It seems on archlinux they changed the default password combination. So if you built your RPi image from scratch with a newer archlinux image:
[archlinuxarm.org] , you might need to use the default user and then "su" to switch to user "root"Quote
Use the serial console or SSH to the IP address given to the board by your router.
Login as the default user alarm with the password alarm.
The default root password is root.