RS 232 Port, 25-pin

The Electronics Industry Association (EIA) has developed standards for data communication. EIA standards where originally marked with the prefix "RS".
"RS" means that it is a recommended standard, but the standards are now generally indicated as "EIA" standards. RS232 was introduced in 1962.

RS232 is a voltage loop interface for two-way (full-duplex) communication represented by voltage levels with respect to system ground (common). A common ground between the PC and the associated device is necessary. The "Data Terminal Equipment" -DTE- (a regular computer) is equipped with a male D-Sub connector. The "Data Communications Equipment" -DCE- is equipped with a female D-Sub connector.

logical "1" (Mark) is represented by a negative voltage of -3V to -25V
logical "0" (Space) is represented by a positive voltage of +3V to +25V
the interface is single ended (connecting only two devices with each other)
the data rate is less than 20 kbps
cable length should be not longer than 16m (50 feet) at the maximum data rate
the connector is a 25 pin D-Sub connector (for possible synchronous communications), or a 9 pin D-Sub connector (the subset for asynchronous communications) With asynchronous communications the serial data bits are not locked to a specific clock at the receiving end. The bits are synchronized by the transmit clock at the sending end.

25 pin Serial Port RS232
1 SHIELD Ground
2 TXD Transmit Data
3 RXD Receive Data
4 RTS Request to Send
5 CTS Clear to Send
6 DSR Data Set Ready
7 GND System Ground
8 CD Carrier Detect
9 n/c (reserved for testing)
10 n/c (reserved for testing)
11 n/c
12 n/c (secondary receive line)
13 n/c (secondary clear to send)
14 n/c (secondary transmitted data)
15 n/c (transmitter signal element timing)
16 n/c (secondary received data)
17 n/c (receiver signal element timing)
18 n/c (local loopback)
19 n/c (secondary request to send)
20 DTR Data Terminal Ready
21 n/c (remote loopback / signal quality detector)
22 RI Ring Indicator
23 n/c (data signal rate select)
24 n/c (transmit signal element timing)
25 n/c (test mode)

9pin Subset Implementation Serial Port RS232
PIN (9pin) PIN (25pin) DESCRIPTION
1 8 CD Carrier Detect
2 3 RXD Receive Data
3 2 TXD Transmit Data
4 20 DTR Data Terminal Ready 
5 7 GND System Ground
6 6 DSR Data Set Ready
7 4 RTS Request to Send
8 5 CTS Clear to Send
9 22 RI Ring Indicator

See also the TIA, standards