Electronics: Logic Gates
The diagrams below are of the commonly used logic gates. They have inputs A and B and an output Y. Logic gates are used to design logic circuits.
Click on the graphic of the gate you are interested in to find out more.
NAND and NOR gates are called universal gates because you can combine them to produce all of the other gates.
Logic Levels
In binary logic there are two levels or states for inputs and outputs. They are called the are logical high and logical low. These correspond to a binary 1 and 0 respectively. Logic levels are usually represented by the voltage difference between the signal and ground (or some other common reference point), although other standards exist. The range of voltage levels that represents each state depends on the logic family being used.
Signals with one of these two levels can be used in boolean logic for digital circuit design or analysis.
Active state
The use of either the higher or the lower voltage level to represent either logic state is arbitrary and may even be changed at different levels within a system. Active-high and active-low states can be mixed at will: for example, a read 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 (see De Morgan's theorem).
An active-high signal represents a binary digit of 1, or asserted state of a logical condition, by the higher of two voltages. The higher voltage represents a binary 1, and the lower voltage represents a binary 0.
An active-low signal represents a binary digit of 1, or asserted state of a logical condition, by the lower of two voltages.
About this site
