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2.2 Logic Gates

The basic symbols for one and two input logic gates are shown in Figure 2.1. Three and more inputs are shown by adding extra inputs (but note that there is no such thing as a three input XOR gate). The ANSI/IEEE symbols can be used instead of the traditional "spade"-shaped symbols, but are "not preferred" according to IEEE Standard 91-1984. As will be seen in Chapter 3, IEEE notation is useful for describing complex logic blocks, but simple sketches are often clearer if done with the traditional symbols. A circle shows logic inversion. Note that there are two forms of the NAND and NOR gates. From de Morgan's law, it can be seen that the two forms are equivalent in each case.

Figure 2.1

Figure 2.1 Logic symbols.

In drawing circuit diagrams, it is desirable, for clarity, to choose the form of a logic gate that allows inverting circles to be joined. The circuits of Figure 2.2 are identical in function. If the circuit of Figure 2.2(a) is to be implemented using NAND gates, the diagram of Figure 2.2(b) may be preferable to that of Figure 2.2(c) because the function of the circuit is clearer.

Figure 2.2

Figure 2.2 Equivalent circuit representations.

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