CAN bus star topology with termination in the middle - bad idea, right? - Page 1
Forum section: electronics / projects

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 16, 2023, 08:47:16 pm
content: If someone were to create a 'hub' PCB for a CAN bus, as per picture below, where all the CAN nodes are all connected to individual H/L pin pairs in the multi-pin connector on the left, a separate 'diag' node connector on the bottom-right, and with the 120 ohm termination resistors positioned as shown, that would be abadidea, right?If I'm interpreting it right, what they've got going on there is effectively creating a star topology with the termination resistors in the middle of the star.This is from a YouTube video I was watching recently on an automotive-related channel, where they were talking about a 'neat' solution they had come up with to tidy up a project car's CAN bus wiring. It involved creating a custom 'hub' box as shown. Apparently this somehow helps with isolating and testing individual CAN nodes, but I don't see how it provides that benefit any more than being able to unplug one or more nodes at their stub connections on a typical bus topology.I suspect theymightget away with this if the branches of their star are short and the CAN bit rate is fairly low.Let's not even talk about the fact that the right-top connector is supposedly ethernet...

author: Benta
author_rank: Super Contributor
timestamp: July 16, 2023, 09:08:12 pm
content: Reminds me of an "automotive tuning expert" YT video showing viewers how to check the engine oil level with the dipstick.Only problem? The engine was running all the time.Just gotta love those "hot rod" kids. But don't let them loose on your own car.

author: PeteH
author_rank: Unknown
timestamp: July 16, 2023, 10:01:23 pm
content: You can do pretty gross things and get away with it. One-offs and home projects are easy, commercializing it and meeting standards (EMI) is a whole other matter.At low bit rates, this is probably fine. But the traces speak to the competency involved here.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 17, 2023, 01:45:41 am
content: The traces look like someone was looking at bus topology diagram and thought the actual implementation must literally look like that.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2023, 06:47:57 am
content: CAN "termination" resistors have two functions;* Termination itself,* Driving the bus (passively) into the recessive state, similar to pull-ups in I2C for exampleLatter is mandatory for a functional system, and 60 ohms anywhere (or justsomething, like 200 ohms, with less noise margin as a result) does that.Whether termination is needed then depends on the stub lengths. Even at 1Mbit/s, CAN is fairly low speed so stubs of several meters work. Note that even the standard allows 0.3 meter stub at 1Mbit/s, and that must have quite some safety margin.That kind of PCB obviously makes no sense, and is a good example of cargo cult engineering. They wanted to make a product with fancy connectors etc., and all of that is unnecessary work.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 17, 2023, 02:39:20 pm
content: I have no idea what bit rate they'll be running the bus at, but it was mentioned that nodes won't be more than 0.5m away - whether that means distance or wiring length, I don't know (because position doesn't necessarily equate to the same in length of wiring). Perhaps it won't be too bad with stubs that length.And you're right, with a pair of resistors arranged like that, they might as well have saved themselves some bother and used a single 60 ohm resistor.At least it's not a commercial product, but a one- or two-off for a project car.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2023, 03:04:41 pm
content: 0.5m stub length works even at 1Mbps. 0.3m is therecommendationin the standard for 1Mbps.