Logic Symbol Interpretation
Each of the logi-gate symbols of figure 3-33 provides a unique interpretation of how the gate operates. Before we can demonstrate these interpretations, we must first establish the concept of active logic levels.
When an input or output line on a logic circuit symbol has no bubble on it, that line is said to be active-HIGH, when an input or output line does have a bubble on it, that line is said to be active-LOW. the presence or absence of a bubble, then, determines the active-HIGH/actIve-LOW states of a circuit’s input and output, and is used to interpret the circuit operation.
To illustrate, Figure 3-34(a) shows the standard symbol for a NAND gate. The standard symbol has a bubble on its output and no bubbles on its inputs. Thus, it has an active-LOW output and active-HIGH inputs. The logic operation represented by this symbol can therefore be interpreted as follows:
The output goes Low only when all the inputs are High, Note that this says that the output will go to its active state only when all the inputs are in their active states. The word ‘all’ is used because of the AND symbol.
The alternate symbol for a NAND gate shown in Figure 3-34(b) has an active-HIGH output and active-LOW inputs, and so its operation can be stated as
The output goes HIGH when any input is LOW.
Again, this says that the output will be in its active state whenever any of the inputs is in its active state. The word ‘any’ is used because of the OR symbol.
With a little thought, it can be seen that the two interpretations for the NAND symbols in figure 3-34 are different ways of saying the same thing.
Summary At this point you are probably wondering why there is a need to have two different symbols and interpretations for each logic gate. Hopefully, the reasons will become clear after reading the nest section. For now, let us summarize the important points concerning the logic-gate representations.
1- To obtain the alternate symbol for a logic gate, take the standard symbol and change its operation symbol (OR to AND, or AND to OR) and change the bubbles on both inputs and output (i.e., delete bubbles where there are none).
2. To interpret the logic-gate operation, first note which logic state, 0 or 1, is the active state for the inputs, and which is the active state for the output. Then realize that the outputs active state is produced by having all the inputs in their active state (if ANd and symbol is used), or having any of the inputs in its active state (if an OR symbol is used).