RE: MCP2515 - Recovering from Error-Passive state

Hi Chuck,

I can tell you from direct experience that the bus length is really
important.  I was right on the edge for the rings project with one cable at
35m I believe.  What I neglected to factor in was the extra meter from the
termination point to the bridges.   That took me over the edge.  It worked,
most of the time but unreliably.

On a day where we had to do maintenance on several water filled lights we
had the guy on the bucket go up and remove each of the 50 lights on several
long segments a few at a time so I could reprogram them with a new version
of the code that allowed the bit rate to be dropped from 1Mbps to 500kbps.
The TEK WCS-10 was then configured over CANOpen to send 1MBps to two
segments and 500kbps to the third.  The main system was dropped to 500kbps
sending into the bridge.

The double rings on the barge all ran 500kbps since that set was designed to
tilt to be transported under a road bridge into the inner harbor so the
cables were all longer than 35m.

Something else to think about.  The 120 Ohm termination at each end is to
match the cable impedance.  The cable impedance is determined by the
inductance of the twisted pairs and the capacitance and is also rated at 120
ohms.  Matching termination is what prevents the reflections of the high
frequency component of the CAN signal edges.   If you are using different
cable you are better off to terminate it with resistors that match the
cable.  But if they are lower in value you have to watch that you no longer
exceed the drive capabilities of the CAN drivers.  Ie.  It reduces the
number of nodes.

The late Steve Corrigan from TI (designer of TI's CAN drivers) wrote a paper
on minimum inter-node spacing.  My nodes on the rings were closer than that
and I have scope photos of the exact issue he wrote about.  I doubt it's a
problem at 125kHz but it could have been at 1Mhz if I hadn't used DeviceNet
Thick cable.

John Dammeyer

> 
> In one location currently the nodes near one end (but separated by maybe
> 300
> feet) are having trouble talking to each other but other nodes on the same
> bus (about 1,500 feet long) seem to be happy.  If it was only the fact
that
> the bus was too long for the data rate (I am looking into this) I would
> expect that the nodes close to the other end would also have trouble (if
the
> problem is 'late' ACKs destroying dominant bits, etc.).
> 
> Cheers,
> 
> Chuck Hackett

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