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60hz interference with CANbus?

J

Joerg

Jan 1, 1970
0
Jim said:
Vladimir Vassilevsky wrote:



After an engine-out over the Atlantic (in a two-engine aircraft) I
became a bit hardened in that domain ;-)

But I do prefer cars with the least amount of electronics. The others
tend to be in the shop too often.

[snip]

That seem to be peculiar to European vehicles... Mercedes, in
particular, has big problems here in AZ.

Out here it runs the gamut. The exception being some Japanese cars. But
even there the number of electric/electronic gizmos is inversely
proportional to reliability. When we came from church last Sunday I
wanted to roll down the rear right window in my wife's car (Toyota).
It's electric. Click - nada. Zilch. Again. Arrgh. Well, at least this
time it didn't quit in the rolled-down position. That'll be $200+ for
the part again I guess (last time it was under warranty). Mine has
cranks and should that ever break I am sure I can fix it with some scrap
metal parts I have in the garage.
 
J

Jim Thompson

Jan 1, 1970
0
Jim said:
Vladimir Vassilevsky wrote:



Joerg wrote:


I was almost going to ask whether you could build cars with less
electronics in there but now I better not ... ;-)


If you would know how all of that software looks like, you would never
drive a car of fly an airplane :)


After an engine-out over the Atlantic (in a two-engine aircraft) I
became a bit hardened in that domain ;-)

But I do prefer cars with the least amount of electronics. The others
tend to be in the shop too often.

[snip]

That seem to be peculiar to European vehicles... Mercedes, in
particular, has big problems here in AZ.

Out here it runs the gamut. The exception being some Japanese cars. But
even there the number of electric/electronic gizmos is inversely
proportional to reliability. When we came from church last Sunday I
wanted to roll down the rear right window in my wife's car (Toyota).
It's electric. Click - nada. Zilch. Again. Arrgh. Well, at least this
time it didn't quit in the rolled-down position. That'll be $200+ for
the part again I guess (last time it was under warranty). Mine has
cranks and should that ever break I am sure I can fix it with some scrap
metal parts I have in the garage.

Maybe it's frozen shut ?:)

...Jim Thompson
 
L

Lanarcam

Jan 1, 1970
0
Joerg :
Out here it runs the gamut. The exception being some Japanese cars. But
even there the number of electric/electronic gizmos is inversely
proportional to reliability.

I seem to remember some formula...
When we came from church last Sunday I
wanted to roll down the rear right window in my wife's car (Toyota).
It's electric. Click - nada. Zilch. Again. Arrgh. Well, at least this
time it didn't quit in the rolled-down position. That'll be $200+ for
the part again I guess (last time it was under warranty). Mine has
cranks and should that ever break I am sure I can fix it with some scrap
metal parts I have in the garage.

When I make that sort of remarks at my workplace, I get blank stares.
I don't say it much anymore, what is the point? ;)

It is annoying when non critical parts break down, but at the very
least they should ensure that safety parts are immune.
 
J

Joerg

Jan 1, 1970
0
Lanarcam said:
Joerg :



I seem to remember some formula...



When I make that sort of remarks at my workplace, I get blank stares.
I don't say it much anymore, what is the point? ;)

It is annoying when non critical parts break down, but at the very
least they should ensure that safety parts are immune.


They do a good job in that domain. But it was a hassle the first time
when the window wouldn't roll up anymore. My wife could not go anywhere
much because you don't want to park a car in a public lot with a window
open.
 
P

Paul Keinanen

Jan 1, 1970
0
Paul Keinanen wrote:




Try to take two CAN nodes and make them work together without connecting
the GND. That may even work at the bench top conditions and for the
moderate speed and the short wire length.

I have been using CAN at 50-250 kbit/s up to several hundred meters in
industrial environments, with dozens of nodes on the bus with or
without a signal ground. I haven't seen big differences one way or the
other. Of course the CAN transceiver on each node is isolated from the
rest of the node.
Practically, the GND has to be
connected for good, and the GND problems is the most common reason for
the CAN not working properly.

