Minecraft wiki entry for Redstone_computers, associated text: Above is a simple 2-bit state, so it has two inputs (left and right) through the repeaters. The output is the redstone line above which will turn OFF when the state is met. The state is whether the redstone input will turn OFF the redstone line above; if so, the state is the redstone inputs. In the above case, the left must be turned OFF (0) and the right (blue) must be turned ON (1) to yield an OFF on the top redstone line. So it expects a state of OFF ON (aka 01 for binary).