CompactPCI Plus

CompactPCI Plus from PICMG Advances Future of CompactPCI

CompactPCI Plus combines the established CompactPCI standard with current fast serial data transfer technologies. Definitions are in place for both migrating existing systems to CompactPCI Plus as well as for building new systems on the up-to-date standard.


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Even the most profound technological developments will evolve over time, as market needs and computing demands change. So it’s no surprise that CompactPCI, originally developed in the mid-1990s, has reached its next milestone. The modular CompactPCI technology is based on PCI bus plug-in boards connected to a passive backplane and uses the proven IEC 1101 mechanics. In addition to its uses in a variety of industrial, telecommunication and transportation applications, the inclusion of conductive cooling enables the use of CompactPCI in military environments, as well.

Table 1 identifies some alternative embedded platforms in use today for comparison purposes. Although each will work with so-called switched fabrics (i.e. additional switches and bridges required to connect each slot with the right interface), these technologies have certain shortcomings that preclude their use in modular embedded systems. Therefore, most modular computers are still based on the CompactPCI standard.

Over the years, CompactPCI has undergone several technology upgrades. The modern interfaces, noted in Table 2, must now coexist and be available directly at the chipset, since these interfaces no longer operate via single controller chips interconnected by a bus. This transition has slowly changed a computer’s structure from a bus-based system to a system with a star topology connected by serial point-to-point connections.

However, the proprietary serial point-to-point connections needed via user-defined pins leads to a growing incompatibility of assemblies from different manufacturers. This type of fragmentation is not helpful to customers or manufacturers. Standardization is the most cost-effective and time-efficient solution.

Enter CompactPCI Plus (, backed by a worldwide subcommittee of 20 participating companies working on a new concept under PICMG, the international organization that also standardized the original CompactPCI bus. Simply speaking, CompactPCI Plus is a basic star topology combined with a full mesh architecture for Ethernet support that functions without switches and bridges.

CompactPCI Plus aims to find a smooth migration path to modern point-to-point connections using the existing CompactPCI standard and the proven, robust 19” technology, with support for all current serial interfaces (i.e. PCI Express, SATA/SAS, USB, Ethernet). Infrastructure costs must remain low, so bridges and switches are the exception, not the rule, and off-the-shelf backplanes, where each slot can be used universally without special routing, need to be available.

Currently, the PICMG subcommittee has defined two standards. The PICMG 2.30 CompactPCI PlusIO is aimed at existing systems currently using CompactPCI (clear migration path). The PICMG CPLUS.0 CompactPCI Plus has been developed for completely new CompactPCI-based computing systems

Migrating from Existing CompactPCI Platforms

PICMG 2.30 CompactPCI PlusIO is based on the basic CompactPCI standard, PICMG 2.0, but complements it by defining the pin assignment and the function of the user pins on the J2 connector for 32-bit system slots. In the basic standard, the signals are defined as backplane I/O signals and not further specified, but CompactPCI PlusIO addresses this problem (Figure 1).

Although the pin number is sufficient for leading four PCI Express x1 links, four SATA, four USB 2.0 as well as two Ethernet 1000BaseT interfaces to the backplane in existing CompactPCI systems, the usual 2 mm connector could not transmit such high-frequency signals with sufficient quality–at least not without many additional ground pins. A new connector was needed, ideally one with mechanics 100% compatible with the current 2 mm connector.

After some industry research, the subcommittee found a connector from 3M that fit the mechanical profile, and offered the necessary signal transmission quality and speed capability–up to 5 Gbits/s. An assembly equipped with this connector remains completely compatible with the original CompactPCI standard, but each pin is shielded individually, independent of the other pins. The connector’s 100 ohm impedance is ideally suited for transmitting single-ended as well as differential signals, and the backplane connector does not change. Forward and backward compatibility is guaranteed without limitations. In addition to the legacy PCI bus, PCI Express, SATA, USB and Ethernet are now available on the backplane.

The new serial interfaces can be either wired via a rear I/O adapter, or new slots can be added to the existing CompactPCI system. Four new slots with identical assignments can be realized with the four PCI Express, SATA and USB interfaces. Ethernet, for example, is used to connect several of these systems. The new slots can be used for many commonly needed functions, such as building hard drive RAIDs, connecting frame grabbers via PCI Express or integrating Mini PCI Express boards for wireless communication.

CPU boards (3U & 6U) supporting PICMG 2.30 remain compatible with the basic standard without limitations and can also be used in existing systems. If a CPU board cannot serve all interfaces for user-defined functions unless all are used to support PICMG 2.30, the result will be a hybrid CompactPCI system. This will typically consist of a number of conventional CompactPCI slots, a PICMG 2.30-compatible (CompactPCI PlusIO) CPU board and several peripheral slots with CompactPCI Plus (e.g., for hard disk drives).

Building New Systems Using the Latest Standards

Geared toward completely new systems, CompactPCI Plus (PICMG CPLUS.0) is an independent specification with an updated, rugged connector that enables a much higher signal density and supports even higher transmission frequencies of more than 12 Gbits/s without special shielding measures. CompactPCI Plus, also based on the mechanics of CompactPCI, still remains compatible with IEC 1101 but supports only modern point-to-point connections.

