A null modem cable differs from ordinary serial cables the same way as Ethernet crossover cables differ from ordinary Ethernet cables. Null modem cables reverse the transmit and receive lines on end to enable direct two-way communication. A null modem cable for PCs ordinarily follows the RS-232 standard and uses the same serial ports as RS-232 cables.
An RS-232 null modem cable transfers data at the rate of 115 Kbps. The fastest null modem cable, based on RS-422, supports up to 450 Kbps.
Today, null modem cables are used primarily by engineers. USB keys, Ethernet crossover cables, and general purpose network routers have effectively made the null modem cable obsolete.