If you split CAN bus in two, can you identify which side is talking?

Dmitriy Analyst Ontario Posted   Latest  

Network buses often contain connectors or splices, which allow to split the bus into two or more parts. If the issue is currently present it is often possible to identify which part of the bus has an issue (example: short to ground). However, if the issue is intermittent and/or requires intact bus to occur other methods have to be used. Non-essential modules can be disconnected or turned off one by one, but this can be a lengthy process. For vital modules this would not work at all; for example, it is impossible to disconnect a PCM and go for a test drive.

Would not it be useful to be able to know which side of the network has a garbled transmission, so that you can focus on the bad side and identify the issue by divide and conquer method? Well, I was playing with one device belonging to a class of CAN bus repeaters and found the following:

  • it splits the CAN bus in two, but glues CAN bus messages back on both sides in real-time (i.e., it is not a gateway that relays messages from one side to another with some delay);
  • if one side of the CAN bus is shorted, the other side still works;
  • if you connect oscilloscope leads to certain pins of the chips inside the repeater, the direction of the message flow can be identified.

diag​.​net/file/f7fvo6f4l…

Sounds like magic, but in my tests it worked. Here is an example of Picoscope capture that shows Can bus 1 messages on the blue trace, but Can bus 2 messages on the red trace:

diag​.​net/file/f25nd7oy0…

Note the serial decoder does not think the packets are valid – because the ACK (acknowledgement) pulse happens on the other line. Neat!

Well, what do you think about this, would you find such a device useful in your arsenal?

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Interesting
Glenn Owner/Technician
Texas
Glenn
 

Thanks for the post Dmitriy, You realize the is in the realm of computer technology and pattern analysis? This is high end dealer training stuff. Bits, Bytes and Parsers are a foreign language to many. This is engineer level testing, but that is where our industry is now.

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Timothy Owner/Technician
Wisconsin
Timothy
 

I'd need a whole lot more education of what the device is actually doing, and the theory behind the system to comprehend what is going on there. You're way out in front of most of us! My training says that it's almost impossible to tell which device is talking on a CAN bus, as most modules send out their own messages, and we aren't told who is sending what. Wouldn't you have to know what the…

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Dmitriy Analyst
Ontario
Dmitriy
   

Timothy, if you split the bus in such a way that one branch contains only one module, this device will show its outgoing messages and acknowledgement pulses. I will attach a drawing shortly. diag​.​net/file/f10ek1xzu… ​

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Tanner Instructor
South Carolina
Tanner
 

I would definitely find something like this useful!

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Michael Technician
Indiana
Michael
 

Dmitriy, This is very interesting. What I would find very useful, is something that would shout out a “who is present” request across the network, then decipher the responses. On the old class 2 systems, the tech 2 had a menu option where it showed who was present on the network. Is there anyway to make that happen on a CAN system. It's my understanding that CAN messages do not include…

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Dmitriy Analyst
Ontario
Dmitriy
 

Michael, this device only monitors existing traffic, I have not thought (yet) about what injecting messages might reveal as well…

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Mike Mobile Technician
South Carolina
Mike
 

This is really interesting. I would live to see it in action and interested to see how consistent it can identify the direction of communication. I'm assuming that is essentially what it does.

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Dmitriy Analyst
Ontario
Dmitriy
 

Absolutely, Mike, I am setting up a real-ish test, which will be putting this device between the DLC in my car and a scantool. I will be looking for messages the scantool sends out and for ECM's messages the scantool acknowledges. Should be fun, and I will post the results here.

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Interesting
Mike Mobile Technician
South Carolina
Mike
 

Awesome! I am looking forward to seeing it!

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Dmitriy Analyst
Ontario
Dmitriy
   

Here is the scantool test. I have an Actron scantool connected to the car through my device. The OBD connector for the scantool intentionally has only CAN bus pins (6 and 14) and power + ground pins populated, so it has to communicate with the car over the CAN bus. This splits the network in two parts: the “car side” and the “scantool side”. diag​.​net/file/f1xp8p32h… ​ I instruct the scantool…

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Mike Mobile Technician
South Carolina
Mike
 

Dmitriy, The tech is a little beyond me but I think I can see where you are isolating the signals and I am excited about the idea of implementing this into diagnostics! A few questions I have: I think, but I am not sure that you are isolating direction of travel of the signals correct? I am assuming that you would need to break the circuit in order to test it? Is there any reason this…

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Dmitriy Analyst
Ontario
Dmitriy
 

Hi Mike, you are correct, I am physically breaking the CAN bus circuit into two sides (say, left and right). Thus, it is an “invasive” method, but sometimes connectors or splices are easy to get to. To be precise, the origin (left or right side) of the message is identified. If one side contains only one module then the direction in which the message has to travel is obvious. In my example, I…

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Mike Mobile Technician
South Carolina
Mike
 

Dmitriy, If I were to think of a time when implementing wht you are describing into a diagnostic I could see it being a very quick way to diagnose LIN bus issues in which the suspected failing modue is not one that allows communication through the scan tool, and does not set no communication codes. As an example, a power window motor module that receives a LIN signal from the switch to tell…

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Dmitriy Analyst
Ontario
Dmitriy
 

Mike, that was a fascinating read. I guess I will try to make a LIN version of the device and see what it can do.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Dmitry, I will comment that Autel MaxiSys Ultra has a topology function and it is awesome to give full picture layout of all onboard modules. If one is not reporting it will be in orange. If it is not on the vehicle, it will be grayed out. This goes a long way to avoiding exploratory access and playing the guess the module or wiring game. What you are showing is more detailed and very…

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Dmitriy Analyst
Ontario
Dmitriy
 

Glenn, I agree that if the module is dead, there are easier ways to find that out. As I have mentioned in the original post, this approach is more geared towards intermittent issues. And, there are plenty of techs here on Diag,Net that understand CAN bus and other advanced topics well, which is what I really appreciate about this site.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Dmitry, I am glad you are sharing the information, it will go a long way to helping many understand. I support the old saying is “enquiring minds want to know”. I am all for what you are doing and support you on your efforts and initiative. There are many out here that think with an open mind, and I am glad for that. There are other sites that hound progress and initiative. (They are still…

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Timothy Owner/Technician
Wisconsin
Timothy
 

Heck, I remember using AD when it came with its own PC and used 5.25 floppies! To me, it was far better than the old Chilton's and … manuals, even way back then. My new Samsung phone came with an owner's manual that is 300+ pages long! This tool does appear to be very interesting…I do a bunch of comm issues, so it'll come in very handy.

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