Dear Friend,
8051 ports are called Quasi bidirectional ports.If u want to use as out port just write . If u want to use as In put u have to write all 1 to the coresponding port bits. This give high always at the out put. It also disables the fetswitch. Then external device can pull it down. Any TTL even Shotkey devices easily pull down the output.If ur using the hcmos just check the data sheets of the corresponding IC Isink current.
bye
ajay