PC/104 Bus Variants Explained: PC/104, PC/104-Plus, PCI-104, PCIe/104 and PCI/104-Express

PCIe/104 is the PCI Express generation of the PC/104 embedded board standard: boards stack through a surface-mounted connector that carries PCI Express links, USB and, on Type 2, SATA, without a backplane. It is one of five bus variants that share the same board footprint. The variants differ only in which expansion connector sits on the board and which bus that connector carries: ISA on the original PC/104, ISA and PCI on PC/104-Plus, PCI alone on PCI-104, PCI Express on PCIe/104, and both PCI and PCI Express on PCI/104-Express. Because the footprint, the mounting holes and the 0.6-inch stack pitch never changed, a board from any generation fits the same stand-offs. What decides whether two boards can talk to each other is the bus.

Choosing the wrong variant is the most common mistake we see in PC/104 designs. An I/O module bought for the ISA connector will not work on a PCIe/104 host, and a PCIe/104 Type 1 module will not find its x16 link on a Type 2 host. This guide sets out what each variant carries, how Type 1, Type 2, Universal and OneBank connectors differ, where each bus is still the right choice and how the standard lets buses be mixed in one stack. For the form factor itself, its history and how it compares with COM Express, start with what PC/104 is and the PC104 form factor.

The years are those generally cited for each specification’s first release. Later revisions have added features, notably the Type 2 connector, without changing the footprint.

The rule is simple: a module needs its own bus to be present at its position in the stack, and only the host board creates buses. Passive connectors on a module simply pass a bus through to the next board. The matrix shows which module types a host can drive without a bridge.

A PC/104-Plus peripheral module talks on PCI only; its ISA connector is a pass-through, so it cannot be used on an ISA-only host. In the same way a PCI/104-Express peripheral talks on PCI Express only and needs a host that provides it.

The original PC/104 connector carries the 16-bit ISA bus of the early PC on 104 pins split across a 64-pin and a 40-pin header. It is slow, at a few megabytes per second, and it needs the address and interrupt settings that ISA always needed, but it is simple. A custom I/O module for an ISA stack can be designed with a handful of parts, which is why thousands of PC/104 modules exist and why research groups and defence programmes still build to it.

The RMS Consortium’s own description is that PC/104 and PC/104-Plus remain manufactured and in use in slower-speed systems that benefit from modularity and ruggedness. In practice that means host boards with Vortex86 or similar low-power x86 processors that still bring ISA out to the connector, running long-established software on programmes that will not be redesigned. Modern chipsets no longer include ISA, so current ISA hosts generate it through a bridge, which is worth testing with older modules. New designs should not start here, but existing ones can be kept supplied for years.

PC/104-Plus added a 120-pin PCI connector on the opposite edge of the board while keeping the ISA connector. PCI at 32 bits and 33 MHz raised the available bandwidth by an order of magnitude and made video capture, faster networking and communications modules practical. Bus loading limits the number of PCI modules in one stack, typically to four, and each module is assigned a slot position by a jumper or rotary switch so that its PCI signals do not collide with a neighbour’s.

PCI-104 removed the ISA connector and kept only PCI. It frees board area, lowers cost and simplifies routing, at the price of needing a host that provides PCI. It is the natural home for modules designed after ISA stopped mattering. A PCI-104 module stacks on a PC/104-Plus host, since the PCI connector is the same, but it cannot stack on an ISA-only PC/104 host.

PCIe/104 replaces the parallel buses with PCI Express and, with it, the bandwidth needed for 10G Ethernet, cameras, NVMe storage and accelerators. The connector is defined in two heights, 0.600 in and 0.866 in, so that the stack pitch of the older buses is preserved while taller heat sinks can be accommodated. The initial specification was adopted in 2008.

Two connector types share a common core. Both carry four x1 PCI Express links, two USB 2.0 ports, the ATX power control signals (+5 V standby, power supply on and power OK), the +3.3 V, +5 V and +12 V rails and an SMBus. Type 1 adds one x16 PCI Express link that can be configured as two x8 links or two x4 links, which suits graphics and the highest-bandwidth I/O. Type 2 replaces the x16 with two x4 PCI Express links and adds two USB 3.0 ports, two SATA ports, an LPC bus and an RTC battery signal, which suits storage and general-purpose I/O. Type 2 arrived with version 2.0 of the specification, so boards from before then are Type 1 or Universal even when they are not labelled as such.

