Null Modem Cable Pinout (DB9-DB25)
A null modem cable is a specially configured RS232 serial cable designed to connect two Data Terminal Equipment (DTE) devices, such as two computers, directly to each other without a modem. Unlike a standard "straight-through" cable, a null modem cable crosses the transmit (TX) and receive (RX) lines-and often the handshaking lines like RTS/CTS and DSR/DTR-so that the output of one device is fed into the input of the other. In the previous epoch, these cables were used for early multiplayer gaming, local file transfers, and connecting terminals. Now, it is used to connect external devices to a computer.
Use this cable between two DTE devices (for instance two computers).
 | DB9 pin D-SUB female to PC1 |
 | DB25 pin D-SUB female to PC2 |
| DB9 | DB25 | ||
|---|---|---|---|
| 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 |
Pin numbers follow the DB9 pinout on the 9-pin side and the RS232 DB25 pinout on the 25-pin side, so the table also serves as a DB25 to DB9 pinout reference. All connector variants (DB9, DB15, DB25, RJ45) are compared on the RS232 connector pinout page.
Null modem (crossed) cable DB9-9 pin
Null modem (crossed) cable DB25-25 pin
Modem (straight) cable (DB9-DB25)
Modem (straight) cable (DB25-DB25)
Modem (straight) cable (DB9-DB15)