Logic level

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 name Q, read "Q bar" or "Q not", represents an active-low signal. The conventions commonly used are:

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.

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 incorporate Schmitt 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.

4-level 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 (see Karnaugh map § Don't cares).