Logisim-evolution Troubleshooting

Quick answers to the problems students hit most often. Each section starts with the fix; details follow.

What do the wire colors mean?

Dark green = 0, light green = 1, blue = unknown, gray = not connected to anything, red = error (conflicting values), orange = bit-width mismatch. When you finish wiring, every wire should be green. Blue or gray wires mean something is not connected yet — that is normal mid-build but not in a finished circuit.

Why are my wires red?

Red means two things are fighting over the same wire. The usual causes: two outputs connected together, or a subcircuit pin left floating while its parent drives it. Fix: find the junction where the red starts, check for double-driven outputs, and make sure every subcircuit pin is either wired or deliberately tied off. Cornell’s CS3410 lab puts it bluntly: a finished circuit should have no red wires.

Why are my wires blue?

Blue means the value is unknown — almost always an unconnected input somewhere upstream. Trace back from the blue wire to the nearest input pin or gate input that has nothing attached. Also try Simulate → Reset Simulation (Ctrl-R): after resets and edits, stale unknown values sometimes linger until you reset.

Why is my clock not ticking?

Because automatic ticking is off by default. Check three things in order: (1) your circuit actually contains a Clock component (Wiring library); (2) Simulate → Ticks Enabled (Ctrl-K) is checked; (3) the tick frequency (Simulate → Auto Tick Frequency) is not absurdly slow. For single steps use Ctrl-T (half cycle) or F2 (full cycle).

Why does my sequential circuit start in the wrong state?

Memory elements (flip-flops, registers, RAM) power up as 0 by default. Real hardware powers up unpredictably. If your design assumes unknown startup values, enable Project → Options… → Simulation → “Memory elements start in unknown/random state (rather than 0)” to test honestly — this is documented in the official docs’ limitations section.

Logisim-evolution will not start / “requires a Java Runtime Environment”

The portable .jar needs Java 21+; the native .msi/.dmg installers bundle their own Java and need nothing. If you downloaded the JAR, install Eclipse Temurin OpenJDK 21, then run java -jar logisim-evolution-5.0.0-all.jar. An UnsupportedClassVersionError means an old Java (8/11) is first on your PATH — check with java -version. Platform guides: Windows · macOS · Linux.

Still stuck?

Ask on the project’s GitHub issue tracker with your .circ file attached. And when you move from simulation to a physical breadboard, a digital multimeter is the fastest way to check what a real circuit is doing.

As an affiliate partner, we may earn a commission on qualifying purchases — at no extra cost to you.

Related tutorials: getting started · subcircuits · all tutorials