By Zhineng Fan, Technologist, Amphenol

PCI-SIG® has enabled PCI Express® (PCIe®) technology to be cost-effective and easy to implement by supporting multiple form factors for a variety of applications. PCIe technology is widely adopted in applications ranging from small, power-constrained IoT sensors and mobile devices to servers and networking and communications equipment. Given the unique requirements of each of these applications, the form factors and connectors that PCI-SIG has delivered need to maintain backwards compatibility with technology leveraging previous specifications. 

The Fundamental Role of CEM Connectors

Card Electromechanical (CEM) connectors are one type of PCI-SIG developed connector. They are by far the longest standing PCIe form factors, with the first iteration delivered in 2000. CEM connectors play a significant role, as they are the connection between a motherboard, an add-in card (AIC) and a riser card. The connector can attach modules such as a solid-state drive (SSD) device for mass storage, a graphics processing unit (GPU) which accelerates the creation of images on a device, a network interface card (NIC) that attaches cables to communicate with external equipment, a machine learning (ML)/deep learning (DL) module or a hybrid computing module. Cards supporting a wide range of functions and performance are available, allowing the customer to customize a solution to best meet their goals. The CEM baseline specification is used as a reference for multiple mechanical form factors throughout the industry. The specification supports forward and backward compatibility to deliver a consistent end user experience.

The PCIe CEM specification defines the pin out for x1, x4, x8 and x16 link width. It also defines the footprint on the motherboard and mating interface, and thus helps the CEM connector match to that and guarantee the interoperability. The specification also determines the connector’s form factors such as height and width, while still giving vendors ultimate flexibility regarding the design of their connectors for performance. The connector has two main parts: metal contacts for electrical transmission and the plastic body which holds each contact into position and provides mechanical strength for the connector. Since the AIC does not have to be on the same CPU, vendors can easily exchange or upgrade the components to meet customer needs and market demand.

Backwards Compatibility with Past Generations

CEM connectors are available in different connector sizes, supporting previous and upcoming PCI Express specifications. As these connectors have evolved for almost two decades, PCI-SIG has continued to execute upgrades that result in enhanced performance and flexibility.

As bandwidth demands increase and vendor companies upgrade from previous specifications, they can make changes depending upon their respective approaches to improve performance with CEM connectors. Each company has proprietary technology that could help them upgrade the technology, but these changes would be made inside the connector. The performance improvement sometimes requires modification on the motherboard footprint and card edge finger dimensions. Even with these changes, the CEM connectors will maintain backwards compatibility on the mating interface. CEM connectors enable the new generation system (with a new generation connector) to accept an older generation AIC and work at the old generation speed. For example, a PCIe 4.0 AIC can plug into a PCIe 5.0 CEM connector on a PCIe 5.0 motherboard and it will run with PCIe 4.0 performance.

PCI-SIG members have access to the form factor specifications and can help develop the next generation of PCI Express architecture. Learn more about PCI-SIG membership here.