RedHat 9 has the serial console and relevant devices already compiled in.
To use the serial console, append to the
kernel line in /etc/grub.conf:
console=ttyS1,115200
The documentation claims that you can define multiple consoles, but it lies. Default is to use the VGA adapter + a keyboard.
You will need an agetty running on ttyS1 if you want to log in on this console, but if you just want to capture the output the agetty isn't necessary. Add this line to /etc/inittab:
S1:2345:respawn:/sbin/agetty -L 115200 ttyS1 vt100
Notice the speed parameters need to match if you are going to use both. The 115200 speed was plenty fast for a 10Gb test with lots of noise and worked fine in the Windows Hyperterm.