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.

[ollien]

Originally Posted by cbale2000

We ran our robots CAN in Star Topology last year and didn't have any issues with it. The only problems we did run into were related to wiring and not the network itself (and the wiring issue was fixed when we went through and properly soldered all the connections).

Having previously had a robot where a single faulty CAN connection disabled the entire robot for the match on several occasions (courtesy of ineffective quick-disconnects) I can say I definitely don't mind the redundancy in the system. I'd much rather loose a collector or a single drive motor than have the entire machine parked in the middle of the field for a whole match.

Hm. I'm curious as to why you didn't run into reflection issues like others have been suggesting (in both this thread and the other). How long were your wire runs?

[FrankJ]

Originally Posted by ollien

Hm. I'm curious as to why you didn't run into reflection issues like others have been suggesting (in both this thread and the other). How long were your wire runs?

Keep in mind that this is rarely works/doesn't work scenario. Below a certain level it will not show up at all. Then it is a noisy line. You start losing packets. CAN is fault tolerant so they get re-transmitted. Once again you may or may not notice. Especially if you don't have a lot of traffic on the bus. Next you start getting quirky non repeatable issues. That might get attributed to other things.

[cbale2000]

Originally Posted by ollien

Hm. I'm curious as to why you didn't run into reflection issues like others have been suggesting (in both this thread and the other). How long were your wire runs?

All of the Talons we used (about 10 total) were more or less their uncut stock length (with the exception of two which had ~6" extensions soldered on). All other CAN devices (PCM, Rio, PDP, etc.) were connected with CAN wires between 6"-12" in length. We also only used one set of wires from each CAN device (the second set from the Talons were bundled up with zip ties, kept as spares in case the main set broke).

The setup used the standard CAN termination with the roboRio and the PDP with no additional resistors added.
All CAN devices were wired to a pair of grounding bars (one for each color wire) we ordered from McMaster and connected using ring terminals. Also, if I remember correctly, the total bus useage was around 60% (though that may have changed as programmers added features).

[ozrien]

Using a star network generally creates more problems on CAN bus than it will solve, and we do not recommend it.

Please read pages 35-37 of the Talon SRX User's Guide.

http://www.ctr-electronics.com/talon...ical_resources

[Ether]

Originally Posted by ollien

Hm. I'm curious as to why you didn't run into reflection issues like others have been suggesting (in both this thread and the other). How long were your wire runs?

Originally Posted by cbale2000

All of the Talons we used (about 10 total) were more or less their uncut stock length (with the exception of two which had ~6" extensions soldered on). We also only used one set of wires from each CAN device (the second set from the Talons were bundled up with zip ties.

I'd love to put a scope on those wires and see what the signal looks like at the nodes.

[cbale2000]

Originally Posted by Ether

I'd love to put a scope on those wires and see what the signal looks like at the nodes.

Not sure if we have a scope that works, but assuming we did, how would you go about doing that? Just connect one probe to green and the other to yellow?

Originally Posted by ozrien

Using a star network generally creates more problems on CAN bus than it will solve, and we do not recommend it.

Please read pages 35-37 of the Talon SRX User's Guide.

http://www.ctr-electronics.com/talon...ical_resources

Does Star topology assume both yellow and green wires are connected to the same hub? If that's the case this is not what we did. Our Topology actually looks similar (in my opinion anyways) to the "master cable harness" topology except rather than using a cable we used two busses, one for each color wire.

[marshall]

Originally Posted by cbale2000

Not sure we have a scope that works, but assuming we did, how would you go about doing that? Just connect one probe to green and the other to yellow?

Not quite but close. You will need a DSO-style scope that can help you look at the signals and do some analysis on them with looking for noise.

I actually think a better idea would be to use some of the tools now available via Linux for looking at the CAN bus for timeouts and retransmits (I believe you can do this on the RoboRIO in code too). I suspect you're going to see more errors with a star topology.

We actually have a scope that can do this and if I had time then I'd like to but I don't honestly have the time to dig into it. If there is a team in NC that wants to do it then I'll volunteer the scope for the research but they would need to come out to our space to use it and bring the necessary test setups with them.

[Ether]

Originally Posted by marshall

...a better idea would be...

It's a good idea, but it's not better, just different.

Depends on what you're trying to accomplish.

You'll need a piercing probe with that DSO to look at the signal at the SRX.

[marshall]

Originally Posted by Ether

It's a good idea, but it's not better, just different.

Depends on what you're trying to accomplish.

You'll need a piercing probe with that DSO to look at the signal at the SRX.

Fair enough. Great minds think differently and all.

Would you though? We poked at ours last year and just did it at the junction point between two SRXs.

[Ether]

Originally Posted by marshall

Would you though?

Only if you want to see what the waveform looks like at the SRX, compared to a proper bus with short stubs. Which is what I'm interested in. It would shed a lot of light on this whole discussion.

[EricVanWyk]

Is there any active circuitry on this board, or is it just connectors?

[marshall]

Originally Posted by Ether

Only if you want to see what the waveform looks like at the SRX, compared to a proper bus with short stubs. Which is what I'm interested in. It would shed a lot of light on this whole discussion.

This is why they pay you the big bucks.

[Ether]

Eric!! Haven't seen you here in almost 3 years!

[EricVanWyk]

Originally Posted by Ether

Eric!! Haven't seen you here in almost 3 years!

Been a little while... I'm a grad student now. Thesis's rocket pitch is: terminate a high speed digital communication bus with biological goop instead of a resistor to make a cheap bio sensor. Time Domain Reflectometry that uses improved math to get away with worse (but cheaper!!) hardware.

Or put another way, I'm using the stuff that make e.g CAN busses fail for good instead of evil.