Encoder#

An encoder is the reverse of a decoder: it takes 2ⁿ inputs, one of them active, and produces the n-bit binary code that names which one. Eight input lines become a 3-bit number, sixteen become a 4-bit number. Where a decoder expands a code into one-hot, an encoder compresses one-hot back into a code.

The Problem of More Than One#

A plain encoder assumes exactly one input is active. If two are high at once, the naïve output is meaningless — it is neither input’s code. Real inputs do not cooperate, so the useful part is the priority encoder: when several inputs are active, it outputs the code of the highest-priority one and ignores the rest, usually flagging separately whether any input was active at all. The standard parts are the 74148 (8-to-3 priority) and 74147 (10-line-to-4-line BCD priority).

Structurally each output bit is an OR across the input lines whose codes have that bit set, with the priority logic layered on top to suppress lower inputs when a higher one is present.

Where It’s Used#

The archetypal use is interrupt handling: many devices each assert their own request line, and a priority encoder turns that set of lines into the binary vector of the most urgent one for the processor to service — resolving simultaneous requests by priority in the process. The same shape appears in keypad and position encoding, in finding the highest set bit of a word, and, in general, wherever a one-hot signal has to be converted back to a compact binary index. Encoder and decoder are the round trip between “which one” as a code and “which one” as a single active line.