Quote:
Originally Posted by Ed Law
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?
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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.