Quote:
Originally Posted by Ari423
From everything I've read and understand about CAN, the bus was designed to be used in series, not parallel. According to some other posts by knowledgeable people that I can't find right now because I'm on mobile, using a star topology can introduce noise and bad things into the network and is generally not recommended. Especially considering any good CAN device should have the CAN leads physically connected so the signal is passed through even if it fails. Can someone correct me on this or is this product pretty useless?
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Honestly, it doesn't make a huge difference.
The CAN bus is, well, a bus. The bus is two wires (CAN-Hi and CAN-Lo) that are connected by a termination resistor (120 ohm) on each end. The CAN-Hi and CAN-Lo signals are equal but opposite, as shown below, so that, when twisted together, they reduce the effects of interference and data corruption.
The termination resistors are put in to prevent signals from 'bouncing back' or 'echoing' inside the bus, such that signals are not repeated or misread.
With this knowledge, you should be able to see what the purpose of these termination resistors are, and why CAN exists in a bus (every 'node' on the network can read and write). You should also notice that the bus is mostly arranged in parallel. See the circuit diagram below.
Something you should notice is that, because of this, it doesn't really matter where you attach a CAN device. You can do it either along a single line, branching off (kinda like a highway), or a single point that branches out to multiple other devices, like a star, with a termination resistor at the end.
Something you should note is that this isn't a true star topology. The RoboRIO has an inbuilt 120 ohm resistor on its CAN bus, which means there must be a 120 ohm resistor on the other end. This functions like a bus, with any devices branched off in a 'star' pattern also belonging to the bus. Despite them all being connected at a single point, it's the same thing as using a bus, but bringing all the connection points closer together. It looks like a star, but in reality it is a bus. The actual name for this arrangement is 'high speed CAN'.
A TRUE star topology is different from a bus in that it doesn't use a termination resistor at all. Instead, it relies on the combined resistance of all of its members. By the ISO 11898-3 spec, the overall resistance should be about 100 ohms, but never less. The actual name for this arrangement is 'low speed, fault tolerant CAN'. This name is misleading as normal CAN is fault tolerant for if devices go down (e.g. an SRX carks it), but the star topology is fault tolerant if a physical connection is severed. You can read more about the differences in this
wikipedia document, under 'Architecture'.
All the images in this post were taken from
this page.