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   tp_himark() - get or set XOFF high-water mark

   nOldMark := tp_himark(nPort, [nNewMark])

   nPort       Serial port number.
   nNewMark    New high-water mark.

   Returns     Original high-water mark.

   tp_himark() gets or sets the flow-control high-water mark setting of
   port nPort.  The high-water mark controls RTS, DTR, and XON/XOFF flow
   control, as well as the tp_onhimark() notification.  It is the
   minimum amount of free space in the input buffer before Telepathy
   will attemp to stop data input by turning off RTS and/or DTR, or by
   sending an XOFF, depending on the flow control options enabled.

   The initial high-water mark setting is 1/4 of the input buffer size.

   The minimum high-water mark setting is 0, and the maximum is one less
   than the input buffer size.  Also, it must be less than the low-water
   mark setting.


   If nNewMark is omitted, tp_himark() just returns the current setting.
   If nNewMark is specified, it changes the setting and returns the
   original value.

   Example:

       tp_open(1, 1000, 1000, 9600, 8, "N", 1)
       tp_himark(1, 100)   // Send XOFF with 900 characters in buffer.