USB to DB9 Serial Adapter: Getting the Wiring Right
DB9 is the connector everyone pictures when they think of serial, even though the standard originally used 25 pins. Getting the correct gender and wiring is the whole job.
Quick Answer
A USB to DB9 serial adapter presents a 9 pin D-sub connector carrying RS-232 signals, usually male. Match the gender to the equipment socket and confirm whether the link needs straight through or null modem wiring. Only pins 2, 3 and 5 carry data and ground; the remaining pins are handshake and status lines that some equipment requires.
DB9 Pinout
| Pin | Signal | Direction from the computer |
|---|---|---|
| 1 | DCD, carrier detect | Input |
| 2 | RXD, receive data | Input |
| 3 | TXD, transmit data | Output |
| 4 | DTR, data terminal ready | Output |
| 5 | GND, signal ground | Ground |
| 6 | DSR, data set ready | Input |
| 7 | RTS, request to send | Output |
| 8 | CTS, clear to send | Input |
| 9 | RI, ring indicator | Input |
Gender and Connector Options
| Option | Use | Note |
|---|---|---|
| Male DB9 | Plugs into most equipment sockets | The usual adapter output |
| Female DB9 | Plugs into equipment with a male socket | Less common, sometimes needed |
| DB9 to DB25 adapter | Older modems and plotters | Pinout conversion, not just size |
| Screw terminals | Bare wire installations | Used in industrial cabinets |
| Right angle DB9 | Tight spaces | Check the direction of the bend |
Straight Through or Null Modem
Straight through
Pins map one to one. Used when the computer connects to a modem, instrument or any device acting as DCE.
Null modem
Transmit and receive are crossed and handshake lines looped, so two DTE devices can talk.
How to tell
Read the equipment manual. If it expects a null modem cable and you use straight through, the link appears dead rather than degraded.
Real-World Applications
- Connecting a scale or label printer to a PC.
- Programming a controller that presents a female DB9 socket.
- Link two controllers that both expect DTE wiring using a null modem adapter.
- Replacing a failed built in serial port on a desktop.
Related Guides
For cabling choices, read USB to serial cable. For port setup, see USB to COM port adapter. For isolated industrial units, start with RS232 to USB adapter.
FAQ
Is DB9 the original serial connector?
No. RS-232 originally used a 25 pin connector. The 9 pin version became standard on PCs.
How many pins are actually used?
Three for basic data: transmit, receive and ground. The rest are handshake and status signals.
Can I use a straight cable where null modem is required?
No. Data directions collide and nothing transfers. Use the correct wiring.
Do all adapters end in male DB9?
Most do. If your equipment has a male socket, buy a female output adapter or a gender changer.