CAN bus termination issue

[TheReelMuhcoy]

I am working on a CAN bus using an Axiomatic ECU and an actuator which has a built in CAN controller. When I first wired up the network and components, I was not aware that the a 120ohm termination was required at the Axiomatic ECU. When I opened communication to the bus I was not able to connect to the Axiomatic unit, but I did connect to the actuator controller. It was suggested to me that the controller had a built in termination resistor.

Now I have spliced in a 120ohm resistor between the H&L just past the connector of the Axiomatic ECU, now when I try to open the communication to the bus i get an error and cannot open the network unless I unplug the actuator controller. This is my first time working with CAN and I honestly have no idea what I am doing, but I want and need to learn in order to complete this project, does anyone have any suggestions where to go from here?

I just tried tried to ground the CAN, and still come up with the same error. I receive the same error I received when I did not have the Axiomatic ECU terminated properly.

I have read that 60ohm resistors may be used to termination in certain situations, could that be used in the actuator controller? therefore giving me an issue when i use a 120ohm at the other end of the network?

I also just wanted to mention this is roughly a 5 foot run.

 

[bigdogguru]

There are three standard methods of terminating a CAN bus:

1. Standard Termination
2. Split Termination
3. Biased Split Termination

Typically the same method is utilized at both ends of the bus.

As such you need to determine which devices on the bus are terminated, the exact method used and whether the termination if present can be disabled.

Are you utilizing a CAN bus adapter to analyze the bus traffic? If so, is the adapter terminated as well?

The following image demonstrates all three methods:


The following documents detail their usage:

AN228 A CAN Physical Layer Discussion

Controller Area Network Physical Layer Requirements


BigDog

 

[TheReelMuhcoy]

I will take a look at the actuator controller to see if i can determine how it is terminated, honestly I am not sure how I will be able to determine this.

I am using a CAN to USB adapter to open the CAN network and read/write to the Axiomatic ECU. I only have one 120Ohm terminating resistor on my CAN H&L wires right now which is 1 Inch away from the Axiomatic ECU, I do not have any other terminating resistors anywhere else except from what I believe is inside that actuator controller.

Thanks much for your help so far!
-Jon

 

[bigdogguru]

Typically, a properly terminated CAN bus using standard termination will indicate approximately 60Ω when measured between the H and L lines with a DMM with the CAN bus and all nodes powered down, due to the two 120Ω in parallel.

You can use method and a DMM, to test for internal termination of individual nodes when not connected to the bus and in an unpowered state.

A properly terminated node would normally indicate a resistance of approximately 120Ω, while an unterminated mode would normally indicate a significantly higher resistance, typically in the 25kΩ to 40kΩ range, although some can indicate a much higher value.

Several of the USB to CAN bus adapters I've encounter are terminated by default and typically offer the ability to disable the termination if required, while an equal number offer no method of internal termination.

Therefore, do not assume any node you attach to the CAN bus is terminated or unterminated, use a DMM to determine its state before attaching it and plan accordingly.

Nodes which are terminated and the termination unable to be disable will, of course, need to be located at the ends of the CAN bus topology.

Resist the temptation to follow the advice of some forum threads to use a single 60Ω termination for CAN bus of short length, that particular advice is erroneous and does not adhere to CAN bus standards.

I have implemented many short CAN buses on the bench, several shorter then 5 feet, using proper termination methods without any issues.

However, you should be aware there is a minimum distance between node required for reliable operation, the distance is largely dependent on the capacitive load of the nodes and the distributed capacitance of the CAN bus media.

Critical Spacing of CAN Bus Connections

Although, in the case of a CAN bus comprised of three nodes spaced over 5 feet it typically shouldn't be an issue.

In most cases the user manual or datasheet of the node device will indicate the termination options of the node device.

Have you examined the manual/datasheets of both the Axiomatic ECU and CAN controller/actuator?

Also, a simple email or phone call to the manufacturers of the node devices can usually provide insight to the termination issue.


What bit rate are you attempting to utilize on the CAN bus?

I would suggest initially using a moderate bit rate until you succeed in establishing communications between the nodes and then increase it to your required rate.



BigDog

 

[TheReelMuhcoy]

BigDog,
Thanks for the information above. I just tried testing the resistance between terminals of the actuator controller and came up with nothing. Again I do not know any of what I am doing here, so I jumped a resistor on the connector and attempted to open the CAN network. Still nothing (by nothing, I mean I receive the same error 'Error 9: network is in a passive state" or something similar, I can insert a print screen if necessary. I checked the baud rate which is locked at 250bits.

I only have a manual/datasheet for the Axiomatic controller. The actuator is a smaller part from another larger system, which I have no information.

Is there any chance I could setup a video call, using skype, or facetime. I can show the setup I have and we can trouble shoot live?

- - - Updated - - -

The CAN software and adapter that I am using are this: **broken link removed**

The controller node that I am using is this : TDAX030530.pdf

The controller/actuator i have no information on except that it is manufactured/engineered by Temic and it communicates with over J1939 CAN bus

 

[TheReelMuhcoy]

This may be a silly questions, but would the wire I am using for the network run cause an issue? I used some spools of 18ga or 16ga hook up wire that I had laying around. Should I be using something specifically designed for CAN networks?