The Complete Handbook of Circuit Decoders & Encoders

Chapter 1: Understanding Circuit Decoders & Encoders

A circuit decoder is a logic circuit that takes a binary code on its inputs and activates one, and only one, of its many outputs based on that code. The encoder does the opposite: it takes an active signal on one of its inputs and produces a binary code on its outputs corresponding to that input. At their heart, these are the circuits that allow a digital system to understand its own binary language well enough to take action in the physical world, or to translate a physical action into a code it can process. Without them, a microcontroller would be unable to select a specific memory chip, a keyboard would be a bundle of confused wires, and a robotic arm would have no idea what angle it was pointing at.

Consider the most classic example of a decoder: the 74LS138. This is a 3-to-8 line decoder. It takes a three-bit binary number as input. Because a three-bit number can represent eight distinct values (from 000 to 111), the 74LS138 has eight outputs, usually labeled Y0 through Y7. When you apply the binary code "010" to its inputs, the output Y2 goes low, and all other outputs remain high. The circuit is literally decoding the binary number "2" and "selecting" output number 2. This is why decoders are also frequently called "demultiplexers" in certain configurations; they can route a single data signal to one of several destinations based on the address presented to