Opendrain

Open-drain is a type of digital output or bus configuration in which a device can pull a line to ground but cannot actively drive it high. When the output is not active, the line is left in an open state and is pulled high by an external pull-up resistor to a supply voltage. Because multiple devices can connect to the same line, any device that drives the line low will force the bus low, while releasing the line allows it to return to high. This behavior makes open-drain well suited for wired-AND or wired-OR style signaling, depending on polarity and pull-up arrangement.

In practice, open-drain outputs are implemented using an N-channel MOSFET or an NPN transistor. Microcontrollers and other devices frequently expose pins that can be configured as open-drain outputs, or as inputs (high impedance) when not actively driving the bus. The external pull-up resistor value is chosen based on supply voltage, desired speed, and allowable current; common values range from a few hundred ohms to tens of kilo-ohms. Higher resistances enable lower static current but slower edge transitions.

Common applications include bidirectional communication buses such as I2C and SMBus, where multiple devices must share lines without risking direct conflicts. Open-drain also supports level shifting between voltage domains by using appropriate pull-ups. While it provides safe multi-device arbitration, it introduces slower rising edges and depends on pull-up integrity; failure to provide a pull-up or excessive leakage can cause the line to sit in an indeterminate state. Open-drain is often contrasted with open-collector, which uses a similar concept with a bipolar transistor.