Source : Free On-Line Dictionary of Computing

Telecommunications Device for the Deaf
     
         (TDD) A terminal device used widely by deaf
        people for text communication over telephone lines.
     
        The acronym TDD is sometimes expanded as "Telecommunication
        Display Device" but is generally considered to be derived from
        "Telecommunications Device for the Deaf", although there is
        some disagreement on this.  The deaf themselves do not usually
        use the term "TDD", but prefer simply "{TTY}" -- possibly the
        original term.  The ambiguity between this and the other
        meanings of "{TTY}" is generally not problematic.  The acronym
        "TTD" is also common [Teletype for the deaf?].
     
        The standard most used by TDDs is reportedly a survivor of
        {Baudot code} implemented {asynchronous}ly at 45.5 or 50
        {baud}, 1 {start bit}, 5 data bits, and 1.5 {stop bits}.  This
        is generally incompatible with standard {modems}.
     
        [Standards docs?  {i18n} issues?]
     
        A typical TDD is a device about the size of a small {laptop
        computer} (resembling, in fact, a circa 1983 Radio Shack
        {Model 100} computer) with a {QWERTY keyboard}, and small
        screen (often one line high, often made of an array of
        {LED}s).  There is often a small printer for making
        transcripts of terminal sessions.
     
        Because of the {Stone Age} vintage of this technology (the
        idiosyncrasy of which drives up the expense of individual
        units), it is thought that TDD standards should transition to
        use of standard modem line settings (e.g., ASCII 2400-8-N-1).
        An obstacle to this is the millions of Baudot-only terminals
        in use (an example of {lock-in} with a high cruft factor).
     
        Another scenario sees the use of TDDs being replaced by the
        use of {personal computers} and {talk} protocols, presumably
        over the {Internet}.
     
        (1998-06-15)