Source : Free On-Line Dictionary of Computing

start bit
     
         A {bit} which signals the start of transmission of
        a character on a {serial line}.  For an {RS-423} signal, the
        line is normally at logical zero which there is no data and
        the start bit is a logical one.  The zero-one transition tells
        the receiver when to start sampling the signal to extract the
        data bits.
     
        [Is this upside-down?]
     
        (1995-02-02)