CAN bus termination resistor - anything special? - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: June 26, 2015, 12:58:23 am
content: Just reverse engineering a a CAN isolator board and the two 120R termination resistors are different to the other 0603 SMD resistors on the board. Is this just for greater electrical ruggedness in case something rude gets connected to the buss or are they perhaps some sort of low inductance resistor or some other special thing? Never touched CAN stuff before.

author: ovnr
author_rank: Frequent Contributor
timestamp: June 26, 2015, 01:13:42 am
content: Probably ruggedness (higher power handling and thermal mass). As it happens, MELF resistors havehigherinductance than their plain SMD counterparts.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: June 26, 2015, 04:28:32 am
content: Probably mostly for the higher power rating, yes. (5 V)^2/120 ohm = 208 mW. Even though this is not continuous power, the designer probably didn't feel comfortable using a 0603 or even 0805 resistor, which are often rated ~1/8 W. And a MELF resistor in this case probably gives a higher rated power dissipation per board area than a 1206 resistor, so that's what was chosen.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 26, 2015, 06:28:59 am
content: They're there for power handling, especially in the event of a fault. If one of the CAN bus lines gets connected to +12V, even briefly, there can be a lot more dissipation in the resistor than usual, and an 0603 would pop.

author: max_torque
author_rank: Super Contributor
timestamp: June 26, 2015, 10:57:29 am
content: You really need to protect your CAN bus against accidental connection to Vbatt!In a car, you need to protect against having approx 14V across the can bus, hence the large rated resistors

author: uoficowboy
author_rank: Regular Contributor
timestamp: June 26, 2015, 03:44:49 pm
content: Quote from: AndyC_772 on June 26, 2015, 06:28:59 amExactly this! Short circuit protection for CAN termination is annoyingly tricky - even these resistors look pretty small.

author: eneuro
author_rank: Super Contributor
timestamp: June 26, 2015, 07:06:38 pm
content: Quote from: max_torque on June 26, 2015, 10:57:29 amIf we are talking about car CAN bus what about car VAN bus-similar termination requirements to CAN bus?I'm especially interested in VAN bus between BSI and Instrument cluster (main controls display like engine RPM, spedometer, etc) eg. in Peugeot 206 MUX version for which I've was able to find wiring diagram like this:BTW: There is CAN bus between BSI and ECU, so termination of CAN bus in this way you descibed is common in automotive industry?

author: Circlotron
author_rank: Super Contributor
timestamp: June 27, 2015, 11:28:51 am
content: My application is for communication between a pair of swing door operators in a building. The doors overlap slightly and they have to talk to each other to ensure that door A always opens first and closes last. Virtually zero risk of anything improper getting on the buss so I used a pair of 62R 0805 size resistors in series.

author: eneuro
author_rank: Super Contributor
timestamp: June 27, 2015, 08:37:55 pm
content: Quote from: Circlotron on June 26, 2015, 12:58:23 amMaybe for lower speed applications in new projects thisATMEL LIN Networkingalso used in automotive industry could be more interesting as serial bus?Eg. nice SO8 package like this one to start with LIN networks?4th generation LIN Bus Transceiver, SO8 and DFN8 packageYep, around $1 for something like this not so bad at allhttp://www.digikey.com/product-detail/en/ATA663211-GAQW/ATA663211-GAQWCT-ND/5213069Do You REALLY Know What LIN Is?This small off topic, but looking for something easier in my automotive solution than CAN/VAN I've found LIN and it is in my interest now, especially if it were possible use it with optic fibers, etcMaybe you find it usefull too.