2-3In some cases, more than one variable is used in a single expression. For example, the expressionAB C D is spoken "A AND B AND NOT C AND D."POSITIVE AND NEGATIVE LOGICTo this point, we have been dealing with one type of LOGIC POLARITY, positive. Let’s furtherdefine logic polarity and expand to cover in more detail the differences between positive and negativelogic.Logic polarity is the type of voltage used to represent the logic 1 state of a statement. We havedetermined that the two logic states can be represented by electrical signals. Any two distinct voltagesmay be used. For instance, a positive voltage can represent the 1 state, and a negative voltage canrepresent the 0 state. The opposite is also true.Logic circuits are generally divided into two broad classes according to their polaritypositivelogic and negative logic. The voltage levels used and a statement indicating the use of positive or negativelogic will usually be specified on logic diagrams supplied by manufacturers.In practice, many variations of logic polarity are used; for example, from a high-positive to a low-positive voltage, or from positive to ground; or from a high-negative to a low-negative voltage, or fromnegative to ground. A brief discussion of the two general classes of logic polarity is presented in thefollowing paragraphs.Positive LogicPositive logic is defined as follows: If the signal that activates the circuit (the 1 state) has a voltagelevel that is more POSITIVE than the 0 state, then the logic polarity is considered to be POSITIVE. Table2-2 shows the manner in which positive logic may be used.Table 2-2. —Examples of Positive LogicAs you can see, in positive logic the 1 state is at a more positive voltage level than the 0 state.Negative LogicAs you might suspect, negative logic is the opposite of positive logic and is defined as follows: If thesignal that activates the circuit (the 1 state) has a voltage level that is more NEGATIVE than the 0 state,then the logic polarity is considered to be NEGATIVE. Table 2-3 shows the manner in which negativelogic may be used.

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