PC/104 is a way to embed the PC architecture in applications with space and power restrictions. The PC/104 standard defines an ultra-compact, modular, and stackable version of the ISA bus. The PC/104 form factor features full software compatibility with the PC bus. PC/104 cards can be used as a standalone SBC system or a mezzanine bus expansion card.

Just as system buses developed from the ISA bus to the PCI bus, the PC/104 standards have kept pace by developing different types of cards. The original version is the PC/104 (PC bus, 104 pins). It defines a modular version of the ISA bus with a 64-pin connector and an adjacent 40-pin connector.

PC/104-Plus (PC/104 plus a PCI-bus connector) followed the progression from ISA to PCI bus. It added a 120-pin connector, similar to the 32-bit PCI connector, opposite the PC/104 ISA connector. The connector may be 5V, 3.3V, or Universal keyed. As with PCI, care must be taken to connect cards only to systems that support their signal voltages.

With space at a premium, the next logical progression was to set a standard for a PC/104 card with only the 120-pin PCI connector, and no ISA connectors, namely the PCI-104 standard.

As the cards are designed to be stacked without a backplane or card cage, each bus type has a choice of bus options -- stackthrough, where the bus pins extend through the card to facilitate stacking, and a slimmer non-stackthrough version. 8-bit, 16-bit, and 32-bit modules can be combines in a stack, provided that similar types of cards are adjacent to each other.

PC/104 cards measure 90 x 96mm (3.6" x 3.8") and are stacked 15mm (0.6") apart.