The AT89C2051 ports, when used as outputs are able to sink higher currents than the standard 8051.
When used as inputs they work, more or less, as the ports of other derivatives. The manufacturer says that the external driver must sink about 0,5 mA to put the input in its low state. But this applies only in the transition to the low state. Once the low state is achieved, the current can be much lower and still keeping the '0' level.

Alfredo del Rio.