Just a thought, are you sure that ttyS0 is a valid comm port on your box? (I know, I know, it should be)
Check your BIOS (CMOS) too, make sure that it isn't disabled. And then try minicom to see what it thinks of ttyS0 (and I would even go so far as to check the device numbers associated with /dev/ttyS0 -- refer to /usr/src/~linux/Documentation/devices.txt for the correct device number).
And what about ttyS1? does it work?
John
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