underside of the PCI connector to run into components on the lower board. Out of Spec Components and Connectors - The PC/104 specifications limit the height of components and connectors that may be placed on either side of the board. A constrained height region is intended to guarantee that boards will not interfere with other neighbors stacked above or below. However, it is not uncommon to find boards which violate these constraints. Heatsinks - The standard spacing between stacked PC/104 boards is 0.600 inches (15.24 mm). Components with significant power consumption (CPUs, GPUs, FPGAs) often require a larger heatsink which does not fit within the traditional board spacing. It is not uncommon to find PC/104 CPU boards with relatively large heatsinks that do not allow for peripheral boards to be stacked above it. Later revisions of the PCI/104-Express and PCIe/104 specification introduced an optional taller 0.866 inch (22.00 mm) connector that is compatible with the traditional height connector. The mechanical interference issues listed above can often be addressed with a Bus Spacer, which allows additional room between the boards. However, Bus Spacers increase overall stack height, and may not be suitable for space-constrained applications. It may also be possible to re-arrange the boards in the stack to remove the interference. Another option is to modify the offending boards to remove the interference (e.g. depopulate a connector), but this may require the vendor to supply a customized version of the board. Potential compatibility issues In theory, PC/104 boards are interoperable. It is possible to assemble a system using boards from several different vendors, subject to the fundamental Bus Structure compatibility issues listed above. However, compatibility issues sometimes appear. PC/104 Keying Pins the PC/104 specification defines two keying pins to prevent misalignment of the connector. These pins are not to be populated on the connector. However, some vendors do not utilize keyed PC/104 connectors. The may cause issues when using boards from multiple vendors. It is often possible to modify the non-keyed board by clipping the offending pins. Required Power Supply Voltages the PC/104 buses provide for several supply voltages (+5V, +3.3V, +12V, etc.). Which voltages are actually used by a board is at the discretion of the board manufacturer. Care must be taken to supply all of the voltages required for the boards in the stack. Power Supply Contention some boards will provide power on the stack (e.g. a CPU board).