A specially designed cable that allows a user to connect two computers directly to each other via their communications ports (RS-232 ports). Null modems are particularly useful with portable computers because they enable the portable computer to exchange data with a larger system. Null modem cables have the Tx (transmit) and Rx (Recieve) lines crossed over, thereby allowing data to be sent from one device to the other. They are similar to a crossover cable where it is the CTS (clear to send) and RTS (ready to send) lines are crossed over. Null-modem cables are used for serial port connections.