Just googling around, so apologies if I'm way off, but is it like this
http://benjeapes.blogspot.com/2010/06/back-when-pad-didnt-even-mean-pad.html ? Is it a
serial sniffer? If so and assuming you know which serial input and standard it's
using you could probably either hook up another serial sniffer/analyzer to it and see what
it's doing or a standard terminal would likely spit something out once you figure out
the proper communication settings. Given no clue if it's DTE or DCE.