An interface defines the communication boundary between two entities, such as a piece of software, a hardware device, or a user. It generally refers to an abstraction that an entity provides of itself to the outside.

An UART, universal asynchronous receiver / transmitter is responsible for performing the main task in serial communications with computers. The device changes incomming parallel information to serial data which can be sent on a communication line. A second UART can be used to receive the information. The UART performs all the tasks, timing, parity checking, etc. needed for the communication. The only extra devices attached are line driver chips capable of transforming the TTL level signals to line voltages and vice versa.

Serial communication is performed by writing output, and reading input registers on a UART. A UART (universal asynchronous receiver / transmitter) is an IC capable of converting parallel information to serial data, transmitting it over a line and receiving it on the other end where the data is translated back to parallel again.

The RS232 connector was originally developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel. In practice, only one serial communication channel with accompanying handshaking is present. Only very few computers have been manufactured where both serial RS232 channels are implemented.

Serial communications with RS232. One of the oldest and most widely spread communication methods in computer world. The way this type of communication can be performed is pretty well defined in standards. I.e. with one exception. The standards show the use of DTE/DCE communication, the way a computer should communicate with a peripheral device like a modem.

Consider the situation where someone is helping you picking apples from a tree. Your helper climbs in the tree and throws all the apples to you. You have to put them in buckets. In the normal situation, you can easily catch all apples, but when one bucket is full and it has to be replaced by an empty one, this action costs more time than is available between two apples thrown by your helper.

The most used computer standard is without doubt ASCII, the American Standard Code for Information Interchange. When people started to develop computers, they had to define a way to represent certain types of information in a digital format. For numbers this was relatively easy, but text representation was far more difficult. Morse code was developed in the 19th century, but could not be easily adapted to the binary system in computers because the codes used for characters have different lengths and there is no obvious sorting method.

RS 232 is well-known due to popularity of today's PC's, unlike the RS422 and RS 485. These are used in industry for control systems and data transfers (small volumes, NO hundreds of Mb/s). 

EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signalling. The difference between the wires’ voltages is what conveys the data. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 volts will allow correct operation of the receiver.

EIA-422 (formerly RS-422), now TIA-422, is a technical standard which specifies the "electrical characteristics of the balanced voltage digital interface circuit"[1]. It provides for data transmission, using balanced or differential signaling, with unidirectional/non-reversible, terminated or non-terminated transmission lines, point to point, or multi-drop. In contrast to RS-485 (which is multi-point instead of multi-drop) EIA-422 does not allow multiple drivers but only multiple receivers.

So far we have introduced RS-232 Communications in relation to the PC. RS-232 communication is asynchronous. That is a clock signal is not sent with the data. Each word is synchronized using it's start bit, and an internal clock on each side, keeps tabs on the timing.

The Serial Port is harder to interface than the Parallel Port. In most cases, any device you connect to the serial port will need the serial transmission converted back to parallel so that it can be used. This can be done using a UART. On the software side of things, there are many more registers that you have to attend to than on a Standard Parallel Port. (SPP).