/sys$common/syshlp/helplib.hlb SET, TERMINAL, Qualifiers, /FULLDUP  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /FULLDUP (default)
       /NOFULLDUP

    Controls whether the terminal operates in full-duplex mode. The
    /FULLDUP qualifier is equivalent to the /NOHALFDUP qualifier.