Introduction
- Electronic gates require a power supply.
- Gate INPUTS are driven by voltages having two
nominal values, e.g. 0V and 5V representing logic 0 and logic 1
respectively.
- The OUTPUT of a gate provides two nominal
values of voltage only, e.g. 0V and 5V representing logic 0 and logic 1
respectively. In general, there is only one output to a logic gate except
in some special cases.
- There is always a time delay between an input
being applied and the output responding.
Logic gates
Digital systems are said to be constructed by using logic gates. These gates are the AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates. The basic operations are described below with the aid of truth tables.AND gate
The
AND gate is an electronic circuit that gives a high output (1) only if all its inputs are high. A dot (.) is used to show the AND
operation i.e. A.B. Bear in mind that this dot is sometimes omitted i.e.
AB
The OR gate is an electronic circuit that gives
a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR
operation.
The
NOT gate is an electronic circuit that produces an inverted version of the
input at its output. It is also known as an inverter. If the input
variable is A, the inverted output is known as NOT A. This is also shown
as A', or A with a bar over the top, as shown at the outputs. The diagrams
below show two ways that the NAND logic gate can be configured to produce a NOT
gate. It can also be done using NOR logic gates in the same way.
This is a NOT-OR gate which is equal to an OR
gate followed by a NOT gate. The outputs of all NOR gates are low if any of the inputs are high. The
symbol is an OR gate with a small circle on the output. The small circle
represents inversion.
EXOR gate
The 'Exclusive-OR' gate is a circuit
which will give a high output if either, but not both, of its two inputs are high. An encircled plus sign (
) is used to show the EOR operation.
The 'Exclusive-NOR' gate circuit does the opposite to the EOR gate.
It will give a low output if either, but not both, of its two inputs are high. The symbol is an
EXOR gate with a small circle on the output. The small circle represents
inversion.
The NAND and NOR gates
are called universal functions since with either one the AND and OR functions
and NOT can be generated.
Note:
A function in sum of products form can be implemented using NAND gates by
replacing all AND and OR gates by NAND gates.
A function in product of sums form can be implemented using NOR gates by
replacing all AND and OR gates by NOR gates.
Table 2: Logic gates representation using the Truth table