# Proper CAN bus termination

I've been struggling with getting CAN bus transmission to work properly. I'm driving the setup with a Parallax Propeller board and the specifics are in another post, but I have a more specific question about the CAN bus wiring and termination:

Each source I've found that describes CAN bus termination does so with something like the top of the diagram shown below - a high and a low wire with 120ohm resistors at each end.

However, in my lab setup what I actually have is equivalent to the lower part of the diagram shown - the resistance is actually in the "middle", between the two devices. I've also tried playing with that resistance value, from taking it off entirely, and around 110ohms, as well as 220ohms and they all suffer from the same problem and the chip I'm using is reporting a transmission error.

Two 120ohm resistors in parallel should be the same as 60ohms resistance. But does the actual position of the resistors along the wire matter? The devices are physically about 6 feet of wire apart and it's running at 500Khz. I know some basics including Ohms Law, and I get that CAN uses voltage differential to transmit, but if the position of the resistors is what is wrong then there's definitely something I don't understand about how this circuit is supposed to work.

simulate this circuit – Schematic created using CircuitLab

• Do the boards you are using share a common 0V? CAN was designed for automotive use so there is an assumption of all boards having a chassis reference. The transmitters and receivers have to be within the common mode bus voltage range. Mar 31, 2017 at 7:54
• The Propeller board, my CAN bus shield, and the ground pins on the ODB-II connector are all tied together, so yes. I'm honestly not sure what "common mode" means in this context (Googling it now). When I put my voltmeter on either of the wires (and the other end on my common ground) it gives the expected recessive voltage of about 2.5v).
– bgp
Mar 31, 2017 at 7:56