Logic Gate Details

Logic Gate Details

AND Gates
A simple logic gate that has two inputs is the AND gate. The output of an AND gate is true if the first input is true AND the second input is true. To draw up a truth table for a gate has two inputs, all of the possible combination's of values for two inputs are listed. You will see that these combination's are the same as binary numbers from zero to three.

OR Gates
The OR gate is another with two inputs. The output of an OR gate is true if one of the two inputs is true OR the other input is true OR both inputs are true. The truth table and logic diagram symbol for an OR gate are shown on the previous page.

NOT Gates
Apart from a buffer, the simplest kind of logic gate is the NOT gate (also called an inverter or inverting buffer). A NOT gate has one input and one output and the output is the opposite (or complement) of the input. If the input is false then the output is NOT false.

NAND, NOR and XOr gates all have two inputs. From the descriptions below, you should be able to draw up the truth tables for each gate. The output of a NAND gate is the opposite of the output of an AND gate. (a NAND gate is a NOT AND gate). The output of a NAND gate is true if, and only if, it is not true that both the inputs are true. The output of a NOR gate is true if, and only if, it is not true that either of the inputs is true.


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