Source : Free On-Line Dictionary of Computing

Universal Asynchronous Receiver/Transmitter
     
         (UART) An {integrated circuit} used
        for serial communications, containing a transmitter
        (parallel-to-serial converter) and a receiver
        (serial-to-parallel converter), each clocked separately.
     
        The parallel side of a UART is usually connected to the {bus}
        of a computer.  When the computer writes a byte to the UART's
        transmit data register (TDR), the UART will start to transmit
        it on the serial line.  The UART's status register contains a
        {flag} bit which the computer can read to see if the UART is
        ready to transmit another byte.  Another status register bit
        says whether the UART has received a byte from the {serial
        line}, in which case the computer should read it from the
        receive data register (RDR).  If another byte is received
        before the previous one is read, the UART will signal an
        "overrun" error via another status bit.
     
        The UART may be set up to {interrupt} the computer when data
        is received or when ready to transmit more data.
     
        The UART's serial connections usually go via separate {line
        driver} and {line receiver} {integrated circuits} which
        provide the power and voltages required to drive the serial
        line and give some protection against noise on the line.
     
        Data on the {serial line} is formatted by the {UART} according
        to the setting of the UART's control register.  This may also
        determine the transmit and receive baud rates if the UART
        contains its own clock circuits or "{baud} rate generators".
        If incorrectly formated data is received the UART may signal a
        "{framing error}" or "{parity} error".
     
        Often the clock will run at 16 times the baud rate (bits per
        second) to allow the receiver to do {centre sampling} - i.e. to
        read each bit in the middle of its allotted time period.  This
        makes the UART more tolerant to variations in the {clock rate}
        ("jitter") of the incoming data.
     
        An example of a late 1980s UART was the {Intel 8450}.  In the
        1990s, newer UARTs were developed with on-chip {buffer} space
        for data.  This allowed higher transmission speed without data
        loss and without requiring such frequent attention from the
        computer.  For example, the {Intel} {16550} has a 16 byte
        {FIFO}.
     
        The term "Serial Communications Interface" (SCI) was first
        used at {Motorola} around 1975 to refer to their start-stop
        asyncronous serial interface device, which others were calling
        a UART.
     
        See also {bit bang}.
     
        [Is this the same as an {ACIA}?]
     
        (2003-07-13)