Open drain
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Open drain is one of the many different electrical input/output standards in digital designs today.
[edit] Definition
The word "drain" in the term "Open-drain" refers to the drain terminal of a MOSFET transistor. (The analogous term for BJT devices is open collector.) Open-drain outputs can be useful for analog weighting, summing, limiting, etc., but we will discuss applications in digital logic only. An open drain terminal is connected to ground in the low voltage (logic 0) state, but has high impedance in the logic 1 state. This prohibits current flow, but as a result, such a device requires an external pull-up resistor which is also connected to the positive voltage rail. Note that in newer microelectronic circuits/devices which utilize open drain (such as microcontrollers), there may be a 'weak' internal pull-up resistor to accomplish the task of connecting the terminal in question to an internal positive voltage source/rail; that source typically being the VCC of the device.
When a device is in the high-impedance state, the pull-up resistor keeps the line at logic 1. The line stays there until the device goes into the logic 0 state, and begins to sink current. This current flow creates a voltage drop across the pull-up resistor, and the line drops to the logic 0 voltage.
[edit] Applications
One useful property is that the external pull-up resistor need not be connected to the same voltage as the chip supply voltage Vcc: a lower or higher voltage can be used instead. Open drain circuits are therefore sometimes used to interface two series of devices that have different operating logic levels (voltages).
Another advantage is that more than one open-drain output can be attached to a single wire. If all outputs attached to the wire are in the high-impedance/logic 1 state, the pull-up resistor will hold the wire in a high voltage state. If at least one of the device outputs is in the ground/logic 0 state, it will sink current and bring the line voltage low. This is analogous to a logical function: it behaves like an AND gate in active-high logic and an OR gate in active-low logic (that is, when a lower voltage indicates an "active" signal.) Such circuit constructions are termed "wired-AND" and "wired-OR", respectively.
Open-drain devices are commonly used to connect multiple devices to a bus (i.e. one carrying interrupt or write-enable signals). This enables one device to drive the bus without interference from the other inactive devices - if open-drain devices are not used, then the outputs of the inactive devices would attempt to hold the bus voltage high, resulting in unpredictable output.
