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9.5: Encoder

  • Page ID
    956
  • [ "article:topic", "authorname:tkuphaldt", "license:gnu" ]

    What is an 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.

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

    14168.png

    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.

    Encoder Design Applications

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

    14169.png

    Our truth table is:

    14170.png

    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.

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    Equation Collection Summary

    The collection of equations is summarized here:

    14171.png

    The Resulting Circuit Diagram

    The circuit is:

    04464.png

    The Resulting Ladder Diagram

    And the corresponding ladder diagram:

    04483 (1).png