Figure 1-5. Exclusive OR symbols
b. One use of the exclusive OR circuit is as a detection circuit in binary operations. For
example, two signals that are always identical during normal operation can be compared by connecting
them to the inputs of an exclusive OR circuit. As long as they are the same (either both high or both
low) the circuit will not be activated. However, if one of the signals fails or is incorrect, the gate is
enabled and the resulting output indicates that an error has occurred.
A circuit that performs an AND function is one that produces an output only when all inputs are
active. Its operation can be described as producing an output either when A AND B are both active or
when all inputs (A AND B AND C etc) are active.
The symbol for this type of gate is shown in Figure 1-6. As in the other symbols, its shape
indicates its function and state indicators tell you which level is considered the active signal. In A,
Figure 1-6, the symbol represents a circuit that produces a high output if and only if all inputs are high.
The one in part B produces a low output if and only if all inputs are low. This symbol (in either form)
performs the Boolean function of AND; its output is expressed as A B or AB.
Since an AND gate requires that all inputs be active before it produces an active output, the last
input signal to become active is called the enable signal. The other input (inputs, if there are more than
two) is called the "prime" signal because it "primes" the gate or gets it ready for the last input signal to
enable the gate. For AND gates that require high at the input, the first high input signal(s) primes the
gate and the last high signal enables it.