Dear god! This is why I don't visit the consoles forum very often.
Since you seemed to not get it and nobody properly answered your question I'll clue you in. SMS pads are NOT serial pads. They are direct pin pads (each input gets a pin like on arcade controls). That goes for atari pads as well as any other 9 pin controller you run across btw.
I know you've moved on, but I'm posting for other's reference.