Solution:

Under non-addressable mode, master 4550 with previous version of firmware wouldn't send message until it meets terminal character . Now the latest firmware modifies it and allow you to send message character by character without terminal character . When user types one character from host PC, slave 4550 would immediately receive it via master 4550, but this function is only for non-addressable mode. The latest firmware of 4550 is V3.9 so far.