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Build Your Own Cat5 Cable Tester - Part 1
Topic:Homebrew Designs   Date: 2002-09-01
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Part 1 - How to wire Cat5 cables, and how they work.

There are many different schemes for wiring Category 5 cables, EIA/TIA 568A and 568B are the most common. We wrote up a couple pdf diagrams for these you can refer to: 568A, 568B . 568B is quite widely used, especially in the US; however, new wiring installs should probably follow 568A. So, if you want to make a Cat5 cable and have a couple RJ-45 connectors, some wire, and a crimper, look at the 568A diagram and match the colors on both plugs. Green/White should be on the left, with the tab facing away from you as you look at the plug.

What does this mean, really? Well, the ethernet devices communicate on two pairs of wires. One pair is on pins 1 and 2. The other pair is on pins 3 and 6. It is important that the pairs are twisted together. You can tell what pairs are twisted together by the fact that they share colors. That is, orange and orange/white are twisted together. Note that orange may be orange with a small white stripe, and orange/white may be white with a small orange stripe. On our cable, orange with a small white stripe is simply orange. All this comes down to the fact that we need to connect pins 1 and 2 on one device to pins 1 and 2 on the other device, and pins 3 and 6 on one device to pins 3 and 6 on the other device. You could use any pair you wanted, really, and this is why it doesn't matter, technically, if you use 568A or 568B style wiring. The problem arises if you work on the wiring later. If you stick to one standard, you don't have to guess what the other end is.

Let's dig a bit deeper. When your NIC (Network Interface Card) sends data to another device on the network, it uses the pair of wires known as the Transmit pair. When your NIC receives data, it uses the Receive pair. The Transmit pair is on pins 1 and 2. The Receive pair is on pins 3 and 6. Hubs and Switches automatically "cross over", so you can wire the Transmit pair to the Transmit pair on the Hub, and the hub will actually connect to the Receive pair (confusing). This is why you can't connect two hubs together without using a special crossover port or cable. The Hub connects the transmit pair to receive. If you hook two hubs together, by connecting pins 1 and 2 of the two hubs together, you are actually connecting the receive pairs of the hubs together. You have to use a crossover cable to cancel two crossovers. (Are you entertained, yet?). For you old-timers, this is similar to RS-232, where there is data communication equipment and data terminal equipment. A terminal would hook up directly to a computer. If you hooked up two computers together, you needed a crossover (null modem connector).

There is a polarity involved with the wires in the pair. That is, one of the wires in each pair is positive, and one is negative. The positive wire on the transmit pair must hook up to the positive wire on the receive pair on the other side. The solid (or dominant) color is negative. That is, orange and green are negative. Orange/White and Green/White are positive. You can't simply connect the pair to any old combination on the other end, you have to match polarity.

These pairs use a kind of transmission that is immune to noise. How this works is that the signal is interpretted as the relative difference between the wires in the pair. If there is an external signal that interferes with the wires, it will affect both wires in the pair, so if the signal is the relative difference between the wires in the pair you still can discern the correct signal.

For a straight-through cable, then, you really only need to connect pin 1 to 1, 2 to 2, 3 to 3, and 6 to 6. If you are connecting two NICS together, you need to connect the transmit pair of one NIC to the receive pair of the other. Hence, you need to connect 1 to 3, 2 to 6, 3 to 1 and 6 to 2. This is called a crossover cable, and is equivalent to using 568A on one end and 568B on the other end.

Notice that the brown and blue pairs are ignored. Well, they don't really do anything besides hold the connector on. For our lab, we find that it is easier to crimp the connectors if we just clip off the brown and blue wires. If everybody follows the standard, the brown and blue pairs could be used for something. 100Base - T4 uses the brown and blue pairs. There will undoubtably be other specifications that use these pairs, but for 100Base-TX and 10Base-T, which we use, there is no reason to worry about the brown and blue pairs.

There are five parts to this article:
Build Your Own Cat5 Cable Tester - Introduction

Part 1 - How to wire Cat5 cables, and how they work

Part 2 - Circuit design

Part 3 - Creating the circuit board

Part 4 - Drilling the PC board and assembling



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