Q: 'serialport in use' quit appliction'

Did you try shutting down the computer and starting it up again?  You could try zapping the PRAM at startup, by immediately pressing/holding down these (4) keys: Command + Option + P + R.  Don't release them until you hear the startup tone a third or fourth time.  You could also open the AppleTalk control panel and simply turn OFF AppleTalk (assuming the Performa isn't networked) or change the Printer Port setting to "Remote Only."