Page 93 - Handout Digital Electronics

Basic encoder logic circuit

A3 A2 A1 A0 F1 F0
0 0 0 1 0 0
0 0 1 0 0 1
0 1 0 0 1 0
1 1 0 0 1 1

The operation of the above logic is such that the active input line number is output, for example when
A0 = 1, then F0 = 0 and F1 = 0. The logic circuit below illustrates this point:

Figure 26: Basic encoder operation
Consider an 8 to 3 encoder truth table below:

X7 X6 X5 X4 X3 X2 X1 X0 Y2 Y1 Yo
0 0 0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 1 0 0 0 1
0 0 0 0 0 1 0 0 0 1 0
0 0 0 0 1 0 0 0 0 1 1
0 0 0 1 0 0 0 0 1 0 0
0 0 1 0 0 0 0 0 1 0 1
0 1 0 0 0 0 0 0 1 1 0
1 0 0 0 0 0 0 0 1 1 1
From the truth table, the logic expressions for Y0, Y1 and Y2 are:

YO = X1 + X 3 + X 5 + X 7
Y1 = X 2 + X 3 + X 6 + X 7
Y 2 = X 4 + X 5 + X 6 + X 7

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