The advantages are still "stacking" up

The original PC/104 format, and the PC/104 Consortium that has extended and evolved that original standard, have been around for a while. In fact, 2012 is the Consortium’s 20th year, which is quite a long time in a technology-based industry. So before we start the party (20 years is definitely worth celebrating), let’s take a minute to look at the whole idea of a “stacking” architecture and all the benefits it provides to system designers

Without question, the most important aspect of the PC/104 formats is the ability to “stack” boards together as a means to connect expansion boards (Figure 1). The stacking concept was undoubtedly born from a desire to support a lot of flexibility in system expansion. Unlike formats before it, the PC/104 format did not provide just a single expansion connector or a mezzanine board, but ran the entire set of bus signals up through the stack. Today this may not seem like a very revolutionarily idea, but it was a powerful concept compared to other form factors designed around motherboards, card cages, and backplanes.

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Figure 1: PC/104’s ability to stack boards together to connect expansion boards can be considered its most important aspect.
(Click graphic to zoom)

(Small) size matters

One of the “big” PC/104 advantages is its size. At only 3.6" x 3.8" (90 mm x 96 mm), PC/104 systems can be used in many applications where other formats simply won’t fit. But its stacking design also brings a big advantage to the Z-axis: height. The height of the stack is up to the user. If a system can be created with only one or two cards, then the enclosure can be low profile (the cards are spaced 0.6" – 15.24 mm – apart). If more expansion is needed, the enclosure can be sized to fit. Unlike backplane-based systems, there are no empty slots that take up space, even when there is no expansion function being used.

Flexible to fit any application

Flexibility is PC/104’s middle name. It’s a modular “building block” system that is always ready to expand or change as the application evolves. The flexibility of PC/104 is especially valuable during system development. There’s no need to purchase a new backplane or order PCB changes to test a new feature; add or change a hardware feature by simply plugging a new card into the stack. This kind of instant gratification is a big help to anyone’s development budget and timeline.

One solid block

The stacking design includes another benefit, which is basically free as it adds nothing to the cost of the board. PC/104 systems excel at physical ruggedness. It’s easy to see that the typical desktop PC expansion card plugged into a motherboard at a right angle is not going to last long in an environment with any vibration or shock. Industrial or embedded form factors that are designed for more rugged use, such as CompactPCI, use expensive chassis to secure each card. In contrast, PC/104 boards bolt directly to each other, forming an extremely rugged cube of boards. In most situations, no chassis or mechanical reinforcement of any kind is needed.

Lower system cost

Putting together a typical PC/104 system doesn’t require any backplanes, chassis, baseboards, NRE fees, or other start-up or overhead costs. Purchasing just the board(s) needed in the system, and plugging them into each other, is all that’s required. When considering the cost of high-speed backplanes and the associated chassis needed around them, the advantage of the PC/104 stacking approach is obvious.

Selecting the right format

It should be noted that there isn’t a single PC/104 standard. There are many related formats available, from the slow-and-steady ISA bus implementations of the original PC/104 bus specifications, to versions with PCI, to the latest PCI Express versions with x16 lane I/O. The single PC/104 specification has given way to a rich family of formats that are all based on the stacking concept. There are literally hundreds of off-the-shelf PC/104 CPU and I/O expansion boards available from a wide variety of vendors. For more information, check out the product search page on the PC104.org website and search PC/104 and Small Form Factors’ product database at www.smallformfactors.com/products before your next project.

PC/104 Consortium 408-337-0904 info@pc104.org www.pc104.org