when to use the cross over cable

We been doing some testing and I want to make sure I really understand this, so some smart person, please help me out?

I want to know all the places we will need to use the cross over cable.

  1. Camera to crio?
  2. (tethered operation) classmate to crio?
  3. (tethered operation) computer (not classmate) to crio?
  4. crio to radio on robot?

please add any configurations I may have missed.

wendy holladay

  1. The cRio does not support auto-detection of crossover. Neither does the camera. Thus, this connection requires a crossover cable.

To my knowledge, all common laptops auto-detect crossover and thus don’t need crossover cables. Switches should always work without crossover cables.

that’s what i thought but does the classmate need it?

in FRC document How to Configure Your cRIO figure 2, shows the cross over cable. is the classmate e-net port different than a regular pc??

thanks for the quick response


I don’t think it’s different.

I’ve tethered many times without a crossover cable.

so have we, hence my confusion with the first documentation. i like to use documentation because i tend to forget if i don’t work with something for even a short while. but looking at the documentation makes me think you need a crossover cable a lot more than you actually do.

we have used a non crossover from both a regular pc and the classmate in tethered mode and have even reimaged the crio succesfully with a non crossover cable.


How do you tell whether a ethernet cable is a crossover cable or not? I know FIRST gave us a 5-foot crossover cable in the KOP each year and they are a different color each year, orange in 2009, pink in 2010 and gray in 2011. I think ours are all mixed up with our many other ethernet cables. Are they labelled differently? Is the only way to connect between the cRIO and camera and see if it works?

The gray crossover cable has the label “XOVER” molded in the plastic cover at each end. I highlighted it in red Sharpie so it would be obvious.

The prior years’ crossover cables we have didn’t come with any sort of label. I had put red electrical tape on the ends of one at some point, but either it didn’t stick well or someone intentionally removed it.

While most crossover cables are labeled as such, to check you can always look closely at the terminals. You’ll be able to see the individual wires that go from the contacts into the wire. There are 4 pairs of wire in each ethernet cable, and each pair is color coded - you’ll have a solid brown wire paired with a brown/white striped wire, for example. Usually you’ll have brown, blue, orange, and green wire pairs.

In normal ethernet cables, you’ll see everything is wired “straight” between each terminal - holding them next to each other, the wire that comes out of the far left pin on one goes into the far left pin on the other. Same for the second, third… eighth pins.

If you have a crossover cable, two of the wire pairs are swapped - specifically, pin 1 is swapped with pin 3, and pin 2 is swapped with pin 6. So, for example, you might see that the green stripped wire is swapped with the orange stripped wire, and the green is swapped with the orange. A quick google search turned this up as a good example:


With that, you can see why it’s important to use the correct cable. The original design for the cables, computers, and the routers between them, was that computers would transmit on certain pins and receive on others. Routers, on the other hand, would receive on the pins that computers transmitted on, and send on pins the computer received on - thus the cables could be kept simple with pin 1 going to pin 1 and so on, and patching multiple cables together to make one longer would be easy. as these things go, however, people eventually wanted to do other things with their ethernet ports, like hook two computers up without a router between them, or hook two routers up together. That’s where the crossover cable comes into play… essentially rerouting the transmit/receive pins for you for that sort of direct communication.

Since then, everything in the computer has gotten smaller and more complex, and someone figured it would be a good idea to have the ethernet port auto-detect which pins it should use. Of course everyone then had to copy it, so now it really doesn’t matter much when dealing with computers.

On our team, we put green tape around both ends of all our crossover cables. That way, we have an easy way of knowing if it’s crossover or not at a quick glance.

Granted I’ve only been in FIRST for 3 years, but I have never had to use a crossover cable to connect to any device. If you are having a problem connecting devices it is much more likely that you have a subnet or ip-address problem. You might want to (with the camera at least), reset using the reset button and then look in the FIRST Kop documentation for the default IP

Thanks Alan and Phil. I did find all 3 crossover cables from our last 3 years. They were all 5 feet long and I didn’t have any other ethernet cable of that length. I took your advice and put red electrical tape on both ends.


When formatting the cRio it is good to connect through a switch or hub. During the process the system resets the port and this can cause some trouble getting everything started back. The switch will always be seen as active when the system reboots after a new format. That is one reason, among several, why the instructions say to never try to format the cRio over the wireless link.

The only place we use a cross over cable is from the camera to the cRio.

All other places will auto detect the connection and work if booted up after connected.

Here is a link with another description of the same issue.