PC/104 and PC/104Plus are embedded computer standards controlled by the PC/104 Consortium which defines both a form factor and computer bus.

The difference between PC/104 and PC/104Plus is that PC/104 supports the ISA bus interface whereas the later PC/104Plus standard supports the PCI bus interface.

The idea of PC/104 is to build a system by 'Stacking' boards one on top of the other. Unlike the popular ATX form factor which utilizes the PCI bus and is currently used for most PCs, the PC/104 form factor has no backplane, and instead allows modules to stack together like building blocks. The stacking of buses is naturally more rugged than typical PCs. The boards are stacked using mounting-holes in the corner of each module which allow the boards to be fastened to each other with standoffs.

Eltech offer a range of Multi-Function PC/104 CPU boards which provide the main components such as processor, memory, LAN, Serial Interfaces, etc.. Applications that require additional I/O such as Analogue Inputs, GPS, etc can then add PC/104 boards supporting the relevant interface by plugging the board on to the PC/104 stack.
The standard size of boards complying to the form factor is 3.55 x 3.775 inches (90.17 x 95.89 mm), while the height is typically constrained to the boundaries of the connectors.