VMS Help SET, TERMINAL, Qualifiers, /READSYNC  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /READSYNC
       /NOREADSYNC (default)

    Controls whether the terminal uses the Ctrl/S and Ctrl/Q
    functions to synchronize data transmitted from the terminal.

                                 CAUTION

       SET TERMINAL/READSYNC should not be used on LAT terminal
       lines. Setting this characteristic may cause unexpected
       results.

    The default is the /NOREADSYNC qualifier; the system does not
    use the Ctrl/S and Ctrl/Q functions to control reads to the
    terminal. The /READSYNC qualifier is useful for certain classes
    of terminals that demand synchronization or for special-purpose
    terminal lines where data synchronization is appropriate.