When you’re troubleshooting cascaded router/hub/switch connection problems, you may want to investigate the crossover cables as a possible cause. However, chances are you don’t have a spare crossover cable lying around that you can use for a quick test.
I recently discovered a solution in a handy tool that you can easily build yourself. It’s called a crossover adapter, and it allows you to instantly turn any regular network cable into a crossover cable. With the crossover adapter in your toolbox, you’ll be able to easily troubleshoot suspect crossover cables. In this Daily Drill Down, I'll show you how to build your own crossover adapter. As I do, I’ll also explain how it works.
The instant two-computer network
A crossover adapter is also extremely handy when you need to transfer data between two computers. Rather than connecting the computers to your regular network hub—which could mean interfering with an existing configuration—you can just connect the two computers together with the crossover adapter and a regular network cable.
A look at the standards
Before we get started with actually building our crossover adapter, let’s take a few minutes to explore the two different wiring standards for network cabling.
As you may know, every company that develops a product for the computer industry tries to establish its product as the standard all others should follow. However, that doesn’t always work out in the best interest of the consumer. In many cases, an independent organization must step in and guide the industry to a standard.
In the case of the wiring standard, the Electronic Industries Alliance (EIA) and the Telecommunications Industry Association (TIA) got together in 1991 and set a standard for network cable wiring called the EIA/TIA-568 Standard for Commercial Building Telecommunications Wiring. The EIA/TIA-568 standard describes the performance and installation specifications for network cabling, covering both the cable itself and how it’s connected to modular plugs and jacks.
As you may have guessed, the EIA/TIA-568 standard’s cable performance specifications include the Category 5 (Cat 5) cabling now recommended for use on all current network wiring plans (The EIA/TIA-568 standard also covers Categories 1 through 4 cabling). Of course, the main objective of the EIA/TIA-568 standard is to give all manufacturers the ability to build equipment and components that will interoperate in a standardized cabling environment.
The EIA/TIA-568 standard has two schemes for how the wire is connected to modular plugs and jacks—T568A and T568B. These standards are essentially the color code patterns that you use for connecting the colored wires in the cable with the connectors in the RJ-45 modular plugs and jacks.
The US Government uses the T568A wiring standard for all network wiring done under federal contracts. However, many corporate networks also use the T568A wiring standard. One reason for this use is that the T568A wiring standard provides backward compatibility with both one-pair and two-pair Universal Service Ordering Code (USOC) wiring schemes. Bell Systems and AT&T put these wiring schemes into place in the 1970s to connect customer premises equipment to the public network. Many of them are still in use.
Most other network configurations use the T568B wiring standard. This standard is identical to the AT&T 258A, which was the preferred and most widely used wiring scheme before the adoption of the EIA/TIA standards. However, this wiring standard only provides backward compatibility with one-pair USOC wiring schemes.
Now that you have a little background information, let’s take a look at the actual color code wiring diagrams for the EIA/TIA T568A and T568B connections. Figure A shows the T568B color code wiring diagram, and Table A contains the T568B wiring specifications. Figure B shows the T568A color code wiring diagram, and Table B contains the T568A wiring specifications. When you compare the diagrams, you’ll see that the only difference between the two color codes is that the orange and green pairs are interchanged.
|A standard EIA/TIA T568B color code wiring diagram|
|A standard EIA/TIA T568A color code wiring diagram|
The crossover cable bridges the two standards
Regardless of which EIA/TIA standard is used in a regular network cable, all the wires run straight through, as illustrated in Figure C and detailed in Table C. However, with a crossover network cable, wires 1, 2, 3, and 6 are crossed over, as illustrated in Figure D and detailed in Table D.
|In a regular network cable, all the wires are connected straight through.|
|In a crossover network cable, wires 1, 2, 3, and 6 are crossed over.|
You can see that a crossover cable is really nothing more than a cable with one end wired with the EIA/TIA T568A scheme and the other end wired with the EIA/TIA T568B scheme. With this in mind, let’s now take a look at the steps for building our crossover adapter.
