Crossover Ethernet cable - is it nessecery?
 

hey !
I did some reading today, and i saw that the Axis camera needs to be connected with a CROSSOVER Ethernet cable instead of a regular one to the cRIO.

Is it that important ? whats the diffrence between regular and crossover cables ?

Thenks !

Re: Crossover Ethernet cable - is it nessecery?
 

The short answer is, yes it is necessary. I don't know the details, but basically, a regular cable has the same wires in the same locations at both ends (pin 1 on the left is pin 1 on the right), where a crossover cable switches positions of the wires (pin 1 on the left might be pin 3 on the right or something like that).

Re: Crossover Ethernet cable - is it nessecery?
 

Right. It is needed because the transmit line (Tx) on the cRio needs to be connected to the receive line (Rx) on the camera and vice versa. Many devices such as routers and switches (and anything that conforms to the IEEE 802.3 standard) can perform automatic crossover, but these devices do not.

Re: Crossover Ethernet cable - is it nessecery?
 

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.

Re: Crossover Ethernet cable - is it nessecery?
 

If thats the case- is there any diffrence about which end of the cable I connect to which device ? (Camera and cRIO)

Re: Crossover Ethernet cable - is it nessecery?
 

If you are using a crossover cable, then no, it doesn't matter which end you connect where.

Re: Crossover Ethernet cable - is it nessecery?
 

No. it dosen't matter which side is connected to which. x-overs connect a device to a device. straight cables connect devices to routers/hubs.

Re: Crossover Ethernet cable - is it nessecery?
 

So the control system manual shows a crossover going from the cRIO to the wireless bridge and also from the cRIO to the classmate. Are both connections required to be crossover ethernet cables, or can one or both be made with a regular ethernet cable?

Thanks!

Alex

Re: Crossover Ethernet cable - is it nessecery?
 

Actually, a straight cable can be used for both the wireless bridge-to-cRIO AND for the Classmate-to-cRIO connections. A crossover cable will also work. I know the FIRST documentation says to use a crossover cable to connect the Classmate with the cRIO, but it doesn't appear to be necessary.

Just the camera requires the crossover cable.

Re: Crossover Ethernet cable - is it nessecery?
 

This is because the Classmate's Ethernet port has the intelligent crossover (IEEE 802.3). Only one of the devices needs to do it in order for either type of cable to work.

Re: Crossover Ethernet cable - is it nessecery?
 

Crossovers are to speak to like devices ie the cRio, strait through are for unlike devices, So it is necessary to yse a crossover otherwise your data will not transfer.

Re: Crossover Ethernet cable - is it nessecery?
 

So our link between the Classmate and the cRIO (untill we will configure the wireless) sould also be over a crossover cable ?

Re: Crossover Ethernet cable - is it nessecery?
 

You can use either type of cable.

Re: Crossover Ethernet cable - is it nessecery?
 

Because the classmate is smart enough to auto-configure itself.

Re: Crossover Ethernet cable - is it nessecery?
 

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 :)