Library /sys$common/syshlp/dtehelp.hlb SET, DTE, /READ_DELAY  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /READ_DELAY=delay

    Specifies the modem port read delay in milliseconds. The modem
    port read delay is the time interval during which data in the
    modem port is transferred into receive buffers at the terminal.

    By default, the modem port read delay is 50 milliseconds. This is
    also the minimum value.