Serial RS232

Serial Ports come in two "sizes", There are the D25 pin type connector and the D9 pin type connector, both of which are male on the back of the PC, thus you will require a female connector on your device. Below is a table of pin connections for the 9 pin and 25 pin D-Type connectors.

DB9	DB25	Abbrevation	Function
1	8	CD	Carrier Detect
2	3	RD	Receive Data
3	2	TD	Transmit Data
4	20	DTR	Data Terminal Ready
5	7	SG	Signal Ground
6	6	DSR	Data Set Ready
7	4	RTS	Request To Send
8	5	CTS	Clear To Send
9	22	RI	Ring Indicator

Abbrevation	Function	Description
CD	Carrier Detect	When the modem detects a "Carrier" from the modem at the other end of the phone line, this Line becomes active.
RD	Receive Data	Serial Data Input (RXD)
TD	Transmit Data	Serial Data Output (TXD)
DTR	Data Terminal Ready	This is the opposite to DSR. This tells the Modem that the UART is ready to link.
DSR	Data Set Ready	This tells the UART that the modem is ready to establish a link.
RTS	Request To Send	This line informs the Modem that the UART is ready to exchange data.
CTS	Clear To Send	This line indicates that the Modem is ready to exchange data.
RI	Ring Indicator	Goes active when modem detects a ringing signal from the PSTN.

Null Modems
A Null Modem is used to connect two serial communication devices. This is commonly used as a cheap way to network games or to transfer files between computers (using Zmodem Protocol, Xmodem Protocol etc). This can also be used with many Microprocessor Development Systems. It only requires 3 wires (TD, RD & SG) to be wired straight through thus is more cost effective to use with long cable runs.

LoopBack
It has the receive and transmit lines connected together, so that anything transmitted out of the Serial Port is immediately received by the same port. If you connect this to a Serial Port an load a Terminal Program, anything you type will be immediately displayed on the screen.

Here datasheet for RS232