Always remember that it's the shape of the signal that tells you the function and in this case the
symbols in Figure 1-4 represent OR gates. The presence or absence of state indicators (circles) only
specifies which voltage level (high or low) is required on each input and which level is produced.
a. Look at A of Figure 1-4. The absence of circles means the active signal is a high. We know,
then, that the inputs require a high to enable the circuit because there are no circles on either of them.
The absence of a circle on the output means that the gate produces a high output when it is active or
enabled. From this we can say that the circuit requires a high in, to produce high out. But a high on
which input? The symbol shape is "OR" so a high on input A or B or both causes a high out. Each of
these conditions is shown in the truth table of Figure 3-17 on page 32.
b. The symbol in B, Figure 1-4, has circles on the input and output but the shape is still the
same so it's still an OR gate. A circle means a low is the active signal so this gate requires a low on A or
B or both to produce a low output. The truth table in part B shows that the same four conditions as in
part A result in different output conditions. Obviously the circuits represented are both OR circuits.
The difference is that one operates as an OR function of the relatively high signal, and the other as an
OR function of the relatively low signal.
It's important that you realize we are discussing the function of circuits without looking at a
schematic and knowing only how they respond to input signals. The circuits could be designed several
different ways and still perform the same function. Thus, the inclusive OR symbol used here represents
any and all inclusive OR circuits.
The exclusive OR is a variation of the OR symbol. The circuit it represents is still an OR circuit
and performs an OR function; that's why the shape of the symbol is the same.
In A, Figure 1-5, for example, the gate requires a high on either input A or B just as the inclusive
OR does. The gate is "exclusive" in that it will not be enabled (produce an output) if both inputs are
high. A complete description of exclusive OR operation is "either A or B but NOT BOTH."
a. Look at either of the truth tables in Figure 1-5. They show that the required output is
produced only when the inputs are not the same (not both high or both low.) The circuit represented by
the symbol in part A produces a high (no circle on the output) when it is enabled. Its truth table
indicates that it is enabled only when the inputs are not the same. When the inputs are both high or both
low, the gate is inhibited and the output is low. The symbol in part B performs the same function but
produces a low when it is enabled. The Boolean expression for the output of this type of gate is written
either AB + AB or A B.