CAN bus termination - split with a capacitor and common mode chokes

[Postal2]

Do it this way:
(see picture)

... The tapped terminator, with CMC, I would expect is generally better than not using it; ....

Your opinion is as always valuable.

[T3sl4co1l]

And just for completeness, the proposed alternative.



(*) Intermediate nodes can't be treated the same way, so default to the base case regardless.  (Note this means the proposal is inclusive, it's a superset of the base case; only the end nodes can be terminated in this way, and it is optional whether one or both do.)

V_N shown to suggest noise; note that in the terminal cases, the split terminators with CMCs are highly effective against interference (large CM attenuation ratio), but the intermediate nodes see no such benefit while in recessive mode.

Also consider the case with terminator inside the CMC (notice the CMC is itself a stub length!!) versus outside, or if the distance between say nodes 1 and 2 is small enough and they share GND so that they can be behind the same CMC, which then realizes the grouped-nodes case I mentioned.

To be perfectly precise, note I'm not saying one way is strictly better than the other; I would need to know all possible use-cases, sources of interference, loads, etc., and that's just not possible, I don't have nearly enough experience in this (and, I would suspect no one else yet in this thread does, either).  But I can claim (with some justification) that the tapped-terminator case is generally better (lower emissions, higher immunity) -- when the optional is available to do so -- and that this will be true for more cases than the default connection is.

Not even sure what circumstances I could craft that would make it strictly worse; maybe in a double-isolated case, where the bus itself is floating, and now the fact that it's ~statically anchored to one ground more than others see a different impedance, and, maybe it's still fine for the given node, but the others are worse or something?  I'm kind of grasping at straws here, honestly.

It's great when you can do it is the point, you can just do it at only two points.

Perhaps it's bad for bulk current injection; automotive heads might offer some insight here.  BCI is a weird test; they don't care how the current gets in, they just crank up the test level until it's met.  Scary, probably wildly unrealistic when impedances are extreme, but if one wants to play against such tests, a lower impedance is desired.  Mind, that's the whole point, the split terminator does better with BCI in and of itself, but the real question is the CMC, does it saturate, does it catch fire, when that much (10s mA? at RF) current is applied?

Tim

[Postal2]

And just for completeness, the proposed alternative. ...

"CAN transceiver" and "ESD protection" are essentially one chip. Diodes are added very rarely. The parameters of the internal protection of the transmitter comply with ISO requirements.
https://cccsolutions.eu/wp-content/uploads/2017/08/ISO-7637-22011E-STANDARD-CCC-Solutions-AB-Sweden-.pdf

[T3sl4co1l]

Well, transceivers are all over the place.  Maybe it's integrated, maybe not.  Certainly, TVS for the purpose remain available in great quantity, implying a need somewhere.

Tim

[Simon]

And just for completeness, the proposed alternative. ...

"CAN transceiver" and "ESD protection" are essentially one chip. Diodes are added very rarely. The parameters of the internal protection of the transmitter comply with ISO requirements.
https://cccsolutions.eu/wp-content/uploads/2017/08/ISO-7637-22011E-STANDARD-CCC-Solutions-AB-Sweden-.pdf

That standard is about the resilience of the device to disturbances in the power input. It won't cover "what do you do if for some unknown reason out of your control on one specific installation due to cabling or equipment not designed or supplied by you the can bus has some static or other transient on it?". For this reason people put TVS diodes, because why have to argue with a customer about their shitty use of your equipment that they now say is failing and it being your fault.

[Postal2]

.... people put TVS diodes, ...

I have seen industrial device with TVS on CAN bus only once. I was very surprised.

I have never seen a 60 + 60 + capacitor terminator. I would wash my hands after touching such a device.

[T3sl4co1l]

Guess you have to wash your hands every time you touch something with Ethernet...

[uer166]

.... people put TVS diodes, ...

I have seen industrial device with TVS on CAN bus only once. I was very surprised.

I have never seen a 60 + 60 + capacitor terminator. I would wash my hands after touching such a device.

