$MOD51
org 0000H
ljmp INITIAL

org 0100H
INITIAL:
setb P0.0
mov TH1,#0FDH
mov TMOD,#20H
setb TR1
clr P0.0
mov PCON,#00H
mov SCON,#0D0H
clr TI
clr RI
MULAI:
setb P0.0
mov A,P1
mov P2,A
mov SBUF,A
clr P0.0
clr TI ;                     ERROR ONE
       jnb TI,$    ;better          ;polling TI :-)
       clr TI         

jb RI,SERIAL_RECEIVED
ljmp MULAI

org 0150H
SERIAL_RECEIVED:
clr P0.0
mov A,SBUF
swap A
setb P0.0
mov P2,A
jmp SERIAL_RECEIVED      ;ERROR TWO 
             jmp MULAI ;better