Logic Diagram for 4-input priority encoder 13.K-Maps for 4-input Priority Encoder 12.Input D3 has the highest priority, so regardless of the values of the other inputs, when this input is 1, the output for xy is 11 (binary 3) 11 The higher the subscript number, the higher the priority of the input.X’s in the output column indicate don’t care conditions, the X’s in the input columns are useful for representing a truth table in condensed form.If all inputs are 0, there is no valid input and V is equal to 0. In addition to two outputs x, and y, the truth table has a third output designated by V, which is a valid bit indicator that is set 1 when one or more inputs are equal to 1.Truth Table of a 4-input Priority Encoder: Inputs D0 D1 D2 D3 0 0 0 0 1 0 0 0 X 1 0 0 X X 1 0 X X X 1.If two or more inputs are equal to 1 at the same time, the input having the highest priority will take precedence.A priority encoder is an encoder that includes the priority function.In that case, only A7 will be encoded and the output will be 111. Let us also assume that input lines A2, A4 and A7 are all simultaneously in logic ‘0’ state. Let us assume that the octal-to-binary encoder described in the previous slide has an input priority for higher-order digits. In this type of encoder, a priority is assigned to each input so that, when more than one input is simultaneously active, the input with the highest priority is encoded. Priority Encoder The encoders available in IC form are all priority encoders. Logic circuit for octal-to binary encoder.Ao’ is not connected to the logic gates because the encoder outputs always be normally at 000 when none of the inputs is LOW 7
For instance, a low at A3’ will produce O2 =0, O1=1 and O0 =1, which is binary code for 3. A low at any single input will produce the output binary code corresponding to that input.Truth table for octal-to binary encoder.This is a basic 10-line-to-4-line encoder. The Decimal-to-BCD Encoder This type of encoder has 10 inputs (one for each decimal digit) and four outputs (corresponding to the BCD code). An encoder is a combinational logic circuit that essentially performs a “reverse” decoder function.