I use serial ports on Linux fairly regularly, and I have found two things that
can bite you quite often:
1. The port is turned off or disabled in the BIOS. I run into this one a lot.
I think some Windoze Modem installation programs will do this. This is the
first thing I check when having a problem with ttyS0 or ttyS1. So get into the BIOS setup at power on time, and make sure those ports are enabled.
2. The labels for the ports are incorrect. Sometimes the header gets plugged
into the wrong port on the back of the case. So what you think is ttyS0 is really ttyS1 (and vice versa).
Hope this helps