8251串行通信自发送、接受，在发送准备好后不能发送数据是什么原因？

问题描述：

发送接收时钟都联在8253方波的输出端，波特率1200
……8253初始化，N=52，clk=1Mhz
mov dx,io8251b
xor al,al
mov cx,03
delay: call out1
loop delay
mov al,40h
call out1
mov al,4eh
call out1
mov al,27h
call out1
waiti: mov dx,io8251b
in al,dx
test al,01
jz waiti
mov ah,01
int 21h
cmp al,27
jz exit
mov dx,io8251a
inc al
out dx,al
mov cx,0F00h
s51: loop s51
next: mov dx,io8251b
in al,dx
test al,02
jz next
mov dx,io8251a
in al,dx
mov dl,al
mov ah,02
int 21h
jmp waiti
exit: mov ah,4ch
int 21h
out1 proc near
out dx,al
push cx
mov cx,0F00h
gg: loop gg
pop cx
ret
out1 endp
code ends
end start
键入字符a之后,收回来一个乱码,或者干脆什么也收不回来
无网络环境

问题解答：

防火墙干扰