Received via email: 11/11/2002
(Sent to a friend by David, as clarification about the wiring diagrams included on www.wtvi.com/teks/exchange/crossovercable.html
The second diagram is a good diagram to use as your reference. The first and second diagrams normally work for the author because they seem to be the order of colors most often used by cable manufacturers. However, it's not the colors that matter, it's the numbering of the wires that matters (i.e. 1 through 8, starting from the left as shown in the RJ-45 picture). I normally number them starting from the opposite end, but to save confusion let's stick with the numbering in this diagram.
In ethernet, only 4 wires are used. You can completely remove the other 4 wires and your ethernet will still work perfect. Keeping this in mind, the only wires that matter are numbers 3, 6, 7, and 8 according to the diagram. For now, just ignore the rest of the wires. Now think of these 4 wires as two pairs of wires. 3 & 6 are one pair, 7 & 8 are another pair. In the first diagram on the website, wires 3 & 6 are colored green and green-stripe respectively because they are paired together. Likewise, wires 7 & 8 are colored orange and orange-stripe. In the second diagram, the coloring is different, but don't let this fool you. It is the numbering that matters, not the colors.
Okay, that's the numbering and the pairs. But what is a pair? A wire pair is just a one-way communication from one computer to the other. Looking at the second diagram and looking only at the left side (let's call the left side computer #1), pair 7 & 8 is the transmit from computer #1 to computer #2, while pair 3 & 6 is the receive from computer #2 to computer #1. Likewise, looking only at the right half of the picture, pair 7&8 is the transmit from computer #2 to computer #1 while pair 3&6 is the receive from computer #1 to computer #2. This is full-duplex communications because both computers can talk at the same time. If you have a 100 Mbit/s ethernet card, both computers talk at 100 Mbit/s simultaneously. That's 200 Mbit/s total. Half-duplex systems only use one pair (only 2-wires) for both computers to talk on. A lot of newer ethernet cards can detect whether a crossover cable is used or not. If no crossover cable is used, it defaults to half-duplex. I'm guessing that this did not work in your case because the cheap PCMCIA adapter I gave you probably did not auto-negotiate.....
Always remember that a pair is a pair is a pair. You can accidentally swap wires 7 & 8 for 8 & 7 and it will still work... for now, let's just work with the diagram the way it is drawn. FYI, keeping in mind the information that I have told you here (numbering matters, not coloring), you can go to the links that the author of the website said didn't work for him and see that they will work just fine. On those links, however, please ignore the +/- signs. As I said before, a pair is a pair is a pair. The +/- is only there for convention.
Thanks,
David Feany Motorola FTW, Software Engineering,CBTSP