CAN Rules

R61 The PDP CAN interface must be connected to the CAN-bus on the roboRIO (either directly or daisy-chained via another
CAN-bus device).

I am reading this correctly that every team is required to use CAN?

Also, R40 defines acceptable positive and negative wire colors, but I see no rules that define CAN wire colors.

Correct. Don’t be afraid. This is a different implementation of CAN than in the past.

Also, R40 defines acceptable positive and negative wire colors, but I see no rules that define CAN wire colors.

CAN wires are signal wires. No signal wires have color requirements.

Though the green and yellow wires in your kit will match the coloring on the connectors so that may be a wise choice.

Of course, but virtually the entire control system uses the Red:+, Black: - convention, and you could use White:+, Blue - if you wanted to.

It is a differential pair, not signal and ground. It should be a twisted pair.

It should be twisted, but does not have to. During beta testing, we were running it using non-twisted to a worse case than using twisted, no issues.

If you want to twist your own cable…

It is not really a “balanced” signal but it is a transmission line. You will need CAN enabled if you wish to use the pneumatics PCM module and turn on your compressor. As always, CAN implementation requires a terminating resistor at each end of the buss. One is provided in the RoboRio (The start of the buss) and another is provided in the PDP (possible end of the buss). If you wish to configure your CAN wiring differently or not use the PDP CAN, then be sure to provide the required termination resistor at the last device in your chain. The majority of CAN problems in the past have centered around the termination resistor.

Per R61, PDP CAN is required.

Thanks Joe, I forgot we included that. This gives the battery voltage reported by the PDP, right?

Just to avoid confusion, CAN to the PDP is required (and to the PCM if you use pneumatics), you can still use PWM for motor control.

CAN is required to talk with the PDP, but the upside is that you get to learn what all your load currents are!

The battery voltage is monitored by the roboRIO. I assume the PDP requirement is so the DS and/or field can monitor currents.

Correct. :slight_smile:

The 4 ft of green/yellow wire is enough for some short runs for PDP and PCM connections. We plan on using CAN elsewhere as well.

I have not found a good source for green/yellow twisted pair. I know it doesn’t have to be the same color, but it’s good consistent reminder and it’s our school colors. :slight_smile:

Does anyone have a good source for the green/yellow twisted pair wire?


And while I’m at it, do teams have any recommendations for connectors for the CAN wires?

We are likely to try out the new Talon SRX. They have wires for CAN. How should We connect the CAN wires together? What kind of connector would be good here?

We’re considering PWM connectors, but they PWM connector notoriously do not lock.

Thanks for the ideas.

Posted this in another thread, but since you ask… Hansen Hobbies sells 2-pin latching polarized PWM connectors. They are what we’ll be using on our 2015 robot.

The connectors form Hansen Hobbies are at a price premium. They are just Molex connectors - try digikey

Also, electricity is color blind to insulation color. Twisting your pairs isn’t absolutely critical on CAN, but not doing so will turn 2 minutes of laziness into two hours of hair pulling. CAN uses a differential signal: both will rest to the same voltage, then when signaling, CAN High gets pulled up a volt or two and CAN low gets pulled down a volt or two and they are compared against each other. If there is noise it’ll effect them the same and that differential will remain intact. Good stuff.

Where you can find another opportunity to pull your hair out is loose connections on the CAN bus. It is a bus, all CAN High connections should be congruent with each other and all CAN Low connections likewise. If you have a bad connection in the middle, you’ll lose not just the other half of devices, but the terminating resistor as well and the whole bus itself… well, in an ideal world. But fortunately theory and reality don’t agree and sometimes you’ll still have an intact bus.

BTW, I’d use WAGO lever locks for the motor controllers.

Yeah, the green/yellow is helpful for other reasons.
-Makes robot inspection and troubleshooting easier.
-The colors are labeled on the roboRIO CAN connector
-The colors are present on the PDP/PCM Weidmuller connectors.
-The colors match the Talon SRX cable harness
It all adds up to a better CAN bus experience.

Very well put.

What we did (temporarily) is just twist the ends together, then wrap with electrical tape. It lets you verify everything works, you can address each Talon, and drive it around.