null modem cable

Definition of null modem cable in The Network Encyclopedia.

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What is Null Modem cable?

A serial cable with cross-pinning that is used for file transfers and for other specialized communication between computers.

How It Works

Null modem cables, also known as file transfer cables, are used to directly connect two computers for transferring files between them - for example, by using the Direct Cable Connection accessory of Microsoft Windows 95 and Windows 98.

The most common form of null modem cable is based on the RS-232 serial transmission interface specifications. Pins 2 and 3 are crossed in a null modem cable to allow you to directly link two RS-232 serial ports on different computers. Other pins can also be crossed depending on the intended purpose of the cable. The most common forms of termination for these cables are DB9 and DB25 connectors.

Serial RS-232 null modem cables support distances of up to about 15 meters and can be used for any communication for which a direct DTE-to-DTE connection or DCE-to-DCE connection is required. If longer distances are needed in serial communication, you can use a line driver to amplify the signal up to several kilometers. A cable connecting two pieces of data terminal equipment (DTE) is specifically called a null modem cable, while a cable connecting two pieces of data communications equipment (DCE) is usually called a tail-circuit cable and has a different cross-pinning. You can obtain null modem cables for other serial interfaces such as V.35 and those with special types of pinning.

NOTE

Parallel data-transfer cables achieve faster data transfer rates for file transfer than serial null modem cables.

TIP

Use a shielded null modem cable to extend distances up to 30 meters without using line drivers.