hi all,

thank you for all idea.

i already check and found that some noise maked this problem and i were modify the code for receive, everything ok now.

i think must check first and make sure the start bit, and after that receive data.

this is code for ref., pls tell me if have any wrong.

regards,
Linh.

receive_from_rack:
        push    03h
        push    04h
        push acc
get_char_2:
        mov     r3,#10
        jb       rxdn,$
get_char_1:
        jb       rxdn,get_char_2
        djnz    r3,get_char_1
        mov     r4,#8
        mov     r3,#10
        djnz    r3,$
        jb      rxdn,get_char_2
get_char:
        mov     r3,#45
        djnz    r3,$
        mov     c,rxdn
        rrc     a
        djnz    r4,get_char
        mov     final,a
        pop acc
        pop     04h
        pop     03h
        ret