Thx for your suggestion, I'll try to test it in computer to see what happens.

For receiving the ">" character, I tried to used the following method

RECEIVE_PROMPT:

HERE1: JNB RI, HERE1
CLR RI
HERE2: JNB RI, HERE2
CLR RI

CALL SHORTDELAY
RET

However, when this part was added, the LED(should turn off when the SENDMSG rountine completed) did't stop. It seems that the program was held in this part.