Ok, used the program Korodny suggested, still the same results: No answer from the SMS pad, get RIGHT+DOWN on joy3, RIGHT+DOWN+FIRE on joy4 with a Mega-Drive pad (really weird)
I am getting 4.9 reading from my multimeter on all the data pins. I tried shortening one of the data pins with ground pins 19 and 20, both times the joytester program read correctly with no problems. So I guess my paralell port is ok.
So the problem is either the cable or the joypads. Time to test the cable again I guess.
By the way, thank you a lot guys, I'd be completely stuck without your guidance
Zetr0: I am always around, I just go into lurking mode sometimes
Last month I even took my A600 to a retrocomputing fair here, connected it to a projector and played some Stardust on a huge screen
EDIT: Well, tested the cable for continuity again, everything is allright. I have no idea of what's going on here. The SMS pad is also working properly on the Amiga normal joy ports.