VMS Help TCPIP Services, Programming Interfaces, Sockets API, $QIO, IO$_TTY_PORT|IO$M_TN_STARTUP, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The IO$M_TN_STARTUP subfunction allows the application to
    communicate over a socket using the terminal driver QIO
    interface. Note that incoming and outgoing data is processed
    by the terminal driver, and that the terminal's characteristics
    may affect the format of the data. Be aware that by default, the
    terminal will echo incoming data back to the sender.
    Once the subfunction completes, the application is free to
    perform all terminal QIO functions on the connection. While the
    socket is bound to a terminal device, it will process neither the
    IO$_READxBLK nor the IO$_WRITExBLK function, and will return the
    error SS$_DEVINUSE.