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Electronic Labs_90

Electronic Labs_90 - 6-4 The Decoder Since the C line is...

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Unformatted text preview: 6-4 The Decoder Since the C line is permanently HI it is the same as having a C input on gates (D), (E), (F) and (G). BCD INPUTS DECIMAL OUTPUTS HIGH NORMAL ABNORMAL If we note that the B and A inputs repeat for the C B A OUTPUT OUTPUT first and second four input combinations, and ' INDICATION C is always present then the 0, 1, 2 and 3 out- puts will each appear twice. A check with a “logic probe” would verify that the codes appearing on gates (D), (E), (F), and (G) inputs repeat twice for the eight input combinations. Summary In this Laboratory Exercise you saw the need for a digital decoder to recognize coded information and provide an output for each coded input com- bination. You saw from the truth tables and cir- cuits that the AND operation is required for the decoder circuit and that a circuit with N input Table 6-3 “bits” will decode or provide 2N outputs. You identified the BCD to Decimal decoder as a system Exercise Procedure composed of AND gates and IN VERTERS giving output states 0 through 9. Also, a four bit BCD input code is used but only ten output com- binations were required. HHOOi—‘l—‘OO i—Ir—Ai—Ai—Ioooo HOP—‘OP—‘OHO D 2. a) Turn off the trainer power, disconnect the wire from INVERTER (C) output and re- connect to a permanently HI (Vcc) circuit point. D b)Turn on trainer power and complete Finally, you increased your troubleshooting ability Table 6-4 for all BCD inputs. by analyzing circuit diagrams. You noted that logic lines are identified by the logic operation per- iii‘él‘iidbé’ha‘iaifiisgitirii‘ZanL’étf 353313;; :32 III-“mg nn-nnnnnnna n-nnnnnmlnn fl-Iflflflflflflflfl Illlllllflflllflflll "lullflflflflllfl Inn-lllllflflfl III-“flflllflflfl Quiz 1. If the input BCD word is 110, the decimal number activated would be: Q4069: 99‘9“!“ 2. The decimal number output 3 corresponds Table 6-4 to the following BCD word: C] c) Compare Table 6-4 with the proper out- a. 00]. puts of Table 6-2. State what has changed and why. b. 010. c. 100. d. 011. 3. If the output of the BCD-to-decimal decoder of Fig. 6-2 was correct except that the deci- mal '7 output always remained HI, which of the following would be the most likely defect? a. INVERTER (D) defective, always at a 0 level output. ...
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