Pinout
RS-232 (DE-9 / DB9) pinout
Pins
- 1: DCD (carrier detect, from the modem)
- 2: RXD (received data — into the PC)
- 3: TXD (transmitted data — out of the PC)
- 4: DTR (data terminal ready, out)
- 5: GND
- 6: DSR (data set ready, in)
- 7: RTS (request to send, out)
- 8: CTS (clear to send, in)
- 9: RI (ring indicator, in)
Specifications
| Contacts | 9 (DE-9) |
|---|---|
| Standard | TIA-232-F |
| Logic 1 (mark) | −3 to −15 V |
| Logic 0 (space) | +3 to +15 V |
| Topology | Point-to-point |
| Duplex | Full duplex |
| Speed | 20 kbit/s by the standard; 115200 baud common in practice |
| Max cable length | ≈ 15 m at 19.2 kbit/s (2500 pF limit) |
| Minimum wiring | Pins 2, 3 and 5 |
| Transceiver | MAX232 (5 V), MAX3232 (3.3 V) |
Notes
- Directions are for the DTE (PC) male connector; the device (DCE) female end mirrors the pins and swaps the directions, which is why a PC-to-PC link needs a null-modem cable crossing 2/3 (and 7/8, 4/6).
- RS-232 voltage levels are typically ±5–15V — NOT compatible with 3.3V/5V TTL logic without a level shifter (e.g. MAX232). A simple link uses only 2, 3 and 5.
