CAN cables

[Mike Copioli]

Below are a couple links to a video tutorial on how to make CAN cables for FRC.

Feedback is appreciated.

http://youtu.be/y-OxhCjmevY
http://www.crosstheroadelectronics.com/Cable.html

[Tristan Lall]

A couple things, more related to the presentation than to the content—because it's definitely a useful video tutorial.

• A quick summary at the beginning, maybe with 5 stills/clips in 15 seconds and a quick voiceover. That way we know what to expect. Something like:
  1. This video will guide you through the assembly of a CAN cable for use with the 2CAN communications module, Jaguar speed controller and other compatible CAN bus equipment.
  2. Cut the cable to length and strip the wires.
  3. Insert the wire, select the polarity and crimp the connector.
  4. Test the cable to make sure it works.
  5. [Demonstration of it in action]
• Title of the video should probably say something about RJ-11 and FRC (or 2CAN, or something). Most industrial CAN cables don't use that connector, so this might be confusing for the general public.
• Your little lightbox is neat....but it's too small, and so some things were too close to the camera. You managed to get some of the demonstrations outside of the frame.
• A little too long. 8 min would be more effective than nearly 12 min.
• Maybe include a summary in the description text field. You could even include your entire script, if you really wanted—but at minimum a summary of the steps and a link to your website.
• Include a title card somewhere identifying the video and the company. (That way, if it ever gets separated from YouTube, such as in an embedded player, it's still obvious whose video it is.)
• Say something about the difference between insulation-displacement connectors and ones where you need to strip wires individually.
• Mention the tradeoffs between cost and quality for connector type (e.g. w/r/t conductor plating). (You brought up gold; didn't mention alternatives.)
• Maybe show a quick example of how to assemble the termination resistor that fits in the back of an RJ-11 housing, and explain the choice of resistance.

Now that you've had one shot at it, and have a script, and know what motions to go through, would you be opposed to re-shooting it? I think you could refine it by shortening it quite a bit with cuts instead of a continuous shot. Voiceover can be used to tie cuts together.

Normally I wouldn't worry about this stuff—because honestly, it's quite sufficient—but since this is a commercial venture, it probably pays to be just a little more polished with it.

[alancop]

Thank you very much for posting this! I was very confused as to how in the world to get started with can on the jags

[otherguy]

You may also find our video an web page on CAN helpful:
team2168.org/index.php/resources/electrical/210-can-jaguars

[Jeff Wider]

Hi all,
This is my first year in the FRC program. So far it has been a lot of fun doing this with my son.

I have a question on CAN implementation. I have searched this forum, but I cannot find a definitive answer. From what I have researched the CAN bus implemented for 1Mb/s is a two wire balanced differential network with a characteristic impedance of 120 ohms. But what is supplied in the KOP, and what is sold by many FRC authorized vendors, are four conductor flat modular cables.

So I am confused somewhat. The CAN spec for the type of network being used is 120 ohms, yet 120 ohm two conductor cable is virtually unattainable. A ground is not required to operate the bus as designed. Having a shielded cable and using the drain wire to go back to one side of a node and connect to ground only there makes sense. Connecting a ground wire along the bus to all nodes does not (ground loops).

We are using the 2CAN so running a ground to it, is not necessary as well seems a little dangerous. I would prefer to keep it isolated from any possible spikes that could arise.

Am I missing something here? This system was designed to only require two conductors for a high, low differential and work in extremely harsh electrical systems (cars). I am scratching my brain on this, I can only guess as to how much most HS students are understanding it.

Also i am unsure of which value for terminations. I have read in TI documents a recommended 100 ohm termination. However, the 2CAN has 120 ohm (following CAN bus spec). So I am recommending we terminate the other end of the bus with 120 ohms. However, if the physical media is not characteristic 120 ohms, and unshielded as well, I question whether 20 ohms of difference at termination will have any real impact to reducing reflections or keeping common mode from be a problem.

My years of working with coax in the CATV industry tells me to use 120 ohm shielded two conductor cable. The AWG would need to be 24/26 to be used in 6p4c RJ plugs. Also to terminate with 120 ohm or split across both wires and use a cap to filter noise. Again, this is my first year doing so if someone with greater knowledge of CAN bus can offer some advise if I am off base I would appreciate it. Thanks.

[Levansic]

Don't over think this.

The CAN specification does not define the physical implementation like wire or connectors. The 2CAN and Jaguars use the common and inexpensive phone cable to make CAN easy to implement for hobbyists.

Your team will not gain anything by reinventing the CAN bus physical interface. The runs are short. Don't worry, use phone line and terminate with either 100 or 120 ohm resistors. Either will do the job.

Don't use a cap to filter anything on this bus. Use caps on motors if you need to, but keep to the recommended cabling for this bus.