-----------                        -----------
 master    |                      |(AT89S52 slaver)
transmiter |-------------->-------|receive
           |                      |
-----------                        -------------
this schematic OK



                    +5v   +5v
                    ---   ---  
                     |     |
             300ohm  R     R  4.7K
                     |     |
                     __    |---->---
        optocoupler  \/   |/        |
                     --   |\        |
                     |     |        |
-----------          |     |        |         -----------
 master    |         |     -        |        |(AT89S52 slaver)
transmiter |----------    gnd       ->-------|receive
           |                                 |
-----------                                   -------------