Thanks for your comment. I will study your tutorial first.
Because RS232 uses inverted logic, so a MAX232 inverts the data - if you're not using a MAX232 you need to invert it in some other way.
1. But when the device (which is TTL 9600 NRZ) is connected to AT89C52 not a PC, why is the signal inverted?
may i make sure something:
2. (P3.3 of AT89) 5V -> (RXDof device) 5V or 0V??
3. (TXD of device)5V -> (P3.2 of AT89) 0V ???
4. Can the setting as set out in the figure2 fulfill the requirement?