A module built for one type will not find the links it expects on a host of the other, so the type is a selection decision, not a detail. The specification makes a mismatch safe rather than silent: the system is held in reset and does not boot, and no damage is done. Universal modules use only the shared x1 links and USB 2.0, so they work on either type. Because the connector is surface-mounted, a host can also fit a different type on each side, such as Type 1 below and Type 2 above.

The full PCIe/104 connector is built in three banks. The first bank carries the shared signals, the four x1 links, two USB 2.0 ports, power and control; the second and third carry the Type 1 or Type 2 signals. OneBank is a single-bank version of the connector that carries only that first bank. It takes less board area and suits small, low-power boards and modules that need nothing more than x1 links and USB 2.0.

OneBank boards share the first-bank signals with full PCIe/104 boards, so they can sit in the same stack, but a OneBank connector cannot pass the second and third banks on. Any module that needs the x16 or x4 links should therefore sit between the host and the first OneBank board. In our range the Diamond Systems Saturn SBC uses a top-side PCIe/104 OneBank socket with four PCIe x1 lanes and two USB 2.0, and the VNS776KL 3.5-inch board has a OneBank Plus socket that combines PCIe/104 and PCI-104 for up to three PCIe x1 modules and four PCI-104 modules. Confirm OneBank compatibility on each module’s datasheet before ordering.

PCI/104-Express carries the PCIe/104 connector and the 120-pin PCI connector on the same board. The specification puts it plainly: PCIe/104 is PCI/104-Express without the PCI bus. A PCI/104-Express host therefore accepts new PCI Express modules and existing PCI modules in one stack, with the host bridging PCI internally, and it is the usual choice for a programme moving from a PC/104-Plus or PCI-104 design that still has PCI modules worth keeping.

Neither PCIe/104 nor PCI/104-Express carries ISA. An ISA module in a stack of this generation needs a bridge module or a different host. Where a stack has no PCI modules at all, a PCIe/104 host saves the cost and board area of the PCI connector.

The host board defines which buses exist in the stack, and each module needs only its own bus to be present at its position. With that rule, a stack can combine generations as long as the host supplies every bus the modules use. A PCI/104-Express host supports PCIe/104 modules and PCI-104 modules together; a PC/104-Plus host supports PCI-104 and ISA modules together. Bridge modules that add PCI or ISA to a PCI Express host are available from some vendors, though they add a layer and a cost that is often better spent replacing the old module.

The stand-off height and the enclosure set the maximum number of layers; a stack that grows past the enclosure is a sign that a host with more on-board I/O, or a 3.5-inch board with PCIe/104 expansion, would be the better base. PCIe/104 links run at the PCI Express generation the specification and the boards support; our guide to PCIe generations explains what each generation delivers per lane.

The PCI/104-Express and PCIe/104 specification has been revised several times since 2008, each revision keeping the board and connector compatible with the last. The milestones below are the ones that most often affect a buying decision.

The PC/104 specification itself is at version 2.6. For a design that has to pass a programme review, cite the specific revision each board was designed to, which the manufacturer’s datasheet or manual usually states.

The PC/104 single board computers we supply cover every generation. Figures in the table are from Diamond Systems’ published information in our catalogue.

The I/O modules follow the same split: analogue, digital and CAN modules on PC/104 and PC/104-Plus, and newer serial modules on PCI/104-Express. Power supplies use the PC/104 connector for power delivery and are indifferent to the data bus, so one supply serves a stack of any generation. Managed switch boards such as the Diamond Systems EPSILON-2400 stack on the same footprint; our guide to board-level switching covers them. When we quote a stack we check every module’s bus against the host before anything is ordered, which is the simplest way to avoid the mismatch this guide opened with.

Pick the bus by working backwards from the I/O the application needs and the modules already in hand, then choose the host board that provides it. For a new design that is almost always PCIe/104 or PCI/104-Express with an Intel Atom or Intel Core host; for a programme in service it is whichever bus the existing modules use, with a bridge or a PCI/104-Express host to carry them forward.

We supply the complete PC/104 range and we specify stacks as a set, with the host, the modules and the power supply checked for bus compatibility before they ship. Where the stack becomes a sealed computer for a vehicle or aircraft, our rugged mission computers show the finished form. Tell us what the stack has to do and what it has to survive, and we will recommend a bus generation and a board set to start from.