A null modem cable will enable you to connect two computers together for serial communications. It is a particularly designed cable that permits anyone to connect two computers directly to each other via their communications ports called RS-232 ports. Serial communications with RS232 is one of the oldest and most widely spread communication methods in computer realm. Null modems are specially helpful with portable computers because they enable the portable computer to exchange data with a bigger machine. Null modem cables have the TD (Transmit Data) and RD (Recieve Data) lines crossed over, permiting information to be sent from one computer to the other. They are similar to a crossover cable where it is the CTS (clear to send) and RTS (ready to send) lines are crossed over. Null-modem cables are particularly used for serial port connections.

9-pin to 25-pin Null Modem Cable Pinout

9 pin female connector

9-pin female connector

25 pin female connector

25 pin D-Sub Female

Signal D-Sub 9 D-Sub 25 Signal
Receive Data 2 2 Transmit Data
Transmit Data 3 3 Receive Data
Data Terminal Ready 4 6+8 Data Set Ready + Carrier Detect
System Ground 5 7 System Ground
Data Set Ready + Carrier Detect 6+1 20 Data Terminal Ready
Request to Send 7 5 Clear to Send
Clear to Send 8 4 Request to Send

25-pin Null Modem Cable Pinout

25 pin female connector

25 pin D-Sub Female

Signal D-Sub 1 D-Sub 2 Signal
Receive Data 3 2 Transmit Data
Transmit Data 2 3 Receive Data
Data Terminal Ready 20 6+8 Data Set Ready + Carrier Detect
System Ground 7 7 System Ground
Data Set Ready + Carrier Detect 6+8 20 Data Terminal Ready
Request to Send 4 5 Clear to Send
Clear to Send 5 4 Request to Send

9-pin Null Modem Cable Pinout

9 pin female connector

9-pin female connector

Signal D-Sub 1 D-Sub 2 Signal
Receive Data 2 3 Transmit Data
Transmit Data 3 2 Receive Data
Data Terminal Ready 4 6+1 Data Set Ready + Carrier Detect
System Ground 5 5 System Ground
Data Set Ready + Carrier Detect 6+1 4 Data Terminal Ready
Request to Send 7 8 Clear to Send
Clear to Send 8 7 Request to Send

This data was contributed by the team at The Hardware Book.