Making a Crossover Coupler

A Cheaper Alternative to a Crossover Cable

This page describes how to make a crossover coupler that joins any two standard 10BaseT cables into a crossover cable.

What's a crossover coupler?

Several types of 10BaseT Ethernet applications require a crossover or null cable. These applications include hubless connection of two Ethernet devices, connection of two hubs that lack an uplink port, or connection of a cable modem to a hub that lacks an uplink port.

A crossover cable is not expensive. They run $7-$12 retail at Fry's and other discount electronic supply houses. If you have spare cable, connectors and a crimping tool, you can make one yourself. If you're like me, however, you have no tools and you're not happy about spending $7-$12 for something that costs a coupla bucks to manufacture. You want an alternative.

Well here it is. What follow are instructions for making a crossover coupler that will make any two standard, or straight-through cables into a crossover cable. Another benefit of using a crossover coupler is that you can replace it with a hub in the future without changing out any of the cables. For example, if you have two computers connected to each other, and you later want to add a third computer or a printer, you can remove the crossover coupler, replace it with a hub, and run one more length of cable to the new device. If you had used a single crossover cable, you'd have to run three new lengths of cable to the location to the new hub.

These instructions may not be perfect for every coupler you might buy, but I'd guess that they're all fairly similar.

Needed materials:

Straight through coupler ($1.69 at Fry's)
Small screwdriver
Needlenose pliers (optional, but helpful)
Time. About 5 minutes.

Disassemble the coupler

If your coupler is like mine, you can disassemble it simply by pulling it apart. It snaps together without any fasteners or glue, so should separate cleanly, without breaking. Once you have it separated, untwist it so it's easier to work with.

Remove the innards from one half of the coupler

	Look inside the coupler and see how it fits together.
	The inner assembly of the coupler plugs into the inside of the housing in much the same way as a cable plugs into the outside. It's held in place by a locking tab on the top and by the leads on the bottom. First, insert the small screwdriver in the slot above the small wires (shown in blue on the right side of the diagram).
	Twist the screwdriver gently while pulling on the small wires. The locking tab on top should release, and the inner assembly should slide part of the way out. The leads on the bottom will continue to hold the inner assembly in place.
	Next, insert an RJ-45 connector into the coupler. The connector will push the leads up and release the entire inner assembly, which should then slide completely out.

	Here it is with the inner assembly removed from one half of the coupler.

How to remove wires

	To remove a wire, bend its corresponding lead up until it sticks straight out of its hole. You can then pull the wire straight back and out through the inner assembly.
	Here it is with a wire removed.

Reassigning pins

To make a crossover cable, we want to swap pins 1 and 3, and pins 2 and 6. When you hold an RJ45 connector by the cable so the metal connectors face toward you, pin 1 is on the left side. If you match the inner assembly to the RJ45 connector, you'll see which is pin 1. In the photo, it's facing the camera and labeled with a little blue 1.

Swap pins 1 and 3, and then swap pins 2 and 6.

Put it all back together

Do everything above in reverse order:

Insert all the leads through their new holes. You may need to use the pliers to pull them all the way in.
Bend the leads down so they roughly match the angle of the other leads.
Insert the inner assembly into the coupler housing.
Snap the two halves together. Give the two halves a twist before you reassemble it, so the wires stay neat inside.

Test the coupler

Use an ohmmeter to test the connections. Plug a normal 10BaseT cable into each end of the coupler and check the connections on the free ends. Pin 1 at one end should connect with pin 3 at the other end and vice versa. Pins 2 and 6 should be similarly connected.

That's it!

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Last updated: 7/30/97

Revision 1.0

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