What is an Open-Collector/Open-Drain Circuit?
Open-collector/open-drain is a circuit technique which allows multiple devices to communicate bi-directionally on a single wire.
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. Many devices can be attached to the signal wire. If all devices attached to the wire are in their non-active state, the pull-up will hold the wire at a high voltage. If one or more devices are in the active state, the signal wire voltage will be low.
An open-collector/open-drain signal wire can also be bi-directional. Bi-directional means that a device can both output and input a signal on the wire at the same time. In addition to controlling the state of its pin that is connected to the signal wire (active, or non-active), a device can also sense the voltage level of the signal wire. Although the output of a open-collecter/open-drain device may be in the non-active (high) state, the wire attached to the device may be in the active (low) state, due to activity of another device attached to the wire.
The bi-directional nature of an open-collector/open-drain device is what makes this circuit so important in interconnecting many devices on a common line. The I2C Bus and SMBus uses this technique for connecting up to 127 devices.
Open-drain refers to the drain terminal of a MOS FET transistor. Open-collector is the same concept on a bipolar device.