12) decoderr prac 12

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Logic Design & Switching Theory

DecodersInstitute of Industrial Electronics

Engineering

Lab Session 12

DECODERS

OBJECTIVE

To Study the basic operation of Decoder.

APPARATUS

Power Supply

Resistors

7-segment LED Display

Jumper wires

DIP switches

BCD-to-7 Segment Decoder (74LS47)

THEORY:

A decoder is a digital circuit that detects the presence of a specifiedcombination of bits or codes on its inputs and indicates the presence of that code by

a specified output level. In its general form, a decoder has n input lines to handle nbits and from 1 to 2nd output lines to indicate the presence of one or more nbitCombinations.

BCD-TO-7-SEGMENT DECODER:

The BCD-to-7 segment accepts the BCD code in its input and provides anoutput to drive 7 segment led Display devices to prduce a decimal reading.

The logic Diagram and logic symbol of 7 Segment decoder is


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Engineering

THE (74LS47) BCD-TO-7 SEGMENT DECODER:The 74LS47 is an example of a decoder that decodes the BCD input and drivesthe 7 segment LED display to show out the decimal result. The 7 segment display driver i.e.

74LS47 has some additional features as by LT , RBI , RBOBI / functions in the logic

symbolas shown.

The bubbles shown in the logic symbol on the output pins (from a-to-g) can be defined

as active-LOW, as are the LT (Lamp Test), RBI (Ripple Blinking Input), RBOBI / (Blinking input/Ripple Blinking Output) functions. The IC can derive the common anode 7-segment Display directly.

LAMP TEST:When a LOW signal is applied to LT input, while RBOBI / is HIGH, then all of the

segments of 7-Segment Display turns on. Actually, Lamp Test is used to verify that Nosegments are burnt out.

ZERO SUPPRESSION:Zero Suppression is a feature used for multi-digit displays to blank out un-necessary

zeros. For example, in a 4 digit display, the number 2.7 may be displayed as 02.70 if the

zeros are not blanked out. Blanking the zeros at the front of a number is called LeadingZero Suppression and blanking the zeros at the back of the number is called Trailing ZeroSuppression.Note that only the unnecessary zeros are blanked out, the essential zeros will remain. For

zero suppression, the functions RBI , RBOBI / are used.

All of the segment outputs of the decoder are non-active (HIGH) if a zero code (0000) is

applied on its input and if RBI is LOW. This causes the display to be blank and produces a

LOW RBO .


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Engineering

While, for the trailing zero suppression, the lowest order digit (Right most) is always blankedif the Zero code is on its BCD inputs because the RBI is Connected to Ground. As shown in

logic diagram Shown below, the RBO of each decoder is connected to the of next highest-

order decoder so that all zeros to the right of the first non zero digit are blanked. In the figureShown below, the two lowest order digits are zeros and therefore are blanked. While, theremaining two digits are displayed.

7-SEGMENT LED DISPLAY:The illustration to the right shows the basic layout of the segments

in a seven-segment display. The segments themselves are identified

with lower-case letters "a" through "g," with segment "a" at the top andthen counting clockwise. Segment "g" is the center bar.Schematic Diagram and configuration of Common anode and CommonCathode displays are as follows.


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Engineering

CIRCUIT DIAGRAM:

PROCEDURE:

Construct the circuit as shown below.

Now close the dip switch and power on the voltage source.

Test the decoder for the Lamp test.

Apply different levels on the input of the decoder in the form of BCD.

Also try to obtain the output with zero suppression and without zerosuppression.

SIMULATION RESULTS:

LAMP TEST:


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Engineering

WITH ZERO SUPPRESSION:

WITHOUT ZERO SUPPRESSION:

OBSERVATIONS:We observe the exact output display at the 7-segment LED display as we apply the BCDinput.

For applying

0001 we get 1.0010 we get 2.0011 we get 3.0100 we get 4.0101 we get 5.0110 we get 6.0111 we get 7.1000 we get 8.1001 we get 9.

CONCLUSION:


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The output result, with 7-segment common anode display, of 7-segment decoder isverified as according to its input.

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12) decoderr prac 12

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