of the MCS-51 Family as listed below Port Pin P3 0 P3 1 P3 2 P3 3 P3 4 P3 5 P3 6 P3 7 Alternate Function RXD (serial input port) TXD (serial output port) INT0 (external interrupt 0) INT1 (external interrupt 1) T0 (Timer 0 external input) T1 (Timer 1 external input) WR (external data memory write str.