Page 61 - Digital Electronics by harish

Decimal input BCD outputs
switch pressed A B C D
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1

For example, when switch „3‟ is pressed, only the OR gates C and D get high input, and
hence the output ABCD will be 0011. Similarly when switch „9‟ is pressed, only the OR
gates A and D get high input and the output will be 1001.

The Decimal to BCD encoder is also available in IC form. The IC number is 74147. It has 16
pins.

2.10 Decoder

Decoder is a logic circuit which converts binary data in one coded form to another coded
form. Decoders have less number of input lines and more number of output lines. Decoders
have no data input but they have only control inputs.

The control input lines are also called address lines. Any one of the output lines will be
enabled depends upon the selection made in the address lines.

3-to-8 Decoder

The 3-to-8 decoder has eight output lines and three control input lines. Any one of the output
will go high (ie. logic „1‟) depends on the conditions of the control lines. It is also called 1 of
8 decoder. The logic diagram and truth table of 3-to8 decoder are given in figure.

61