DIGITAL IC CONVENTIONThe theorems of Boolean algebra are applied to theAND, OR, or NOT logic gates, or any logic gates, onthe basis that only two possibilities exist as far as anystatement of their outputs is concerned. Theirstatements are either true or false. A 1 symbol is trueand a 0 symbol is false. In digital logic circuits, the 1and the 0 are represented by different voltage levels andthe particular logic convention must be specific. Whenthe logic levels for a computer are defined, the twovoltages will be relative to each other when determiningif it is positive or negative logic.Digital computers can use either positive ornegative logic.There are advantages anddisadvantages to both types. Depending on itsapplication in a system, the specific logic convention isconsistent throughout the entire computer. The conceptof positive and negative logic is more than a matter ofvoltage levels. Positive logic indicates that the voltagelevel for a 1 will be more positive than the voltage levelfor a 0. Negative logic indicates that the voltage levelfor a 1 will be more negative than the voltage level fora 0. The following examples are given:When it is necessary to use a piece of test equipmentin performing maintenance on the computer, you willneed to know the logic convention the computer uses.DIGITAL LOGIC GATE INPUT ANDOUTPUT WAVESHAPESThe waveshapes of the inputs and outputs of digitallogic gates are important when analyzing the operationof digital logic gates. They can provide you valuableinformation when you perform maintenance. All dig-ital logic gates produce waveshapes on the input or theoutput of the gate(s). The input and the output can bemonitored individually or they can be monitored at theFigure 4-6.—Examples of waveshapes: A. Non-symmetrical;B. Symmetrical.same time. Learn what the waveshapes mean and learnhow to analyze them. Remember, the clock pulses andtiming signals play an important role in the operation ofthe digital logic gates, combinational and sequential.Waveshapes come in two types: non-symmetrical andsymmetrical. Refer to figure 4-6 for examples.Three characteristics of waveshapes can play animportant role in your understanding of computers.You can use them to monitor and/or analyzewaveshapes. The following examples of each will helpyou see how they are calculated:l Pulsewidth(PW) —PW is the time intervalbetween specified reference points on the leading edgeand trailing edges of the pulse waveform. Pulse widthsare usually further defined as a positive PW and anegative PW. Refer to figure 4-7 for an example.l Pulse-repetitiontime(PRT) —The PRT of asignal refers to the time period from the starting pointof a repeating waveshape until the starting point of thenext repetition.Refer to figure 4-8 for an examplemeasurement.Figure 4-7.—Examples of pulse width (PW) measurements.4-8