DE-9 to DE-9, DE-9 to DE-25 cables. Use this cable to connect two devices equipped with serial RS-232 interface. This cable may be used to connect any RS-232 equipped device to computer, connect two computers via COM serial port and so on. RS232 cable colors included.

The purpose of a null-modem serial cable is to permit two RS-232 devices to communicate with each other without modems or other communication devices 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).



 

RS-232 serial cable (Null modem) DE-9 to DE-9 with handshake

RS232 signal

 
D-Sub 1


 
RS-232 cable wire  color* D-Sub 2 RS232 signal
Receive Data (RxD) 2 brown 3 Transmit Data
Transmit Data (TxD) 3 red 2 Receive Data
Data Terminal Ready (DTR) 4 orange 6+1 Data Set Ready + Carrier Detect
System Ground (Ground) 5 yellow 5 System Ground
Data Set Ready + Carrier Detect (DSR+CD) 6+1 green+black 4 Data Terminal Ready
Request to Send (RTS) 7 blue 8 Clear to Send
Clear to Send (CTS) 8 purple 7 Request to Send
Ring Indicator (RI) 9 white n/c  

*There is no standard color scheme.

Some 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 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.



 


 

RS-232 serial cable (Null modem) DE-9 to DE-9 without handshake

RS232 signal

 
D-Sub 1 Cable wire  color* D-Sub 2 RS232 signal
Receive Data (RxD) 2 brown 3 Transmit Data
Transmit Data (TxD) 3 red 2 Receive Data
System Ground (Ground) 5 yellow 5 System Ground

*There is no standard color scheme.


 







 

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

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
According to 13 reports in our database (11 positive and 0 negative) this pinout should be correct.

Is this pinout
RS-232 serial cable visual pinout:
RS-232 serial cable diagram
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