Black RS232 DTE Null Modem Cable (DB-9 Female)
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.
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 thats 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.
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.
DB-9 Female Pins Connections
- 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