>CommSpy is a Macintosh utility that shows the data being
>sent and received on the serial ports.  It performs the
>function of a hardware line monitor, but does it strictly in
>software. CommSpy can monitor multiple serial ports
>simultaneously.
.....
>
>Mel Patrick - theWabbitGuy - mel@...

What I have done is to tap into the serial line and then just monitor the
line using any old comm program.  I used Versaterm and just saved to disk.
Admittedly I was monitoring communications between a piece of HVAC
equipment and a PC laptop so my computer was dedicated to the task.  It
might work to run your program from the com port and run Versaterm on the
printer port.  If not it should work if you can dedicate another machine to
the task.  You will also have the problem of synchronizing I vs O but such
is life.

Not very sophisticated but it got my job done.

Ps.  Talked to a local EE, he said that a good low threshold Zener diode on
each leg of the sampled serial line would all you to watch both sides at
the same time.

            ------------|-------------->    Serial out \
			|				\   Lines to monitor
	    <-----------)-----|---------    Serial in   /
			|     |
			Z1    Z2
			|     |
		      --|-----|-------->   Serial in to computer

-----------------------------------------------------------------------------
Bryan Bremner              Washington State University  (509) 335-9074
Facilities Administration  Systems & Programming Professional
bremner@...            http://www.physplt.wsu.edu/ppweb.html