HI guys. I did all an tested the minimum system, it's ok, with my small program I can write to P3, but with this form

............
MOV A,#55H
MOV P3,A
...........
this instruction set 10101010 at P3, then the microntroller can read the EPROM and execute the program.
When I try to write to P3 with this

........
MOV A,#55H
MOV SBUF,A
...........

do not send any signal out by P3.1. I am talking send to SBUF but with another program where I set all parameters correctly, the program send continously ASCII characters to buffer.
I read, when I write to SBUF the microntroller active TXD automaticly if you set it correctly.
When I runs the program at hardware it looks like need some intruction or something to write to P3.1 pin. I tested with AT89C52 and 8051, its same.
Hey you guys, the experts in microcontrollers, what I doing wrong ? I