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   tp_isri() - return RI status

   lRI := tp_isri(nPort)

   nPort       Serial port number.

   Returns     TRUE if RI set, FALSE if cleared.

   Returns the status of the RI input: TRUE if RI is asserted, FALSE if
   RI is negated.  RI is the Ring Indicator input.  It is usually
   asserted by a modem when it detects an incoming call.

   Normally you would not use tp_isri() to detect an incoming call, but
   set the modem to auto-answer (for Hayes-compatible modems, this is
   done by sending the modem an ATS0=n command, where "n" is non-zero),
   use tp_waitfor() to look for the word "RING", or use tp_onmodem() to
   post a request for notification when RI changes.

   Example:

       if tp_isri(1)
           ? "RI is high"
       endif