VMS Help Sys Parameters, TTY_ALTYPAHD  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    TTY_ALTYPAHD sets the size of the alternate type-ahead buffer.
    Use this parameter to allow the block mode terminals and
    communications lines to operate more efficiently.

    The default value is usually adequate. Do not exceed the maximum
    value of 32767 when setting this parameter.