Function:   Dtr()

               This function provides some control over the
               Modem, see AtModem() for details and usage.

   Syntax:     Dtr(<lOnOrOff>[,<nPort>])

   Arguments:  <lOnOrOff> a Logical indicating whether Dtr should be
               On or Off, <nPort> as in AtModem().

   Returns:    Nothing.

   Usage:      See the AtModem() example.