Gathering your equipment
To build your crossover adapter, you’ll need the following components:
- · An RJ-45 jack
- · An RJ-45 plug
- · A 1-port unloaded keystone surface mount box
- · A length of Cat 5 cable; anywhere from 6 to 12 inches long
These four components are pictured in Figure E.
|The four components you’ll need to build your crossover adapter|
Chances are you have an RJ-45 plug and a short length of Cat 5 cable lying around. If not, you can find both at your local computer store. While you can use any brand of RJ-45 jack and unloaded keystone surface mount box, I’ll be using the Belkin RJ-45 jack and unloaded keystone surface mount box for my example crossover adapter. Again, you should be able to find these components at your local computer store or online for under $10. Of course, you’ll also need an RJ-45 crimping tool. If you don’t have one, you can get one for as little as $15.
Constructing the plug
Constructing the plug end of the crossover adapter is a pretty straightforward procedure. To begin, strip off about one inch of the cable jacket/insulation, then untwist and fan out the eight wires. As you do, be sure to arrange them according to the T568B color code wiring scheme.
Working with inch-long wires makes it easy to untwist and fan out the eight wires; however, that’s too long for a proper crimp with an RJ-45 plug. Therefore, once you have the wires arranged, grasp them firmly and cut them down to half an inch. Then insert them into the plug, as shown in Figure F, making sure that they stay in the proper order as you guide them into the eight slots in the plug.
|Grasp the wires firmly between your thumb and index finger and slide them into the eight slots in the plug.|
As you push the cable into the plug, slightly wiggle the cable back and forth. This will help ensure a good connection by working the wires all the way to the end of the plug, as shown in Figure G.
|To ensure a good connection, make sure that the wires reach the all the way to the end of the plug.|
At this point, closely inspect the plug to verify that all the wires are still in the correct order. Then, insert the plug into the crimping tool and squeeze the handles together to crimp the plug onto the cable.
Constructing the jack
Constructing the jack end of the crossover adapter should be very easy. Since we wired the plug according to the T568B wiring scheme, we’ll wire the jack according to the T568A wiring scheme. To begin, you’ll strip off about one inch of the cable jacket/insulation, and then untwist and fan out the eight wires. Then set the cable aside for a moment.
Now, if you look at the jack, you’ll see that it has a wiring guide sticker on it that shows the arrangement of the wires for both the T568A and T568B standards. As you study the wiring guide, keep in mind that this wiring guide won’t necessarily correspond to the exact color code wiring schemes shown earlier. Instead, the wiring guide on the sticker and the actual jack itself are designed to make it easy for you to insert the wires. As long as you follow the wiring guide on the sticker, the connector itself will take care of routing the wires to the correct locations in the receptacle end of the jack.
Once you identify the T568A color code wiring scheme for the jack, separate the first four wires, flip open the cover on back of the RJ-45 jack, and insert the wires firmly into the slots, as shown in Figure H. Then, close the cover halfway—just enough to seat the wires into place. This will ensure that the first four wires stay in place as you seat the other four.
|You’ll have better success and less frustration if you only work with four wires at a time.|
Next, flip open the cover and insert the other four wires, as shown in Figure I. At this point, close the cover by pressing firmly until it snaps into place, and insert the locking clip.
|Once the first four wires are securely in place, inserting the second set of four wires will be easy.|
Place the jack into the keystone surface mount box, as shown in Figure J. Then, to complete the operation, snap the cover on.
|Once the jack is complete, insert it into the keystone surface mount box.|
Completing the project
As the finishing touch, I suggest that you label the crossover adapter. When you’re finished, you’ll have a nice little tool, as shown in Figure K, for your troubleshooting toolbox.
|With a crossover adapter in your toolbox you’ll be ready for any troubleshooting situation.|
Now, whenever you need a crossover cable, simply plug a regular network cable into the jack on your adapter.
Greg Shultz is a freelance Technical Writer. Previously, he has worked as Documentation Specialist in the software industry, a Technical Support Specialist in educational industry, and a Technical Journalist in the computer publishing industry.