It follows a brief (not deep) description of the cuasi-bidirectional ports used in the 8051. For details please read the handbooks.

1) Ports 1, 2 and 3. Each pin state is '1' (high level) only if:

Your code has not written a '0' to it.
AND
No external circuit is setting the pin to '0'.

Otherwise, the pin state is '0'.

If you want to use a pin as an input, do not write a '0' to it. When you read that pin you get a '1' unless an external circuit is driven it to '0'.

If you want to use a pin as an output, write the level you need '0' or '1'. Be sure the external circuit where the pin is connected does not impose a severe load on the 8051 port capabilities.

NOTE:
In order to operate this way, the 8051 uses an output circuit similar (but not equal) to an output drain with a pull-up resistor.

2) Port 0 is similar but without pull-ups.

Alfredo.