CPU board, power supply board, and one or more peripheral boards. The maximum number of boards supported by a PC/104 stack will depend on the which buses are used by the peripheral boards. ISA Bus - There is no strict limit to the number of ISA boards which can coexist in one system. However, there is a limited number of Base Addresses, IRQs, and DMA channels which may become a limiting factor. ISA boards may be stacked on either side of the CPU board. PCI Bus - The PC/104-Plus and PCI-104 specifications allows for four PCI "slots". This imposes a hard limit of four PCI peripheral boards per system. All PCI peripheral modules must connect consecutively on one side of the controller due to the signaling requirements of the PCI bus. PCI Express - The total number of PCI Express peripheral boards will depend on the number of PCIe links provided by the CPU board. For example, if the CPU board provides four x1 PCIe links, then a maximum of four x1 PCIe peripheral cards may be installed. If one or more of the peripheral boards provide PCIe link repopulation, additional modules may be installed. PCIe links on the upper side of the CPU board are independent from the links on the underside. Installing a PCIe peripheral under the CPU board will not consume one of the top side links. The quantity and width of PCIe links available may vary between the top and bottom connectors of the CPU board. USB and SATA - The PCI/104-Express and PCIe/104 Specification provides signals for USB and SATA, which may be used by peripheral boards in the stack. Peripheral which use USB and SATA will be limited to the number of links provided by the CPU board. USB peripherals may provide link repopulation by incorporating an onboard USB hub. Regardless of the buses used, the maximum number of boards of a PC/104 stack may be limited due to size, weight, and power restrictions for the target application. Mechanical interference between boards When stacking PC/104 boards together, mechanical interference between adjacent boards is a concern. Bus Connectors - When assembling a system composed of multiple Bus Structures, it is possible for the bus connectors to interfere with components on an adjacent board. For example, if a PC/104-Plus board is stacked on top of a PC/104 board, it is possible for the pins on the