LOL... I guess you have to wash your hands every time you drive a modern car

[Postal2]

LOL... I guess you have to wash your hands every time you drive a modern car

I don't really have any experience with car automotive electronics for some reason, it would be nice to see a photo of the unit with such a terminator if you have one.

[ifrythings]

LOL... I guess you have to wash your hands every time you drive a modern car

I don't really have any experience with car automotive electronics for some reason, it would be nice to see a photo of the unit with such a terminator if you have one.

Here’s a gateway module from a Ford truck, this has up to 6 CAN buses going to it. They also have 27v TVS on each CAN wire.
Transceiver “TJA1042”
TVS marking “27A”

[Postal2]

... Here’s a gateway module from a Ford truck, ...

This is the first time in my life I see this!

Thanks for the photo, very useful.

Here is the CAN of the Liebherr port crane, 4xTJA1050.

[uer166]

... Here’s a gateway module from a Ford truck, ...

This is the first time in my life I see this!

Thanks for the photo, very useful.

Here is the CAN of the Liebherr port crane, 4xTJA1050.

Tesla AP ECU. Check top left, 5 CANs populated. 62R split to cap and CMCs..

[Postal2]

... Tesla AP ECU. Check top left, 5 CANs populated. 62R split to cap and CMCs..

I see 3 out of 5 have a terminator. It looks like it's an automated design system that has had this termination scheme added.

[T3sl4co1l]

More likely they're put on by default, and then depending on where along each bus the module is wired, the terminators are used (or not).  No need for jumpers because Tesla's buses are all designed (famously eschewing wiring for a printed/fabbed bus).  Again, termination works fine for terminal nodes, N/A for middle nodes.

Like, I'm not just saying a bunch of things, there's a reasoning process underlying all this; and that reasoning is universal, it's not just read arbitrarily out of an arcane tome that everyone seems to have but no one seems to know (but, there are those that come close, like Ott's Electromagnetic Compatibility Engineering), it is possible to learn these ground truths (electromagnetic, transmission line and a little network theory) and derive further ones (how to design a bus) from that.

Tim

[Postal2]

It turns out that in the past there were only fools, and the bus was connected in one way, and then 20 years later, especially gifted people appeared and "improved" it. Well, maybe.

[Siwastaja]

It turns out that in the past there were only fools, and the bus was connected in one way, and then 20 years later, especially gifted people appeared and "improved" it. Well, maybe.

You know, the world isn't all black and white. It's a continuum. Maybe a simple resistor termination passes EMC in 99% of cases. Maybe split termination passes with 2dB more margin, and in 99.4% of cases. Therefore it probably isn't "wrong" to do it either way.

[Simon]

Well most of the discussion or papers I found online say 4.7nF -100nF for the capacitor. But I have seen 1µF and 10µF in some designs.

What I am actually considering now as I have two CAN bus controllers at my disposal is to have one "fast" CAN bus and a slower single wire CAN bus. As I think about other projects I need to work on and the evolution of dealing with each project by having a collection of little boards that all talk to each other I will need a board to just connect a trigger control to the system, at this point a transceiver on every board starts to become cumbersome. I would assume that this would make my transceiver a simple open collector transistor with a pull up resistor at each end. This makes small local systems easier with a "fast" CAN bus connection on one device to connect the local system to other parts of a larger system.

Sort of the LIN bus and CAN bus combo logic as why write all that LIN code, my employer would kill me, they are still getting over me digging my feet in the ground and insisting on doing CAN bus (I have not told them that I am rewriting that hehe )

[floobydust]

In the Tesla ECU pic, I also note a pair of capacitors on both sides of the CM choke.
I imagine an EV is an (electrically) extra noisy environment, each motor/VFD might be CAN.
One thing is never assume CANBUS is perfectly balanced. I had some crappy shielded cable with non-symmetrical capacitance to shield, on the twisted pair. Drove me nuts but it was the insulation thickness different between the conductor's two jacket colours.
The SOT-23 might be a Bourns TVS for CANBUS and they use split term in that app note.

[Postal2]

...Therefore it probably isn't "wrong" to do it either way.

