CompactPCI Serial - The Next Generation CompactPCI

What is CompactPCI® Serial?

2009-01-14, Manfred Schmitz

CompactPCI® Plus supports star architecture

CompactPCI® Serial is based on CompactPCI®, an established standard for building cost-effective, reliable industrial computers. Today's users appreciate the modularity, robustness and economic efficiency of CompactPCI®. CompactPCI® Serial offers all these advantages plus fast serial data transfer - including the migration path.

In 1994/95 Jim Medeiros from Ziatech and Joe Pavlat from Pro-log as well as others presented a concept for modular computers based on PCI bus plug-in Eurocards connected to a passive backplane. The concept was called CompactPCI® and soon raised interest at companies like Motorola, Radisys (Intel®) and Lucent who contributed to its standardization. The CompactPCI® bus was successful and soon reached a high penetration of the market for industrial, reliable systems. It became the standard in telecommunications but soon also conquered markets in which the STD bus or the VMEbus were predominant. These are many applications in industrial control as well as medical engineering, measurement and transportation. CompactPCI® adeptly uses the IEC 1101 mechanics standards known and proven with Eurocards and 19" systems. Finally, solutions for conductive cooling were included so that the CompactPCI® bus could also be used in military applications.

During the last years the parallel PCI bus technology has more and more been complemented by fast serial point-to-point connections. Depending on the peripheral board and the function, however, different interface standards are used. SATA or SAS, for example, is the interface for mass storage devices like hard drives. USB has been established for WiFi components and loosely coupled peripheral devices like keyboards, touches, external hard drives etc. Beside the conventional network technology Ethernet is also being used as interface for multiprocessing and as field bus for controlling local I/O. PCI Express® is used for closely coupled peripherals. These interfaces coexist in a modern computer because they are all needed. Unlike in the past, however, they are not operated via single controller chips which are interconnected by a bus but they are all available directly at the chipset. For this reason, the structure of a computer slowly changes from a bus-based system to a system with a star topology connected by serial point-to-point connections.

Naturally, modular computers are still needed. As there are many different serial interfaces new standards were created depending on the industrial range - optimized for the market and the application. For example, AdvancedTCA has been created especially for telecommunication applications, whereas standards as VPX/ VITA-46 are focused on military applications.

Retroactively it was tried to make specialized standards universally usable again. One of these was µTCA. For µTCA a computer system was defined based on the mezzanines standardized at ATCA, the AMC. Due to the fact that originally this concept did not consider passive backplanes for the mezzanines, there were high infrastructure costs for emulating the AMC carrier board. What is more, the mechanics are completely incompatible to IEEE 1101 (Europe boards and 19" technology). The complements which were necessary later to make µTCA suitable for rugged environments also present certain problems.

Another standard exclusively based on PCI Express® - CompactPCI® Express - was not able to penetrate the market. This standard uses the IEC 1101 but the focus on PCI Express® as the only interface type is not sufficient.

The aforementioned system concepts all work with so-called switched fabrics, i.e. additional switches and bridges which are required to connect each slot with the right interface. This causes higher costs for the infrastructure, requires special software for the configuration and does not prevent that customer-specific backplanes are needed. As there are a multitude of different options it is rarely possible to exchange plug-in boards of different manufacturers 1:1.

For this reason, most modular computers are based on the CompactPCI® standard which has proven itself the world over. The serial point-to-point connections are realized as needed proprietarily via user-defined pins. Unfortunately, this leads to a growing incompatibility of assemblies from different manufacturers. What's the solution?
A subcommittee with 20 participating companies works on a new concept under the PICMG, the organization which has also standardized the CompactPCI® bus: CompactPCI® Serial. It aims to find a concept which allows for a smooth migration from the current standard to the modern point-to-point connections based on the existing CompactPCI® standard and the proven and no doubt robust 19" technology. All important serial interfaces shall be supported on an equal footing, i.e. PCI Express®, SATA/SAS, USB and Ethernet. The costs for the infrastructure have to remain low, which means that bridges and switches may only be used as an exception. It has to be possible to offer off-the-shelf backplanes, i.e. each slot has to be usable universally without special routing.

The PICMG subcommittee defines two standards for this:
PICMG 2.30 CompactPCI® PlusIO and PICMG CPCI-S.0 CompactPCI® Serial.

CompactPCI® PlusIO

PICMG 2.30 CompactPCI® PlusIO is based on the basic CompactPCI® standard PICMG 2.0. The basic standard PICMG 2.0 is complemented by PICMG 2.30. This complement defines the pin assignment and the function of the user pins on the J2 connector for 32-bit system slots. In the standard the signals are indicated as BP(I/O) signals and not specified further. CompactPCI® PlusIO makes up for this now. 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. However, the usual 2-mm-connector is not suited for transmitting such high frequency signals with sufficient quality, at least not without many additional ground pins. For this reason a new connector had to be found.

