A crossover cable, sometimes refered to as a x-over or a crossconnect is an Ethernet cable that in addition to allowing the direct connection of NICs also allows hubs and switches to be directly connected.

Here's the pinout for an x-over with a standard RJ-45 jack.


Also note that only pins 1,2,4, and 6 are needed in a 10-BaseT half duplex environment. I usually just dike the other four wires out before crimping. It makes it easy to tell your crosses from your standard Ethernet patch cables.

How about the different types of cables:

  1. ALPHA
    1. white green
    2. green
    3. white orange
    4. blue
    5. white blue
    6. orange
    7. white brown
    8. brown
  2. BRAVO
    1. white orange
    2. orange
    3. white green
    4. blue
    5. white blue
    6. green
    7. white brown
    8. brown

Remember: Tits down!

this was originally posted to comp.os.ms-windows.networking.misc by Steve Winograd (winograd@ibm.net). I have just copied it verbatim, since it is such a nice explanation of what a crossover cable is.

"10/100BaseT Ethernet cards use two pairs of wires: one pair for transmitting, and one for receiving. If you connect two cards together using a straight cable, the transmit circuits are connected to each other and the receive circuits are connected to each other. It's as though you and I are trying to talk on the phone but one of us is holding the handset upside down -- neither of us will hear what the other is saying. A crossover cable reverses the transmit and receive wires between the ends:

Straight cable:

Card 1 Transmit ------ Card 2 Transmit
Card 1 Receive ------ Card 2 Receive

Crossover cable:

Card 1 Transmit ------ Card 2 Receive
Card 1 Receive ------ Card 2 Transmit

A hub has internal circuitry which does the transmit/receive reversal."
Crossover cables are required in some situations because there are two types of RJ45 ports: MDI and MDI-X. As has been explained before, each port has a pair of wires for transmitting, and a pair for recieving. An NIC usually has an MDI port, and a hub, router, switch, or whatever you use (from now on i'll just say H/S/R) will probably have an MDI-X port. The transmit pair on MDI is the same as the recieve pair on MDI-X, and the transmit pair on MDI-X is the same as the recieve pair on MDI, so the signals go from the transmitter to the reciever.

However, when you connect an MDI to an MDI or an MDI-X to and MDI-X, this won't work, for reasons explained above. This is why we have crossover cables - they connect each pair on one end to the other pair on the other end. Some H/S/Rs will have a way to change a port to MDI or MDI-X, but usually when connecting one H/S/R to another, and to connect one computer to another - I have yet to see an NIC with an option to use MDI-X - you have to use a crossover cable.

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