UART

A UART (Universal Asynchronous Receiver/Transmitter) is the microchip with programming that controls a computer's interface to its attached serial devices. Specifically, it provides the computer with the RS-232C Data Terminal Equipment ( DTE ) interface so that it can "talk" to and exchange data with modems and other serial devices.

• As part of this interface, the UART also:
• Converts the bytes it receives from the computer along parallel circuits into a singleserial bit stream for outbound transmission
• On inbound transmission, converts the serial bit stream into the bytes that the computer handles
• Adds a parity bit (if it's been selected) on outbound transmissions and checks the parity of incoming bytes (if selected) and discards the parity bit
• Adds start and stop delineators on outbound and strips them from inbound transmissions
• Handles interrupt s from the keyboard and mouse (which are serial devices with specialport s)
• May handle other kinds of interrupt and device management that require coordinating the computer's speed of operation with device speeds

More advanced UARTs provide some amount of buffering of data so that the computer and serial devices data streams remain coordinated. The most recent UART, the 16550, has a 16-byte buffer that can get filled before the computer's processor needs to handle the data. The original UART was the 8250. If you purchase an internal modem today, it probably includes a 16550 UART (although you should ask when you buy it). According to modem manufacturer US Robotics, external modems do not include a UART. If you have an older computer, you may want to add an internal 16550 to get the most out of your external modem.

http://whatis.techtarget.com/definition/UART-Universal-Asynchronous-Receiver-Transmitter