How to Buy Serial Crossover Cable: 5 Steps - MakeSureHow
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Although they're seldom used since the advent of USB connections, many computers still have one or more of the older-style serial communications ports installed. These were originally used for general-purpose connections to modems, printers, mice and other external devices. They could also be used for computer-to-computer connections, enabling direct communication or file transfers. That's still a convenient option, but you need a special-purpose serial "crossover" cable.


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    What it Is: Serial communications cables typically take a straight-through configuration. This means the pin arrangements at one end are the same as those at the other end. However, that won't do for communication between computers. It would be like two people trying to converse by plugging their ears and talking simultaneously. A crossover cable changes the assignment of two wires, connecting the number 3 (transmit) pin at one end to the number 2 (receive) pin at the other end. It's called a crossover cable because it flips that pair of wires. It's also sometimes called a "null modem" cable, because it permits communication without a modem.
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    Cable Length: Before you run out to your nearest store and buy a cable, think carefully about how you plan to use it. If you're bringing a laptop to a table a few feet away from your main computer, you might only need a three or six foot cable. For a longer or more permanent connection, pulling cable through your house might be necessary. Remember, if the cable's going to stay in place you need extra length so you can route it into the corners where it won't be a tripping hazard. As a rough rule, doubling your length halves communication speed.
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    Cable Quality: Serial cables are manufactured with nine- or 25-pin connectors, and up to 20 wires inside. Cable quality depends on a number of factors, including the gauge of the internal wires connecting each pin. Standard cables typically use 24-gauge wire, though longer cables and higher data rates are possible with heavier wire. Cables with double or triple shielding and higher-quality connectors also allow faster communications and longer cable runs. Gold-plated connections provide the best connectivity and avoid corrosion.
  4. 4
    Adaptors: Some computers have a 25-pin serial port, while others have a smaller nine-pin arrangement. Look at the back of each computer and identify the serial port or ports, and note which connectors you have. For example, if both use a nine-pin male connector, you'd need a cable with nine-pin female connectors at each end. In some instances, if you're unable to purchase a null modem cable with the correct connections, it's sometimes cheaper to buy an adaptor or "gender changer" rather than have a custom cable made. If you have an existing serial cable, you can convert it to a crossover cable by using a null modem adaptor at one end.
  5. 5
    Buying Your Cable: Once you've measured your location and determined how long a cable you need, and written down which connections you need at each end, it's time to shop for your cable. Standard RS-232 cables are typically manufactured in lengths up to 50 feet, but high-quality, heavily-shielded cables can be longer. They'll also have to be custom manufactured though, which drives up the cost. A computer store or electronics retailer such as RadioShack is your best bet for adaptors and gender changers, and most carry a limited range of premade cables in short lengths. Computer stores or online cable specialists can make longer cables to order.

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Categories: Networking

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