This can be useful when several outputs share a single line as would happen on a bus. Different Voltage Interfacing Since the exact voltage of the pullup resistor is not critical, an open-collector output can be used to interface one logic voltage level with another. Example interface between 5V logic and a higher voltage. You may also see the term open-drain output. The value of resistor used to pull up an open-collector is not critical.
Smaller values offer faster switching times at the price of higher current consumption.
Typical values range from a few thousand to a few hundred thousand Ohms.