What is PC104?

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Even before the turn of the century, the US military could drive a small UAV through the sky at 600 mph to act as a decoy and confound enemy resources. Control came from a PC/104 stack embedded within its 7 in. diameter body. Although inevitably designed around the ISA bus technology available at the time, the stack was nevertheless compact, powerful, energy-efficient, rugged and economical enough to perform successfully within this conspicuously demanding role.

The example also demonstrates the original – and continuing – motivation behind the PC/104 form factor and standard. These were developed for embedded systems designers who have always needed to handle real-world inputs and outputs, often in large quantities and different types, and always in application-specific mixes. They also wanted their central processor to benefit from the same economies of scale and proven design standards as enjoyed by desktop PCs.

Evolution of the PC/104 standard

In 1987 the first PC/104 form factor products were introduced to meet these needs. Based on the x86, they provided a standard PC architecture and ISA bus electrical interface, implemented on small form factor boards with high quality, stackable 104-pin connectors. Designers could run PC software on a light, compact, economical stack configured to include only the I/O that they actually needed for their application. PC/104 became an internationally recognized standard under the auspices of the PC/104 Consortium in 1992.
However PC technology has steadily evolved to remain competitive, with ever-improving performance and denser functionality – and the PC/104 standard has always kept pace with these advances. The PC/104-Plus extension appeared in 1995; this included a PCI connector at the top of the board, adding high speed expandability to the ISA connector, which remained. This was followed by PCI-104, a PCI-only implementation which dispensed with the ISA connector.

Nevertheless pressure for dense functionality kept board real-estate at a premium, which made parallel, large pin count buses including PCI as well as ISA unattractive. The PC world, and PC/104 with it, turned to PCI Express. This is a high speed serial bus standard that can reduce pin count to as little as six. In 2008 the PC/104 Consortium, in recognition of growing silicon availability worldwide, released two standards to exploit PCI Express; PCIe/104 and PCI/104-Express. These two standards differ in that PCI/104-Express boards include both PCI and PCI Express connectors while PCIe/104 boards have just PCI Express. PCIe/104 boards free up real estate by eliminating the PCI connector, the routing to it and the PCI Express to PCI bridging chip. Both types are PCI Express Gen 2 and Gen 3 compatible.

The PCIe/104 stackable connector comprises three 52-pin banks, 156 pins in all. For maximum flexibility, this connector is available as either a Type 1 or a Type 2. Type 1 offers the highest performance with a x16 PCI Express Link, while Type 2 provides extra I/O in the form of USB 3.0, SATA, LPC Bus and an RTC battery. In February 2015 the PC/104 Consortium announced a further space- and cost-saving development called OneBank. This uses a smaller, lower cost version of the PCIe/104 connector that comprises just one 52-pin bank instead of three. With OneBank, enough performance and functionality remains for most applications, while 0.513 in2 of real estate is freed up on each side of the board – enough to site three extra I/O connectors, for example. Additionally, OneBank connectors are backward-compatible with the full PCIe/104 versions.

pc104 evolution

PC/104 modules, systems and benefits

Early PC/104 stacks sometimes contained up to seven boards, as less functionality than today was integrated into the CPUs and chipsets of the time. Over time integration levels have steadily increased – aided by developments such as PCI Express and OneBank – and board counts have tended downwards; a recent survey revealed the average number to be 1.2. There is however still a need for specialist I/O such as high performance video or fast networking, and OEMs can also design their own application-specific I/O solutions.

As a result, PC/104 implementations are more compact than ever, while providing dense functionality from a rugged, low cost form factor. The Standard’s demonstrable longevity, together with backward compatibility and standardization provide designers with a stable system environment – yet continuous review and development allow them to move forward with the latest in PC design and topology. PC/104 provides the same performance and functionality as backplane-based chassis systems at a fraction of the cost and size. And, unlike COM modules that require baseboards, PC/104 stacks are ready for direct connection to their real world I/O.

PC/104 applications

Military, transportation and energy remain as one of PC/104’s largest markets today. The products are also found in many other applications around the world that demand their small, rugged, cost-effective form factor. Examples include defense, broadcasting, heavy lifting and positioning, remote platforms, pipeline inspection, traffic monitoring, mining, security and surveillance, networking, communications and industrial control.

Want to learn more?

We recommend that designers of PC/104 boards and associated products as well as systems integrators wishing to use them refer to the PC/104 Consortium. This is the focal point for the entire PC/104 industry including manufacturers and OEMs. It provides a forum for information on current specifications, product offerings, news, and events, and a place to advance and develop specifications that are consistent and stable for long term use.