The signal ground may help in some situations or it may worsen the
situation, especially when a separate wire is running parallel to the
twisted pair carrying the data (i.e. not cancelling any external
field).
CAN transmitter is pulling in one direction only. If there is a CM,
there will be a huge CM glitch with ringing when the dominant level is
changed to the recessive.

When isolated transceivers are used, they usually tolerate 0.5-2.5 kV
common mode voltages. In order to cause data integrity problems, the
common mode voltage would have to be translated to a differential
voltage. When the transmitter goes to recessive state, the current
stops flowing and no voltage drop is generated across the termination
resistance.

Only if the stray capacitance from CAN-H to external ground is much
different from the stray capacitance from CAN-L to external ground, a
voltage difference could be generated across the termination
resistance, when current from the wire with lower stray capacitance
flows through the termination resistance to the other side with a
larger stray capacitance.
CAN is bidirectional bus with the arbitration at the bit level. It is
fairly sensitive to the fast CM glitches. The OP question was about
operating CAN in the proximity of the powerful alternator.

Such alternators generate very strong slowly varying magnetic fields,
but it will generate high frequency interference only if the slip
rings are sparking (assuming synchronous generator).

If a separate signal ground is used, use a wire from the same quad
pair in order to cancel out any voltage induced between the signal
pair and the ground wire. However, when using a quad pair, the balance
in the actual CAN-H and CAN-L lines may be worse than when using an
ordinary twisted pair.

Paul
 
According to the J1939 standard, there are two configurations for
cable: shielded twisted pair and un-shielded twisted pair. We are
using a shielded configuration with two wires and a shield. The
standard further states that the shield should only be grounded at a
single point on the bus, at the point of the best ground. In our
case, the shield is grounded at the engine ECM and the other end (at
our box) is unterminated. So it seems this should be correct as per
the standard.

The question is: is it reasonable to expect interference from the
power alternator on the CANbus cable if it is run in very close
proximity, or would this only happen when there is high frequency
emissions from the sparking rings? If there is a reasonable
expectation of a problem, what is the best way to avoid it and be
consistent with the J1939 standard?
 
J

Joerg

Jan 1, 1970
0
According to the J1939 standard, there are two configurations for
cable: shielded twisted pair and un-shielded twisted pair. We are
using a shielded configuration with two wires and a shield. The
standard further states that the shield should only be grounded at a
single point on the bus, at the point of the best ground. In our
case, the shield is grounded at the engine ECM and the other end (at
our box) is unterminated. So it seems this should be correct as per
the standard.

The question is: is it reasonable to expect interference from the
power alternator on the CANbus cable if it is run in very close
proximity, or would this only happen when there is high frequency
emissions from the sparking rings? If there is a reasonable
expectation of a problem, what is the best way to avoid it and be
consistent with the J1939 standard?

Please post below quoted text. Makes it easier for most of us.

Maybe you could assess the situation at the receiving end where you get
errors. Hang a fast dual-channel scope from CAN-H to board ground and
CAN-L to board ground. These are the signals the chip actually sees. But
be careful, if the noise spikes are extremely bad you could blow the
scope inputs. So you might have to initially do that with protection
diodes against VCC and board ground. If you see noise with full swings
against the rails that means trouble.

I found that in situations like this common mode filtering was required.
This wasn't CAN bus but similar buses in the presence of large switched
loads.
 
J

jasen

Jan 1, 1970
0
Vladimir Vassilevsky wrote:

Why does it require DC coupling to allow a hub-less design?

for a bus topology you need a high-impedance input that can switch to a
low impedance output, the former is hard to do with a transformer in the
input. 10base2 used expensive isolated powersupply modules to get power
to the high impedance input
NRZ would
mean more signal analysis to detect a contention but that's cheap these
days. It's been a bit long ago but at my first job (80's) we had a coax
LAN. There was no hub, just a really long line of RG58 coax which
everyone tapped into.

rg58: sounds like 10base2 (aka 'thin' ethernet), see above.

the 10base2 card I have in front of me reads 0 ohms from the coax shield to
pins 9 and 10 of the DP8392CN chip, again and from the centre conductor to
pin 14
No DC-coupling.

