Code Converters in Logisim-evolution

Different parts of a digital system speak different codes: BCD for displays, Gray code for encoders, Excess-3 for old-school arithmetic. A code converter translates one into the other — pure combinational logic, and a standard lab exercise.

BCD to Excess-3 converter

Step 1 — Write the truth table. Excess-3 is just BCD + 3. For each 4-bit BCD input (0000–1001), the output is the 4-bit binary of (input + 3). Inputs 1010–1111 are don’t-cares.

Step 2 — Let Logisim-evolution build it. Go to Project → Analyze Circuit, fill in the truth table on the Table tab, then click Build Circuit. Logisim-evolution minimizes the logic and drops a finished converter subcircuit onto your canvas. See our Combinational Analysis tutorial for the full workflow.

Step 3 (optional) — Build it by hand. Derive each output bit with Karnaugh maps and wire it from AND/OR/NOT gates. More work, but this is usually what the lab report asks for.

Binary to Gray code converter

Gray code changes only one bit between consecutive values — handy for rotary encoders and clock-domain crossings. The conversion is beautifully simple: the MSB passes through unchanged, and each lower Gray bit is the XOR of two adjacent binary bits. Chain three XOR gates (Gates library) and you are done. Verify with the truth table view.

Go further

Feed your converter into a 7-segment display, or pair it with an encoder on the input side.