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Solved–Lab #8: Decoder– Solution

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Objectives:   In this experiment, you will use the decoder to implement some logic functions.   IC chips: One 74LS139 chip (DUAL 2 4 decoder with negated output and zero-enable).   One 74LS20 chip (DUAL 4-input NAND gates).   The decoder chip you will use in this lab is the 74LS139, which contains two sets…

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Objectives:

 

In this experiment, you will use the decoder to implement some logic functions.

 

IC chips:

  1. One 74LS139 chip (DUAL 2 4 decoder with negated output and zero-enable).

 

  1. One 74LS20 chip (DUAL 4-input NAND gates).

 

The decoder chip you will use in this lab is the 74LS139, which contains two sets of 2 4 decoder with negated outputs and zero-enable. The pin configuration and the function table are given below.

 

Vcc Function Table of 74LS139
16 15 14 13 12 11 10 9
INPUTS OUTPUTS
Enable Select
A B Y0 Y1 Y2 Y3
G B A Y0 Y1 Y2 Y3
G
1 X X 1 1 1 1
G 0 0 0 0 1 1 1
0 0 1 1 0 1 1
A B Y0Y1Y2Y3
0 1 0 1 1 0 1
1 2 3 4 5 6 7 8 0 1 1 1 1 1 0
GND 1 = High, 0 = Low, X = don’t care

 

 

The 74LS20 chip contains two 4-input NAND gates. The pin configurations are shown below.

 

14  13  12  11  10  9  8

 

Vcc

 

 

 

 

GND

 

1  2  3  4  5  6  7

 

74LS20

 

 

 

Propagation delay:

 

Assume that the propagation delay for every NAND gate in 74LS20 is 15 ns (nano-seconds), and the propagation delay for each decoder in 74LS139 is 35 ns.

 

 

 

 

 

AY2019/20 Semester 2                                                   – 1 of 3 –                                                                 CS2100 Lab #8

 

Procedure:

 

  1. Given a 3-variable function S(P,Q,R) = (P + Q + R) (P + Q’) (P’ + R), write this function in the product-of-maxterms form, using the M

 

S(P,Q,R) =      M ( ____________________ )

 

 

  1. Complete the truth table for S below.

 

P Q R S
0 0 0 0
 
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1

 

  1. With the truth table above, implement function S using the 2 4 decoders in the 74LS139 chip, and NAND gates in the 74LS20 chip. Connect the inputs P, Q, and R to SW7, SW6 and SW5 on your logic trainer respectively.

 

Complete the following logic diagram.

 

2  4

DEC Y0

 

B(MSB) Y1
A Y2
G Y3

 

2  4

DEC Y0

 

B(MSB) Y1
A Y2
G Y3

 

 

 

 

 

  1. Show your implementation to your lab TA before you proceed to implement function F in step 7.

 

  1. What is the propagation delay of your circuit above? _______

 

 

 

 

 

 

 

 

AY2019/20 Semester 2                                                   – 2 of 3 –                                                                 CS2100 Lab #8

 

  1. Given a 4-variable function F(W,X,Y,Z) = M(3, 7), write out the simplified SOP expression for F

 

Simplified SOP expression: F = ____________________

 

  1. Implement F using only ONE 2 4 decoder, without any additional logic gate. Connect W, X, Y and Z to SW7, SW6, SW5 and SW4 respectively. (You may find that you do not need to use all 4 inputs.) You circuit is considered wrong if it uses any additional logic gate, even though it produces the correct output.

 

Complete the logic diagram below.

 

2  4

DEC Y0

 

B(MSB) Y1
A Y2
G Y3

 

 

 

  1. Show your implementation to your lab TA.

 

  1. Using one 4:1 multiplexer and one 2 4 decoder with 1-enable as shown below, show how you might implement function G(A,B,C,D) = m(0, 6, 9, 15) without using any additional logic gate. Complete the diagram below.

 

Note that the selector lines for the 4:1 multiplexer have been fixed to AB, and you must not change it.

 

You do not need to implement this since you are not given any multiplexer in this lab.

 

 

 

0 4:1  
1 MUXY   G
 
2 S1 S0  
3
2  4  
DEC Y0  
A B  
I1 Y1
I0 Y2
Y3
EN

 

1

 

 

 

 

 

Marking Scheme: Report (18 marks), Circuit (7 marks); Total: 25 marks.

Your graded report will be returned to you at the next lab.

 

 

 

 

 

 

AY2019/20 Semester 2       – 3 of 3 – CS2100 Lab #8