This is extremly useful is you have taken the time to run a long RJ45 cable around your house, only to discover that you now should have run two cables as you need another one ... doh!
The 10/100 Mbit/spec only actually uses 4 of the 8 available wires as so :-
1. White/Orange (used - Tx Pair 1)
2. Orange/White (used - Rx Pair 1)
3. White/Green (used - Tx Pair 2)
4. Blue/White - not used.
5. White/Blue - not used.
6. Green/White - (used - Rx Pair 2)
7. White/Brown - not used.
8. Brown/White - not used.
So all we need to do is wire the 2 unused pairs (4 wires) into the same connections as you would have done normally in the 2nd RJ45 plug at both ends of the RJ45 wire - ie
(was 5 now 1) White/Blue (now used, Tx Pair 3)
(was 4 now 2) Blue/White (now used, Rx Pair 3)
(was 7 now 3) White/Brown (now used, Tx Pair 4)
(was 8 now 6) Brown/White (now used, Rx Pair 4)
There you have it, 2 Ethernet connections down a single RJ45 wire.
There are a few different ways to practically implement this
A.) Use a RJ45 faceplates, where you can simply wire the 'fixed' cabling as above, keeping the RJ flyleads the same. You'll need to buy 2 x RJ45 dual faceplates but it will look much more professional ...
B.) Buy some RJ45 plugs and an RJ45 crimper - not the most cost effective option if you are just doing a couple of connections though ...
C.) Lastly, cut the ends off existing RJ45 patch cables and connect the wires together. It's important to try and keep the 'twists' going, especially for longer runs of cable. Ideally solder the wires together and use heat shrink, but if you're really on a budget, then electrical tape and twisted wires should work ok ..
Last couple of points -
1. This will NOT work for Gigabit Ethernet - that uses all 4 pairs.
2. Make sure you identify the pins correctly, the colour codes I've given 'may' not be valid for your cables (there are two different standards.. )
Look here to check the RJ45 Pinout (courtesy datalinkcom.net)
Edited by RBJTech, 09 March 2007 - 10:04 AM.