Mindsenors CAN Splitter

[FrankJ]

Originally Posted by asid61

After going through the TI whitepaper (which only briefly mentions stub length and doesn't explicitly mention the words "star topology" at all, although it was a good read), running star topology doesn't seem too bad.

Yep. it is more about physical layer than specially about star topology. I edited the post to reflect that. It doesn't mention Star topology because the recommended topology is a line structure.

The main problem with stubs seems to be that the lack of termination resistor can cause reflections. The solution is to simply make the stubs no longer than "1/3 of the line's critical length".

This is explained in the following passage:
"The critical length of a bus line occurs at the point where the down-and-back propagation delay (tprop(total)) of a signal through a line equals the transition time(tT) of a signal (the greater of the rise or fall times). Network Critical Length = tT = tprop(total) Therefore, a typical CAN driver may have a 50 ns transition time, and when considering a typical twisted-pair transmission line prop delay of 5 ns/m, the down-and-back delay for one meter becomes 10ns/m. The critical length becomes 5 m (50 ns / 10ns/m = 5 m), and the max un-terminated stub length for the network is 1/3rd of the critical length, or 5/3 m (1.67 m)."

You should include the next paragraph.

When critical length is taken into consideration, driver slew-rate control becomes a valuable design asset.
The Standard recommends a maximum un-terminated stub length of 0.3 m with a 1 Mbps signaling rate,
but with slew rate control, reduced signaling rate, and careful design, longer stub lengths are easily
obtained.

Without knowing the specs of your transceivers and going deeper into the physics than most of us want to, you are better off sticking with the standard. Standards are usually comfortably conservative.

I'm not sure what the transition times or propagation delays are for a regular FRC system, but just judging from their example (which seems reasonable) a star topology should be more than doable.

As others have said, CAN is pretty robust. You can get away with a lot. But when you are troubleshooting issues, it gives you one more problem spot to look at.