Symbols for logic gatesA logic gate is an electronic switch that performs a logic operation. The earliest logicgates were built using vacuum tubes; later, transistors and diodes were employed. Mod-ern logic gates are fabricated on integrated circuits (ICs), with hundreds or even thou-sands of gates per chip.An inverter (NOT gate) has one input and one output. Other logic gates usuallyhave two inputs and a single output. The symbols for the various logic gates are shownin Fig. 30-3.Complex logic operations56130-3Symbols for logic gates: NOT (A), AND ( ), OR (C), XOR (D), NAND (E),NOR (F), and XNOR (G).It is possible for AND, NAND, OR, or NOR gates to have three or more inputs. Foran AND gate, the output is high if and only if all of the inputs are high. For an OR gate,the output is low if and only if all the inputs are low. The multiple-input NAND and NORgates perform the conjunction or disjunction operation first, followed by negation. Fig-ure 30-4 shows schematic symbols for four-input AND four-input NOR gates.The XOR and XNOR operations are hard to define when there are three or more in-puts. These fall into the category of complex logic operation.Complex logic operationsThe term “complex,” when used to describe a logic operation, does not necessarilymean complicated. A more apt adjective might be composite. But there are cases whenlogic operations are indeed quite complicated.No matter how messy a particular logic operation might appear, it can always bebroken down into the elementary operations defined above.