Hi Joe, Sorry to reply you late.
Thank you for your help. I figured it out yesterday and work late, so I didnot get a chance to reply you.
The problem was the master IP setting. I though the master's IP will be only for master; slave IP for slave. So I set both the master, which is the PLC and the VFD at IP 1. Also, the manual of the VFD gave me the wrong command address. I figured that out with trying some address around the address from the manual.
Thank you again for your help. You replied real fast.
Regards,
Hang