Writing "from" another terminal

[Dave Jenks]

I have a 3b1 with one free serial port.  For irrelevent reasons I need
to have input come _from_ that port  (as if it were typed there).  It
has to be correct enough that the data would echo back onto the
terminal (if echo is on).

I know that this is possible because I saw a true unix wizard do this
many years ago on a vanilla v7 system.  Unfortunately, I have not been
able to reach him.  All I recall him saying was, "*chuckle* it's just
a 17-line program!", and he suggested that I figure it out.  Well, 7
years later, I am swallowing my pride/ego.

Any ideas?
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	    "Pro is to con, as progress is to Congress..."
                                                                      
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