What is an "open drain" ?

[J. Mark Wolf]

An open-drain can only drive a signal low (sink current). It cannot drive a signal high (source current).

If you ever need an open-drain pin to go high, you must have a pull-up resistor connected between the open drain pin and a voltage. This resistor is what would give you the high voltage at the open drain pin.

Think of it his way, if you want to light an LED with an open drai pin, the anode of the LED must be tied to say, +5 volts, through a series resistor, and then you can drive the cathode of this LED low (lighting the LED) with an open-drain pin. This is called "sinking" current.

You cannot, however, tie the cathode of the LED to ground through a series resistor, and drive the anode of the LED with the open drain pin. That would be called "sourcing" current, and an open-drain pin cannot do this.