Null modem

Null modem is a communication method to directly connect two DTEs (computer, terminal, printer, etc.)

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.

[5] 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.

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).

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.

If devices require Carrier Detect, it can be simulated by connecting DSR and DCD internally in the connector, thus obtaining CD from the remote DTR signal.

[1] Because of the compatibility issues and potential problems with a simple null modem cable, a solution was developed to trick the software into thinking there was handshaking available.

The software would work thinking it had hardware flow control but could suddenly stop when higher speeds were reached and with no identifiable reason.

There are some advantages to this: For instance, DOSBox has allowed older DOS games to use virtual null modems.

Another common example consists of Unix pseudoterminals (pty) which present a standard tty interface to user applications, including virtual serial controls.