The dawn of stackable PCI Express

The PCIe/104 and PCI/104-Express specification will provide an avenue for the rugged PC/104 architecture markets to expand.

The time has come! For the past two years, the PC/104 Consortium has been engaged in an effort to bring PCI Express to the stackable PC architecture. Numerous complex technical and marketing hurdles had to be overcome. Thorough debates amongst industry leaders had to take place to ensure that any stackable PCI Express roadmap was in line with PC/104's reputation for stability and technically sound specifications that last for many years.

With the approval vote by the Consortium's members to adopt EPIC Express and its subsequent placement into a "hold" status due to a discovered deficiency in the way the original connector was being used, the stackable PCI Express effort was moved into the Technical Committee last spring so that a full debate and solution could be achieved. The Technical Committee, with Jim Blazer as its chairman (who also chaired development of the PC/104-Plus specification), was chartered to draw on the expertise of the Consortium's membership to hammer out all the pitfalls and technical challenges of stackable PCI Express. Twenty-two voting and nonvoting members answered the call.

The Technical Committee's efforts were defined by the Board of Directors' desire to create a roadmap of advancing technologies and specifications that had synergy with the current specifications of PC/104, PC/104-Plus, PCI-104, EBX, and EPIC. Key to this was bus placement based on mechanical considerations and the ability of add-on cards to mate with any of the three form factors. This would maximize interchangeability for board-level manufacturers and OEMs, which is at the heart and strength of the PC/104 community.

The Board of Directors also directed that priority consideration be given to the roadmaps belonging to the major CPU chip vendors that drive the PC industry. Historically, this began with only the ISA bus (PC/104), then ISA and PCI (PC/104-Plus), then PCI only (PCI-104), then PCI and PCI Express (PCI/104 Express), and eventually PCI Express only (PCIe/104).

By following these roadmaps, embedded users can leverage the entire PC industry's hardware and software development efforts. Adhering to these roadmaps also allows easy base-level component development, speeds time to market for full-up OEM systems, and ensures that the developed architecture will continue to complement future technologies supported by the CPU chip vendors, thereby providing a place for expansion and growth.

The new specification is the result of bi-weekly Technical Committee meetings during the past year. Several key technical issues were revisited multiple times. The connector technology ultimately chosen came from an independently sponsored effort to create a connector suitable for a next-generation PC/104 Consortium specification. The result was a modified version of the high-density Samtec Q2 connector optimized for the 0.600" (15.24 mm) board separation of the PC/104 architecture. This solution keeps emphasis on ruggedness in a stackable, surface-mount environment and optimizes for maximum signal bandwidth and integrity while keeping Gen 2 PCI Express in sight.

To test the integrity of the specification, highly advanced test boards were designed to study stacking height limits, breakout patterns, PCI Express switch effects, and via and trace length effects. Several manufacturers, willing to take the financial risk that changes to the specification could occur before final ratification, have also built working modules.

These efforts resulted in the PCIe/104 and PCI/104-Express Specification, which as of press time has received approval votes from both the Consortium's Technical Committee and Board of Directors and is headed for a vote by the voting members. Also included in the specification are details on how the buses are integrated into the larger EBX and EPIC form factors.

If approved by the membership, the PCIe/104 and PCI/104-Express specification will provide an avenue for the rugged PC/104 architecture markets to continue to expand and provide organic growth opportunities for all members of the embedded industry.

PC/104 Embedded Consortium
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