Lab 5 Decoder and Encoder ICs: 7400(NAND), 7404 (Inverter), 7427(3-input NOR), 74155 (decoder), 74147(Encoder), 7447(BCD-to-7-segment decoder), a 7-segment display, seven 47042 resistors for the 7-segment display For each experiment, when the circuit is working, have the lab instructor che ck it. 1. Decoder is a circuit that converts binary information from n input lines to a maximum of 2″ unique output lines. The purpose is to generate the 2″ minterms. It is also used in conjunction with other code converters, such as BCD-to-seven segment decoder. A seven-segment indicator is used to display
Lab 5 Decoder and Encoder ICs: 7400(NAND), 7404 (Inverter), 7427(3-input NOR), 74155 (decoder), 74147(Encoder), 7447(BCD-to-7-segment decoder), a 7-segment display, seven 47042 resistors for the 7-segment display For each experiment, when the circuit is working, have the lab instructor che ck it. 1. Decoder is a circuit that converts binary information from n input lines to a maximum of 2″ unique output lines. The purpose is to generate the 2″ minterms. It is also used in conjunction with other code converters, such as BCD-to-seven segment decoder. A seven-segment indicator is used to display any one of the decimal digits 0 through 9. Usually, the decimal digital is available in BCD. A BCD-to-seven-segment decoder accepts a decimal digital in BCD ad generates the corresponding seven-segment code, as shown below. f b g 0123456789 d (a) Segment designation (b) Numerical designation for display Fig. P4-9 (a) Fill out the following truth-table Input Output 7-segment code d BCD code ? B #input Digital Display D A a b ? e f 0 1 2 3 4 5 6 7 8 9 not lit = 0, lit = 1 segment lit = 0, segment not lit = 1 (b) The following figure shows the connections necessary between the decoder and the display. The 7447 IC is a BCD-to-7-segment decoder/driver that has 4 inputs for the BCD digit. Input D is the most significant and input A is the least significant. The 4-bit BCD digit is converted to a 7-segment code with outputs a through g. The outputs of the 7447 are applied to the inputs of the seven-segment display. This IC contains the 7 LED 2. Encoder: An encoder is a digital circuit that performs the inverse operation of a decoder. An encoder has a number of inputs, only one of which is activated at a given time, and produces an output code, depending on which input is activated. That is, the outputs of an encoder generate the binary code corresponding to the input value. IC 74147 is a decimal-to-BCD encoder as shown below. Note that the inputs and outputs of the chip are Active-LOW, and it is a Priority encoder – highest input has priority. (a) Fill out the following priority encoder truth-table LSB 1 2 Table 6.18 Truth table of 74147 Active-low decimal inputs Active-low BCD outputs 3 4 5 6 7 8 9 D ? B A 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 0 1 1 1 B 0 1 3 4 5 BCD output Decimal input X 0 MSB 8 9 X X ? X 0 1 0 1 0 X X X X X 0 1 1 (b) Connect each out ? 0 1 1 and then to an LE 1, as shown below. LOMCI WC muus on 1414/ W SWITICS, (luw) avuve umy unc input at a time. Observe the LED and 7-segment display to see if the corresponding output is right. Connect from 7404 to lad, then free led to 7447 Anode +5 Decoder Encoder 74147 7447 segments on top of the package. Construct the circuit. Apply the 4-bit BCD digits through 4 switches, and observe the decimal display from 0 to 9. Inputs 1010 through 1111 have no meaning in BCD. Keep this circuit for the next experiment. Vcc B B 1 C Vcc 16 f 15 g 14 ? 2 LT 3 4 BI RBI g PLETEL 7447 D 5 6 O D A 13 b 12 11 c 10 d e 9 IND 7 8 d a f NC 14 13 Vac b 3 ? 11 10 9 8 O NC d 7 3. A combinational circuit has 3 inputs: x, y and z, and 3 outputs: F1, F2 and F3. The simplified Boolean functions for the circuit are F1 = AC + A’B’C’ F2 = A’B + AB’C’ F3 = AB + A’B’C Implement and test the combinational circuit, using a 74155 decoder IC and some external logic gates. The block diagram of the decoder and its truth table are shown below. The 74155 can be connected as a dual 2×4 decoder or as a single 3×8 decoder. When a 3×8 decoder is desired, inputs C1 and C2, as well as G1 and G2, must be connected together, as shown in the block diagram. G is the enable input and must be equal to 0 for proper operation. The 8 outputs are labeled with symbols given in the data book. The 74155 uses NAND gates, with the result that the selected output goes to 0 while all other outputs remain at 1. a. Verify the function table of the decoder 74155. b. Derive the truth-table for F1, F2 and F3 c. Draw the logic diagram for implementing F1, F2 and F3 using a 3×8 decoder and some external logic gates. d. Use 74155, 7427 and 7404 to verify your implementation 16 Vac 280 9 1 C1 10 15 281 ? C2 11 2Y2 3 12 B B 2Y3 13 74155 7 A 1Y0 A 6 2 141 G1 5 1Y2 14 4 G G2 1Y3 GND 8 ? X G 1 0 0 0 0 0 0 0 0 Inputs ? B X X 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1 1 1 0 1 0 1 0 1 Truth table Outputs 270 271 272 273 190 191 192 193 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 0 Fig. 11-7 IC Type 74155 Connected as a 3 x 8 Decoder
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