Hi JuanB,
Well I haven't tryed with pull up/down resistors. Now I connected 680E: A line to Vcc and B line to ground and unfortunately it's still not working.
Have you got an option to test this library on 2 Arduinos? I don't know what can cause this problem, but hardware part looks ok, since everything looks normal when watching signals on osciloscope.
Thank you for your time!
Jakob