Tuesday, May 8, 2012

Crossover Cables | Modular Jacks and Plugs

One of the most frequently asked questions on wiring newsgroups and forums is "How do I make a crossover cable?" Computers that are equipped with 10Base-T or 100Base-TX network adapters can be connected "back-to-back"; this means they do not require a hub to be networked together. Back-to-back connections via crossover cables are really handy in a small or home office. Crossover cables are also used to link together two pieces of network equipment (e.g., hubs, switches, and routers) if the equipment does not have an uplink or crossover port built-in.
A crossover cable is just a patch cord that is wired to a T568A pinout scheme on one end and a T568B pinout scheme on the other end. To make a crossover cable, you will need a crimping tool, a couple of eight-position modular plugs (a.k.a. RJ-45 plugs), and the desired length of cable. Cut and crimp one side of the cable as you would normally, following whichever wiring pattern you desire, T568A or T568B. When you crimp the other end, just use the other wiring pattern.
As mentioned several times elsewhere in this book, we recommend that you buy your patch cords, either straight through or crossover, instead of making them yourself. Field-terminated patch cords can be time-consuming (i.e., expensive) to make and may result in poor system performance.
Table 1 shows the pairs that cross over. The other two pairs wire straight through.
Table 1: Crossover Pairs 
Side-One Pins
Wire Colors
Side-Two Pins
1 (Transmit +)
3 (Receive +)
2 (Transmit –)
6 (Receive –)
3 (Receive +)
1 (Transmit +)
6 (Receive –)
2 (Receive –)

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