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.Download PCI/104 Specification