Thank you for your direction. I found the web page after looking around your web site before see your latest reply.
The main problem I have right now is that I can see the log of the booting progress came out from 703n but when I see "autoboot after one second" or something like that, I cannot stop the boot process by type tpl . When I push the key "p" I noticed some strange character come out so it did something but the boot continues. I don't know if this because you are using PL2103 but I use a Nokia cable with PL2303. Maybe the level are not exactly what 703n expected. I read that some people using a 10k resister for pull up. However, they are using it for tx not the rx and my tx pin works fine because I can see the output. In any case, thank you for your detailed explanation and let me know if there's anything I can try.