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60hz interference with CANbus?

E

EnigmaPaul

Hi All,

I am working on an electronic engine controller with CANbus at
250kpbs. It is used on a large 200KW generator set, 480V-3phase.
Getting some strange CANbus missing data but very intermittent and
only seems to happen in the testing lab and not in other situations.
The only difference I can think of is that in the lab the main power
wires are connected to a load bank to run under load testing. I also
noticed that the CANbus wire is shield twisted-pair but it running
directly on top of the AC alternator and current transformer so its
practically living next to the main power outputs of the generator
set.

I'm wonder if its conceivable that 60hz electric or magnetic field
interference could effect the CANbus at all?

Thanks,

Paul
 
V

Vladimir Vassilevsky

EnigmaPaul said:
Hi All,

I am working on an electronic engine controller with CANbus at
250kpbs. It is used on a large 200KW generator set, 480V-3phase.
Getting some strange CANbus missing data but very intermittent and
only seems to happen in the testing lab and not in other situations.
The only difference I can think of is that in the lab the main power
wires are connected to a load bank to run under load testing. I also
noticed that the CANbus wire is shield twisted-pair but it running
directly on top of the AC alternator and current transformer so its
practically living next to the main power outputs of the generator
set.

I'm wonder if its conceivable that 60hz electric or magnetic field
interference could effect the CANbus at all?

CAN is quite robust to the air coupled junk. Most likely you are
experiencing the strong common mode interference along the CAN line.
Make sure there is no bouncing between the ground potentials on the both
ends of the bus, and there is no grounding currents via CAN.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
 
J

Joerg

Vladimir said:
CAN is quite robust to the air coupled junk. Most likely you are
experiencing the strong common mode interference along the CAN line.
Make sure there is no bouncing between the ground potentials on the both
ends of the bus, and there is no grounding currents via CAN.

If it is common mode noise Paul could try some clamp-on ferrites on the
lines. Either order from a place like Amidon or, with lots of luck, a
Radio Shack here or there might have some. A couple years ago I lucked
out when I had a similar situation at a client. Their local Radio Shack
had four of them (down to zero when I left...).

For hardcore CM noise: Get some large #43 and some large #77 or J
ferrites. 1" O.D. or more. Then pull the cable through as many times as
possible without force or cable damage. If that doesn't change a thing
then most likely it ain't CM.

Generators produce a lot more than just 60Hz. Besides lots of harmonics
there may be commutation spikes etc.
 
V

Vladimir Vassilevsky

Joerg wrote:

If it is common mode noise Paul could try some clamp-on ferrites on the
lines.

The CM input impedance of CAN is at the order of kOhms. Besides, the
proper termination of CAN on the transceiver should have a decent common
mode choke. The external ferrite can't really help.
Generators produce a lot more than just 60Hz. Besides lots of harmonics
there may be commutation spikes etc.

CAN is very reliable indeed. However there are two typical mistakes when
it works with glitches:

1. No common ground connection between the ends of the bus at all.
2. The huge current along the bus ground.

I believe it can be either one of those two reasons. An optocoupled CAN
can help both cases however it looks like an overkill for this application.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
 
E

EnigmaPaul

The CAN cable is shielded and is grounded at one end only, which is at
the engine ECM. Both ends are terminated with a 120ohm resistor and
the other end of shield is unterminated.
 
M

martin griffith

top posting corrected
The CAN cable is shielded and is grounded at one end only, which is at
the engine ECM. Both ends are terminated with a 120ohm resistor and
the other end of shield is unterminated.
try 100nF to ground on the unterminated shield


martin
 
J

Joerg

Vladimir said:
Joerg wrote:




The CM input impedance of CAN is at the order of kOhms. Besides, the
proper termination of CAN on the transceiver should have a decent common
mode choke. The external ferrite can't really help.

They can. I have used them successfully on other kinds of buses that
have inherent CM rejection. Ethernet as well. I would definitely try.

CAN is very reliable indeed. However there are two typical mistakes when
it works with glitches:

1. No common ground connection between the ends of the bus at all.
2. The huge current along the bus ground.

I believe it can be either one of those two reasons. An optocoupled CAN
can help both cases however it looks like an overkill for this application.

The 2nd problem can be helped with additional CM chokes if it is noise
at higher frequencies that rides on that ground. I agree on the
opto-solution. It's often done in RS232 but that is almost like using a
Smith&Wesson to kill a fly. I think in "Once upon a Time in the West" a
guy did that...
 
V

Vladimir Vassilevsky

They can. I have used them successfully on other kinds of buses that
have inherent CM rejection. Ethernet as well. I would definitely try.


CAN is *not* differential. It is pseudo differential. CAN requires that
the both ends of the bus should have the common reference ground. CAN is
three wires, not two. CAN-H, CAN-L and GND. That makes it very
different from the TP Ethernet, for example. The most common reason for
the problems with CAN is the bouncing between the reference grounds at
the ends.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
 
J

Joerg

Vladimir said:
CAN is *not* differential. It is pseudo differential. CAN requires that
the both ends of the bus should have the common reference ground. CAN is
three wires, not two. CAN-H, CAN-L and GND. That makes it very
different from the TP Ethernet, for example. The most common reason for
the problems with CAN is the bouncing between the reference grounds at
the ends.

