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You can see an open collector o/p used in IC. In same way we have open drain o/p in 8051 port0. The output essentially acts as either an open circuit (no connection to anything) or a connected to ground. we use external pullup resistor to provide high volt (1) rather than providing open circuit output.
Because the pull up resistor is external and need not be connected to the chip supply voltage, a lower or higher voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating logic voltage levels or to control external circuitry that requires a higher voltage