open drain circuit

attach a pull up resistor.
active low or high impedance state

in reference to:

"Open-collector/open-drain devices sink (flow) current in their low voltage active (logic 0) state, or are high impedance (no current flows) in their high voltage non-active (logic 1) state. These devices usually operate with an external pull-up resistor that holds the signal line high until a device on the wire sinks enough current to pull the line low."
- Open-Collector/Open Drain FAQ (view on Google Sidewiki)