What Is PC/104? The PC104 Form Factor and Stacking Standard Explained

The PC/104 form factor is a standard for small embedded computer boards that stack on top of each other instead of plugging into a backplane. Each board measures 3.550 × 3.775 in (90 × 96 mm), carries the same four corner mounting holes and connects to its neighbours through pin-and-socket bus connectors that pass every signal up the stack. A single board computer forms the base, I/O modules and a DC/DC power supply bolt on above it on 0.6-inch stand-offs, and the result is a self-contained computer that fits in the hand and tolerates the shock and vibration that would shake a card out of a slot. The name comes from the 104 pins of the original ISA bus connector, and the format has been a published standard since 1992.

The reason it persists is that the PC/104 form factor solves a problem boxed computers do not. It lets a designer put exactly the processor, I/O and power the application needs into a package sized for a vehicle chassis, an avionics bay or a piece of test equipment, and then change any single layer years later without redesigning the rest. This guide explains the form factor rules, how a stack is built, where the standard came from, what goes in a stack, where PC/104 is used, how it compares with COM Express and 3.5-inch boards, and how to carry a legacy stack forward.

A PC/104 board is defined by a handful of mechanical and electrical rules. Meet them and a board stacks with any other board that uses the same bus; break one and it may not fit, or may fit and not talk. The table sets out the rules a designer or buyer checks.

The PCI Express connector is surface-mounted, which lets a board use a different connector type on its top and bottom sides. That flexibility, and the details of Type 1, Type 2 and OneBank connectors, are covered in our guide to PCIe/104 and the PC/104 bus variants.

A PC/104 stack starts with a host board, usually a single board computer with the processor, memory, storage interfaces and a set of on-board I/O. Its bus connector has pins on the top and sockets on the bottom, so a module placed above it plugs straight in and presents the same connector again to the next board. Four stand-offs at the corners take the mechanical load, and the specification fixes the spacing so that every board, heat sink and tall component is designed to fit inside the same layer height.

Modules add what the host lacks: analogue input for sensors, digital I/O for switches and relays, serial ports for legacy equipment, CAN bus for vehicle networks, extra Ethernet ports, or a managed switch. A DC/DC power supply on the same footprint converts a vehicle or battery input into the regulated rails the stack needs and delivers them through the bus connector, so there is no power harness inside the enclosure. A typical stack is three to five boards; larger ones exist, limited by bus loading and by the height of the enclosure.

Cooling is almost always passive. Boards are specified for wide temperature operation, and conduction-cooled variants add a heat spreader that carries processor heat to the enclosure wall. The finished stack is usually bolted into a sealed aluminium housing with the connectors brought out through MIL-DTL-38999 or M12 connectors, at which point it becomes a rugged mission computer rather than a set of boards.

PC/104 began in 1987 as a compact, stackable version of the IBM PC bus, devised by Ampro, and became an open standard when the PC/104 Consortium formed in 1992 to publish it. Each new bus generation since has kept the footprint and added a connector, which is why the family reads as a sequence rather than a set of competing standards.

The larger EPIC and EBX board formats, and their Express versions, use the same stack connectors so that PC/104 modules can be added to a bigger host board. The years above are those generally cited for each release; the consortium’s own documents are the authority.

Yes. The format is more than thirty years old, and newer standards exist for most of the things it does. It stays in use because five properties come together in one package.

• Rugged by construction. Pin-and-socket connectors and corner stand-offs hold a stack together under vibration in a way that card-edge slots do not. There is nothing to work loose.
• Small. A complete computer with I/O and power occupies roughly the footprint of a hand and grows only in height as modules are added, which matters wherever size, weight and power are constrained.
• Modular. Any layer can be replaced or added without touching the others, which is how a stack designed a decade ago can gain a newer processor board or a 10G switch board.
• Interoperable. Hundreds of modules from many manufacturers are built to the same specification, so a designer is not tied to one supplier for every board in the stack.
• PC-based. Most host boards use x86 processors and run standard Linux or Windows, so drivers, tools and application software carry over from desktop development.

The trade-off is bandwidth. The original ISA and PCI buses are slow by modern standards, and PCIe/104 is limited to the PCI Express generations its specification defines. Where an application needs the fastest interconnect available, a COM Express or COM-HPC module on a custom carrier is the better fit; where it needs rugged, expandable I/O in a small space, PC/104 remains a strong choice.

The host board sets the bus and the processor. PC/104 single board computers range from low-power Vortex86 and AMD Geode boards on the ISA bus, which keep long-running programmes supplied, to PCIe/104 and PCI/104-Express boards with Intel Atom and 11th Gen Intel Core processors for current designs. Some carry a COM Express module on a PC/104 carrier, so the processor can be upgraded by swapping the module while the carrier and its I/O stay in service. Many include data acquisition on the board itself.

