Direct computer-to-computer communication requires a special cable because you can’t connect a DTE device directly to another DTE device. When you connect two DTE devices with serial data ports, you connect the output on one computer to the output on the other computer, and the input to the input, so neither computer will actually receive any data. Therefore, you must flip the connections, so each output connects to an input. A cable or adapter that connects output pins to input pins is called a null modem.