The optional Duplex argument specifies whether the line
    is full or half duplex.  Possible duplex-type values are:

    full      The line has full-duplex capabilities.

    half      The line has only half-duplex capabilities.

    The default value is taken from the device capability.
    If that is unknown, the default is full.

    Example:

    create modem connect line DDCMP-0 -
         communication port DMB-0-0, duplex full