The NOR gate, which is composed of two or more inputs and a single output, also has a universal property. The term NOR is formed by the concatenation NOT-OR and implies an OR function with an inverted output. The standard symbol for the NOR gate is shown in Figure 1-7 and its truth table listed in Table 1-5. The logical operation of the NOR gate is such that the output is HIGH (1) only when all the inputs are LOW.
Figure 1-7 Standard logic symbol for NOR gate
INPUT |
OUTPUT |
|
A |
B |
|
0 |
0 |
1 |
0 |
1 |
0 |
1 |
0 |
0 |
1 |
1 |
0 |
Table 1-5 Truth table for NOR gate
The Exclusive-OR (XOR) and Exclusive NOR (XNOR) gates
These gates are usually formed from the combination of the other logic gates already discussed. However, because of their functional importance, these gates are treated as basic gates with their own unique symbols. The truth tables for the XOR and XNOR gates, shown in Figure 1-8, are listed in Table 1-6. The Exclusive-OR is an "inequality" function and the output is HIGH (1) when the inputs are not equal to each other. Conversely, the Exclusive-NOR is an "equality" function and the output is HIGH (0) when the inputs are equal to each other.
Figure 1-8 Standard logic symbols for: (a) XOR (b) XNOR
|
INPUT |
XOR OUTPUT |
XNOR OUTPUT |
|
A |
B |
|
|
0 |
0 |
0 |
1 |
0 |
1 |
1 |
0 |
1 |
0 |
1 |
0 |
1 |
1 |
0 |
1 |
Table 1-6 Truth table for XOR and XNOR logic gates
