Signs for TTY, TDD
Meaning: a telecommunication device that lets Deaf and Hard-of-Hearing people use the telephone to communicate in text (English), by typing messages back and forth to one another instead of vocally talking and aural-listening; common use in the 1980s till gradually obsolete from the 2000s or so when videophone emerged.
TDD for Telecommunication Device for the Deaf.
Q: "What is the difference between TTY and TDD?"
Kind of a culture-specific difference. The term TTY (teletyewriter) is commonly used among Deaf people in the Deaf community whereas the term TDD (Telecommunication Device for the Deaf) is generally used by hearing people in services.
Deaf Culture and tidbits
A common definition of TTY/TDD involves "a special device." I wouldn't say it's special. It's just some variety of technology that is no more special than a telephone, a smartphone, etc.
Did you know that...? Deaf history
"Canadian-born American Deaf inventor William E. Shaw (1869-1949) had invented several things, including clocks, alarms, phones, etc. He had worked for Thomas Edison for half a decade.
Headline: 'Talkless Phone' Invented by Deaf Mute. Story: "One of the most recent devices for use in communication between deaf mutes is a 'talkless phone' that conveys messages by means of an alphabet printed on electric-light bulbs. As the operator presses the keys of a special typewriter wired electrically, the corresponding letters are lighted, spelling out the message." Source