The original application of a null modem was to connect two teleprinter 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 DTEs (or two DCEs) needed to communicate with each other, then a null modem was necessary. Null modems were 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. On the
Amiga computer, a null modem connection was a common way of
playing multiplayer games between two machines. The popularity and availability of faster information exchange systems such as
Ethernet made the use of null modem cables less common. In modern systems, 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). An example of embedded systems that widely use null modems for remote monitoring include
RTUs, device controllers, and smart sensing devices. These devices tend to reside in close proximity and lend themselves to short run serial communication through protocols such as
DNP3,
Modbus, and other IEC variants. The Electric, Oil, Gas, and Water Utilities are slow to respond to newer networking technologies which may be due to large investments in capital equipment that has useful service life measured in decades. Serial ports and null modem cables are still widely used in these industries with Ethernet just slowly becoming a widely available option. == Types of null modem ==