The company 3M produces a connector which is capable of transmitting these signals with sufficient quality without additional ground pins. The connector's mechanics are 100% compatible to the current 2-mm-connector. The difference lies within the connector. Each pin is shielded individually and independent of the other pins. The impedance is 100 Ohm so that it is ideally suited for transmitting single-ended as well as differential signals. The connector on the backplane does not have to be changed. If an assembly is equipped with this connector it remains completely compatible to the original CompactPCI® standard. These boards can be used in all existing systems without limitations. The new connectors make PCI Express®, SATA, USB and Ethernet available on the backplane - beside the legacy PCI bus. Forward and backward compatibility is guaranteed without limitations.

To use the new serial interfaces they can either be wired via a rear I/O adapter or new slots can be added to the existing CompactPCI® system. Four new slots with identical assignment 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 e.g. for building hard drive raids, the connection of frame grabbers via PCI Express® or the integration of Mini PCI Express® boards for wireless communication - functions which are constantly needed today and which are often realized after a fashion with customer-specific solutions.

CPU boards which support PICMG 2.30 (3U as well as 6U) remain compatible to the basic standard without limitations and can also be used in existing systems. If a CPU board cannot serve all interfaces, the remaining pins can still be used specifically. The new connector allows for transmission of signals with up to 5 Gb/s, mechanically it remains 100% compatible to the standard.

CompactPCI® Serial

CompactPCI® Serial, in contrast to CompactPCI® PlusIO, is a new independent basic standard. PICMG CPCI-S.0 is the designation of the new basic specification. This standard introduces a completely new connector. This enables a much higher signal density and supports even higher transmission frequencies of 12 Gb/s and more. CompactPCI® Serial is based on the mechanics of CompactPCI®, so it remains compatible to IEC 1101, but it only supports modern point-to-point connections. As the mechanics are 100% compatible with IEC 1101, all standard 19" system solutions can be used without limitations. The dimensions of the backplanes are identical to those of the CompactPCI® backplanes and are fixed in the same way. Front panels and handles have not been changed either. The well-proven hot plug mechanics - the switch in the handle - remain the same and acquires a new meaning for hard disk raids. Only the connector is replaced by a modern type which is able to support the high frequencies.

The CompactPCI® Serial standard uses connectors produced by the company FCI. The connector family is distributed under the marketing name AirMax and supports transmission frequencies of 12 Gb/s and more without special shielding measures. AirMax offers space for up to 184 pin pairs on a 3U board. On the other hand, the high pin number, which is important because of the star topology of modern computers for the system slot, allows for using a multitude of free pins for customer-specific rear I/O on peripheral assemblies. The mechanics of the connector meet the requirements of the IEC 1101 and the look-and-feel is very much like that of the proven 2-mm-connector of the CompactPCI® standard. An important point for choosing the AirMax connector for the standard was that FCI agreed to accept the conditions of the PICMG concerning their patent policy. According to American law it is not permissible to specify a component in a standard which may only be produced by a single manufacturer. It is not allowed to use a standard to build or support a cartel. A second source for the AirMax connectors is Amphenol TCS.

Compared with CompactPCI®, the male and female connectors are exchanged. The male connector is located on the plug-in board, the female connector on the backplane - as in VMEbus systems. This means that there is no danger anymore to crimp or distort pins on the backplane. And if a pin should be damaged contrary to expectations, it is much easier to replace a plug-in board than a backplane or a complete system. The AirMax connector has especially been developed for use in rugged environments. There are models of the male connector, i.e. the connector on the plug-in board, with only two walls which can be mounted side-by-side without any clearance and models where the pins are completely enclosed by four walls. Especially for CompactPCI® Serial an additional model is being developed, a male connector with three walls. All in all CompactPCI® Serial uses three male connector types and 2 female connector types from the wide range of AirMax connectors. A connector with four walls and six pin rows is always used on the assembly. This is 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. In this fashion a protected male connector with a cross wall in the middle is built. This cross wall supports the stability of the plugged-in board and prevents that a CompactPCI® Serial board is accidentally plugged into a conventional CompactPCI® slot so that pins are distorted.

As mentioned before, CompactPCI® Serial is based on a simple, complete star topology equally for PCI Express® (optionally also Serial Rapid IO, SRIO), SATA resp. SAS and USB. The system slot supports up to eight of these peripheral slots. No bridges or switches are needed for a system with up to 9 slots. In principle all peripheral slots are identical. Only one is connected via an extra wide PCI Express® link, the Fat Pipe. This slot can be used for a high-end graphics extension, for example.

Ethernet is not wired as a star, but in a full mesh. In full mesh architectures each of the new slots 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 copper connections 100/1000/10GbaseT. This offers the advantage of unlimited interoperability even for different data rates. Electrical isolation is possible at least optionally, which ensures the absence of feedback from the boards. This is important for redundant, safety-critical systems. Besides, several systems can easily be connected to each other.

