As I have quite some trouble every time I need grub via serial console, here’s just my personal reminder on how to do it right:

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# cat /boot/grub/grub.conf
serial --unit=0 --speed=38400
terminal --timeout=15  console serial

title	hardened-sources-2.6.20-r1
root	(hd0,0)
kernel	(hd0,0)/boot/vmlinuz-2.6.20-hardened-r1 root=/dev/hda1 console=ttyS0,38400 console=tty0

Additionally the agetty entry for ttyS0 in /etc/inittab needs to be uncommented and changed accordingly to the serial speed

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# cat /etc/inittab | tail
s0:12345:respawn:/sbin/agetty 38400 ttyS0 vt100