RE: Long, Low speed CAN bus

Hi Chuck,
I've not worked with longer than 500' so I may not be able to answer all =
your questions.

Remember the CAN drivers always need at least one termination resistor =
for the drive circuit to function.  I believe, but perhaps someone can =
correct me, the impedance of a cable is usually listed at a specific =
frequency like 1MHz or 10MHz and for CAN bus it's normally 120 Ohm.

A quick and easy way is with a pulse generator and scope.
https://www.edn.com/electronics-blogs/scope-junction-blogs/4431974/Quick-=
--dirty-cable-length---impedance-measurement

What you will find is that if you don't have the cable properly =
terminated the reflection may or may not impact your messages.  =
Generally the reflection is only an issue when it disrupts messages at =
specific points along the cable. =20

Think of that pebble you throw in the pond.  The ripples that spread out =
are like a sine wave slowly decreasing in amplitude.  If you measure the =
depth of the pond at the mid sine wave point at any spot it will be the =
same depth.    But move to the crest or the trough of the wave and the =
pond depth changes because the water is lower or higher than the =
midpoint.

The reflection off the end of the transmission line is like that crest =
and trough.  As it travels back along the network cable, if it happens =
to have a crest or trough at a point where a CAN device is measuring the =
bits, you will end up with a damaged message.  That node may be really =
unreliable while one 3' further down works perfectly.

What's worse is it's not just a pulse but a sequence and all but one =
message may be received perfectly but one will be intermittent. Very =
hard to find the reason.

So I think you do need to look at termination as important.

Another thing often overlooked is the CAN DC ground.   For those kinds =
of distances and cable resistance you have to address common mode =
difference between the signal (relative to ground) and the signal at the =
other end relative to its own ground.    Remember, unlike RS485 or RS422 =
the current doesn't change direction for a 0 verses a 1.  So although =
the receivers may well measure the difference they are also referenced =
to ground.

So for those kinds of distances using isolated CAN drivers where the =
power supply for the drivers accompanies the signal may work better.  =
Ie. Opto isolators on the TTL side.  The CAN devices are driven from a =
24V or 48V power supply regulated down to 5V for the driver.   =
Individual node driver current is small enough that the difference in =
ground between the ends of the cable won't cause common mode issues.

You see that approach showing up in POE systems.

So take your cable.  Start with a 5V 80uS pulse and look for the =
reflection to determine the impedance and termination needed.  And look =
at the edge rounding.  Next transmit 6.25kHz (80uS ON, 80uS OFF) and =
look at the signal at the other end with respect to rounding of the =
edges.  Maybe use a CAN driver to do this instead.

If you don't have isolated CAN power/signal then make sure the testing =
you do has a DC current load simulating the real system. =20

This should give you an idea of where your sample point should be.  The =
80% might be too far into the bit period.  Or maybe it's perfect. =20

In either case Mike Schofield's BitTime software shows you can get 8500 =
meters with 83% sample point.  So distance for the ACK bit to return =
isn't an issue.

Just some ideas.

John Dammeyer




> Environment:
> Very low speed. ~ 12,500 bps.
> Controller: MCP2515
> Driver: MCP2551
> Clock source 14.7456 MHz crystal divided by 2 my ATMega1284 processor =
*
> Lightning/Surge protection using GDT, TBU, TVS ***
> Current MCP2515 bus timing parameters:
> 	BRP		24
> 	SJW		2 **
> 	Samples	1
> 	PSEG		8
> 	PSEG1		4
> 	PSEG2		3
> 	My understanding is that the above values result in a sample point
> of 80%
>=20
> * New board design uses separate 7.3728 MHz crystal for the CAN =
drivers
> (new
> board has two controller/driver sets).
>=20
> ** Never have understood this value well enough to intelligently pick =
a
> value.
>=20
> *** The TBU adds about 11 ohms of resistance to each of CH & CL.  This =
is in
> series with the driver at each location, not in series with the cable =
from
> site-to-site.
>=20
> My Issue ...
>=20
> The bus is very long.  I have measured the round-trip cable resistance =
at
> about 85 ohms per 1,000 feet.
>=20
> In one bench test using a 564' length of CAT-5 to simulate field =
cable, four
> trips through the cable resulted in a length of 2,256 feet and a =
round-trip
> resistance of about 191 ohms.
>=20
> Using 120 ohm terminators the signal level (using a scope) was below =
1.2v
> (differential) so the driver did not decode the signal.
>=20
> I removed the terminators and it was obvious on the scope that that =
was bad
> because there was no resistance to discharge the cable capacitance =
(large RC
> discharge time).
>=20
> I then tried 1k terminators and all worked fine.  For the final test I =
had 7
> controllers at one end and two at the other generating a total of =
about 15
> messages per second.
>=20
> I could see that there was some RC discharge curve effects but, I =
assume,
> because I am using very low speed and sampling at 80% this was ok.
>=20
> Above 2k total (two 4k resistors) RC effects started cause errors =
(recessive
> did not drop low enough, fast enough).
>=20
> Showing voltage levels from near (higher) and far (lower) ends of the =
cable
> (20ms/div, all traces 1v/div):
> https://tinyurl.com/yywjxkyc
>=20
> Showing the RC effects (500 us/div):
> https://tinyurl.com/y38gz52y
>=20
> My Conclusion ...
>=20
> In my usage (low bps), cable mismatch, reflections, etc. caused by =
wrong
> terminators, stubs, etc. are not as important to getting maximum =
distance at
> is the signal amplitude.
>=20
> Questions ...
>=20
> Am I right in my conclusion (proper cable termination not important to =
me,
> for maximum distance use larger 'terminators')?  There is still a =
distance
> limit due to timing as a result of the travel time of the ACK bits =
from the
> far end of the cable.  There is also a limit on how large the =
'terminators'
> can be because the RC discharge time will become significant at some =
point.
>=20
> Do my bus timing values look reasonable?  (sometimes seems like a =
'black
> art')
>=20
> Can someone give me more insight into what value I should use for SJW? =
 My
> understanding is that this addresses clock jitter but I doubt that's =
an
> issue with my low bps rate.
>=20
> Regards,
>=20
> Chuck Hackett
> Tampa, Florida, USA
>=20
> --
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