Digital circuits are electric circuits based on a number of discrete voltage levels. In most cases there are two voltage levels: one near to zero volts and one at a higher level depending on the supply voltage in use. These two levels are often represented as L and H.
| Technology | L voltage | H voltage | Notes |
|---|---|---|---|
| CMOS | 0V to VCC/2 | VCC/2 to VCC | VCC = supply voltage |
| TTL | 0V to 0.8V | 2V to VCC | VCC is 4.75V to 5.25V |
It is possible to construct digital circuits in forms other than electronic. In principle, any technology capable of representing two discrete states and performing Boolean operations could be used to build a logic circuit. Hydraulic, pneumatic and mechanical versions of logic gates exist and are used in situations where electricity cannot be used. The first two types are considered under the heading of fluidics. One application of fluidic logic is in military hardware that is likely to be exposed to a nuclear electromagnetic pulse (nuclear EMP, or NEMP) that would destroy any electrical circuits.
Logic systems can be constructed from diverse systems including optical, magnetic, chemical, biochemical and quantum systems. In each case, the desired logic function can be found in the interactions of the physical components. For example if two particular enzymes are required to prevent the construction of a particular protein, this is the equivalent of a biological "NAND" gate.
Digital circuits are the most common hardware realisation of Boolean algebra and are the basis of all digital computers. (See also logic gate.)
They can also be used to process digital information without being connected up as a computer. Such circuits are referred to as "random logic".
Digital circuit technological innovations have benefitted from the discovery of superconductivity. Most recently, attempts are being made to construct optical computing systems capable of processing digital information.
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