Experiment# 6 Combinational Logic Circuits

Experiment # 6

Decoderand Multiplexer circuits

OBJECTIVES

  1. Understanding the construction and operational principles of digital BCD-to-7segment decoder, and Multiplexer circuits.
  2. Understanding how to implement functions using multiplexers.

BACKGROUND

BCD-to-7segment decoder

the idea of a seven-segment indicator for representing decimal numbers. Each segment of a seven-segment display is a small light-emitting diode (LED) or liquid-crystal display (LCD), and - as is shown below - a decimal number is indicated by lighting a particular combination of the LED's or LCD's elements:

Binary-coded-decimal (BCD) is a common way of encoding decimal numbers with 4 binary bits as shown below:

Decimal digit / 0 / 1 / 2 / 3 / 4
BCD code / 0000 / 0001 / 0010 / 0011 / 0100
Decimal digit / 5 / 6 / 7 / 8 / 9
BCD code / 0101 / 0110 / 0111 / 1000 / 1001

Your job for this lab is to design and test a circuit to convert a 4-bit BCD signal into a 7-bit control signal according to the following figure and table:

b3 b2 b1 b0 / a b c d e f g
0 0 0 0 / 1111110
0 0 0 1 / 0110000
0 0 1 0 / 1101101
0 0 1 1 / 1111001
0 1 0 0 / 0110011
0 1 0 1 / 1011011
0 1 1 0 / 0011111
0 1 1 1 / 0 0 0 1 1 1 1
1 0 0 0 / 1 1 1 1 1 1 1
1 0 0 1 / 1 1 1 00 1 1

Notice that the truth-table corresponds to a seven-segment device whose display elements are active high. That is, each element will be active when its corresponding input is '1', the 7448 IC is of this type.There is another type of seven-segment device whose display elements are active low, That is, each element will be active when its corresponding input is '0', the 7447 IC is of this type.

Multiplexer Circuit

Multiplexer, or MUX, is a logic circuit that select and route any number of inputs to single output. One of the multiple inputs are selected by the selector inputsand rout to the single output. The number of selector inputsdetermine the capacity of multiplexer. For example, if a certain MUX has only one selectorinput, it is referred as a “2 line-to-1 line MUX” because one selector can only select one from two inputs. A MUX with 3 selector inputsis called “8 line-to-1 line MUX”, since 3 selectors a capable of selecting an output from 8 inputs (23=8). MUX is also referred to as “Data Selector” because it selects one output from among many inputs.

Function expression, such as F(CBA)=Z(0, 1, 2, 6, 7), can be easily executed on MUX. The function “F” in fig.6.2 generates the sum of products (CB+CB) from states 0, 1, 2,7. Refer to the 4 line-to-1 line MUXin fig.6.2, the output is determined by states selectors A, Band input C. When CBA=000, 001,010, 110, 111 the output F is1. In all other states F=0.

PRELAB:

BCD-to-7segment decoder

  1. Use data sheet to draw the schematic (pin diagram) of the 7448 aBCD-to-7segment decoder and write down its function table.

Multiplexer

  1. Write down Boolean expression and truth table for the output F3in 2 by 1MUX shown in fig.6.4.
  2. Use data sheet to draw the schematic (pin diagram) of the 74151 an8 to 1 MUX and write down its function table.
  3. Implement the function F= Σ(0,2,4,5,7,8,10,11,15) using 74151 MUX. Show necessary connections in block (symbol) diagram, use A,B,C,D as inputs where D is the most significant bit. Derive the truth table for F.

EQUIPMENTS REQUIRED

KL-31001 trainer kit, Module KL-33005 / KL-33006

PROCEDURES:

Part I: BCD-to-7segment decoder

INPUTS / 7- Segments / Display
D / C / B / A / a / b / c / d / e / f / g / number
0 / 0 / 0 / 0
0 / 0 / 0 / 1
0 / 0 / 1 / 0
0 / 0 / 1 / 1
0 / 1 / 0 / 0
0 / 1 / 0 / 1
0 / 1 / 1 / 0
0 / 1 / 1 / 1
1 / 0 / 0 / 0
1 / 0 / 0 / 1

a)Connect inputs A, B, C, D of U5 (7448) on block b of module KL-33005 to Data Switches SW3, SW2, SWI, SWO respectively. The 7448 is a BCD-to-7-segment decoder/driver.

b)Connect the input (LT) to DIP1.0 and se it to logic 1. Follow the input sequences for D, C, B, A in Table 6.1 and record outputs of the 7-segment display. Compare your results with the function table of the 7448 in your prelab.

c)Set the input (LT) to logic 0. is there any different from step b?

Part II:Constructing a 2-to-I Multiplexer

a)Block e of module KL-33006 will be used as a 2-to-I MUX.

C / B / A / F3
0 / 0 / 0
0 / 0 / 1
0 / 1 / 0
0 / 1 / 1
1 / 0 / 0
1 / 0 / 1
1 / 1 / 0
1 / 1 / 1

b)Connect inputs A, Bto SW0 and SW1. Connect input C to SW3Follow the input sequences in Table 6.2 and record states of F3. Which input (A or B) determines the output? What is the effect of input C?

Part III: Constructing an 8 to 1 Multiplexer Circuit with TTL IC

a)U6 (74151) on block f of module KL-33006 will be used in section of the experiment.

b)Connect inputs DOD7 to DIP Switch 1.01.7; inputs C, B, A to DATA Switches SW2, SWI, SWO. Follow the input sequences in Table 6.3, adjust DOD7 and record output states. Determine on which input among DOD7 does F depend on. Compare results with function table of 74151.

C / B / A / F
0 / 0 / 0
0 / 0 / 1
0 / 1 / 0
0 / 1 / 1
1 / 0 / 0
1 / 0 / 1
1 / 1 / 0
1 / 1 / 1

Part IV: Using Multiplexersto create functions

a)Use block f of module KL-33006 to build the circuit you have implemented in your prelab step 4. Place connection leads according to your block diagram.

b)Connect inputs D, C, B, A to Data Switches SW3, SVV2, SWI, SWO respectively. Connect output Y to Logic Indicator LO. Follow the input sequences and verify the truth table you have derived in your prelab.

Exercise:

1)In this experiment you have learned about multiplexers. Define what is a demultiplexer, search the net to find the number of an 8-output demultiplexer, and draw its pin diagram and function table.

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