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   tp_lomark() - get or set XON low-water mark

   nOldMark := tp_lomark(nPort, [nNewMark])

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

   Returns     Original low-water mark.

   tp_lomark() gets or sets the flow-control low-water mark setting of
   port nPort.  The low-water mark controls RTS, CTS and XON/XOFF flow
   control.  It is the minimum amount of free space in the input buffer
   before Telepathy will allow data input to resume after turning it off
   when it reached the high-water mark.

   The initial low-water mark is 3/4 of the input buffer size.

   The minimum low-water mark is 1, and the maximum is the input buffer
   size. Also it must be greater than the high-water mark setting.

   If nNewMark is omitted, tp_lomark() 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_ctrlrts(1, 2)    // Enable RTS handshaking.
       tp_lomark(1, 900)   // Raise RTS when 100 character left
                           //   in buffer.