Long CAN bus

This message is partially a test to see if my unsubscribe of one email
address and subscribe from another worked, but, I thought that I would also
contribute:

In my application (outdoor ride-on railroad signal system,
http://MiniRailSolutions.com) I use CAN bus to connect control nodes around
the railroad.  These busses are very long (over 1,500 feet) and low speed
(less than 25 kbps) using MCP2551 drivers and 120 ohm terminators.

As the bus length got longer I was having reliability issues.  I started
looking at the bus with an o-scope and discovered that my issue was not
noise or reflections but rather simply that the signal amplitude was very
low.  After doing some bench testing with a couple 1,000 feet of CAT5 under
my bench, I discovered that when I switched to using 1k terminators the
signal was much better.  I think I now have field bus segments up to 2,000
feet long running fine.  So, it appears that, when using very low speeds,
amplitude is more important than reflections, etc.

Since several customers require bus runs of well over 4,000 feet I have
developed a "dual-port" version of my node which supports 2 CAN busses.
These are used as message bridges between segments and allow splitting the
bus into shorter segments.

I have also had to go to isolated CAN drivers and an isolated DC-DC supply
to address ground potential differences and lightning protection (my test
area, Florida, USA, seems to be the lightning capitol of the world) - hint
TVS diodes are not good enough!  I can tell you horror stories of the
effects caused by lightning arrestors on commercial power lines (shunt
strikes to local ground).  One railroad is located right next to a 500,000
volt substation - fun.

I sure have learned a lot in dealing with it over the past 10 years - and
have the scars to prove it.  Part of the story here:
http://www.minirailsolutions.com/living-with-lightning/

Cheers,

Chuck Hackett
UP Northern 844, Mitch-Cal Shay #2
Owner MiniRail Solutions (http://MiniRailSolutions.com)



--
Archives and useful links: http://groups.yahoo.com/group/CANbus
Subscribe and unsubscribe at www.vector.com/canlist/
Report any problems to <[email protected]>