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."

Now "immune" is maybe a bit of a stretch here but that should leave
available options for common mode debugging. of course, it's not really
isolated like LAN but at least differential. And a 0.9V hysteresis is
pretty good.
 
V

Vladimir Vassilevsky

Joerg wrote:

Supposedly it is to be ground-bounce insensitive. Here is a link:
http://www.embedded.com/showArticle.jhtml?articleID=13000304

The typical CAN transceiver is operable if the CM is in the range of
+/-12V or so. It is not too much, especially if there is an electric
power equipment in proximity. The fact is that the CAN is unstable if
there is no good solid ground, and this is a major source of trouble
with it.

VLV
 
J

Joerg

Vladimir said:
Joerg wrote:




The typical CAN transceiver is operable if the CM is in the range of
+/-12V or so. It is not too much, especially if there is an electric
power equipment in proximity. The fact is that the CAN is unstable if
there is no good solid ground, and this is a major source of trouble
with it.

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.
 
V

Vladimir Vassilevsky

Joerg wrote:

Yes, when it hits the rail it'll fall apart. You seem to be quite
knowledgeable about CAN:

I work for the automotive.
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.

CAN is a common bus architecture. CAN is pretty cheap and simple, and it
runs via any pair of wires. The neat feature of CAN is the bus
arbitration and the collision avoidance at the hardware level. This
implies the DC coupling. With the AC coupled architecture, the star
configuration with hubs would be required. Also, that would require some
sort of network management. So it would be rather expensive and
complicated, especially at the time when CAN was introduced.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
 
V

Vladimir Vassilevsky

Joerg wrote:

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.

P.S.

The initially proposed CAN standard was also about the fail safe
operation. The bus was supposed to be fully operable if either CAN-H or
CAN-L is broken or short to the GND or Vcc. That implies the connection
to the ground.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
 
P

Paul Keinanen

Vladimir Vassilevsky wrote:

Traditional 20 mA current loop, properly terminated RS-422/485 and CAN
bus systems are very closely related and work without a separate
ground wire.

In all these systems, one end of the transmitter sends a current into
the loop, which flows through some kind of load or termination
resistance and return to the other end of the transmitter.

At the other end the voltage drop across the load/termination
resistance is sensed (either directly or as the light intensity in the
optoisolator in an isolated 20 mA current loop).

RS-422/485 are bipolar current loops, while the "20 mA" and CAN are
unipolar with nominally 20 mA in the Mark state or 10-70 mA Dominant
state and practically 0 mA in the Space or Recessive state and hence
no voltage drop across the load/termination resistance.

The only thing that might justify the separate ground wire is the
receiver common mode voltage problem and the receiver transistor
biasing. By using galvanic isolation with a floating transceiver power
supply, the high resistance receiver transistor biasing resistors will
force the floating transceiver power supply around the actual line
potential.

Some clamping diodes may be needed between the line and the floating
transceiver Vcc and Gnd, if there is a large stray capacitance between
the floating electronics and some kind of physical ground, since if
there would be a very fast common mode interference on the line, the
receiver biasing transistors would not be able to charge that stray
capacitance fast enough.

Some RS-232/422/485 converter manufacturers have even removed the
signal ground terminal from the RS-422/485 side of their never
converter versions.
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.

While the Ethernet hardware these days is dirt cheap, the situation
was quite different in the 1980s. The original Ethernet vampire tap
transceivers were very expensive, the first Ethernet I built used AUI
cabling to avoid the transceivers. Even the first external Thin Wire
Ethernet transceivers were quite expensive (but at least comparable to
the actual Ethernet AUI card in the computer).

The CANbus is from the same era.

The 10baseT Ethernet became popular much later.

Paul
 
V

Vladimir Vassilevsky

Paul Keinanen wrote:

Traditional 20 mA current loop, properly terminated RS-422/485 and CAN
bus systems are very closely related and work without a separate
ground wire.


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. Practically, the GND has to be
connected for good, and the GND problems is the most common reason for
the CAN not working properly.

In all these systems, one end of the transmitter sends a current into
the loop, which flows through some kind of load or termination
resistance and return to the other end of the transmitter.

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.
At the other end the voltage drop across the load/termination
resistance is sensed (either directly or as the light intensity in the
optoisolator in an isolated 20 mA current loop).

RS-422/485 are bipolar current loops, while the "20 mA" and CAN are
unipolar with nominally 20 mA in the Mark state or 10-70 mA Dominant
state and practically 0 mA in the Space or Recessive state and hence
no voltage drop across the load/termination resistance.

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.

VLV
 
J

Joerg

Vladimir said:
Joerg wrote:




I work for the automotive.

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

CAN is a common bus architecture. CAN is pretty cheap and simple, and it
runs via any pair of wires. The neat feature of CAN is the bus
arbitration and the collision avoidance at the hardware level. This
implies the DC coupling. With the AC coupled architecture, the star
configuration with hubs would be required. Also, that would require some
sort of network management. So it would be rather expensive and
complicated, especially at the time when CAN was introduced.

