"interface circuit from AT89C51RC2 to Serial port"

It is built into the MCU and terminates in pins called TXD and RXD.

To convert this TTL level signals to RS232 levels you just need a MAX232 chip or something similiar.

That is all the interface you need.

By the way do you happen to have a copy of the datasheet for the MCU that you are talking about ? If YES read it. If NO get it . NOW.

Raghu