The system slot supports a total of 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, 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 consumptin 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. As mentioned, each slot can support up to eight Ethernet interfaces for building up the full mesh network. The interfaces are all accessible at the same time. This is important for standards like Mini PCI Express® cards for example, which expect USB as well as PCI Express® support. The peripheral slot is also supplied with 12V and has a maximum power consumption of 60W.

The CompactPCI® Serial, a simple star combined with a complete mesh for Ethernet, functions without switches and bridges. There is a system slot and up to eight peripheral slots. The pin assignment of the system and the peripheral slot is congruent. A pin informs the plugged board whether it is located in a system or peripheral slot. This way 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, of course - usually SATA. The Ethernet interfaces are always supported, however. So CompactPCI® Serial supports symmetrical multiprocessing with up to nine CPUs in one system - without bridges and without special backplane. With bridges even more are supported.

The AirMax VS® connector system by FCI

The AirMax VS® connector family introduced by FCI is the first backplane connector system that requires no interleaving shields between the signal pins but can nevertheless be used to transmit high speed signals with up to 12.5 Gbit/s or more.

This new connector system enables a high signal density and shows advantageous insertion loss and crosstalk characteristics. The electric performance is independent of the shields now, which opens up more design possibilities for the system developer. The users can allocate differential, single-ended or low voltage signals to single pins in a connector module depending on the system requirements. As there are no shields it is relatively easy to increase the pitch of the connector in order to accommodate more signals paths in a PCB layer. In applications which do not require the maximum signal density, fewer layers are necessary so that the board costs are lower. Thanks to the modular structure of the connector, the number of the contacts can also easily be modified, thus it is possible to react quickly to the customer's wishes.

At the moment, AirMax VS® connectors are available in the configurations Co-Planar, Mezzanine and Daughter Card to Backplane (receptacle on backplane or header on backplane).

Migration Path

An easy migration path to CompactPCI® Serial is of special importance for applications in which CompactPCI® is already used. In most cases the migration will be accomplished via CompactPCI® PlusIO (PICMG 2.30). Such a CompactPCI® hybrid system will consist of a number of conventional CompactPCI® slots, a CPU board which is compatible to PICMG 2.30 and several peripheral slots with CompactPCI® Serial - e.g. for hard disk drives. In the next phase there might be a 2-slot CPU board which is system slot on the CompactPCI® as well as on the CompactPCI® Serial and connecting legacy PCI with the serial future.

CompactPCI® Serial and CompactPCI® PlusIO is a further development of the proven CompactPCI® standards. CompactPCI® Serial is based on proven interface standards which it uses in a modular computer system. CompactPCI® Serial is defined universally and independent of the application. Proven, cost-effective and robust mechanics in combination with the latest serial interconnects like PCI Express® 2.0, SATA 3.0, USB 3.0 and Ethernet 10GbaseT make CompactPCI® Serial the platform for modular computers in the next decade.

PICMG 2.30 CompactPCI® PlusIO complements the CompactPCI® basic standard PICMG 2.0 by defining the function of the user pins on the J2 connector. This way 4 PCIe® x1, 4 SATA, 4 USB 2.0 and 2 Ethernet 1000BaseT can be led to the backplane.

CompactPCI® Plus Hybrid
Hybrid systems facilitate the migration from CompactPCI® to CompactPCI® Serial. The figure shows a double-width CPU board which is system slot on the CompactPCI® as well as on the CompactPCI® Serial bus, thus connecting the legacy PCI with the serial future.

CompactPCI® Plus Star
PICMG CPCI-S.0 CompactPCI® Serial supports star architecture for PCI Express®, SATA und USB. Up to eight peripheral slots with identical pin assignment can be controlled without needing complex bridges, switch fabrics or special backplanes.

CompactPCI® Plus Mesh
PICMG CPCI-S.0 CompactPCI® Serial uses a full mesh architecture for 100/1000/10GBaseT Ethernet. With up to 9 boards in the system no bridges, switch fabrics or special backplanes are needed either.

CompactPCI® Plus Connector Set-up
PICMG CPCI-S.0 CompactPCI® Serial uses the robust connectors of the FCI AirMax family. On each 3U board 184 pin pairs on 6 male connectors transmit signals with 12.5 Gbit/s or more. In some of the connector types used the pins are enclosed by three or four walls, in other types there are only two walls so that the pins can be mounted side-by-side.

Manfred Schmitz, Technical Director MEN Mikro Elektronik, kindly supported by Marco Pagnin, FCI Deutschland

German version on ElektronikPraxis: Ursprung und Konzept des Bus-Standards CompactPCI Plus

What is CompactPCI® Serial?

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