9 pin DE9 male connector at the computer.
10 pin IDC male connector at motherboard.
RS-232 (standardised as EIA/TIA-232, today TIA-232-F) is the long-established asynchronous serial interface used by the PC “COM” port. On a PC the port is a 9-pin D-sub (DE-9, commonly called DB-9) wired as Data Terminal Equipment (DTE), connecting to Data Circuit-terminating Equipment (DCE) such as a modem. Besides transmit (TXD) and receive (RXD) data, it provides hardware handshaking lines (RTS/CTS, DTR/DSR) and status lines (DCD, RI). Signals are single-ended and bipolar — a logic 1 (mark) is a negative voltage and a logic 0 (space) a positive voltage, typically around ±5 to ±12 V — so they are not compatible with TTL levels without a converter. Internally the port is often brought out to a 10-pin IDC header.
| DE9 Pin | IDC 10 Pin | Name | Dir | Description |
|---|---|---|---|---|
| 1 | 1 | CD | IN | Carrier Detect |
| 2 | 3 | RXD | IN | Receive Data |
| 3 | 5 | TXD | OUT | Transmit Data |
| 4 | 7 | DTR | OUT | Data Terminal Ready |
| 5 | 9 | GND | - | System Ground |
| 6 | 2 | DSR | IN | Data Set Ready |
| 7 | 4 | RTS | OUT | Request to Send |
| 8 | 6 | CTS | IN | Clear to Send |
| 9 | 8 | RI | IN | Ring Indicator |
Notes
- Direction is DTE (Computer) relative DCE (Modem)