitation of the buss?  Can you do something
funky like daisy-chain the pairs?  What I mean by this is: pairs 1 &
2 connect to pairs 3 & 4 at each of the ends of the star
segments.  At the hub, connect pairs 3 & 4 of the 1st star segment
connect to pairs 1 & 2 of the next.  And so on.

What you wind up with is a buss that loops through each segment of
the star.  The downside is that total buss length is about 2x the sum
of all the star segments.  Is that too long?

This also ignores the possible effects of the different twist rates
within the cat5 cable.  Dunno if that a problem.

dwayne

--
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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2006\02\22@212259 by Vitaliy

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"Mauricio Jancic" wrote:
>            I have an already installed wiring in a factory with CAT 5 UTP
> cable. The wire is not used at the moment and its installed in a star
> topology. At the center of the wiring I would like to put a CAN device and
> there would be another can device on each end of the cables.
>
>            I understand that this will not work for CAN since it would
> need
> a BUS with the shortest possible stubs.

I don't see why not. What's the data rate, and how long is each segment?

> Now, how can I make (or where can I
> buy) a hub for this type of connection? Anyone sell it? Oh, I'm using
> micrchip's MCP2551 as the physical interface.

I don't understand exactly what the problem is, and why you want to use a
hub to solve it.

Best regards,

Vitaliy

2006\02\23@064631 by Mauricio Jancic

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part 1 2360 bytes content-type:text/plain; (decoded 7bit)

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
infospamKILLspamjanso.com.ar
http://www.janso.com.ar
+54 11 4542 3519
> {Original Message removed}

2006\02\23@070822 by Howard Winter

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Mauricio,

On Thu, 23 Feb 2006 08:46:17 -0300, Mauricio Jancic 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'm not a CAN expert, but looking at your diagram you could do it with 2 CAN busses if this is acceptable,
using the two-way technique that was suggested earlier:

Host-CAN2-Host-CAN3-Host-CAN1 (total 260m)

and

Host-CAN5-Host-CAN4 (total 200m).

The first one is a little over-spec, but not much!

Cheers,


Howard Winter
St.Albans, England


2006\02\23@071600 by YAP

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On 2/23/06, Mauricio Jancic <.....infoKILLspamspam.....janso.com.ar> wrote:
{Quote hidden}

Hi Mauricio,

I don't know how cost sensitive your application is but can't you just use
five adapters like the CANUSB (http://www.canusb.com/) or similar (CAN232
http://www.can232.com/ if you don't have an USB capable host). Use 120 ohm
terminations on each segment.  On the host use a daemon app that handle the
interfaces. If you have a host that  lives on WIN32 or UNIX my canald daemon
(http://www.vacp.org) will do this out of the box for you.

Another solution is to build a board with multiple MCP2515 which would be
fairly easy to do. I have one such board in progress and it will be
available this spring on the VSCP site.

You can of course also lower speed and daisy chain the nodes. If you want a
bullet proof solution I recommend the first approach.

Regards
/Ake

>
>
>
>

2006\02\23@133259 by Gerhard Fiedler

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YAP wrote:

> Another solution is to build a board with multiple MCP2515 which would be
> fairly easy to do. I have one such board in progress and it will be
> available this spring on the VSCP site.

Yes, that's the "hub solution". You need to mirror the traffic from each
branch to all other branches. Maybe that's even possible without the
MCP2515? Just with a bunch of MCP2551 and some logic?

Gerhard

2006\02\23@145943 by Richard Prosser

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On 24/02/06, Mauricio Jancic <infospamspam_OUTjanso.com.ar> wrote:
{Quote hidden}

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

2006\02\23@164046 by Mauricio Jancic

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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
KILLspaminfoKILLspamspamjanso.com.ar
http://www.janso.com.ar
+54 11 4542 3519

> {Original Message removed}

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