Hi,

I'm working with a CAN bus based on CANOPEN protocol. My bus lenght
is about 200m and the speed is 250Kb/s. I have 10 nodes on this bus,
the master is at one end of the bus, on the other end I've got the
resistance of 120 Ohm. The system works in a curious mode. Sometimes
the master receives messages with RTR bit at 1, when it should be 0,
for example in SDO messages, and sometimes not. Even if I reduce the
speed at 125, the same thing happens! So, I don't think is a speed
problem. I made a test, and it works: if I put a resistance also on
the second last node, I do not receive unexpected RTR bit at 1, and
the bus works correctly from the first to the last node.
Can somebody explain this to me?

Giulia

In article <1187797827.661554.301610@i13g2000prf.googlegroups .com>,
ciani.giulia@gmail.com says...
resistors at each end of the line???

ciani.giulia@gmail.com wrote:
I know nothing about the CAN bus, but 120 ohms seems a highly
peculiar line termination value. 70, or 100 ohms is more likely.
Check it. Do you have both ends of the bus terminated?

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>



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ciani.giulia@gmail.com wrote:
There are three distinct issues.

1. CAN must be terminated at each end of the main bus.

2. There is a maximum stub length to each device.

3. The controller does not sit at the 'end' of the bus in a multidrop
system.


Violate any of the above and your CAN bus may not operate as advertised.
The max speed depends on the length of the system.


Cheers

PeteS

In article <46CC62AB.39F692DB@yahoo.com>, CBFalconer says...
That's the usual termination value. The original description appears to
be of a bus with only a single terminator.

It's also worth noting that there is no master on a CAN bus.

Robert

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<ciani.giulia@gmail.com> wrote in message
news:1187797827.661554.301610@i13g2000prf.googlegr oups.com...
There should be a resistor at BOTH ends of the bus.

Meindert

This morning I insert a resistor at the other side of the bus, near
the master, and it works! Thank you!

ciani.giulia@gmail.com wrote:
That's nastily close to the absolute maximum of 250m at that bit rate.
It's right on the rule-of-thumb limit of 50e6 (m*bit)/s. You will have
to be quite careful about cabling, transceivers and oscillator
tolerances to make that work reliably.

There is no such thing as "the" master on a CAN bus. CAN is a
multi-master bus.

Bad. Stretching the bus length this close to the limit, you absolutely
need terminators at both ends.

Robert Adsett wrote:


In the original post Giulia mentioned that she is using CANopen on top of
CAN, CANopen has an master. It is called Network Management (NMT) master.
It's only a a logical behavior, of course no influence of the physical and
Data Link Layer protocol as such.

Heinz

In article <faplmf$127$02$1@news.t-online.com>, Heinz-Jürgen Oertel
says...
Ah, Missed that.

Robert

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