The Free On-line Dictionary of Computing (30 December 2018):

Telecommunications Device for the Deaf
Telecommunication Display Device
TTD

    (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".  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.  It uses asynchronous transmission of 1400 Hz
   and 1600 Hz tones at 45.5 or 50 baud, with one start bit,
   5 data bits and 1.5 stop bits.  This is generally
   incompatible with standard modems.

   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
   LEDs).  There is often a small printer for making
   transcripts of terminal sessions.  An acoustic coupler
   connects it to the telephone handset.

   With the falling cost of personal computers and the
   widespread use of Internet talk systems, there is now
   little reason to use this Stone Age technology.

   [Standards?  i18n?]

   (2006-10-08)