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by Dinesh Thakur Category: Computer Network

A null modem cable is a cable that plugs into the communication port (the serial port, the one where you would plug in the cable for a modem) of two computers that are pretty close to each other. Then the two computers can share information back and forth, even faster than if they were connected by a modem and phone lines. If you want to be able to connect your portable computer directly to your bigger computer without modems, you need to hook them together with a null modem cable. Or you can use a standard serial cable and get a null modem adaptor, which is just a plug with a female jack on one side and a male jack on the other.

 More technical info: The reason you need a null modem cable, not a standard serial cable, is that the port where you plug in on the modem is wired as a mirror image of the port on a computer-the computer might send signals on wire 2 and receive them on wire 3, while vice is versa at the modem end. Since the ports on both computers are wired the same way, the signals get crossed if you use a regular serial cable. In a null modem cable, the wire going to pin 2 of one computer is crossed so it goes to pin 3 of the other, and so on (only 3 pairs of wires need to be reversed).

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