In the early days of Unix, teletypewriters or ttys were used by users to communicate over a serial line with the mainframe computer. The mainframe had no keyboard or monitor as it has now and so during those early days, all commands were issued through the tty. The output of the commands was typed on the printer that was part of the tty. The tty terminals had no processing power of their own and hence were known as “dumb” terminals.

Every character that was typed on the tty was sent character by character over the serial line. While some ttys had the capability to send and receive mixed case letters, others did not. The mainframe responded to the type of characters that it received when the user logged on to the system. If the user has used small case to logon, the mainframe recognized that the tty was capable of mixed case and sent the output to the tty in mixed case as well. On the other hand, if it received only upper case characters from the user, it responded with upper case only.

Some ttys sported special features like the ability to use two colored ribbons: the standard black ribbon and a special red ribbon for typing words that carried special emphasis. The printers could also type bold face by typing a character twice in the same place.

Very soon, CRTs were added to the terminal as the standard output device eliminating the need to rely on the printer. These CRTs were predominantly monochrome with a green color character output. If the need arises, regular PCs can also be connected to a mainframe computer with the aid of a “terminal emulation” software. This software enables the PC to send requests to the mainframe and receive the output to be displayed on the monitor. tty is also the name of a Unix command that prints the name of the file connected to standard input on the standard output device.

Another electronic device that helps people with hearing impairment or speech disabilities to communicate over a regular telephone is also sometimes called a tty.
It is also called the telecommunications device for the deaf (TDD). The device helps people to carry on a conversation by typing text much similar to the way that text chat is done using Internet Messengers these days except the tty uses a regular telephone line for
the call.

This tty has a cradle and special acoustic cups to place the telephone receiver in order to convert each character typed on the keyboard to a unique sound. This sound travels through the telephone line to its destination, where the sound is again deciphered to its corresponding character and displayed as output. ttys are normally needed at both ends to carry on a text-based conversation. But a normal person wishing to communicate with another tty user can make use of a Telecommunications Relay Service (TRS) where an intermediate telephone operator listens to the voice of the phone user and types the message to the tty user and reads the feedback from the tty user back to the phone user.

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