CompactPCI Serial: The new CompactPCI "base" specification

CompactPCI Serial (PICMG CPCI.S-0) has added a number of features and benefits while maintaining backward compatibility to create a foundational spec for serial-based CompactPCI technologies.

6Adopted in 2011, the new CompactPCI Serial specification revealed key innovations in connectors, interconnects, power supply, mechanical construction, and conductive cooling to enable the CompactPCI family of specification to extend into the age of point-to-point serial interconnects. These innovations, along with backward compatibility with legacy technologies, offer developers intriguing opportunities for system development. In the first half of this article, Alexander reviews the innovations of CompactPCI Serial, while the second installment will cover the costs and benefits of using CompactPCI Serial over other serial-based architectures such as xTCA and VPX/OpenVPX.

CompactPCI (PICMG 2.0) began in 1995 with the "base" specification that combined the Euromechanics standard (IEC 60297) with a PCI bus (Figure 1). At that time, data exchange between modules was provided by a 32-bit PCI bus with a throughput capacity of about 1 Gbps, and this bus fulfilled the function of a universal interconnect for PICMG 2.0 systems by providing data exchange with peripheral and storage modules as well as for collaborative computing. Over the next 10 years, two further CompactPCI specifications were issued, each of which added serial interconnects to the parallel PCI bus: the CompactPCI Packet Switching Backplane (PICMG 2.16) introduced two Ethernet channels, while CompactPCI PlusIO (PICMG 2.30) afforded 4 x1 PCIe channels and two additional Ethernet channels per slot.

Figure 1: The inter-modular data exchange interconnects of the PICMG CompactPCI specification family have evolved multiple times since the adoption of the original specification in 1995.

In 2011, the CompactPCI Serial (CPCI-S.0) specification was adopted, which supplemented the CompactPCI family of specifications by renewing the data exchange interconnects between modules to ensure a modular systems solution for the next 15-20 years. CompactPCI Serial substantially increased throughput capacities in all types of interconnects, adding PCIe and Ethernet for data exchange, SAS/SATA for storage systems, and USB 2.0/3.0 for general profile peripheral I/O.

Serial innovations establish a "base" specification for next-generation CompactPCI systems

On one hand, CompactPCI Serial looks like the logical continuation of a step-by-step transition from parallel buses to point-to-point serial interconnects. On the other, if looked at more deeply and with keen attention to detail, CompactPCI Serial can be viewed as the next-generation equivalent of the original CompactPCI "base" specification. The novelty and significance of CompactPCI Serial, as well as the reasons for establishing it as the "base" specification of the CompactPCI family moving forward, are very important considerations for both industry players and developers of embedded systems alike.

CompactPCI Serial incorporates five key innovations that have enabled it to become the base specification for serial-based CompactPCI systems:

  • High-density connectors
  • High-speed interconnects
  • Mechanical structure of 6U CompactPCI Serial blade modules and 3U/6U backplane implementation options
  • New power supply scheme
  • Conduction cooling

High-density connectors

CompactPCI Serial uses new high-density connectors for data transfer, both on the side of blade modules and on the side of motherboards (Figure 2). The structure of these connectors enables blade modules to be applied in a fashion consistent with that of mezzanines, allowing them to be connected directly to motherboards. Testing of these connectors showed high-quality signal transmission up to 12.5 GHz.

Figure 2: A view of the FCI's CompactPCI Serial AirMax VS connectors.

High-speed interconnects

For controlling and monitoring system service parameters, CompactPCI Serial outlines four interconnect pin (pinout) designations and one I2C bus on the system controller and peripheral connectors. Specifically, the four interconnect pinouts outlined are:

  • Eight PCIe channels (two x8, six x4)
  • Eight Ethernet Base-T channels
  • Eight SAS/SATA channels
  • Eight USB 2.0 or USB 3.0 channels

At the physical level, each channel consists of two differential pairs that provide data transfer from source to receiver and back. As all CompactPCI Serial interconnects are serial, the specification also describes their topology, defining PCIe, SATA, and USB in star topologies with the system controller as a host, and Ethernet in a full mesh or "each-to-each" topology (Figure 3).

Figure 3: The inter-modular interconnect topologies of the CompactPCI Serial specification define PCIe, SATA, and USB pinouts in a star topology (a) and Ethernet in a full mesh topology (b).

Mechanical structure of 6U CompactPCI Serial blade modules and 3U/6U backplane implementation options

Under the CompactPCI Serial specification, all module connections to the backplane are located on the upper half of the 6U modules. The lower half of the modules remains free for direct connection to rear I/O modules and/or installation of connectors according to customer requirements (Figure 4). The specification also describes an additional connector on the system controller for 6U modules, providing two additional Ethernet channels that can be used for ensuring compatibility with the PICMG 2.16 switching architecture and an additional power supply. If organization of the 6U system does not require support for PICMG 2.16 and an additional power supply, 3U host processing modules can be used in such systems.

