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   tp_int6b() - configure port to use INT 6Bh

   tp_int6b(nPort, nUnit)

   nPort       Serial port number.
   nUnit       Logical port number.

   Configures a serial port to use INT 6Bh network services instead of
   accessing the hardware port directly.  nUnit is the network logical
   unit number for the port you wish to use, ranging from 0 to 2.

   INT 6Bh services are provided by many network communication servers,
   including Ungermann-Bass NETCI, Novell NASI, and Network Products
   Corporation NCSI.  These provide basic serial I/O services, but some
   Telepathy features are not available through INT 6Bh.

   Efficiency.  Sending or receiving data generates network packets,
   which means a lot of overhead for each individual call.  Telepathy
   will take care of this on input (see 'Input Buffers' below), but when
   sending data, you should try to work with large blocks of up to about
   500 characters rather than with single characters.  (In fact, the
   magic number is 498 characters.  Above that size there will be no
   gain in efficiency.)

   Output Buffers.  Ports opened using INT 6B have no output buffer.
   Buffering is handled by the driver.  Because there is no buffer,
   neither tp_flush() nor tp_clearout() work, and tp_outchrs() and
   tp_outfree() do not return any useful information.

   Input Buffers.  INT 6B ports do have input buffers, but they work
   differently than for normal ports.  The driver has its own input
   buffer, and Telepathy moves data from that buffer to its own whenever
   you call a function that reads characters or looks at the input
   buffer.

   Handshaking.  The only handshaking option you can control with INT 6B
   is XON/XOFF flow control.  XON/XOFF cannot be turned on in one
   direction only; it must be both or neither.  If you try to turn on
   just one direction, it will not be enabled.  DCD handshaking does not
   work, and you cannot detect the states of any modem status lines, or
   control DTR or RTS.  If you depend on dropping DTR to hang up a
   modem, or sensing DCD to tell whether the modem is online, you're out
   of luck.  However, closing a port will usually drop DTR for you.
   Also note that the 'lNoReset' option to tp_close() does not work;
   when you close a port, it's closed and that's the end of it.

   Serial Parameters.  You must specify valid serial parameters (baud
   rate, etc.) when you open a port, but they have no effect.  You
   cannot change any of those parameters except by reconfiguring the
   server.  As with INT 14, the baud rate you specify is the one used by
   tp_szmodem() to decide on what subpacket size to use.

   Notifications.  Forget it.  Not available with INT 6B.

   BREAKs.  tp_break() works a little differently with INT 6B.  If you
   leave off the second (BREAK time) parameter or specify a non-zero
   value, it sends a BREAK but the server decides how long it is to be.
   If you specify 0 as the second parameter, tp_break() tells the server
   to put the current virtual circuit on hold and return control to the
   command interpreter.  This is the only way to get from the connected
   state back to the interpreter without closing the port.


   Opening Ports.  For technical reasons, Telepathy has to send some
   data out the port when opening it.  We've chosen to send the two
   character sequence <space><backspace>, on the assumption that you'll
   be connecting to the server's command interpreter, and that sequence
   will be pretty harmless.

   Reopening Ports.  Reopening ports works the same as for INT 14 with a
   non-FOSSIL driver.  See the discussion under tp_int14().

   Detecting Ports. tp_isport() simply checks whether an INT 6B driver
   is installed.