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A null modem is a specially designed cable that allows a “head-to-head” connection between two nearby serial devices (computers) through their communication ports (RS-232). Having a length limitation of up to 30 feet, it is most commonly used to connect PCs within the same room for gaming and other purposes such as sending and receiving files.
A null modem is also known as a crossover cable.
A null modem uses a modem protocol with Tx (transmit) and Rx (receive) lines only suitable for serial connections. An RS-232 serial communication interface is the standard communication channel through the data terminal equipment (DTE) — usually a personal computer — and data communications equipment (DCE), or the modem. Sending and receiving is done by separate lines, each of which has a function that enables the communication. A DTE sends data on the line which is the receiving line for the DCE. Sometimes in the absence of a DCE interface, the null modem makes the DTE interface of a PC look like a DCE interface in order to facilitate communication.