What is PC/104?
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 over a hundred different PC/104 modules introduced by more than three dozen manufacturers. Like the original PC bus itself, PC/104 is thus the expression of an existing de facto standard, rather than being 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.