Originating with the IBM PC AT, a 9 pin D connector has become a common connector for RS-232 async applications.
For the male connector (DTE), the pin assignments are as follows:
Pin Signal Equivalent pin on DB-25
----- ------ -----------------------
shell ground 1
1 DCD 8
2 RXD 3
3 TXD 2
4 DTR 20
5 signal ground 7
6 DSR 6
7 RTS 4
8 CTS 5
9 RI 22