null modem/modem eliminator

A null modem simulates a modem connection. It is a connection cable or adapter with a serial RS-232 interface in which the lines for the transmit data( TxD) and the receive data( RxD) are crossed. Via such a null modem, two data terminal devices can be coupled and exchange data with each other, without the need for a modem.

Null modems cannot be used with modems because of their crossed lines.