The Need for an Embedded-PC Standard:

Over the past decade, the PC architecture has become an accepted platform
for far more than desktop applications. Dedicated and embedded
applications for PCs are beginning to be found everywhere! PCs are used as
controllers within vending machines, laboratory instruments, communications
devices, and medical equipment, to name a few examples.

By standardizing hardware and software around the broadly supported PC
architecture, embedded system designers can substantially reduce development
costs, risks, and time. This means faster time to market and hitting critical
market windows with timely product introductions. Another important advantage
of using the PC architecture is that its widely available hardware and
software are significantly more economical than traditional bus architectures
such as STD, VME, and Multibus. This means lower product costs.

For these reasons, companies that embed microcomputers as controllers within
their products seek ways to reap the benefits of using the PC architecture.
However, the standard PC bus form factor (12.4" x 4.8") and its associated
card cages and backplanes are too bulky (and expensive) for most embedded
control applications.

The only practical way to embed the PC architecture in space- and power-
sensitive applications has been to design a PCÑ chip-by-chip Ñdirectly into
the product.

But this runs counter to the growing trend away from "reinventing the wheel."
Wherever possible, top management now encourages out-sourcing of components
and technologies to reduce development costs and accelerate product design
cycles.

A need therefore arose for a more compact implementation of the PC bus,
satisfying the reduced space and power constraints of embedded control
applications. Yet these goals had to be realized without sacrificing full
hardware and software compatibility with the popular PC bus standard. This
would allow the PC's hardware, software, development tools, and system design
knowledge to be fully leveraged.

PC/104 was developed in response to this need. It offers full architecture
and hardware and software compatibility with the PC bus, but in ultra-compact
(3.6" x 3.8") stackable modules. PC/104 is therefore ideally suited to the
unique requirements of embedded control applications.

A Proposed Extension to IEEE-P996:

Although PC/104 modules have been manufactured since 1987, a formal
specification was not published until 1992. Since then, interest in PC/104 has
skyrocketed, with numerous PC/104 modules introduced by more than one hundred
manufacturers of PC/104 compatible products. Like the original PC bus. PC/104
is thus the expression of a de facto standard, rather than the invention and
design of a committee.

In 1992, the IEEE began a project to standardize a reduced form factor
implementation of the IEEE P996 (draft) specification for the PC and PC/AT
buses, for embedded applications. The PC/104 Specification has been adopted as
the "base document" for this new IEEE draft standard, called the P996.1 Standard
for Compact Embedded PC Modules.

The key differences between PC/104 and the regular PC bus (IEEE P996) are:

 Compact form factor. Size reduces to 3.6 by 3.8 inches.

 Unique self-stacking bus. Eliminates the cost and bulk of backplanes and card
cages.

 Pin-and-socket connectors. Rugged and reliable 64- and 40- contact male/female
headers replace the standard PC's edgecard connectors.

 Relaxed bus drive (6 mA). Lowers power consumption (to 1-2 watts per module)
and minimizes component count.

By virtue of PC/104, companies embedding PC technology in limited space 
applications can now benefit from a standardized system architecture complete 
with a wide range of multi-vendor support.

Two Ways to Use PC/104 Modules:

Although configuration and application possibilities with PC/104 modules are
practically limitless, there are two basic ways they tend to be used in embedded
system designs:

 Standalone module stacks: As shown in Figure 2, PC/104 modules are self-stacking.
In this approach, the modules are used like ultra-compact bus boards, but without
needing backplanes or card cages. Stacked modules are spaced 0.6 inches apart. (The
three-module stack shown in Figure 2 measures just 3.6 by 3.8 by 2 inches.)
Companies using PC/104 module stacks within their products frequently create one or
more of their own application-specific PC/104 modules.

 Component-like applications. Another way to use PC/104 modules is illustrated in
Figure 3. In this configuration, the modules function as highly integrated components,
plugged into custom carrier boards which contain application-specific interfaces and
logic. The modules' self-stacking bus can be useful for installing multiple modules
in one location. This facilitates future product upgrades or options, and allows
temporary addition of modules during system debug or test.

About the PC/104 Consortium:

The purpose of the PC/104 Consortium is to establish PC/104 as a broadly supported
industry standard architecture for embedded-PC applications. The PC/104 Consortium
maintains and distributes the PC/104 Specification and other PC/104-related documents,
serves as a liaison to standards bodies such as IEEE P996.1, and engages in a variety
of public relations activities on behalf of PC/104. Consortium membership is open to
companies who offer or use PC/104 modules, as well as to companies who provide products
that target PC/104 applications. 

"What is PC/104?" reprinted courtesy of the PC/104 Consortium.
PC/104 and the PC/104 Logo are Trademarks of the PC/104 Consortium