What is an encoder? Draw the logic circuit of Decimal to BCD encoder and explain its working.
Ans. Encoder: It is a combinational logic circuit which converts alphanumeric characters in Binary codes. It has 2n (or less) input lines and n output lines. It may be Decimal to Binary or Hexadecimal to Binary or Octal to BCD etc.
Decimal to BCD Encoder: It has 10 inputs (for decimal numbers 0 to p) and 4 outputs for the BCD number. Therefore this is a 10 line to 4 line encoder. Table (a) shows the decimal digits and the equivalent BCD numbers. By this table, we can get the relationship between decimal digit and BCD bit. MSB of BCD bit is Y3. For decimal digits 8 or 9, Y3 = 1. Hence we can write OR expression for Y3 bit as Y3 = 8 + 9
Likewise, Bit Y2 is 1 for decimal digits 4,5,6 and 7. Therefore we can write OR expression
Y2 = 4 + 5 + 6 + 7
Y1 = 2 + 3 + 6 + 7
Yo = 1 + 3 + 5 + 7 + 9
The logic circuit for the expressions (Y0, Y1, Y2, Y3) is demonstrated in fig. (b). As a High shows on any of input lines the equivalent OR gates provide the BCD output. For example, if decimal input is 8, High shows only on output 3 (and LOW on Y0, Y1 , Y2), hence giving the BCD code for decimal 8 as 1000. Likewise, if decimal input is 7, then High shows on outputs Y0, Y1, Y2 (and LOW on Y3), hence providing BCD output as 0111.
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Decimal
Digit
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BCD Code
Y3 Y2 Y1 Y0
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0
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0 0 0 0
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1
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0 0 0 1
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2
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0 0 1 0
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3
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0 0 1 1
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4
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0 1 0 0
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5
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0 1 0 1
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6
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0 1 1 0
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7
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0 1 1 1
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8
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1 0 0 0
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9
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1 0 0 1
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Truth Table of Decimal to BCD encoder
Fig.(b) Logic diagram for Decimal to BCD Encoder