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PC Serial Port Connections
Until the advent of the IBM/PC most RS-232 connections were by means of a 25 way D-type connector. The PC observed this convention but also gave rise to the alternative, and now popular, 9 way configuration.
The following table details the signals found on both the 9 way and the 25 way variants of the PC's serial ports.
Note: Both connectors are males at the back panel of the PC.
| 9 pin | 25 pin | Name | Signal Flow at PC | |
| 1 | 8 | DCD | Data Carrier Detect | Input |
| 2 | 3 | RxD | Receive Data | Input |
| 3 | 2 | TxD | Transmit Data | Output |
| 4 | 20 | DTR | Data Terminal Ready | Output |
| 5 | 7 | GND | Signal Ground | |
| 6 | 6 | DSR | Data Set Ready | Input |
| 7 | 4 | RTS | Request To Send | Output |
| 8 | 5 | CTS | Clear To Send | Input |
| 9 | 22 | RI | Ring Indicator | Input |