2-4Table 2-3.—Examples of Negative LogicNOTE: The logic level LOW now represents the 1 state. This is because the 1 state voltage is morenegative than the 0 state.In the examples shown for negative logic, you notice that the voltage for the logic 1 state is morenegative with respect to the logic 0 state voltage. This holds true in example 1 where both voltages arepositive. In this case, it may be easier for you to think of the TRUE condition as being less positive thanthe FALSE condition. Either way, the end result is negative logic.The use of positive or negative logic for digital equipment is a choice to be made by designengineers. The difficulty for the technician in this area is limited to understanding the type of logic beingused and keeping it in mind when troubleshooting.NOTE:UNLESS OTHERWISE NOTED, THE REMAINDER OF THIS BOOK WILL DEALONLY WITH POSITIVE LOGIC.LOGIC INPUTS AND OUTPUTSAs you study logic circuits, you will see a variety of symbols (variables) used to represent the inputsand outputs. The purpose of these symbols is to let you know what inputs are required for the desiredoutput.If the symbol A is shown as an input to a logic device, then the logic level that represents A must beHIGHto activate the logic device. That is, it must satisfy the input requirements of the logic devicebefore the logic device will issue the TRUE output.Look at view A of figure 2-1. The symbol X represents the input. As long as the switch is open, thelamp is not lit. The open switch represents the logic 0 state of variable X.

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