Abstract: In a modular tower building block system for containing computing system devices, an I/O bus is incorporated into the modular blocks of the building block system by using a printed circuit board to carry the I/O bus in each modular block. The printed circuit board is mounted and positioned in each modular block to electrically connect with a printed circuit board in a next adjacent modular block when two modular blocks are stacked on each other. Also, there are a plurality of I/O buses on the printed circuit boards and only one I/O bus is distributed from each modular block. The printed circuit board is precisely located in each modular block at a predetermined position. Alignment pins and receivers provide alignment between stacked modular blocks to precisely position one modular block to the other modular block. This also aligns electrical connectors on the printed circuit boards of the stacked blocks so that the connectors on the printed circuit boards from two blocks mate when the two blocks are stacked.