ORG 000H
        JMP     MAIN
ORG 023H
        JMP     SERIAL
ORG 030H
MAIN:
        MOV     SCON,#50H
        MOV     TMOD,#20H
        MOV     TH1,#-24
        SETB    TR1
        SETB    EA
        SETB    ES
        MOV     A,#'A'
        MOV     SBUF,A
        JMP     $

SERIAL:
        JB      RI,RECEIVE
        CLR     TI
        INC     A
        CJNE    A,#'Z'+1,SKIP
        MOV     A,#'A'
SKIP:
        MOV     SBUF,A
RETI
RECEIVE:
        CLR     RI
        MOV     A,SBUF
        CPL     A
        MOV     P1,A
RETI

END