CompactPCI® Serial Overview

The CompactPCI® Serial standard, PICMG CPCI-S.0, is the successor to the popular CompactPCI® specification PICMG 2.0. First released in 2011, the CompactPCI Serial architecture supports high-speed interconnects using PCI Express as well as Ethernet, SATA, and USB. This flexibility permits use of a wide array of commercial solutions, making CompactPCI Serial one of the most versatile, high-performance, and low-cost modular open standard for embedded computing.

New Features in CompactPCI Serial Revision 3

With the new CompactPCI Serial Revision (CPCI-S.0 R3.0) in 2024 some new features are included in the specification:

• Support of PCI-Express Gen4 (16GT/s) on all 8 peripheral slots
• 25GBaseKR single star or full mesh support (in addition to 10GBaseT)
• Definition of PCIe Gen4 Channel and Test Paddle Card requirements
• Rear IO for instrumentation applications
• Backward compatibility to CPCI-S.0 R2.0

CompactPCI Serial Form Factors and Conduction Cooling

CompactPCI specifies 3U and 6U sizes. Convection and conduction cooling are supported for both form factors.

The IEEE standard underlying the CompactPCI specification defines a specific design for conduction-cooled boards, making it difficult to cool a board using both convection and conduction without additional effort. Since conduction-cooled boards are typically produced in low quantities, they tend to be costly.

CompactPCI Serial takes a different approach. Any convection-cooled card can be easily converted into a conduction-cooled card using a custom aluminum block. This method reduces development costs for specialized conduction-cooled boards, making advanced cooling technology more accessible and cost-effective.

Standard CPCI board inside a CCA frame (left) and conduction-cooled housing (right).

CompactPCI Serial Backplane Topology

CompactPCI Serial uses a star topology for PCI Express, with optional support for Serial RapidIO (SRIO), SATA/SAS, and USB. The system slot can manage up to eight peripheral slots, eliminating the need for bridges or switches in systems with up to nine slots.

Ethernet, however, is configured as a full mesh network. In this setup, each of the nine slots is directly connected to every other slot through dedicated point-to-point connections. This design ensures that even if a CPU board supports only two Ethernet interfaces, three slots can still be fully wired without the need for switches or additional routing. Transmission is based on the proven standards for copper connections up to 10GbaseT to allow configuration of redundant, safety-critical systems. This is important for demanding mil/aero, transportation, security, and medical applications.

CompactPCI Serial uses a star architecture for PCI Express connections (left)

CompactPCI Serial Backplane Architecture

All peripheral slots are fundamentally identical, with the exception of two that are connected to the system slot via an extra-wide PCI Express link known as a Fat Pipe. These slots can be used for higher-bandwidth connections to demanding peripherals like high-end graphics cards. Each peripheral slot supports 1 PCI Express link, 1 SATA/SAS, and 1 USB 2.0/3.0 interface. All interfaces are accessible simultaneously.

Slots in a CompactPCI Serial system.

Hot Swap and Multiprocessing with CompactPCI Serial

The pin assignment of the system and the peripheral slot is congruent. In this way it is possible to plug a system slot board into each peripheral slot to support symmetrical multiprocessing. This is a significant advance over other 3U/6U standards. Due to hot swap functionality during operation it is also well suited for mission-critical systems.

CompactPCI Serial Connector

CompactPCI uses an AirMax connector that enables high signal density and transmission speeds of 12 Gbit/s and beyond. The connector accommodates up to 184 pin pairs on a 3U board.

This high pin count is particularly crucial for the system slot due to the star architecture of modern computers. It also provides plenty of free pins for customized rear I/O on peripheral assemblies. The connector’s design meets IEEE 1101 standards, and its appearance and feel closely resemble the reliable 2-mm connector used in the CompactPCI standard.

In contrast to CompactPCI, the positions of the plug and receptacle connectors are reversed. The plug connector is now on the plug-in card, while the receptacle connector is on the backplane, similar to the configuration in VMEbus systems. This design eliminates the issue of twisted pins on the backplane.

Compared to CompactPCI, the CompactPCI Serial plug and receptacle are swapped.

CompactPCI Serial and CompactPCI Hybrid Systems

CompactPCI Serial can be seamlessly integrated with existing CompactPCI boards by using a CompactPCI PlusIO (PICMG 2.30) system slot card. These hybrid systems provide a transition from parallel to serial connections. A board that supports CompactPCI PlusIO remains fully compatible with CompactPCI and can be used in existing systems without any limitations.

Hybrid backplanes from various manufacturers allow for flexible configurations, with peripheral slot boards placed on either side of the system slot board. These setups support up to seven parallel and four serial boards without needing a bridge.

Application-specific backplane with two system slots for CompactPCI and CompactPCI Serial.

Hybrid system with CompactPCI PlusIO CPU, and CompactPCI and CompactPCI Serial peripheral boards.

Typical Applications for CompactPCI Serial

CompactPCI Serial is suitable for a wide range of applications that benefit from hot plugging and hot swap capabilities. That’s why hot plugging is a core feature of CompactPCI Serial. These applications include everything from pluggable hard disks and RAID systems to complex multi-system setups.

• Industrial automation: CompactPCI Serial is intended as a general-purpose industrial standard. As such, it is suitable for robotics, machine control, automation, and energy sector applications. Other markets that can benefit from the architecture include research, medical equipment, ground transportation, and shipboard computing.
• Avionic and mission-critical: CompactPCI Serial’s proven Eurocard mechanics, real-time OS support, and conduction cooling make it well-suited for modular, mission-critical military, aerospace, and avionics applications. The CompactPCI Serial Space variant meets extreme environmental requirements for space, designed for use on satellites and ground stations. Regular CompactPCI Serial products can be combined with Space variants for developing test and simulation systems, making it a versatile solution for both space and Earth-based applications.
• Multiprocessing and cluster: CompactPCI Serial supports complex multiprocessing with up to nine CPU boards in a system without bridges, enabling high data rates in clustered systems. Central communication via the system slot eliminates the need for special hot-plug controllers. Ethernet-based full mesh networks allow direct card-to-card communication, removing the system slot as a single point of failure. This architecture matches the needs of high-availability clusters and compact systems with powerful computing, such as those found in telecommunications.

Revision 2, released on June 16, 2015 provides minor and incremental improvements:

• New connector, RJ6, is optional and provides an additional 48 pins of rear I/O from the System Slot. The total available I/O pins from the System Slot is now 58.
• Conduction cooling mechanicals improved (maximum  handle size is now defined)
• The System Slot can now be located on the right side of the card rack as well as the left side.
• I2C management bus further defined
• Location of SATA coupling capacitors clarified

+5 and +12 volts power are normally provided by an in-rack or external power supply.