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

    Default: full              Value: full or none

    Indicates whether the interchange circuits are to be monitored and
    used. The value none means that only the data leads are monitored.

    The value full must be used when the value of the characteristic
    duplex is half. You can modify this this characteristic only when
    the entity is disabled. This characteristic is supported only if
    the characteristic connection type is non-switched.