2-5Figure 2-1. —Logic switch: A. Logic 0 state; B. Logic 1 state.Closing the switch (view B), represents the logic 1 state of X. Closing the switch completes thecircuit causing the lamp to light. The 1 state of X satisfied the input requirement and the circuit thereforeproduced the desired output (logic HIGH); current was applied to the lamp causing it to light.If you consider the lamp as the output of a logic device, then the same conditions exist. The TRUE(1 state) output of the logic device is to have the lamp lit. If the lamp is not lit, then the output of the logicdevice is FALSE (0 state).As you study logic circuits, it is important that you remember the state (1 or 0) of the inputs andoutputs.So far in this chapter, we have discussed the two conditions of logical statements, the logic statesrepresenting these two conditions, logic levels and associated electrical signals and positive and negativelogic. We are now ready to proceed with individual logic device operations. These make up the majorityof computer circuitry.As each of the logic devices are presented, a chart called a TRUTH TABLE will be used to illustrateall possible input and corresponding output combinations. Truth Tables are particularly helpful inunderstanding a logic device and for showing the differences between devices.The logic operations you will study in this chapter are the AND, OR, NOT, NAND, and NOR. Thedevices that accomplish these operations are called logic gates, or more informally, gates. These gates arethe foundation for all digital equipment. They are the "decision-making" circuits of computers and othertypes of digital equipment. By making decisions, we mean that certain conditions must exist to producethe desired output.In studying each gate, we will introduce various mathematical SYMBOLS known as BOOLEANALGEBRAexpressions. These expressions are nothing more than descriptions of the input requirementsnecessary to activate the circuit and the resultant circuit output.

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