Minecraft wiki entry for Redstone_computers, associated text: There are five busses in the CPU, each to carry information from one component to the next. Busses are channels of redstone connecting each component. Since we are building a 4-bit computer, we only need four channels in our bus. These are the red and blue lines connecting the components inside the CPU. Notice that the blue buses have less than four lines, this is because they do not carry data. Since busses can only carry data one way (in Minecraft, due to repeaters only working one way), there are two buses connecting the CPU to the outer computer.