Page 62 - Digital Electronics by harish

Control Input Output
C B A Y 7 Y 6 Y 5 Y 4 Y 3 Y 2 Y 1 Y 0
0 0 0 0 0 0 0 0 0 0 1
0 0 1 0 0 0 0 0 0 1 0
0 1 0 0 0 0 0 0 1 0 0
0 1 1 0 0 0 0 1 0 0 0
1 0 0 0 0 0 1 0 0 0 0
1 0 1 0 0 1 0 0 0 0 0
1 1 0 0 1 0 0 0 0 0 0
1 1 1 1 0 0 0 0 0 0 0

In the logic diagram, the AND gates are numbered from 0 to 9. When the control input lines
th
CBA are 000, all the inputs of 0 AND gate are 1. Hence, Y 0 will be „1‟. But all other
AND gates will have at least one input as „0‟. Hence, their outputs are „0‟. When the control
st
input lines CBA are 001, all the inputs A of 1 AND gate are 1. Hence, Y 1 will be „1‟. But
all other AND gates will have atleast one input as „0‟. Hence, their outputs are „0‟.

Similarly, the outputs for all other control input combinations can be found. The 3-to-8
decoder is available in IC form also. The IC number is 74138.

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