Basic logic operations

Chapter Basic logic operations

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Positive versus negative logicUsually, logic 1 is high and logic 0 is low. This is known as positive logic. But thesestates can be reversed, and the results will be just as good. When logic 1 is low and logic0 is high (so that a bit 1 is “off” and a bit 0 is “on”), negative logic isbeing used.In the rest of this chapter, positive logic is the rule. This is simply to minimize con-fusion.Basic logic operationsPerhaps you’ve taken sentenial logic, also called propositional logic or propositionalcalculus (it sounds a lot more sophisticated than it really is) in high school or college.Electronic digital logic is basically the same thing.In logic, there are three basic operations. These are depicted in Table 30-2.Basic logic operations559Table 30-2. Logic operations.OperationDescriptionSymbologyNOTNegation or complement of X–XXANDConjunction of X, YXYXYORDisjunction of X, YX YNOTThe logic NOT, also called negation or inversion, involves only a single logic variable.It changes the value of a bit from 1 to 0, or from 0 to 1, as shown in the truth table forlogic operations (Table 30-3). It is represented by a minus sign in front of a capital let-ter, usually from the end of the alphabet (W, X, Y, or Z). It might also be represented bya squiggly dash before the letter, an apostrophe after the letter, or a line over the letter.Table 30-3. Truth table for logic operations.XY-XXYX+Y00100011011000111011ANDThe logic AND, or conjunction, involves two variables. The resultant is high if, but onlyif, both variables are high. If either variable is low, or if both are low, then the resultant is