Decoder Tutorial for Logisim-evolution
A decoder turns an n-bit binary code into 2n outputs, with exactly one output active at a time. It answers the question “which input combination is this?” — the basis of address decoding in memories and display drivers.
Using the built-in Decoder
Open the Plexers library and place a Decoder. The default is a 2-to-4 decoder: a 2-bit select input on the west side, four outputs on the east. Wire a 2-bit input pin to the select input and four LEDs to the outputs. With the Poke tool, step the input through 00, 01, 10, 11: exactly one LED lights for each code.
Example: labeled outputs
The LED demo above is already a complete example. Use the Text tool (the “A” icon in the toolbar) to label each LED with the code that activates it, so the circuit documents itself.
Example: driving a 7-segment display
The I/O library includes a 7-Segment Display. A classic classroom exercise: take a 4-bit input, work out which digit (0–9) it represents, and light the right segments. The clean way to build it is to write the segment patterns as a truth table — one output column per segment a–g — then choose Project → Analyze Circuit and press Build Circuit to generate the segment logic. See Combinational Analysis for the full workflow.
Decoder vs. demultiplexer
They look similar but differ in one thing: a decoder has no data input — the select code itself decides which output goes high. A demultiplexer (also in the Plexers library) takes one data input and routes it to the selected output. Use a decoder when the code is the message; use a demultiplexer when you need to steer a signal.