Basic Circuit Diagram for PC to Microcontroller Communication

Using the Serial Port
8051 provides a transmit channel and a receive channel of serial communication. The transmit data pin (TXD) is specified at P3.1, and the receive data pin (RXD) is at P3.0. The serial signals provided on these pins are TTL signal levels and must be boosted and inverted through a suitable converter(MAX232) to comply with RS232 standard.
All modes are controlled through SCON, the Serial CONtrol register. The SCON bits are defined as SM0, SM1, SM2, REN, TB8, RB8, TI, RI from MSB to LSB. The timers are controlled using TMOD, the Timer MODe register, and TCON, the Timer CONtrol register.