In the past, serial interface varied commercially and it presented unnecessary costs when connecting DTE and DCE of different manufacturers. Electronics Industries Association (EIA) tried to standardize the serial interfacing for DTE and DCE and came up with the RS-232 interface which is more formally named as Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange. Basically, RS-232 specification identify the mechanical, electrical, functional and procedural descriptions for the interface between DTEs and DCEs.

The RS-232 interface specifies a cable with two connectors which consists of a sheath containing 25 wires with a DB25p-compatible male connector on one end and a DB25P-female connector on the other. The standard also specifies limitations on the voltage levels that the DTE and DCE can output onto or receive from the cable. The list of the functions of some pins are as follows:

Pin 2 – transmit data. Serial data on the primary data channel are transported from the DTE to the DCE on pin 2.

Pin 3 – receive data. Pin 3 is the second primary data pin. Serial data are transported from DTE to DCE on pin 3.

Pin 4 – request to send. For half-duplex data transmission, the DTE uses pin 4 to request permission from the DCE to transmit data on the primary data channel.

Pin 5 – clear to send. It provides handshake between DTE and DCE

Pin 6 – data set ready. Control signal sent from the DCE to DTE to signify availability of communication channel.

Pin 7 – signal ground. Signal reference for all data, control and timing signals.

Pin 8 – receive line detect. The DCE uses this pin to signal the DTE when it determines it is receiving a valid analog carrier.