DB9 / RS-232 (DE-9) Connector Pinout
The 9-pin serial connector — RS-232 data and modem-control signals (DTE).
Overview
The 9-pin D-sub (properly DE-9) is the classic RS-232 serial port. The pinout below is for a DTE (a PC/terminal); a DCE (modem) swaps the data and handshake directions. RS-232 uses ±3 to ±15V signalling — NOT 3.3/5V logic, so you need a level converter (e.g. MAX3232) to talk to a microcontroller.
For a basic 3-wire link you only need TXD (3), RXD (2), and GND (5); the rest are hardware flow-control / modem-status lines.
Pinout
| Pin | Name | Function | Description |
|---|---|---|---|
| 1 | DCD | DATA | Data Carrier Detect (input). |
| 2 | RXD | DATA | Receive Data (input). |
| 3 | TXD | DATA | Transmit Data (output). |
| 4 | DTR | DATA | Data Terminal Ready (output). |
| 5 | GND | GND | Signal ground. |
| 6 | DSR | DATA | Data Set Ready (input). |
| 7 | RTS | DATA | Request To Send (output). |
| 8 | CTS | DATA | Clear To Send (input). |
| 9 | RI | DATA | Ring Indicator (input). |
Notes
- 3-wire minimum: TXD (3), RXD (2), GND (5).
- RS-232 is ±12V-ish — never wire it straight to an MCU; use a MAX3232 level shifter.
- A null-modem cable crosses TXD↔RXD (and RTS↔CTS, DTR↔DSR) for DTE-to-DTE links.
Reference: RS-232 DE-9 pinout· verified 2026-06-21