wiki:Linux/Ubuntu/Kernel/SerialConsole

Linux Kernel Boot Serial Console

(see also  Kernel console configuration)

Edit the grub kernel boot line and add :

title           Ubuntu Hardy 64-bit, kernel 2.6.24-16-generic
root            (hd0,4)
kernel          /vmlinuz-2.6.24-16-generic root=UUID=bb2c3a14-1588-4fb9-8411-71f114b568b4 ro console=tty0 console=ttyS0,115200n8
initrd          /initrd.img-2.6.24-16-generic
quiet

This will write to the screen and the serial port, but input will be taken from the last console listed.

screen session to connect via a USB-to-Serial adapter from a laptop to capture the kernel boot messages:

screen -L /dev/ttyUSB0 115200,cs8,-ixon,-ixon,istrip

Note: -L logs to the screen.0.log in the current directory.