FB pixel

Encoder

An encoder is a circuit that changes a set of signals into a code. Let's begin making a 2-to-1 line encoder truth table by reversing the 1-to-2 decoder truth table.


reversing the 1-to-2 decoder truth table

This truth table is a little short. A complete truth table would be


complete reversing 1-to-2 decoder truth table

One question we need to answer is what to do with those other inputs? Do we ignore them? Do we have them generate an additional error output? In many circuits this problem is solved by adding sequential logic in order to know not just what input is active but also which order the inputs became active.



A more useful application of combinational encoder design is a binary to 7-segment encoder. The seven segments are given according


7-segment encoder

Our truth table is:


7-segment encoder truth table

Deciding what to do with the remaining six entries of the truth table is easier with this circuit. This circuit should not be expected to encode an undefined combination of inputs, so we can leave them as "don't care" when we design the circuit. The equations were simplified with karnaugh maps.


remaining six entries of the truth table
remaining six entries of the truth table
remaining six entries of the truth table

The collection of equations is summarised here:


collection of equations

The circuit is:


circuit diagram

And the corresponding ladder diagram:

ladder diagram
 
Lessons In Electric Circuits copyright (C) 2000-2020 Tony R. Kuphaldt, under the terms and conditions of the CC BY License.

See the Design Science License (Appendix 3) for details regarding copying and distribution.

Revised November 06, 2021

 
Use left and right arrow keys to change pagesUse left and right arrow keys to change pages.
Swipe left and right to change pages.\Swipe left and right to change pages.
Make Bread with our CircuitBread Toaster!

Get the latest tools and tutorials, fresh from the toaster.

What are you looking for?