Answer to Question #244768 in Electrical Engineering for shenyue

Question

•  Using the 74LS148 IC, DIP switches and 3 LEDs, draw and construct the 8-to-3 line priority encoder. Record the outputs to complete the truth table below

Answer 1

Unlike a multiplexer that selects one individual data input line and then sends that data to a single output line or switch, a Digital Encoder more commonly called a Binary Encoder  takes ALL its data inputs one at a time and then converts them into a single encoded output. So we can say that a binary encoder, is a multi-input combinational logic circuit that converts the logic level “1” data at its inputs into an equivalent binary code at its output.

Generally, digital encoders produce outputs of 2-bit, 3-bit or 4-bit codes depending upon the number of data input lines. An “n-bit” binary encoder has 2n input lines and n-bit output lines with common types that include 4-to-2, 8-to-3 and 16-to-4 line configurations.

The output lines of a digital encoder generate the binary equivalent of the input line whose value is equal to “1” and are available to encode either a decimal or hexadecimal input pattern to typically a binary or “B.C.D” (binary coded decimal) output code.

One of the main disadvantages of standard digital encoders is that they can generate the wrong output code when there is more than one input present at logic level “1”. For example, if we make inputs D1 and D2 HIGH at logic “1” both at the same time, the resulting output is neither at “01” or at “10” but will be at “11” which is an output binary number that is different to the actual input present. Also, an output code of all logic “0”s can be generated when all of its inputs are at “0” OR when input D0 is equal to one.

One simple way to overcome this problem is to “Prioritise” the level of each input pin and if there was more than one input at logic level “1” the actual output code would only correspond to the input with the highest designated priority. Then this type of digital encoder is known commonly as a Priority Encoder  or P-encoder for short.