CAN Bus Impedance Matching - Page 1
Forum section: electronics / projects

author: SparkFly
author_rank: Regular Contributor
timestamp: December 12, 2016, 08:14:30 am
content: Hi all,I am looking into designing a CAN bus, with just two nodes, operating at 1Mb/s. I've noted that whilst the CAN spec itself doesn't mention the physical layer, there are a few documents around from manufacturers which detail requirements set out by ISO 11898. What interests me the most is the requirement for a 120Ohm impedance along the bus. However, whilst I understand how to design/source 120Ohm cable, I am perplexed a little at which (if any) connectors are 120Ohm impedance? I've seen a variety of connectors employed in existing designs, from DB9 to OBD II, but no specs saying they are 120 Ohm.Am I simply looking in the wrong place, and there are 120Ohm connectors, or is there something else going on? I would've thought that everything would need to be matched to avoid reflections...Many thanks for your time, it is most appreciated.

author: bktemp
author_rank: Super Contributor
timestamp: December 12, 2016, 08:26:11 am
content: At 1Mbits/s the connector doesn't care, because it is very short compared to the wavelength of the frequencies involved.

author: kaz911
author_rank: Super Contributor
timestamp: December 12, 2016, 08:26:35 am
content: Quote from: SparkFly on December 12, 2016, 08:14:30 amYou have a 120 Ohm resistor in each end between CAN-H and CAN-Lhttp://www.ni.com/white-paper/9759/en/

author: kaz911
author_rank: Super Contributor
timestamp: December 12, 2016, 08:32:11 am
content: and for cable many twisted pair cables will work. In a pinch I use CAT5e cable. I like the marine version (NMEA-2000) as it supplies up to 4A power as well. So a 5 wire cable with 2 sets of twisted pair - one for CAN-H/CAN-L - one for power - and a shield.

author: tatus1969
author_rank: Super Contributor
timestamp: December 12, 2016, 08:33:36 am
content: Quote from: bktemp on December 12, 2016, 08:26:11 am+1. they care of wave impedance because CAN allows for long cable runs like tens of meters.

author: SparkFly
author_rank: Regular Contributor
timestamp: December 12, 2016, 08:34:08 am
content: Thanks very much for the quick replies.I've designed the 120Ohm resistors on the pcbs anyway, it is good to know the reasoning. Bktemp, I will endeavour to find some further sources on the matter! Thanks for your help once again.

author: cat87
author_rank: Regular Contributor
timestamp: December 12, 2016, 10:42:27 am
content: Because CAN is designed to operate in noisy environments (industrial, automotive) the cabling doesn't really matter that much, as long as it's a twisted pair (ok, not 30 gauge, nor 5 gauge).What does matter on the physical layer, is the impedance of the bus. Normally, CAN bus has an impedance of 60 Ohms, therefore the two 120 Ohms resistors at each end.However, be careful in this matter. If YOU are designing both nodes, then the 120 Ohms resistors are indeed necessary and it's OK that you included them on your board. But if you design only a part of the bus and use something off the shelf, read the documentation of the bought part/device to see what kind of termination is has (if any)Some manufacturers put the 120 Ohms resistor in their product, some do not.

author: SvanGool
author_rank: Regular Contributor
timestamp: December 12, 2016, 11:45:17 am
content: To make the picture complete, on the major points:You need twisted pair cable (recommended AWG 20 or 22), no shield needed, the wiring can be tenths of meters long.The main CAN connection from A to B needs to have one terminator of 120 Ohm at each end (not more and not less), at doubt you can measure for 60 Ohm between CAN-Hi and Lo.You don't need special "60 Ohm" connectors, the speed is that low in frequency, that the impedance of the connector is not important, as long as you adhere to the terminator requirements.In between A and B (mandatory), if you have more than two devices, you can have so called multiple "stubs", which are twisted pair "T"-connections, but such stubs should not be longer than 1 meter (40") and should not have a terminator.If you have more than two devices, you have to look which ones already have terminators (fixed or removable) and you should design a structure from "A to B", using above"stubs", which meets the "2-terminator" rule by adding or removing terminators.Last Edit: added connector description

author: Jeroen3
author_rank: Super Contributor
timestamp: December 12, 2016, 12:43:49 pm
content: You also need to keep the common mode in check. Often using split termination.eg: two series 60 ohms with ground in the middle via a cap.If the bus is isolated...