Talon SRX Motor Controller Problem

[FRC Team CC]

Originally Posted by Bkeeneykid

Also, while I believe this setup may work, I'm not sure it's you best option. Your best option is to connect the green of one to the green of another, like you see in the SRX manual.

Why is daisy-chaining a better option than connecting the wires to a central "hub"? If one of the CAN cables in the daisy-chain stops functioning, don't the rest of speed controllers also stop receiving inputs?

Thanks for your help,
Charging Champions

[Bkeeneykid]

Originally Posted by FTC Team CC

Why is daisy-chaining a better option than connecting the wires to a central "hub"? If one of the CAN cables in the daisy-chain stops functioning, don't the rest of speed controllers also stop receiving inputs?

Thanks for your help,
Charging Champions

Well for one, that's not FRC Legal. *Standard warning about 2016 rules may not apply to 2017* See rule R70:

Each Jaguar or Talon SRX must be controlled with signal inputs sourced from the roboRIO and passed via either a PWM (wired per R68) or CAN-bus (either directly or daisy-chained via another CAN-bus device) signal, but both shall not be wired simultaneously on the same device. If the CAN-bus configuration is used, the firmware on the device must be updated to at least the following versions:

You can only either directly connect to the roboRIO or daisy chain through another device.

[dirtbikerxz]

Originally Posted by FTC Team CC

Why is daisy-chaining a better option than connecting the wires to a central "hub"? If one of the CAN cables in the daisy-chain stops functioning, don't the rest of speed controllers also stop receiving inputs?

Thanks for your help,
Charging Champions

As stated by many people above. The input and output CAN wires in a talon are soldered to the same point. So even if a motor controller were to die, the rest of the can devices would still receive a signal, because the actual can wire itself would still be connected. The only reason why a can chain might stop functioning, is if one of the can wires got physically cut somewhere along the chain. And there is no reason why that should happen, especially during a match.

EDIT: I didn't even realize this, but as Devin stated above, your setup would also be illegal.

[frcguy]

Originally Posted by Bkeeneykid

Well for one, that's not FRC Legal. *Standard warning about 2016 rules may not apply to 2017* See rule R70:



You can only either directly connect to the roboRIO or daisy chain through another device.

I'm not sure about the legality, but I know for sure some teams ran with a star topology during this past season. However, I've heard that it's not a great method for connecting everything. Check out this post for more info on why it isn't recommended: https://www.chiefdelphi.com/forums/s...2&postcount=12.

[dirtbikerxz]

Originally Posted by frcguy

I'm not sure about the legality, but I know for sure some teams ran with a star topology during this past season. However, I've heard that it's not a great method for connecting everything. Check out this post for more info on why it isn't recommended: https://www.chiefdelphi.com/forums/s...2&postcount=12.

If I understand R70 correctly, star topology is technically illegal. I guess many people (including me) didn't realize that.

[FRC Team CC]

Originally Posted by Bkeeneykid

Well for one, that's not FRC Legal.

Ok, thanks for the clarification. Do you guys just solder the cables to each other if they run from one SRX to the next?

Also, do you have one cable on the last SRX in the daisy chain just sticking out and not connected to anything? Or does that connect back to the RoboRio/PDP?

Thanks,
Charging Champions

[Ari423]

Originally Posted by Bkeeneykid

Well for one, that's not FRC Legal. *Standard warning about 2016 rules may not apply to 2017* See rule R70:

You can only either directly connect to the roboRIO or daisy chain through another device.

Not suggesting that a star topology is the better way to go, but the way I read R70 it would be legal. Each Talon is connected directly to the roboRIO, and no Talon has both PWM and CAN running simultaneously. The rule never says that the tree can't branch out, only that each "leaf" (Talon) must be connected to the "roots" (roboRIO) either directly or through a daisy-chain. Let me say it again: not better, but legal (according to me, who has absolutely no bearing in what is actually considered legal)

[dirtbikerxz]

Originally Posted by FTC Team CC

Ok, thanks for the clarification. Do you guys just solder the cables to each other if they run from one SRX to the next?

Also, do you have one cable on the last SRX in the daisy chain just sticking out and not connected to anything? Or does that connect back to the RoboRio/PDP?

Thanks,
Charging Champions

My team solders the cables together, because we like clean runs. But do what suits your team best (snap connectors etc.). Also there should never be a can wire hanging out in the open. It should always be connected to something. The general rule is that the CAN chain STARTS at the RoboRIO and ENDS at the PDP. This is because the PDP has a "terminator" which basically terminates the CAN connection.

[ozrien]

Originally Posted by FTC Team CC

Ok, thanks for the clarification. Do you guys just solder the cables to each other if they run from one SRX to the next?

Also, do you have one cable on the last SRX in the daisy chain just sticking out and not connected to anything? Or does that connect back to the RoboRio/PDP?

