How to use /dev/ttyAMA0 to connect external serial device

[xuniuer]

Hello,

I want to connect an external device (serial port) with '/dev/ttyAMA0' to drive it to speak.
Model B with archlinux installed is used, and it has only one `offical` serial - /dev/ttyAMA0.
I need to use it.

As described in document for "wheezy" PI, I changed /boot/cmdline.txt, but fail to modify inittab file
because there is not the file in archlinux PI. At last reboot PI.

However, after connecting my device to the board, I do not still drive it by serial. I use pyserial library.
Of course, I have tested that the device works in Windows with pyserial by usb-serial connection.
And by usb-serial connection, I can also drive it on PI to speak.

I guess `/dev/ttyAMA0` can not work... Gooooo...ooooogle it, no result

Maybe you have knowledge of this question, then share with me, please.

Thanks a lot!
~Regards