COM Module Technology: PCIe/104 & COM Express

October 14, 2021

In embedded computer systems, COM technology enables the use of an off-the-shelf computer module with a custom, application-specific carrier board, or boards, to fulfill the I/O requirements. This approach allows for a faster time to market, is cost-effective and offers scalability for future upgrades. Products with a COM-based design also foster long-term availability as well as superior life cycle management.

A Brief History of PC/104 and COM Express Standards

Sealevel PCIe/104 Design

The first specification for PC/104™ modules was published in 1987, though they had been manufactured for five years by that time. PC/104 included an ISA connector, followed by PC/104-Plus™ which added a PCI connector. Then, PCI/104™ featured the PCI connector only, followed by PCI/104-Express™ which added a PCIe connector. The most recent evolution, PCIe/104™, only utilizes a PCIe connector.

The COM Express standard was first released in 2005. Subsequent revisions were published in 2010, 2012 and 2017. While there are eight different pin outs defined, the 2017 revision (Rev. 3) removed Types 1, 2, 3, 4 and 5. It is recommended that all new designs use Type 6, Type 7, or Type 10. COM Express has four standard form factors: mini, compact, basic and extended.

The Differences Between PCIe/104 and COM Express

While both PCIe/104 and COM Express approaches enable companies to reach their design – and manufacturing – goals faster and with more long-term availability, key differences help determine the best option for a given application.

In COM Express design, the connectors are soldered directly to the board with the enclosure built directly around the board. With PCIe/104, there aren’t connectors to the outside, but rather headers on the board to bring the specified I/O out. Additionally, with COM Express, engineers are working with a single vendor to supply the COM Express board whereas with a PCIe/104 stack, multiple vendors may be involved. This can be challenging when navigating the supply chain and obsolescence but can also be beneficial as with PCIe/104 you can change a single board within the stack rather than redesigning entirely.

Sealevel COM Express Design

PCIe/104 and COM Express design allow for compact design and increase the likelihood of meeting SWaP challenges. PCIe/104 achieves this through a vertical stacking approach while with COM Express, the size of the board can increase (or decrease) horizontally to meet the desired footprint.

Both approaches allow for the application of FPGA to the COM module or carrier board, further future-proofing the design as I/O can easily adapt as technology evolves without the need to redesign the entire system.

Choosing PCIe/104 or COM Express

As PCIe/104 and COM Express offer similar benefits in terms of time to market, flexibility and scalability, evaluating the operating environment and available footprint can be key determining factors. Selecting an engineering partner with experience in both approaches can also help companies navigate the best solutions for their specific requirements.

Embedded Computing at Sealevel
Sealevel’s award-winning team of electrical, mechanical, software, test and compliance engineer have first-hand experience with COM Express and PCIe/104 to meet custom design requirements. We would welcome the opportunity to guide you through our COTS computing systems or discuss your application-specific needs.

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