There's a known fault where Appletalk will seize control of a serial port and refuse to let it go, even after disabling, switching off, or removing Appletalk services altogether. You have to either trash the preference file for Appletalk and I think one other and reboot, or download a script or small app that takes care of that. I am a little vague on the details, sorry. Hopefully this will ring a bell for someone else, or further searching will reveal an answer.