I see it here

Bye.
Jasen
 
P

Paul Keinanen

Jan 1, 1970
0
The question is: is it reasonable to expect interference from the
power alternator on the CANbus cable if it is run in very close
proximity, or would this only happen when there is high frequency
emissions from the sparking rings? If there is a reasonable
expectation of a problem, what is the best way to avoid it and be
consistent with the J1939 standard?

Have you looked at the signal lines with a battery powered (floating)
oscilloscope in differential (A-B) mode in order to determine, if this
is a 50/60 Hz issue due to the magnetic field or some high frequency
issue due to the slip rings ?

The twisted pair relies on cancelling induced interference during each
turn, so make sure that a good quality twisted pair cable is used with
a constant turns/m ratio. Also avoid sharp bends (created by an
enthusiastic installer making a "pretty" wiring with sharp bends),
which would destroy the symmetry.

If everything else fails, there are various "CAN extenders" using
fiber optics, but these create extra propagation delay, thus reducing
the maximum distance. However, at 250 kbit/s (J1939) this is usually
not a problem.

Paul
 
P

Paul Keinanen

Jan 1, 1970
0
Maybe you could assess the situation at the receiving end where you get
errors. Hang a fast dual-channel scope from CAN-H to board ground and
CAN-L to board ground. These are the signals the chip actually sees. But
be careful, if the noise spikes are extremely bad you could blow the
scope inputs. So you might have to initially do that with protection
diodes against VCC and board ground. If you see noise with full swings
against the rails that means trouble.

When trying to measure differential voltages riding on a large (>10x)
common mode voltage, it is important that the test equipment is truly
floating (e.g. battery powered). Keep the A and B channel test leads
close to each other and away from any grounded objects and put the
test equipment away from any grounded objects (e.g. on a wooden chair)
and do not touch the instrument or the test leads during the
measurements.

Touching or even being close to one signal conductor only may increase
the stray capacitance by more than 100 pF. This is a serious problem
with high impedance systems, but with properly terminated
RS-422/485/CAN systems with 50-120 ohm termination resistance, this is
only an issue for common mode interference in the MHz range. The
unbalanced stray capacitance will cause a current to flow in the load
resistor and convert common mode interference to differential
interference, which should be avoided.

Paul
 
J

Joerg

Jan 1, 1970
0
jasen said:
for a bus topology you need a high-impedance input that can switch to a
low impedance output, the former is hard to do with a transformer in the
input. 10base2 used expensive isolated powersupply modules to get power
to the high impedance input

For the bus one would have to define what constitutes a bus access. It
can be done without DC coupling. But I guess once a protocol is in place
it'll be too late. WRT power transfer we do isolated power transfers in
medical electronics all the time. It isn't at all expensive. Except a
one-time cost for agency certification but that's something med gear has
to undergo anyhow.

The parts of the system that have patient contact typically need to be
100% isolated from everything that is mains connected. Including the bus.
rg58: sounds like 10base2 (aka 'thin' ethernet), see above.

the 10base2 card I have in front of me reads 0 ohms from the coax shield to
pins 9 and 10 of the DP8392CN chip, again and from the centre conductor to
pin 14




I see it here

I'll have to see whether I still have an old card somewhere and measure it.
 
R

Robert Adsett

Jan 1, 1970
0
Joerg said:
Yes, when it hits the rail it'll fall apart. You seem to be quite
knowledgeable about CAN: Why didn't they select an architecture like
Ethernet where it is transformer-coupled? From a cost POV those LAN
transformers are a dime a dozen these days.

I thought the bi-phase mode was supposed to be transformer coupled?
OTOH I've never heard of anyone actually using that mode and not all
controllers support it. I do seem to remember it neing psrt of the
original chips though.

Robert
 
N

Nico Coesel

Jan 1, 1970
0
Joerg said:
Supposedly it is to be ground-bounce insensitive. Here is a link:
http://www.embedded.com/showArticle.jhtml?articleID=13000304

About half way down that article: "Information is carried on the bus as
a voltage difference between the two lines. If both lines are at the
same voltage, the signal is a recessive bit. If the CAN_H line is higher
than the CAN_L line by 0.9V, the signal line is a dominant bit. There's
no independent ground reference point for these two lines. The bus is
therefore immune to any ground noise, which in a vehicle can be
considerable."

