A teletypewriter is a special device that was typically known as the Telecommunications Device for the Deaf or TDD. These devices act as typewriters that can carry special signals to provide text based communications over voice telephone lines. This was the beginning of using data over the telephone lines.
This technology was invented in the 1960s, nearly a century after Alexander Graham Bell attempted to invent the telephone for this audience, which obviously went for the masses who could speak or hear. The Teletypewriter (known as TTY) was initially referred to leased line that provided automatic printing such as news wire services or even for a console access to mainframes. (In fact, if a customer was very frugal dumb terminals for some were too expensive so computing would result in the typewriter that would automatically “printout” what was coming off the “screen”. A user would respond to commands by typing it into the typewriter which then would relay it to the computer.)
TDD/TTY uses a standardized protocol that basically sends various tones through the telephone lines so the other end would receive the message. According to Wikipedia (use with caution) the change to IP in the traditional phone cloud, would make these devices unusable (including the 500 Rotary telephones) because the complexity of converting landline networks to work in “native” cyber (or packet) worlds. While VOIP enables some analog communications, the move to IP would be easier to service providers to tell users to just send iMessages, SMS, Tweet, IM or other text based technologies instead.
According to various sources, TDD is separate from TTY in the technological sense, and speech impaired users that may have acquired brain disorders; a non verbal, but high functioning form of the autism spectrum disorder; cerebral palsy or other types of speech disorders) are basically non existent users. Or so you think. It’s not to say that people who aren’t deaf can’t use this technology and people with speech impairments have ether used TTY or all the other IP based communications like IM, Web, social media and text messaging.
Because of the similarities of the two platforms, here at The Museum, I believe the term should be referred as TTY as non verbal people are actually disabled users to any telephony technologies as well. Using “TTY” in the generic sense would enable inclusion to all disabled users of this technology.