Logic Gates and Combinational Logic
...................................................................................................................................
Gate Types and Truth Tables
The basic logic gates are AND, OR, NAND, NOR, XOR, INV, and BUF. The last two are not standard terms; they stand for "inverter" and "buffer", respectively. The symbols for these gates and their corresponding Boolean expressions are given in the Figure
All of the logical gate functions, as well as the Boolean relations discussed in the next section, follow from the truth tables for the AND and OR gates. We reproduce these below. We also show the XOR truth table, because it comes up quite often, although, as we shall see, it is not elemental.
With the convention known as "negative true", the label would be changed to "switch
closed" with a bar over it: switch closed. Our statement becomes:The labelled voltage is Low (High) when the label's stated function is True (False). So in the figure, the stated function (switch closed) is true when the voltage is low. The bar is meant to envoke the boolean inversion operation:
