A null modem is used to connect two DTE devices together. If you were to use a straight RS232 cable between two DTE devices, then the Transmit line on each device would be connected together and the Receive line on each device would be connected together. In order for a RS232 connection to work, the Transmit line of one device must be connected to the Receive line of the other device and vise versa. A null modem cable or null modem adapter crosses these lines. In addition to the transmit and receive lines, the DTR & DSR handshaking as well as the RTS & CTS flow control lines are also crossed in most, but not all null modem cables and adapters. Some null modems go even one step farther and connect the Carrier Detect (CD) line to the Data Set Ready (DSR) line at each end of the cable or adapter. This is done to fool the computers into thinking there is an active modem connected to the serial port. This complete null modem connection is shown below: 1,6-- 4; 2 --- 3; 3 --- 2; 4 -- 1,6; 5 --- 5; 7 --- 8; 8 --- 7