Why does it require DC coupling to allow a hub-less design? 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. No DC-coupling. The only time this LAN ever
collapsed was when someone needed a 50ohm barrel terminator and "found"
one under the table and the end of "some" piece of coax. He was
sentenced to bringing popcorn for all the next day.
 
J

Joerg

Paul said:
Traditional 20 mA current loop, properly terminated RS-422/485 and CAN
bus systems are very closely related and work without a separate
ground wire.

In all these systems, one end of the transmitter sends a current into
the loop, which flows through some kind of load or termination
resistance and return to the other end of the transmitter.

At the other end the voltage drop across the load/termination
resistance is sensed (either directly or as the light intensity in the
optoisolator in an isolated 20 mA current loop).

RS-422/485 are bipolar current loops, while the "20 mA" and CAN are
unipolar with nominally 20 mA in the Mark state or 10-70 mA Dominant
state and practically 0 mA in the Space or Recessive state and hence
no voltage drop across the load/termination resistance.

The only thing that might justify the separate ground wire is the
receiver common mode voltage problem and the receiver transistor
biasing. By using galvanic isolation with a floating transceiver power
supply, the high resistance receiver transistor biasing resistors will
force the floating transceiver power supply around the actual line
potential.

Some clamping diodes may be needed between the line and the floating
transceiver Vcc and Gnd, if there is a large stray capacitance between
the floating electronics and some kind of physical ground, since if
there would be a very fast common mode interference on the line, the
receiver biasing transistors would not be able to charge that stray
capacitance fast enough.

Some RS-232/422/485 converter manufacturers have even removed the
signal ground terminal from the RS-422/485 side of their never
converter versions.




While the Ethernet hardware these days is dirt cheap, the situation
was quite different in the 1980s. The original Ethernet vampire tap
transceivers were very expensive, the first Ethernet I built used AUI
cabling to avoid the transceivers. Even the first external Thin Wire
Ethernet transceivers were quite expensive (but at least comparable to
the actual Ethernet AUI card in the computer).

The CANbus is from the same era.

The 10baseT Ethernet became popular much later.

Ok, this was in the 80's: We needed ECG trigger inputs for our
ultrasound machines so we figured we might as well design a simple ECG
unit and put it in there. Must be isolated to IEC/EN (and the old UL544
back then). As usual, this extra feature was supposed to cost next to
nothing. So, someone had the idea to use what was then called "Star-LAN"
transformers. I had a hard time talking the group out of that because
they were only available up to 1.5kV breakdown which isn't enough for
medical. But I remember those were really cheap, a few dimes at the most
and that was in low quantities.

Custom-made could be much cheaper, provided you need some large 6-7
digit quantity. Around 1990 I designed a switcher where I needed one.
Not happy with the 50c price tag we asked a Taiwanese company. They made
a custom part for us at a fraction of that.
 
V

Vladimir Vassilevsky

Joerg said:
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 :)

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

Detecting of the collisions of the AC signals at the bit level wouldn't
be very trivial.

NRZ would
mean more signal analysis to detect a contention but that's cheap these
days.

It would be difficult to distinguish the dominant and recessive levels
with the transformer coupled signal.
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.

In the coax Ethernet, the tranceivers are DC coupled at the bus side. It
detects the collisions by the DC level. There were bus-type LANs with
the AC coupling, however they detected the collisions by the packets
destroyed, and/or used the centralized management to eliminate the
contentions. You can afford that if you have a lot of bandwidth and
computing power, however this is not the case with CAN.
No DC-coupling. The only time this LAN ever
collapsed was when someone needed a 50ohm barrel terminator and "found"
one under the table and the end of "some" piece of coax. He was
sentenced to bringing popcorn for all the next day.

I can remember that too. The coax ethernet was a hassle: every time a
cable was jammed somewhere, the whole network was down. I had to check
the cabling section by section. An enlightened installer used some
pieces of 75 Ohm cable intermixed with 50 Ohm, which also contributed to
the reliability...



VLV
 
J

Joerg

Vladimir said:
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.

Detecting of the collisions of the AC signals at the bit level wouldn't
be very trivial.

Ideally that would have to be at the signal signature level. I guess
that could be called "sub-bit level".
It would be difficult to distinguish the dominant and recessive levels
with the transformer coupled signal.

Yes, that is certainly true.

In the coax Ethernet, the tranceivers are DC coupled at the bus side. It
detects the collisions by the DC level. There were bus-type LANs with
the AC coupling, however they detected the collisions by the packets
destroyed, and/or used the centralized management to eliminate the
contentions. You can afford that if you have a lot of bandwidth and
computing power, however this is not the case with CAN.



I can remember that too. The coax ethernet was a hassle: every time a
cable was jammed somewhere, the whole network was down. I had to check
the cabling section by section. An enlightened installer used some
pieces of 75 Ohm cable intermixed with 50 Ohm, which also contributed to
the reliability...

I have yet to see a client where I don't find a mix of 50/75 coaxes in
the lab that look deceptively similar.
 
J

Jim Thompson

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.

...Jim Thompson
 
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