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. The standard evolved over the years and had the third revision in 1969 (RS-232C). The fourth revision was in 1987(RS-232D also known as EIA-232D).
RS232 is identical to the standards CCITT V.24/V.28, X.20bis/X.21bis and ISO IS2110.

RS 232 serial port (9-pin) DTE-device (PC)
male connector, female cable connector
 1 Data Carrier Detect
 2 Received Data
 3 Transmitted Data
 4 DTE (Data Terminal) Ready
 5 Signal Ground
 6 DCE (Data Set) Ready
 7 Request to Send
 8 Clear to Send
 9 Ring Indicator

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. Maximum serial cable length is defined: 75 feet at 9,600 bps, but today cables up to 1,000 feet are used successfully.

The interface is single ended (connecting only two devices with each other), the data rate is less than 20 kbps.

- 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

Devices that communicate over a serial interface are divided into two classes: DTE and DCE. The most important difference between these types of devices is that the DCE device supplies the clock signal that paces the communications on the bus. All DTE-DCE cables are straight through, the pins are connected one to one. DTE-DTE and DCE-DCE cables are cross cables.

DTE - DCE is called a 'Straight Cable'
DTE - DTE is called a 'Null-Modem Cable'
DCE - DCE is called a 'Tail Circuit Cable'

DTE (Data Terminal Equipment) devices are: computers, terminals, printers, CSU/DSU, Multiplexers. DCE (Data Communications Equipment) devices are: modems and other equipment.
Hubs and Routers could be either DTE or DCE.

The Data Terminal Equipment -DTE- (e.g. regular computer) is normally equipped with a male D-Sub connector, Data Communications Equipment -DCE- equipment with a female connector.

This difference determines, which signal a device expects on which pin. Any device is configured as either a DTE or a DCE device. Cables between DTE and DCE are different from cables between DCE and DCE devices. 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.

RS232 Data Structure:

25 pin Serial Port RS232
male connector, female cable connector
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)

Null-Modem Cable, two female 9-pin D-Sub Connectors, DTE-DTE Connection

nullmodem cable

Null-Modem Cable without Handshaking, two female Connectors, DTE-DTE Connection(for DTE to DCE connection a straight cable will be used)

Null-Modem Cable with Loopback-Handshaking, two female Connectors, DTE-DTE Connection

Null-Modem Cable with Full Handshaking, two female Connectors, DTE-DTE Connection

Nullmodem Cable 9pin D-Sub to 9pin D-Sub 
D-Sub 1, female D-Sub 2, female
2 Receive Data 3 Transmit Data
3 Transmit Data 2 Receive Data
4 Data Terminal Ready 6+1 Data Set Ready + Carrier Detect
5 System Ground 5 System Ground
6+1 Data Set Ready + Carrier Detect 4 Data Terminal Ready
7 Request to Send 8 Clear to Send
8 Clear to Send 7 Request to Send

Nullmodem Cable 25pin D-Sub to 25pin D-Sub 
D-Sub 1, female D-Sub 2, female
2 Transmit Data 3 Receive Data
3 Receive Data 2 Transmit Data
4 Request to Send 5 Clear to Send
5 Clear to Send 4 Request to Send
6+8 Data Set Ready + Carrier Detect 20 Data Terminal Ready
20 Data Terminal Ready 6+8 Data Set Ready + Carrier Detect

Serial 8-pin DIN Connector
8-pin DIN Corresponding 25-pin D-Sub Signal Function
3 2 TD Transmitted data
5 3 RD Received data
6 4 RTS Request to send
2 5 CTS Clear to send
4,8 7 SG Signal ground
7 8 DCD Data carrier detect
1 20 DTR Data terminal ready

RS232 to 3.5mm Mini-Jack (walkman headphone style) connection used by many digital cameras to download data to PC
9-pin Connector Function Mini-Jack
2 transmit from device Tip
3 transmit to device Ring
5 Ground Sleeve

RS232 over RJ45 (RS232D)
Description RJ45 Pin
Signal Ground 4
Transmitted Data 6
Received Data 5
Request To Send 8
Clear To Send 7
DCE Ready, Ring Indicator 1
DTE Ready 3
Received Line Signal Detector 2

RS232 - DB25 (not the full set)
Pin 1 Protective Ground
Pin 2 Transmit Data
Pin 3 Received Data
Pin 4 Request To Send
Pin 5 Clear To Send
Pin 6 Data Set Ready
Pin 7 Signal Ground
Pin 8 Received Line Signal Detector (Data Carrier Detect)
Pin 20 Data Terminal Ready
Pin 22 Ring Indicator

RS232 on RJ45
Pin No. Name Notes/Description
1 DSR/RI Data set Ready/ring indicator
2 DCD Data Carrier Detect
3 DTR Data Terminal Ready
4 SGND Signal Ground
5 RD Receive Data
6 TD Transmit Data
7 CTS Clear to Send
8 RTS Request to Send