If you want to print to a Com, or serial communications, port you must have the following commands in your Autoexec.bat file:
MODE LPT1=Com1
No baud rate statement is needed, the default settings used should work fine.
If all your other Windows applications can print to the printer connected to a COM port but FileMaker Pro reports that it is unable to print to LPT1 then you must edit the Claris.ini file to read:
Printer=
Deleting the data after the equals sign will cause FileMaker Pro to use the Windows default.