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Electronic Labs_81

Electronic Labs_81 - Dual Gating Functions Symbolic...

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Unformatted text preview: Dual Gating Functions, Symbolic Notation and Practical Gate Applications 4-3 two will be a majority. We can say that a majority has voted “FOR” if A and B or A and C or B and C vote “FOR”. From this statement we can say that three 2-input AND gates and one 3-input OR gate are required. We would like to use the same type gates with a minimum of inverters for our system. If we define our votes “FOR” as a 1 state at the inputs and use NAND gates, the input gates (1 through 3) will give a 0 at their outputs (E, E, W) when inputs A and B, A and C, B and C respectively were 1. All other combinations would give a 1 output. If we now wanted to use an OR gate with 1 inputs, three inverters would be required. Instead, look at the basic NAND gate symbols of Table 4-1 and see that when the input signals are L0 as indicated by the circles, the OR function will be performed. The output Y will be H] (no circles) for L0 inputs. Therefore in using the NAND gate for gate (4) any or all LO inputs from gates (1-3) will produce a HI for the required OR function AB + AC + BC indicating a majority vote “FOR” at the display module. Note that the NAND gate outputs are symbolically indicated as JET AND functions by the circle and the bar (AB). Con- versely, the OR function output is symbolically shown HI by the absence of a circle. Finally, to indicate a majority vote “AGAINST”, we re— quire a 1 when gate (4) output is 0. A simple INVERTER is shown with a circle at the input indicating a 0 will provide a 1 or HI output. Note that the “AGAINST” output is the complement of the “FOR” output (FOR). The output inversion can be an actual INVERTER or a NAND gate with the inputs tied together. AandBmA and CmBandC mallollhemhavevoted"FOR” nun m . AGMNST '5 Ch TO man-w INPUTS ‘5 FROM SWITCHES { ABONBEC T0 DISPth Fig. 4—3 Exercise Procedure Cl 1. :1) Connect the MAJORITY VOTE system shown in Fig. 4-3 using NAND gates indicated by the 7400 and 7420 numbers. E] b) Turn on the trainer power. Cl c) Complete truth Table 4-2. null“.- III“.- ulna- “III-u III“.- III—I. III-I- Table 4-2 The table should indicate a 1 in the “FOR” column when two or more voters were “FOR”. A 0 should be indicated for all other vote com- binations. The “AGAINST” column should be the complement of the “FOR ” column. NOTE: If proper results were not obtained, the next procedure will be an aid to trouble- shooting. Leave your circuit connected and pro- ceed with Objective B. Objective B. Troubleshoot the MAJORITY VOTE system utilizing functional logic symbology and a “logic probe”. Preparatory Information. If a digital system is not operating properly, a signal tracing technique similar to that used in servicing radios or TV. can be employed. The presence or absence of a logic 1 can be detected with a logic probe. A simple “logic probe” exists Within the trainer in the LED Display. That is, the presence of a logic 1 at one of the inputs will light the respective lamp and conversely a logic 0 will not. By following the functional nota— tion of the logic circuits shown in Fig. 4-3 of Ob- jective A, the required logic states can be verified or their absence detected with the probe. Exercise Procedure D 2. a) Connect a long interconnecting wire to an unused lamp input on the Display. The length should be adequate to probe any point on your MAJORITY VOTE system. B b) Refer to Fig. 4-3 for this and the remaining steps of this procedure. Observe that gate number (1) is shown as a NAND gate and the NOT function is desired. That is, a 1 on the ...
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