Crossover Ethernet cable - is it nessecery?

[Jon Stratis]

Originally Posted by Tytus Gerrish

there are two ways to terminate an 8P8C aka RJ45 Jack. a scheme and B scheme

A is white/orange, orange, white/green, blue, white/blue, green, white/brown, brown


B is white/green, green, White/Orange, blue, white/blue, orange, white/brown, brown

to make a crossover you just terminate A on one end and B on the other end. to make a normal cable terminate with similar ends.

Just nit-picking here... but your lettering is backwards.

TIA/EIA-568-B describes two standards, T568A and T568B:

T568A is white/green, green, white/orange, blue, white/blue, orange, white/brown, brown

T568B is white/orange, orange, white/green, blue, white/blue, green, white/brown, brown


The confusion probably comes from the somewhat twisted (no pun intended) history of the standard. AT&T's 258A standard matches T568B, and is what most people still use for straight cables, mostly out of tradition. In fact, some organizations don't even recognize T568A.

Surprisingly, TIA/EIA-568-B addresses these standards on only one page - the entire document is over 400 pages long. Yet this one page gets more attention than the rest of the pages combined.

For purposes on our robot, all that matters is that you use one standard for one end, the other standard for the other when making a cable to go between the camera and cRio - and as it's been suggested, a straight cable works just fine for the other connections (cRio to wireless adapter, laptop to cRio, etc). However, if you're every doing wiring on a broader scale, the standards do matter. Horizontal cable runs should terminate with T568A, while vertical runs should terminate with T568B, officially.


And finally, a short description on why all of this is necessary, with an emphasis on history. Back when all of this was new, it was pretty expensive to do any autodetection/switching- in fact, no one did it. Instead, they basically defined the ethernet jacks on computers to "expect" transmission on one pair, reception on the other. Since most computers were then plugged into a hub, they physically swapped it on the ethernet jacks on the hub - this way, a straight cable would work between the computer and a hub. The hub expected the reception on the same pins as the computer was transmitting, and vice-versa. But sometimes people wanted to connect computers straight to other computers, or hubs to other hubs. In order to do that, the crossover cable was invented, where the reception/transmission lines were swapped in the cable, instead of in the port the cable was plugging into.

Of course, some devices can now autodetect what the device on the other end is doing and adjust. but that's more expensive to do, so some devices, like the Axis camera, don't bother. The assumption made when they designed the camera was that it would always be plugged into a router. Turns out that's not a great assumption when FIRST picks it up