8bit-micro.com RS-232C PIN Definitions

PROT. GND. (pin 1): This connection is usually to earth, or chassis, ground and it's function is exactly as described,
a protective ground. This pin is normally connected to the shield of an RS-232C cable (if it has one).

TXD (pin 2): This pin has the transmitted data from the DTE (terminal) to the DCE (modem).

RXD (pin 3): This pin has the data received from the DCE.

RTS (pin 4): This pin has a signal asserted by the DTE when it wishes to send data to the DCE.

CTS (pin 5): This is a signal that the DCE uses to tell the DTE that it is OK to send data (usually in response to an RTS [above] from the DTE).

DSR (pin 6): This is a signal sent by the DCE to tell the DTE that it is ready to operate.

SIG. GND. (pin 7): This is the actual signal common. Do not confuse this with the Protective Ground (pin 1), they are
NOT the same. An RS-232C connection will function without the pin 1's being connected, but will not function without  the signal grounds being connected.

DCD (pin 8): This is a signal asserted by the DCE to inform the DTE that a valid data carrier is being detected.

DTR (pin 20): This is a signal asserted by the DTE to inform the modem that it is ready to receive data and decode the
other status signals.

RI
(pin 22): This signal is asserted by the DCE (usually a modem) to let the DTE know that the phone is ringing.