The whole point of the CAN bus is the ability to shift the ground. I don't understand how a capacitor can have a positive effect on different potentials. Let's assume for simplicity that the ground shift is 5 volts, as if it were a snapshot of interference. Then the CMC coil is already unbalanced, and it can be thrown away. The signal is modulated, the fronts are shifted. What's the advantage here?

[dietert1]

I'd guess the split termination with capacitor is meant to reduce common mode RF noise from the bus, while the common mode choke is meant to reduce emissions that result from an asymmetric delay in the bus driver output. It's digital, basically no failure caused by noise. Maybe some retransmission.
CAN is somewhat similar to USB, except lower bandwidth. Often the bus connectors include power and ground and often the bus cable is similar to a USB cable: shielded with four wires of controlled impedance. Power on the bus can be used for isolated bus interfaces.

Regards, Dieter

[Simon]

The whole point of the CAN bus is the ability to shift the ground.

Is it? I thought that is was a differential signal. You will notice that any digital signal of some frequency or having to travel some distance is differential over a twisted pair, this is so that the inexpensive cable will carry the signals easily. The twisted pair means that for complementary or differential signals the inductance of the cable is greatly reduced so the signal can be of a higher frequency and/or travel further. The drivers and receivers themselves are pretty much single ended but act together.

The only way you can guarantee different ground potential is with isolation as for any differential bus type. Most transceivers have a common mode rating of several volts as an absolute max rating to make sure they deal with power voltage spikes. Typically you see something like Vcc+7V, well 5V is the normal supply for CAN bus and putting 7V on that takes you coincidentally to 12V.... the supply voltage of most automotive systems, even on an EV there is a 12V subsystem.

[Doctorandus_P]

I'm using RS485 for my home network (common mode very similar to CAN). and I connect both power (+24Vdc + GND) and data through CAT5 cable. (Still have two spare pairs) On long cables and high load current (bunch of nodes on the far end) there can be a few volts lost over the cable. The data is low current, so it does not have this shift. As a result, Some nodes receive date shifted by a few volts up compared to their local GND, while other nodes receive data that is a few volts shifted in the negative direction, because each node that sends data uses it's own local GND reference.

The current though the power supply wires does not only drop the +24V wire, but it also shifts the GND wire, and thus the local GND for all nodes connected to the cable. And this is the reason why both RS485 and CAN have a common range for their data signals 7V outside of their GND and (usually) 5V power supply. (Some RS485 and CAN transceivers work with a 3V3 power supply. Data transmission only needs 100mV or so differential so this hardly changes reliability.

[Simon]

Yes there is some allowance for common mode shift, but then in the same way any digital input will have a tolerance. My micro controller inputs will see a 1 if the input is more than 0.7*Vcc, this means that any voltage drops on the ground as well as from the high side driver are allowed for.

I don't think you normally want to have a signal ground also carrying enough power to significantly shift the reference of the data lines. Again, for this reason most people use isolated connections when this is a high risk and again they do this because most people are designing something for mass implementation where the exact installation of every single device cannot be predicted. So they take a significant step to make their devices resilient to this if there is a risk of ground shifts that are significant. The ability of the transceivers to deal with some offset is usually reserved for the unexpected situations. If you use all of your margin for normal use case you have nothing left for when you have an unexpected situation.

It's kind of why I created the topic and asked. Because so far what I have done has always worked, but I not know how marginal my designs have been. You have created a bespoke system in your house where you know the exact characteristics. Would you sell the same design to the mass market to do what they liked with it?

[Postal2]

The whole point of the CAN bus is the ability to shift the ground.

Is it? ...

Yes.

[dietert1]

For example if they would use the car chassis as a common ground to distribute energy in an EV like they do for the starter of a conventional car, this could become a disaster for a CAN bus.
I am currently testing a USB device that needs about 120 mA and its 5 V supply drops to only 4.3 V over the cable from the hub. Half of the voltage drop - that is 0.35 V - could be along ground, unless the cable uses its shield to strengthen the ground connection.
CAN transceivers might use voltage dividers on their inputs in order to extend the common mode input range beyond supply rails.

Regards, Dieter