Indigital circuits, alogic level is one of a finite number ofstates that adigital signal can inhabit. Logic levels are usually represented by thevoltage difference between the signal andground, although other standards exist. The range of voltage levels that represent each state depends on thelogic family being used.Alogic-level shifter can be used to allow compatibility between different circuits.

In binary logic the two levels arelogical high andlogical low, which generally correspond tobinary numbers 1 and 0 respectively ortruth valuestrue andfalse respectively. Signals with one of these two levels can be used inBoolean algebra for digital circuit design or analysis.

The use of either the higher or the lower voltage level to represent either logic state is arbitrary. The two options areactive high (positive logic) andactive low (negative logic). Active-high and active-low states can be mixed at will: for example, aread only memory integrated circuit may have a chip-select signal that is active-low, but the data and address bits are conventionally active-high. Occasionally a logic design is simplified by inverting the choice of active level (seeDe Morgan's laws).

The name of an active-low signal is historically written with a bar above it to distinguish it from an active-high signal. For example, the nameQ, readQ bar orQ not, represents an active-low signal. The conventions commonly used are:

• a bar above (Q)
• a leading slash (/Q)
• a leading exclamation mark (!Q)
• a lower-casen prefix or suffix (nQ, Qn or Q_n)
• an upper-caseN suffix (Q_N)
• a trailing# (Q#), or
• an _B or _L suffix (Q_B or Q_L).^[1]

Many control signals in electronics are active-low signals^[2] (usually reset lines, chip-select lines and so on). Logic families such asTTL can sink more current than they can source, sofanout andnoise immunity increase. It also allows forwired-OR logic if the logic gates areopen-collector/open-drain with a pull-up resistor. Examples of this are theI²C bus,CAN bus, andPCI bus.

Some signals have a meaning in both states and notation may indicate such. For example, it is common to have a read/write line designated R/W, indicating that the signal is high in case of a read and low in case of a write.

The two logical states are usually represented by two different voltages, but two differentcurrents are used in some logic signaling, likedigital current loop interface andcurrent-mode logic. High and low thresholds are specified for each logic family. When below the low threshold, the signal islow. When above the high threshold, the signal ishigh. Intermediate levels are undefined, resulting in highly implementation-specific circuit behavior.

It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and occur only in a fault condition or during a logic-level transition. However, few logic circuits can detect such a condition, and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. Some logic devices incorporateSchmitt trigger inputs, whose behavior is much better defined in the threshold region and have increased resilience to small variations in the input voltage. The problem of the circuit designer is to avoid circumstances that produce intermediate levels, so that the circuit behaves predictably.

Nearly all digital circuits use a consistent logic level for all internal signals. That level, however, varies from one system to another. Interconnecting any two logic families often required special techniques such as additionalpull-up resistors or purpose-built interface circuits known as level shifters. Alevel shifter connects one digital circuit that uses one logic level to another digital circuit that uses another logic level. Often two level shifters are used, one at each system: Aline driver converts from internal logic levels to standard interface line levels; a line receiver converts from interface levels to internal voltage levels.

For example,TTL levels are different from those ofCMOS. Generally, a TTL output does not rise high enough to be reliably recognized as a logic 1 by a CMOS input, especially if it is only connected to a high-input-impedance CMOS input that does not source significant current. This problem was solved by the invention of the 74HCT family of devices that uses CMOS technology but TTL input logic levels. These devices only work with a 5 V power supply.

Though rare,ternary computers evaluatebase 3three-valued or ternary logic using 3 voltage levels.

Inthree-state logic, an output device can be in one of three possible states: 0, 1, or Z, with the last meaninghigh impedance. This is not a voltage or logic level, but means that the output is not controlling the state of the connected circuit.

Four valued logic adds a fourth state, X (don't care), meaning the value of the signal is unimportant and undefined. It means that an input is undefined, or an output signal may be chosen for implementation convenience (seeKarnaugh map § Don't cares).

IEEE 1164 defines 9 logic states for use inelectronic design automation. The standard includes strong and weakly driven signals, high impedance and unknown and uninitialized states.

In solid-state storage devices, amulti-level cell stores data using multiple voltages. Storing n bits in one cell requires the device to reliably distinguish 2ⁿ distinct voltage levels.

Digitalline codes may use more than two states to encode and transmit data more efficiently. Examples includealternate mark inversion and4B3T from telecommunications, andpulse-amplitude modulation variants used byEthernet over twisted pair. For instance,100BASE-TX usesMLT-3 encoding with threedifferential voltage levels (−1V, 0V, +1V) while1000BASE-T encodes data using five differential voltage levels (−1V, −0.5V, 0V, +0.5V, +1V).^[8] Once received, the line coding is converted back to binary.

• Logic family
• Digital current loop interface

• Positive Logic (active-high) and Negative logic (active-low )
• Simple MOSFET-based logic level conversion or level-shift based on work done by Herman Schutte at Philips Semiconductors Systems Laboratory in Eindhoven

• Digital electronics

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