As the mechanics are 100% IEC 1101-compatible, all standard 19” system solutions can be used without limitations. The backplane dimensions are identical to CompactPCI and are fixed in the same way. Front panels and handles don’t change either, and the well-proven hot plug mechanics–the switch in the handle–remain the same. This is especially important for hard disk RAID systems that require the exchange of single hard disks without powering the system down in the same manner as boards are hot-swapped.

The CompactPCI Plus standard uses AirMax connectors (currently available from FCI and Amphenol TCS), designed specifically for rugged environments and offering space for up to 184 pin pairs on a 3U board. The high pin number–an important feature because of the star topology of modern computers for the system slot–also enables a multitude of free pins to be used for customer-specific rear I/O on peripheral assemblies. The connector’s mechanics meet IEC 1101 requirements with a look and feel similar to the CompactPCI 2 mm connector. A deciding factor in selecting the connectors for PICMG 2.30 and PICMG CPLUS.0 was, respectively, 3M’s and FCI’s agreement to adhere to PICMG’s IP policy as stated in the organization’s bylaws Article XII, Section 12.1: Rights on Intellectual Property (

Differing from CompactPCI, the CompactPCI Plus male and female connectors swap locations. The male connector goes on the plug-in board and the female connector on the backplane–as in VMEbus systems–so there is no danger of crimping or distorting pins on the backplane. And, if a pin is somehow damaged, it is much easier to replace a plug-in board than a backplane or a complete system.

Overall, CompactPCI Plus uses three male connectors and two female connectors from the wide range of AirMax connectors. Variations of the connectors include a male connector with only two walls, mountable side-by-side without any clearance as well as models where the pins are completely enclosed by four walls. An additional male connector with three walls is being developed specifically for CompactPCI Plus.

A connector with four walls and six pin rows is always used on the CompactPCI Plus assembly, complemented by several side-by-side mountable types with two walls and eight rows. A 3U board is completed by a connector with three walls and eight rows. A protected male connector with a cross wall in the middle supports the plug-in board’s stability to prevent a CompactPCI Plus board from being accidentally plugged into a conventional CompactPCI slot and distorting the pins.

Two Topologies for Exceptional Performance

As mentioned before, CompactPCI Plus is based on a simple, complete star topology that supports PCI Express (optionally also Serial RapidIO, SATA, SAS and USB). The system slot supports up to eight of these peripheral slots, and no bridges or switches are needed for a system with up to nine slots. In principle, all peripheral slots are identical (Figure 2). Only one is connected via an extra wide PCI Express link, the Fat Pipe, which can be used for functions such as a high-end graphics extension.

Ethernet is not wired as a star, but in full mesh (Figure 3), where each new slot is connected to each of the other eight slots via a dedicated point-to-point connection. The wiring pattern has been chosen in such a way that, if a CPU board only supports two Ethernet interfaces, three slots are completely wired–even without switches, routing, etc.

Contrary to other concepts, the Ethernet transmission on the backplane is based on the proven standards for 100/1000/10GbaseT copper connections. Not only does this structure enable the easy connection of several systems to one another, it offers unlimited interoperability, even for different data rates. Electrical isolation is possible, at least optionally, to ensure the absence of feedback from the boards, which is very important for redundant, safety-critical systems.

The system slot supports seven PCI Express links with up to four lanes each, one link with up to 16 lanes, eight SATA/SAS interfaces, eight USB 2.0 as well as eight USB 3.0 ports, eight Ethernet interfaces and a number of signals for supporting these interfaces and for general system management (reset, IPMB, hot plug, geographical addressing, etc). A 12V power supply is available with a maximum power consumption of 60W for one 3U slot.

Accordingly, a peripheral slot offers one PCI Express link with up to 16 lanes, one SATA/SAS interface, one USB 2.0 and one USB 3.0 interface. Each slot can also support up to eight Ethernet interfaces to build up the full mesh network. These interfaces are all accessible simultaneously, an important consideration for standards like Mini PCI Express, which expects USB as well as PCI Express support.

The peripheral slot, also supplied with 12V and drawing a maximum of 60W, offers a pin assignment congruent to the system itself. Each pin informs the plugged board whether it is located in the system or via the peripheral slot, so it is possible to plug a system slot board (as a rule, a CPU board) into each peripheral slot. In this case, some interfaces are not supported–usually SATA–but Ethernet interfaces are always supported. So, CompactPCI Plus supports symmetrical multiprocessing with up to nine CPUs in one system–without bridges and without a special backplane. If bridges are used, even more CPUs can be supported.

Recognizing CompactPCI’s long legacy and proven benefits as a modular, robust and economical system platform, PICMG has made the commitment to revamp this standard for both new and legacy systems. This approach ensures that the investment companies have already made in CompactPCI are protected, while providing an up-to-date version of a proven standard for new systems that encompasses the latest serial technologies.

As technology advances and older architectures start to fall behind current system needs, an undertaking such as the CompactPCI Plus initiative helps to ensure that the market will have access to the latest technologies as well as upgrade paths for existing legacy-based systems.

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