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10BaseT Crossover Cable

Connector 1
Connector 2
[Diagram of crossover pinout]
4Not used
5Not used
7Not used
8Not used

Crossover Cable for 10BaseT and 100BaseT networking

This section describes how to wire an RJ-45 10BaseT cable so that two Ethernet adapters can communicate directly, without an intermediate hub, in a two-node network.

Why You Need a Crossover Cable

A 10BaseT connection requires two pairs of unshielded twisted pair wires, a total of four wires. One pair of wires is called the transmit pair (TD+ and TD-) and the other pair is called the receive pair (RD+ and RD-). Because adapters and hubs use opposite pairs to transmit and receive, standard 10BaseT cables use straight-through wiring. This allows PCs connected to 10BaseT hubs to communicate without conflict.

When you connect two notebooks together, or a Handheld PC or Palm-size PC to a notebook, or a hub to a hub, you must use a crossover cable. This connects the pins used as the transmit pair at one end of the cable to the pins used as the receive pair at the other end. The easiest way to make a crossover cable is to start with a standard cable and replace the RJ-45 connector at one end. The pinouts for a crossover cable are shown below.

Note: Although the functional equivalent of a crossover cable is required to connect two hubs to one another, many hubs have a crossover switch that allows you to use a straight-through cable for this purpose.

Pin Assignments for a Crossover Cable

Pin 1 of the RJ-45 connector is on the left as you look straight at the connector with the pins down and the cable behind. This is shown in the picture below:

Swap the pins on one connector according to the diagram below:

Pin 1 TD + <-----------------> Pin 3 RD +
Pin 2 TD - <------------------> Pin 6 RD -
Pin 3 RD + <-----------------> Pin 1 TD +
Pin 6 RD - <------------------> Pin 2 TD -

It is a good idea to put red tape around your new crossover cable so people don't confuse it with a straight-through cable.

Remember: You cannot use a crossover cable to network your PC to a hub, and you cannot use a standard straight-through cable to network your PC directly to another PC. You can only use a crossover connector with a 10BaseT (twisted pair) network &emdash; not a 10Base2 (coax) network.

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Last revised: March 30, 2003