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Nullmodem (9-25) cable


Use this cable between two DTE devices (for instance two computers)

9 pin D-SUB female
9 pin D-SUB female connector  to the Computer 1

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 TD signal of one device must be connected to the RD input of the other device (and vice versa).

Also, however, many DTE devices use other RS-232 pins for out-of-band (i.e., "hardware") flow control. One of the most common schemes is for the DTE (the PC) to assert the RTS signal if it is ready to receive data (yes, it DOES sound backwards, but that"s how it works), and for the DCE (the modem) to assert CTS when it is able to accept data. By connecting the RTS pin of one DTE to the CTS pin of the other DTE, 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.

D-Sub 9 D-Sub 25
Receive Data22Transmit Data
Transmit Data33Receive Data
Data Terminal Ready46+8Data Set Ready + Carrier Detect
System Ground57System Ground
Data Set Ready + Carrier Detect6+120Data Terminal Ready
Request to Send75Clear to Send
Clear to Send84Request to Send

Note: DSR & CD are jumpered to fool the programs to think that they are online

25 pin D-SUB female
25 pin D-SUB female connector  to the Computer 2

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