PCI Bus Technology Evolves from Legacy to Serial Interfaces

PCI Bus Technology Evolves from Legacy to Serial Interfaces

The increasing use of fast serial point-to-point connections in peripherals and embedded functions over the last several years has yielded a slow, but sure, change in the structure of computing systems.

While quite a few new serial-based standards that satisfy the fast serial point-to-point requirement have been developed, most have been optimized for special markets or applications during their early development phase.

However, two of these recent standards have integrated the established 19-in technology including the single (3U) or double (6U) Eurocard format. Both standards - CompactPCI Serial (PICMG CPCI-S.0) and VPX (ANSI-VITA 46.0) – have contributed to the creation of a migratory path from legacy technology to today’s serial interfaces.

Table Of Contents


Introduction To PCI Board Technology

As far back as 1981, a consortium led by Motorola and Philips developed the VMEbus, which became standardized as ANSI/IEEE 1014-1987 by the IEC. Originally, VMEbus was intended for use with the 6800 Motorola processor family in control systems.

However, VMEbus soon supported both RISC (e.g., PowerPC) and CISC (e.g., x86) architectures and established itself in industrial automation, medical engineering, telecommunications, aerospace and especially in military engineering.

The first VMEbus was equipped with a 16-bit data bus and a 24-bit address bus. The advanced VME64, which came next, offered a bus width of 64 bits with 80 Mbit/s while VME320 (2eSST) then provided a higher data rate of 320 Mbit/s. VMEbus established itself as the preferred standard for robust, modular bus-based systems with a passive backplane.

Not until 1994 was a concept introduced for modular computers based on PCI bus plug-in cards in Eurocard format connected to a passive backplane. Presented by Ziatech and Pro-log, the concept was called CompactPCI (PICMG 2.0).

Soon however, more companies such as Motorola, Radisys (Intel) and Lucent showed interest, participating in the standardization of CompactPCI – one of the first ever PICMG standards.

The success of the CompactPCI bus quickly resulted in a high market penetration for industrial, reliable systems. Shortly after, the telecommunication industry established CompactPCI as its most important standard. And, its adoption didn’t stop there. CompactPCI soon moved into markets traditionally occupied by the STD bus or the VMEbus, such as medical engineering, measurement and transportation.

Finally, conductive cooling solutions were also included, which enabled military applications to benefit from CompactPCI. CompactPCI was cleverly based on the IEC 1101 mechanical standards for Eurocards and 19-in systems known and proven from the VMEbus, which, undoubtedly, contributed to its rapid, wide-spread adoption.

Achieving Backward Compatibility with Hybrid Solutions

The increasing use of fast serial point-to-point connections in peripherals and functions has also resulted in the adoption of a number of different interface standards such as SATA and SAS for mass storage devices, like hard disks. Similarly, USB has established itself for WiFi components as well as loosely coupled peripheral devices, including keyboards, touch screens and external hard disks.

Besides traditional network technology, Ethernet is used as an interface for multiprocessing as well as a field bus for decentralized I/O. And PCI Express is used for connection of closely coupled computer peripherals.

These interfaces are able to coexist because they each have a unique range of special applications, and also because a modern computer needs all of them.

However, unlike in the past, these interfaces are not connected to separate controller chips, in turn interconnected by a bus. Rather, in modern chipsets, all these interfaces are directly available at the chipset. This is where that transformation from a bus-based system to a system with a star topology connected by serial point-to-point connections takes place.

The challenges these technologies presented to accommodate serial interfaces involved their application to industrial systems and their need to maintain modularity. And extremely important was the need for a smooth migration that would not obsolete the large inventory of installed systems. Preserving costly hardware and software investments in the field required an evolution rather than a revolution.

For VMEbus, VXS (VITA 41) ensures backward compatibility, as boards are mechanically and electrically compatible to VME and VME64 systems. They conform to the 19-in and Eurocard format with 0.8-in board distance and maintain P1 and P2 connectors. Using a 7-row multigig RT2 connector, VXS boards offer fast serial communication via Infiniband, Serial Rapid I/O, Aurora, PCI Express and Gigabit Ethernet. The theoretical bandwidth is up to 3 Gbit/s per slot.

CompactPCI uses the same newly defined connector in its PICMG 2.16 extension – calling it J3, while VXS calls it P0. However, this extension is limited to 6U boards for VXS and PICMG 2.16, so compact solutions cannot be implemented.

In addition, SATA and USB are excluded in VXS, since Ethernet and PCI Express are the widely used serial interconnects in the PC world. VXS systems also need a switch board or a corresponding slot at the backplane, with star and double star configurations differentiated.

True Path to Hybrid Solutions

For 3U boards, CompactPCI Express (PICMG EXP.0) was introduced some years ago, but the adoption of CompactPCI PlusIO (PICMG 2.30) that followed in 2010 quickly superseded this interim specification.

CompactPCI Express was limited to a single serial interface type – PCI Express – and was not compatible to CompactPCI due to new connectors. In contrast, CompactPCI PlusIO learned valuable lessons from the standards of the early 2000s.

As the name suggests, PICMG 2.30 is an extension of the original PICMG 2.0, fixing the pin assignment of the J2 connector that had been free before, while providing four PCI Express, four USB, four SATA and two Gb Ethernet interfaces.

The new, perfectly-shielded connector supports differential signals with over 2.5 Gbit/s, while being 100% compatible with the previous 2-mm connector, making CompactPCI PlusIO backwards compatible to CompactPCI and supporting all current serial interfaces. Plus, it can be used both for 3U and 6U board formats.