PC/104 I/O modules add the channels the host lacks: 16-bit analogue input and output with autocalibration, digital I/O with counters and timers, optoisolated inputs and relay outputs, RS-232/422/485 serial ports, isolated CAN bus channels and extra Ethernet ports. PC/104 power supplies convert a wide vehicle or battery input to regulated rails, with versions that add a UPS and battery charger or filtering to the requirements of MIL-STD-461, MIL-STD-704 and MIL-STD-1275. Where the system needs networking, a managed Ethernet switch board on the same footprint adds Gigabit and 10G ports; our guide to board-level switch modules explains that option.

Storage and wireless usually go on the host board’s own mini PCIe or M.2 sockets rather than on a stack layer, which keeps the stack short and the antennas close to the enclosure wall.

The format appears wherever a computer has to be small, tolerant of its surroundings and in service for a long time. Military ground vehicles are the largest single market: a PC/104 stack with a MIL-STD power supply sits behind a display, runs a sensor or controls a turret, and stays in the fleet for the vehicle’s lifetime. Aircraft and helicopter avionics use it for mission and data-recording computers where weight and size are constrained. Unmanned aircraft, ground vehicles and underwater vehicles use it for flight and payload computers for the same reasons.

Outside defence, PC/104 is common in railway and transport equipment, in industrial machinery and process control, in test and measurement rigs that need many analogue channels, and in research platforms where a custom I/O module is easier to design for a stack than for any other form factor. In each case the buying pattern is the same: a programme selects a stack architecture once and then supports it, with incremental upgrades, for ten years or more.

The finished stack often becomes one of our rugged mission computers, sealed and cabled for the platform.

Several board-level approaches cover most rugged embedded designs, and they answer different questions.

COM Express separates the processor from the I/O. The computer-on-module carries the CPU, memory and chipset; the designer builds a custom carrier board with exactly the connectors the product needs. It gives the most freedom and the highest bandwidth, at the cost of designing and qualifying a carrier. Our COM Express range, the COM-HPC vs COM Express guide and our comparison of SMARC, Qseven, OSM and COM Express cover that route.

3.5-inch single board computers put a processor and a fixed set of I/O on one board, ready to run with no carrier and no stack. They are the fastest route to a working system when the on-board I/O is enough. The 3.5-inch SBC range includes boards with a PCIe/104 connector, so a 3.5-inch host can still take PC/104 modules.

PC/104 sits between the two. There is no carrier to design, but the I/O is not fixed either: the stack grows to fit the application, and any layer can be changed later. The hybrids show how close the approaches have become. A PCI/104-Express board that carries a COM Express Type 6 module gives an upgradeable processor on a stackable base, and a 3.5-inch board with PCIe/104 expansion gives fixed I/O plus a stack.

Many PC/104 systems in service were designed around ISA I/O modules and processors that are long out of production. The format’s modularity is what makes them maintainable: the host and modules can usually be replaced one at a time, provided the bus is respected. The table sets out the common routes.

Software is usually the larger task. ISA modules rely on fixed addresses and interrupts, so a new host board needs those resources available, and drivers written for an old operating system may need porting. Diamond Systems states that its GPIO-MM-XT digital I/O module keeps the register maps of the older boards it replaces, which is the kind of detail worth checking before any migration.

PC/104 has been an open, multi-vendor standard since the PC/104 Consortium was formed in 1992. On 16 October 2025 the consortium renamed itself the RMS Consortium, for Rugged, Modular, Stackable, to reflect a remit that now extends beyond the PC/104 family. It continues to publish the PC/104, PC/104-Plus, PCI-104, PCIe/104 and PCI/104-Express specifications, along with the larger EPIC and EBX board formats that use the same stack connectors, and it has a new specification, UxV/35, in development for unmanned systems. The PC/104 specification is at version 2.6, and version 3.0 of the PCI/104-Express and PCIe/104 specification was published in 2015.

For a buyer, the practical consequence is that the mechanical and electrical rules are public and stable. A board that meets the specification from one vendor stacks with a board from another, and a module designed today fits a host built fifteen years ago, provided the bus matches.

PC/104 is the right starting point when an application needs a small, rugged computer with I/O that can be specified now and changed later, and when the programme will run long enough for that flexibility to pay back. Where the deciding factor is raw bandwidth or the smallest possible footprint with fixed I/O, another form factor fits better.

We supply the full PC/104 range: host boards on every bus generation, the I/O modules and power supplies that stack on them, and switch boards for the network. We start from the application rather than a part number, recommend a host board and bus generation that fit the I/O and the environment, and build, image and validate the complete stack before it ships. Where the requirement points elsewhere, we say so and suggest a rugged single board computer or a COM Express design instead.