Stacks
A system composed of PC/104, PC/104-Plus, or PCI-104 modules is often referred to as a "stack". Although many stacks include modules which are all the same form factor, it is not uncommon to find PC/104 modules in a stack with PC/104-Plus modules.
Each stack must contain at least one motherboard or CPU, which acts as a controller for the peripheral components. The motherboard is often referred to as a single-board computer (SBC), for it often has interfaces for all standard PC components (i.e. keyboard, mouse, serial ports, etc.). This controller must support the signaling buses used on all add-in modules. It's possible, however, that a peripheral card may perform a stand alone function without requiring a separate motherboard to control it.
There is no strict limit to the number of PC/104 cards which can coexist in one system. However, as more modules are added, the stack height increases, and signaling requirements may not be maintained. A PC/104 stack will usually have a motherboard controller which is also PC/104. Peripheral PC/104 cards can reside on either side of the CPU.
A stack which has any PC/104-Plus modules must be controlled by a PC/104-Plus motherboard controller. Not counting the PC/104-Plus controller, the number of PC/104-Plus peripheral cards in a stack may not exceed four module slices. This is due to the PCI specification, which allows four PCI components in a system. (More PCI devices may be added if a bridge device is used.) The same rule applies to PCI-104 stacks.
When the PCI bus connector is used (PC/104-Plus or PCI-104 modules), all peripheral PC/104-Plus modules must connect consecutively on one side of the controller due to the signaling requirements of the PCI bus. Each card with a PCI bus should include a mechanism to assign its position in reference to the controller. Note that this is not required for traditional backplane motherboards, because a card "knows" which slot it is in. A PC/104-Plus or PCI-104 system may also have PC/104 cards, which may be positioned on either side of the CPU farthest away from the PC/104-Plus card(s) (so the PCI bus is not broken).
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