PC/104 Standard Overview

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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 architects 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 PC - chip-by- chip - directly into the product. This runs counter to the growing trend away from "reinventing the wheel." Wherever possible, top management now encourage outsourcing 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 application. 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, 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 the more than 125 manufacturers of PC/104-compatible products. Like the original PC bus itself, PC/104 is thus the expression of a defacto 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 th PC and PC/AT buses for embedded applications. The PC/104 Specification has been adopted as the "base document" for this n IEEE draft standard, called the P996 I Standardfor 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 (4 mA). Lowers power consumption (to I- 2 Watts per module) and minin-dzes 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.

Basic mechanical Dimensions (8 bit Version)
Basic mechanical Dimensions (8 bit Version)

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.

    Standalone Module Stacks
    Standalone Module Stacks
  • 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 a system debug or test.
    Component-like Applications
    Component-like Applications
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