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CAN bus termination?

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ToBeFrank:
I'm developing a pcb that will be a node on an automotive CAN bus. From what I understand, only the ends of the CAN bus need to be terminated with 120 Ohm resistors. The following image illustrates what I've been reading about termination. Am I correct that I shouldn't have the 120 Ohm termination in my circuit?

Bored@Work:

--- Quote from: ToBeFrank on September 02, 2011, 03:50:08 am ---From what I understand, only the ends of the CAN bus need to be terminated with 120 Ohm resistors.
--- End quote ---
60 Ohm resistors (or 56 Ohm resistors), not 120 Ohm resistors. 2x 60 Ohm = 120 Ohm.


--- Quote from: ToBeFrank on September 02, 2011, 03:50:08 am ---Am I correct that I shouldn't have the 120 Ohm termination in my circuit?
--- End quote ---

Yes, so it depends on what is already in the car, and where you will connect your ECU to (e.g. an until now unused (diagnosis, CAN gateway) CAN bus in a point-to-point config might not have the termination on your side).

However, if you use one of the de-facto standard connectors for CAN I would always leave the termination off the board. For the unlikely case that you later figure you need a termination you can just add an external one, like the first item on this page http://www.ixxat.com/can_accessories_en.html

Uncle Vernon:
The CAN bus physical layer consists of a two wire (CAN-H and CAN-L) differential bus and a signal
ground for reference.

Your circuit is correct. Most implementations will only use a single 120 ohm resistor between H & L. If you are designing a board I'd suggest you add a pin link to your board so the terminating resistor can be switched in and out depending on what other terminations exist in your real world implementations.

ToBeFrank:

--- Quote from: Uncle Vernon on September 02, 2011, 05:54:42 am ---If you are designing a board I'd suggest you add a pin link to your board so the terminating resistor can be switched in and out depending on what other terminations exist in your real world implementations.

--- End quote ---

Can you explain what you mean by "pin link"? I'm thinking a software controlled termination, but I have no idea how to implement that.

Uncle Vernon:

--- Quote from: ToBeFrank on September 02, 2011, 06:24:16 am ---Can you explain what you mean by "pin link"? I'm thinking a software controlled termination, but I have no idea how to implement that.

--- End quote ---
One of these!

Hardware only, No Software required.

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