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9 pin D-SUB female connector layout
9 pin D-SUB female connector
at the Computer 1
25 pin D-SUB female connector layout
25 pin D-SUB female connector
at the Computer 2
Use this cable between two DTE devices (for instance two computers)

The purpose of a null-modem cable is to permit two RS-232 DTE devices to communicate with each other without modems or other communication devices (i.e., DCE's) between them.

To achieve this, the most obvious connection is that the TxD signal of one device must be connected to the RxD input of the other device (and vice versa).

Many DTE devices use other RS-232 pins for flow control. One of the most common schemes is for the DTE (the PC) to assert the RTS signal if it is ready to send data, and DCE (the modem) to assert CTS when it is able to receive data. By connecting the RTS pin of one device to the CTS pin of the other device, we can simulate this handshake.

Also, it is common convention for many DTE devices to assert the DTR signal when they are powered on, and for many DCE devices to assert the DSR signal when they are powered on, and to assert the CD signal when they are connected. By connecting the DTR signal of one DTE to both the CD and DSR inputs of the other DTE (and vice versa), we are able to trick each DTE into thinking that it is connected to a DCE that is powered up and online. As a general rule, the Ring Indicate (RI) signal is not passed through a null-modem connection.

Null modem DSUB9 to DSUB9 cable

  D-Sub 1 D-Sub 2  
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


 

Null modem DSUB9 to DSUB25 cable

  D-Sub 9 D-Sub 25  
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


 

Null modem DSUB25 to DSUB25 cable

  D-Sub25 1 D-Sub25 2  
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
Note: DSR & CD are jumpered to fool the programs to think that they are online
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