Active high and active low

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Active-high signals are used in electronic logic circuits to represent binary values by means of two different voltages. The higher voltage represents a binary 1 or "mark", and the lower voltage represents a binary 0 or "space".

By modern convention, digital signals are always active high unless explicitly stated to be active low.

Active-low signals are used in electronic logic circuits to represent binary values by means of two different voltages. The higher voltage represents a binary 0 or "space", and the lower voltage represents a binary 1 or "mark".

The name of an active-low signal is written with a bar above it to distinguish it from an active-high signal. For example, the name , read "Q bar" or "Q not", represents an active-low signal. Other conventions commonly used are a leading slash (/Q), a leading lower-case n (nQ), a trailing # (Q#), or an "_B" suffix (Q_B). The slash convention is also used with signals that have a meaning in both states. For example, it is common to have a read/write line written R/W, indicating that the signal is high in case of a read and low in case of a write.

RS232 signaling, as used in computer COM ports, uses active-low signals.

Most control signals in electronics are active-low signals (usually reset lines, chip select lines and so on). This stems from the fact that most logic families can sink more current than they can source, so fanout and noise immunity increase. (The reason for this is ultimately related to the fact that electrons are negatively charged.) It also allows for wired-OR logic if the logic gates are open-collector/open-drain with a pull-up resistor. Examples of this are the I²C bus and Controller Area Network (CAN).