Yes and no.. CAN is specified to keep working with one signal wire
shorted to ground or 12V. If it where a pure differential bus, that
wouldn't be possible.
 
J

Joerg

Jan 1, 1970
0
Robert said:
I thought the bi-phase mode was supposed to be transformer coupled?
OTOH I've never heard of anyone actually using that mode and not all
controllers support it. I do seem to remember it neing psrt of the
original chips though.

Hey, that would be interesting. If CAN does offer a somewhat "official"
AC-coupled mode I might be able to dig into it. Have to google for it...

If the controllers don't support it, oh well. Most of the time we do
such buses sans controller anyhow. But it would be nice not to stray too
far from the officially blessed modes of operation.
 
R

Robert Adsett

Jan 1, 1970
0
Joerg said:
Hey, that would be interesting. If CAN does offer a somewhat "official"
AC-coupled mode I might be able to dig into it. Have to google for it...

See http://www.nxp.com/pip/SJA1000_N1.html for NXP's CAN controller that
supports the mode. Philips previous CAN controller also supported it
IIRC.

I've never heard of the mode being used but the fact that Philips
retained the mode in the new chip may indicate someone uses it, or maybe
it was just cheaper to keep it.

Robert
 
J

Joerg

Jan 1, 1970
0
Robert said:
See http://www.nxp.com/pip/SJA1000_N1.html for NXP's CAN controller that
supports the mode. Philips previous CAN controller also supported it
IIRC.

I've never heard of the mode being used but the fact that Philips
retained the mode in the new chip may indicate someone uses it, or maybe
it was just cheaper to keep it.

But it seems to be over $2 in higher quantities. Ouch. And the datasheet
link leads to a zip file. What were they thinking?

If I'd place a serial bus handler above a Dollar my clients would have
me flogged.
 
H

Hans-Bernhard Bröker

Jan 1, 1970
0
Nico said:
Yes and no.. CAN is specified to keep working with one signal wire
shorted to ground or 12V.

To be precise: that's the specification for the low-speed, a.k.a.
fault-tolerant CAN PHY layer. Which only works up to a certain speed
(250 kBits/s if memory serves).
If it where a pure differential bus, that wouldn't be possible.

High-speed CAN is pure differential, low-speed isn't.
 
V

Vladimir Vassilevsky

Jan 1, 1970
0
Joerg said:
Hey, that would be interesting. If CAN does offer a somewhat "official"
AC-coupled mode I might be able to dig into it. Have to google for it...

CAN is almost perfect protocol for the plastic fiber. If the ideal
isolation is a goal, you can consider going optical. It is standardized.
If the controllers don't support it, oh well.

I don't know of this mode or any controllers supporting for it.

Most of the time we do
such buses sans controller anyhow. But it would be nice not to stray too
far from the officially blessed modes of operation.

Why would you need CAN for your applications?

VLV
 
J

Joerg

Jan 1, 1970
0
Vladimir said:
CAN is almost perfect protocol for the plastic fiber. If the ideal
isolation is a goal, you can consider going optical. It is standardized.



I don't know of this mode or any controllers supporting for it.

Robert mentioned one from NXP in his post.

Most of the time we do



Why would you need CAN for your applications?

We don't need it. But we always try to stick closely to some kind of
established protocol. A nice feature of an established protocol is that
many uC contain a multi-purpose comms block that supports 2, 3 or
sometimes even 4 methods.
 
V

Vladimir Vassilevsky

Jan 1, 1970
0
Hans-Bernhard Bröker said:
To be precise: that's the specification for the low-speed, a.k.a.
fault-tolerant CAN PHY layer. Which only works up to a certain speed
(250 kBits/s if memory serves).

Fault tolerant CAN works up to 125k. Actually, this is how the
originally proposed CAN standard looked like.
High-speed CAN is pure differential, low-speed isn't.

Fault tolerant CAN is is rather queer mode of operation, still it is
used sometimes. It's advantage is the very low EMI.

VLV
 
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