Library /sys$common/syshlp/nclhelp.hlb set, modem_connect, line, maximum_disable_transmit_timer  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: 500

    Maximum time, in milliseconds, that clear to send can remain
    asserted before the line is disconnected after Request to Send
    is deasserted. This characteristic is not supported if the
    characteristic modem control is none.