Gift for a Geek

Posted: 12/12/2009 in How to...., Tutorial

RJ45 Ethernet Loopback Cuff link/Keychain

I know that this is the time of year when we are all looking for gifts for our friends and love ones so if you have a geek friend or are even married to one here is a gift that can’t go wrong.

RJ45 Cuff Link RJ45 Cuff Link

First things first, you need some materials! Obviously, you’ll need at least two crimp-on RJ45 plugs and 6-8″ of twisted pair cable. I’m using Category 5 because I am a cheap bastard. Then, you’ll also need an RJ45 crimper. Also, you’ll need a wire-cutter if your crimper doesn’t have one built-in.

Cat5 cable, crimper, RJ45 plugs

Take all the cable out of the sheath. Throw away the sheath and the thread inside the sheath. We won’t need them.

Loose pairs out of the sheath

We will only need two wires for a simple loopback capable of 10/100. I opted for the Blue and White-Blue. You will need four wires for a full loopback (I’ll cover that in a moment). It’s easier if you use all four solid colors for that, although I opted for the Blue and Green, and their white pairs for my example here. Fold them in half, then cut them with a wire cutter so you have four or eight similar lengths of wire.

Wires bent in half

Cutting wires in half

The below diagram shows how to make a full loopback. Looking at the plug with the pins facing you and facing up, the pins are numbered left to right.
Diagram

For a normal loopback, you only need to connect Pin 1 to Pin 3 and Pin 2 to Pin 6, as shown below:

Half loopback

A full loopback looks like this, but the wiring is hard to follow from the photo. Use the diagram above for reference.
Full Loopback

Now, carefully insert the plug into the crimp tool without disturbing the wires. Crimp the connector, and you’ve got a loopback tester!
Crimping

Make another one just like it, and you can use them as cuff links.  To fasten them, just pass the wires through the button-holes from the outside of the cuff, then spread the wires out to hold everything in place. This part will be facing you most of the time and isn’t likely to get noticed at all.

Cuff Link

Alternatively, you can put the loopback tester on your keychain.
Loopback tester keychain

Using the ethernet loopback tester:
As I already mentioned, a loopback tester simply connects an ethernet adapter to itself so that it thinks it’s on a network. This is good for troubleshooting physical layer stuff. For example, you could use this to determine if the port on the ethernet switch, the port on the adapter, or the cable between them is to blame for connectivity problems.  Some switches and ethernet cards (particularly auto-crossover cards) aren’t fooled by this trick and some switches will see this adapter and report a “Partition” on the port.  Here’s what happens on my Sun Ultra 5 when I unplug it from the network (errors) and plug the crossover into the port in its place:

Crossover works!

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