Learning Event 4:
IDENTIFY THE OR AND THE AND SYMBOL
A circuit that performs an OR function is one that has at least two separate input connections and
is activated or enabled by any one of them. The term "enable" is a common way to describe the action
of the signal that activates a gate. In binary logic, input signals don't usually force gates to produce an
output; rather the input signals allow or enable a gate to produce an output. We usually think of input
signals as either inhibiting a circuit or enabling it. Any signal that prevents a gate from producing its
active output is called an inhibit signal; and, the signal that ultimately allows a gate to produce its active
output is called an enable signal.
a. The standard logic symbol for an OR circuit is shown in Figure 1-4. In the case of two inputs
(A, B) to an OR gate, the circuit is activated or enabled by a specified input level to either input A or
input B. When the circuit is enabled it produces a specified output level. The Boolean expression for
the output is A + B.
Figure 1-4. OR circuit symbols
b. OR gates with more than two inputs connections are very common and we describe them in
the same way as the two input OR gate. The OR gate is activated if input A or B or C etc. is active. If
more than one input is active the result is the same, the circuit still produces an output. From this
characteristic we determine that this symbol represents an inclusive OR, the conditions that cause an
output include the condition where more than one input is active. Another way of describing the circuit
operation is "it produces an output if any one or more inputs is active."