68300

Hello List,
 
I am currently using the MC68376 (using a 20 MHz external clock oscillator
chip) along with Phillips TJA1050 CAN transceiver and the typical 120 ohm
terminating resistors on the bus.  Currenly, I have a machine set up with a
1 MHz bit rate, 35 nodes (in a linear network with no stubs), and a length
of about 85 feet, but the CAN bus starts going nuts.  It seems that if I
shorten the network to 31 nodes (with about 13 feet less cable length), then
the CAN bus appear to be working.  If I try a different combination of 31
nodes out of the 35, it also works again.  If I take the 31 node network and
remove one 1 foot interconnect cable and insert a 26 foot cable, the bus
goes nuts again.
 
At this point I am beginning to believe that the problem is simply the cable
length of the network.  The obvious solution may be to drop the bit rate
down to, say, 500 kHz.  However I have a few questions first.
 
1.  I can set up the parameters in the CAN module one of two ways.
Currenlty, I am using:
 
        _TOUCAN.CANCTRL1 = 0x87;        // Low tolerance clock, 1MHz CAN bus
        _TOUCAN.PRESDIV = 0x00;
        _TOUCAN.CANCTRL2 = 0xF3;
 
The other option is:
 
        _TOUCAN.CANCTRL1 = 0x04;        // High data rate, 1MHz CAN bus
        _TOUCAN.PRESDIV = 0x01;
        _TOUCAN.CANCTRL2 = 0x49;

Which would be better?
 
2.  If I drop the bit rate, does should I set the paramters more for a
higher data rate mode or for more of a less accurate clock mode?
 
3.  Does anyone have a chart showing recommended cable length, nodes, and
various bit rates for the CAN bus?
 
4.  Using a split terminating resistor with a center tapped capacitor tied
to ground, would help with noise issues, but I am thinking this is not the
main cause.  Do you agree?
 
5.  I am currently using 22AWG twisted shielded cable with the shield being
termiated at one end only.  I can increase the wire size to 20 AWG which
should help the bus length somewhat according to one application note that I
found, but I do not believe it to be a significant contributor to the
problem.  Any opinions?
 
6.  In the early days of investigating CAN bus implementations, I thought I
saw that the maximums were 1 MHz bit rate at a 50 meter length and about 110
nodes.  I thought that these three things could be achieved at the same
time.  Am I right or wrong about this?  Are there certain other things that
I need to do to meet this description?
 
Thanks in advance,
Jim
 
 


[Non-text portions of this message have been removed]