Sometimes when connecting computer systems and peripherals, you need to create a particular kind of cable for Serial connections.

The “Null-Modem Cable” is so called because it is used on the serial ports without a modem, for machine to machine communication. The cable simply has the Rx (receive) and Tx (Transmit) lines crossed - so the two machines talk directly to each other.

You may find this necessary to connect to some MicroControllers and accessories.