CompactPCI PlusIO is an extremely money-saving option, since the systems do not need switch boards, and the high-speed connector is low-cost. CompactPCI PlusIO system slot boards can be used in both CompactPCI and hybrid systems with CompactPCI Serial peripheral slot boards. In stark contrast to VXS, CompactPCI PlusIO is ideally suited for applications with mixed parallel and serial communication requirements.

The Emergence of Pure Serial System Standards

A number of serial-only system standards with high visualization, image capture, memory and multiprocessing capabilities were developed just in time to serve data hungry applications.

However, only two of these standards are modular and robust enough for harsh environments without limits. VITA adopted VPX in 2007 and more recently PICMG introduced CompactPCI Serial (PICMG CPCI-S.0).

Both rely on the proven IEEE 1101 mechanics – 3U and 6U formats as well as 19" technology – that provide important functions for safety and reliability such as hot-plug/hot-swap and good heat dissipation, including conductive cooling. Much of the success of these two standards is derived from the inherent modularity and robustness built into them rather than implemented as an afterthought, as in standards such as ATCA and MTCA.

VPX, based on VMEbus, is the successor of VXS with the robust 7-row multigig RT2 connector as its most striking innovation. For 6U boards, VPX provides 464 signal pins, 32 of which are differential pin pairs for serial interconnects and 128 pin pairs for customer-specific I/O. The complex connector, built up in a 16-layer wafer technology, supports transfer rates of up to 6.25 GHz with less than 3% crosstalk.

It not only works with different interconnects, but also with different communication architectures requiring certain switches or bridges depending on the overall system build-up – except for some mesh configurations, which can do without a central switch.

OpenVPX (VITA 65, adopted in 2010) tries to lift the separate parts of the VPX specifications from the board level to the system level, in an attempt to rectify the multiple interoperability problems. Even though VPX is not backward compatible to VMEbus, customer-specific hybrid backplanes allow connection with VME64 and VXS.

CompactPCI Serial has been developed in parallel with CompactPCI PlusIO for hybrid systems based on CompactPCI. Here, too, a more dense connector with 184 pin pairs (368 pins) per 3U board (AirMax from FCI or Amphenol TCS) guarantees transfer rates of up to 12 Gbit/s and more, with less than 3% crosstalk.

The new codable CompactPCI Serial connectors resemble the old 2-mm types, but up to six connectors per 3U board can be used. Four are used on the G22 (see figure). And, with their own walls, they’re much more robust and protected against incorrect plug-in that damages the connectors. Headers placed on the board avoid twisted pins on the backplane, and the board dissipates up to 60 W with a voltage supply of +12 V.

Figure 1. The 3U G22 CompactPCI Serial single board computer delivers 12 Gbit/s of backplane bandwidth with full mesh capabilities without additional configuration overhead.

In addition to mezzanine boards, such as PMC and XMC, used with CompactPCI Serial, special mezzanines can be plugged directly into the backplane via AirMax connectors that are turned 180°. The base CPCI-S.0 specification explains the complete system architecture including rear connection, the build-up of transition modules and the mechanics for systems with conductive cooling.

Choose Wisely

While both standards – CompactPCI Serial and VPX – are the best available today due to robustness and reliability, users need to carefully evaluate which system features will best serve each specific application with respect to cost and performance.

In certain industrial and civil applications, the higher cost of a VPX system may be justified, particularly when the non-recurring engineering (NRE) purchase costs are already high. However, users would be prudent to understand the ramifications of dealing with numerous VPX specifications including VITA 46.0, 48.0, and VITA 65.0 through 68.0 plus all the sub-specifications. The advantages of implementing the single base specification of CompactPCI Serial may dwarf the benefits of the VPX system.

Be aware also that even at the board level, higher costs are incurred due to the complex VPX connector, which provides fewer signals and offers no advantages regarding speed, safety or robustness compared to the CompactPCI Serial connector.

On the other hand, CompactPCI Serial offers no special interconnects such as Serial RapidIO or Aurora – but does offer the whole range of serial interfaces just like any PC.

There are two other cost factors to consider. First, the use of management controller hubs, switches and bridges make systems more complex and more expensive to maintain. CompactPCI Serial can operate without these components, even in complex multicomputer systems. The second involves overhead for software adaptations, which are not required with the CompactPCI Serial system.

The high number of VPX system configurations normally will cause problems with interoperability. While OpenVPX attempts to reduce these issues, the multitude of options still makes it very difficult to exchange plug-in boards of different manufacturers one-to-one. And, with VPX systems, backplanes are application-specific as a rule rather than the standard backplanes used by CompactPCI Serial.

To expand upon the cost-effective standardization of components with CompactPCI Serial, the strictly standardized pin assignment of CompactPCI Serial enables most applications – simple or complex – to be built mainly of standard boards and backplanes with no or very few NRE costs.

When a civil application requires effective cooling, CompactPCI Serial also specifies a CCA frame for boards and the corresponding infrastructure for conductive cooling systems. To save costs, standard assemblies do not have to be redesigned for conductive-cooled environment, which would reduce available space on the printed circuit board. Instead, they are simply equipped with a CCA frame.

Yet another cost factor involves the power supply unit (PSU). For its 5 V/12 V/48 V strategy, VPX requires more complex PSUs. Conversely, CompactPCI Serial defines a single 12 V supply and can use commercial off-the-shelf (COTS) PSUs.

While there are many similarities between VPX and CompactPCI Serial, the two systems differ substantially with respect to cost and complexity. A military and defense application may require and therefore justify the higher costs of a VPX system; but there are other more cost-conscious applications that could perform quite well with a less costly CompactPCI Serial system.

Certainly any complex computing architecture will cost, but CompactPCI Serial’s proven price/performance ratio should be a primary consideration in the evaluation of any embedded computing application.

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