A teleprinter (teletypewriter, Teletype or TTY) is an electromechanical typewriter that can be used to send and receive typed messages from point to point and point to multipoint over various types of communications channels. They were adapted to provide a text-based user interface to early mainframe computers and minicomputers, sending typed data to the computer with or without printed output, and printing the response from the computer. Some models could also be used to create punched tape for data storage (either from typed input or from data received from a remote source) and to read back such tape for local printing or for transmission of previously stored messages and data. These punched tapes were also compatible with higher speed tape readers and punches directly attached to computers.
Teleprinters could use a variety of different communication media. These included a simple pair of wires, dedicated non-switched telephone circuits (leased lines), switched networks that operated similarly to the public telephone network, and radio and microwave links. A teleprinter attached to a modem could also communicate through standard switched public telephone lines. This latter configuration was often used to connect teleprinters to remote computers, particularly in time-sharing environments.
Teleprinters are now largely obsolete, though they are still widely used in the aviation industry (AFTN and airline teletype system), and variations called Telecommunications Devices for the Deaf (TDDs) are still used by the hearing impaired for typed communications over ordinary telephone lines. In computing teleprinters have been replaced by fully electronic computer terminals which usually use a display screen instead of a printer, though the term "TTY" is still occasionally used to refer to them, such as in Unix systems.
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