A Null modem cable is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using an RS-232 serial cable. The RS-232 standard is asymmetrical as to the definitions of the two ends of the communications link so it assumes that one end is a DTE and the other is a DCE e.g. a modem. With a null modem connection the transmit and receive lines are crosslinked. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by a standard.
The original application of a null modem was to connect two teletype terminals directly without using modems. As the RS-232 standard was adopted by other types of equipment, designers needed to decide whether their devices would have DTE-like or DCE-like interfaces. When an application required that two DTE's (or two DCE's) needed to communicate with each other, then a null modem was necessary.
Null modems are commonly used for file transfer between computers, or remote operation. Under the Microsoft Windows operating system, the direct cable connection can be used over a null modem connection. The later versions of MS-DOS were shipped with the InterLnk program. Both pieces of software allow the mapping of a hard disk on one computer as a network drive on the other computer. No Ethernet hardware (such as a network interface card or a modem) is required for this.
The popularity and availability of faster information exchange systems such as Ethernet made the use of null modem cables less common. Nowadays, such a cable can still be useful for kernel mode development, since it allows the user to remotely debug a kernel with a minimum of device drivers and code (a serial driver mainly consists of two FIFO buffers and an interrupt service routine). KGDB for Linux, ddb for BSD and WinDbg or KD for Windows can be used to remotely debug systems, for example. This can also provide a serial console through which the in-kernel debugger can be dropped to in case of kernel panics, in which case the local monitor and keyboard may not be usable anymore (the GUI reserves those resources and dropping to the debugger in the case of a panic won't free them).
Another context where these cables can be useful is when administering "headless" devices providing a serial administration console (i.e. managed switches, rackmount server units and various embedded systems).