68300

Hello again, Jim, seems like you've made a lot of progress since last year.

You're right about the length issue.  Running at 1Mbps, you're doing about
as well as possible (and probably pushing it) with 72 feet.  I don't know
the exact spec, but I do know that the spec for 500Kbps is 30 meters (98
feet) per SAE J2284.

1.  The only difference in your given choices is 10 or 20 time quanta per
bit.  Data rates, sample points, and resync tolerances are the same in both
your choices.  So, 20 Tq per bit (what you've called Low tolerance clock)
is better because you resync to the edges more accurately.  What you've
called High data rate allows greater chance for errors (greater chance for
retransmissions) due to clock drift.  (In reality you won't have errors
with either setting.)

2.  Again, stick with more Tq per bit.

3.  Sorry, don't have one.

4.  You're right that noise is probably not an issue.

5.  You're right that your cable is just fine now.  I don't think you can
gain any appreciable length by using larger guage conductors.  This is
outside my knowledge base, though.

6.  1 Mhz at 50M sounds too good to be true.

Jeff





"Godiska, Jim" <jgodiska@...> on 08/28/2002 11:11:21 AM

Please respond to 68300@yahoogroups.com

To:    68300@yahoogroups.com
cc:
Subject:    [68300] 68376: CAN bus rate and length issue


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]



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