mmmpfff no ! no soundcard after all... assuming the rise/fall time caused by the input capacitor an all the a/d blahblah... not a good solution

Well one thing i know, is that its to replace a load of old unit based on first generation Z80. The customer already tried to grab the data with hyperterminal and other program on various baud rate, bit mode, parity or not, inverted or not... all of those test failed once they tried to send the same data as grab with these programs.
It's a home-brew system made in the 80's and the egineer who did it is dead, no traces of it, no code, no schematic, no nothing...

I guess it's a home made protocol. It's 2 wire communication : ground and signal, no clock, no balanced lines... so 4800 Bauds is certainely the fastest speed.... still a guess once again