Thanks,
Charging Champions

Hi FTC Team CC,
This is all explained in our documentation. Please read section 1.3.2 of the Talon SRX User's Guide. I think you will find answers much faster this way instead of posting a question and waiting for responses at each step.

We do not recommend star-topology, the CD forum is filled with many threads explaining why. But ultimately it can create failure-modes that are difficult to root-cause for the inexperienced. It's not clear to me that star-top is explicitly illegal from reading R70, but again we don't recommend it.

The alternative to daisy chaining would be to create a single bus harness that the CAN nodes "tap" into via short splices. This is how CAN buses are wired in cars. However in FRC its generally more convenient to daisy chain via connectors for high-serviceability during competition season.

Many teams have wired the Talon SRX on CAN bus in the fashion we've documented with great success.

There are no crash symptoms in WPILIB caused by unpowering a Talon, nor does it impact communication with sibling Talons on the CAN bus. This is explained in section 16.6 of the Talon SRX Software reference manual. In fact this is true for all of CTRE CAN products.

[FRC Team CC]

Thank you all so much for your help. We will probably just stick with CAN connections daisy-chained to each other.

Thanks,
Charging Champions

[GeeTwo]

Combining a lot of what has already been said with a bit more, here's the CAN plan we use, and why:

• CAN is designed as a bus, not a star topology. Doing a modestly deep dive, it appears that wiring in a star topology will work just fine until it fails, when it is most likely to do so with no useful diagnostic clues. Bottom line: star topology is bad juju for CAN.
• From a practical standpoint, CAN devices come in two main flavors: incorporated wires, or wire clips. (male vs female). To the extent you can, alternate the male and female devices along the bus to minimize the number of connections. This is just a specific application of KISS.
• Putting the PDP at the end of the bus is great because it has an internal terminator, but if your design has a device (e.g. the PCM on our 2016 robot) well away from the main control board, don't be afraid to use your own 120Ω resistor.
• Don't put any permanent connectors, cuts, or solder on the CAN wires. This allows you to re-use them from year to year assuming the rules don't change drastically. We had considered the new molex 2-pin genderless connectors for CAN, but when we realized how often we really needed wire ends to go in connectors on female devices, we decided not to do this - ever.
• If you have too many "male" devices compared to "female", use the VEX CAN connectors to make the necessary links. They are specifically designed for CAN impedance, and are really inexpensive - use them.
• If you have too many "female" devices compared to "male", use approx 20 AWG wire, preferably stranded, and most definitely twisted to at least "CAT 3" specs, that is a twist every 3 inches.

[FRC Team CC]

Thank you all for your help. We greatly appreciate it.

Thanks,
Charging Champions

[FRC Team CC]

One more quick question. Some of the replies on this thread have recommended using Wago Lever Nuts to connect the cables (https://www.amazon.com/Wago-222-415-...AO9N0B7PPXTIQ). Does this count as star topology (and is it therefore illegal)? Would you recommend this over the CAN connectors sold by CTR (http://www.ctr-electronics.com/adapt...-5-pack.html)? Our concern is that daisy-chaining all of the motor controllers will be more prone to failure if even one of the motor controllers in the beginning of the daisy chain fails.

Thanks,
Charging Champions

[GeeTwo]

Originally Posted by FTC Team CC

One more quick question. Some of the replies on this thread have recommended using Wago Lever Nuts to connect the cables (https://www.amazon.com/Wago-222-415-...AO9N0B7PPXTIQ). Does this count as star topology (and is it therefore illegal)? Would you recommend this over the CAN connectors sold by CTR (http://www.ctr-electronics.com/adapt...-5-pack.html)? Our concern is that daisy-chaining all of the motor controllers will be more prone to failure if even one of the motor controllers in the beginning of the daisy chain fails.

Thanks,
Charging Champions

I believe that bobbysq meant these wago lever nuts, just two connections. These will probably work OK, but the CAN connectors are more appropriately sized to CAN sized wire, making them easier to work with and less likely to reflect signals.

I understand that the motor controllers have a robust pass-through of the CAN lines; it would take a catastrophic motor controller failure to disrupt the daisy chain.

[FRC Team CC]

Originally Posted by GeeTwo

I believe that bobbysq meant these wago lever nuts, just two connections. These will probably work OK, but the CAN connectors are more appropriately sized to CAN sized wire, making them easier to work with and less likely to reflect signals.

I understand that the motor controllers have a robust pass-through of the CAN lines; it would take a catastrophic motor controller failure to disrupt the daisy chain.

Oh ok, that makes sense. Would you recommend using these at all: https://www.amazon.com/Wago-222-415-...AO9N0B7PPXTIQ? Or is it preferable to stick with the two connection options mentioned above?

Regarding the motor controllers, will the CAN connection work even if one of he motor controllers stops receiving power? Does transmission of the CAN signal rely on the motor controllers to be on? Or does the CAN signal get transmitted though the motor controllers even if it does not receive power?

Thanks once again so much.

- Charging Champions