Figure 4: Examples of the front and rear I/O card connections to CompactPCI Serial backplanes in 3U (a) and 6U (b) systems.

It is important to note that the CompactPCI Serial specification does not impose restrictions on the configuration of the backplane, leaving it at the discretion of customers and backplane manufacturers. In this regard, some systems can be designed to connect only one application or peripheral module, while others can connect up to 24 application modules.

Figure 5 illustrates the different implementation options of monolithic and composite backplanes in CompactPCI Serial and hybrid systems. Monolithic backplanes can be implemented in 3U or 6U CompactPCI Serial systems with eight peripheral serial slots to create pure serial systems. Composite backplanes, on the other hand, can be equipped by 6U CompactPCI Serial system developers to create both pure serial and hybrid systems: 8 serial peripheral slots can be leveraged for pure serial, while a combination of serial peripheral slots and slots supporting PICMG 2.0 blade modules can be instituted for a hybrid CompactPCI configuration. The advantages this variety creates are twofold. First, in the case of 6U systems driven by 3U host processors, the price of backplanes is reduced because they become simpler to manufacture. Second, the support for legacy PCI (PICMG 2.0) and Ethernet (PICMG 2.16) CompactPCI interconnects enables the creation of hybrid systems, which is beneficial for extending product lifecycles.

Figure 5: Illustrated below are examples of composite (b, d) and monolithic (a, c) backplanes. For pure serial configurations, 3U/6U CompactPCI Serial systems can be outfitted with a monolithic backplane that supports eight serial peripheral slots (a/c), or 6U composite backplanes with eight serial peripheral slots can also be instituted (b). For hybrid configurations, a 6U composite backplane with support for 2 CompactPCI 2.0 peripheral blade modules and 6 CompactPCI Serial peripheral slots can be leveraged (d).

In addition to the above-mentioned improvements, one mechanical element in 3U modules and two mechanical elements in 6U modules serve to center and direct boards when connecting to backplanes, simplifying the process of system assembly and module replacement.

New power supply scheme

The new connectors also allowed for improvement of the power supply system. CompactPCI Serial requires only one +12 V power supply with a maximum input power of 79.8 W per 3U module and 171 W per 6U module. This supply provisions power to 8 HP or 12 HP high-performance modules, or modules with conductive or liquid cooling."

CompactPCI Serial also provides the system controller with the ability to control the power supply in event occurrences such as Wake-On-LAN (WOL) or Wake-On-Modem (WOM), for example. Therefore, functions implemented in modern processors and chipsets can be used in an embedded system.

Conduction cooling

The CompactPCI Serial specification offers developers the simple option of conduction cooling, which entails packing a standard module in a metal enclosure. That concept requires an extension of pitch between system modules from the traditional 4 HP to 5 HP, thus requiring a special backplane. However, this does not require a special module design, which is overall more expensive.


Very often, the success of a new technical solution is based on the support of hereditary technologies. A striking example is the success of the x86 processor architecture, which supports the operation of previously written program code. As previously mentioned in the description of various backplane implementations, CompactPCI Serial provides system developers with wide integration possibilities for PICMG 2.0 and PICMG 2.16. It also supports CompactPCI PlusIO (PICMG 2.30), as PICMG 2.30 peripherals and application modules use identical connectors and are completely compatible with CompactPCI Serial. Cumulative information on the compatibility of standard modules within the CompactPCI specification family is provided in Table 1.

Table 1: System controllers and peripheral modules are compatible across much of the CompactPCI range of specifications.

In a 3U format, a hybrid system can be implemented by means of a PCI/PCIe bridge module connected to the system controller and two backplanes, with the PCI segment located on one backplane and the CompactPCI Serial segment on the other. In one such option, offered by FASTWEL, the bridge is implemented on the left side of the mezzanine and connects to the CPC510 system controller.

Ecosystem of central processors for CompactPCI Serial

The original CompactPCI specification defined an input power of only 50 W for both 3U and 6U modules that often required proprietary methods when installing server-grade CPUs. The increase in input power for CompactPCI Serial system controllers to 79.8 W and 171 W for 3U and 6U systems, respectively, allows for installation of the lion's share of embedded x86 or PowerPC CPUs."

Also, because of the previously discussed advantages of CompactPCI Serial's mechanical structure, double-width modules can be outfitted not only with soldered CPUs from mobile roadmaps, but also server-grade socketed processors and vertically installed DIMM or SO-DIMM memory.

Selecting CompactPCI Serial

The informed reader may ask, "Why was it necessary to develop a new specification based on serial interconnects if two such specifications already exist in the PICMG consortium (AdvancedTCA and MicroTCA), as well as a third in VITA (VPX/OpenVPX)?" This is a good question, and the answer is important in understanding the advantages of system development with a particular architecture or for a specific application. This will be discussed in detail in the second installment.

Alexander Buravlev is Technical Director of Aquarius Ltd. and a Fastwel technical expert.