The addition of PCI Express to the CompactPCI standard allows CompactPCI Express to utilize the latest PC market components; and in turn extends the useful lifespan for the bus architecture for at least the next decade.

Benefits of CompactPCI Express

CompactPCI Express brings several benefits to traditional CompactPCI applications, including:

• Higher Performance:
• Bus transfer speeds increase 10-50X
• PCI Express connections are all point-to-point, eliminating arbitration delays
• CPU-to-CPU transfers can occur at the full bus bandwidth (No need to run relatively slow Ethernet through the backplane)
• Compatibility with legacy boards and chassis
• Hybrid systems that include a mix of CompactPCI Express as well as legacy CompactPCI slots are easily implemented
• Since the boards are the same form factor and include the same face plates and other mounting hardware, existing chassis need only upgrade their backplane to be used in CompactPCI Express
• Low costs through access to the latest commodity components
• The latest I/O components that include native PCI Express interfaces can be easily designed into new CompactPCI Express I/O boards
• New capability to run PCI Express over cable
• Allows attached PCs to control a CompactPCI Express sub-system at full speed and with full S/W transparency
• Bus Unification, improved multi-processing and fault-tolerance
• CompactPCI Express supports both CPU-to-I/O and CPU-to-CPU communications over the common backplane
• This provides a cost-effective upgrade path for PICMG 2.16 applications

CompactPCI Express Basics

CompactPCI Express replaces the P1 and P2 connectors used in CompactPCI with four connectors that provide the new PCI Express bus as well as enhanced power capabilities to each board, as shown is figure 1.   The bottom connector provides high current connections for incoming power; the second and third connectors provide the PCI Express differential pairs for multiple PCI Express busses to be routed from each board to the backplane.  The top connector provides utility pins for user-defined rear I/O, PXI extensions for instrumentation and power input for the low-cost Type II I/O modules.

System slot boards within CompactPCI Express drive either 2 or 4 PCI Express buses onto the backplane.  This allows the direct connection of up to 4 I/O slots.  For larger configurations, a switch is needed to expand the PCI Express fan out to additional I/O slots.  An 8-port switch board is shown in figure 2.

A bridge board can connect one PCI Express port to traditional PCI, enabling hybrid systems that contain a mix of CompactPCI Express and legacy CompactPCI slots.  Figure 3 shows a 6U CompactPCI Express backplane that supports this hybrid architecture.

Internal or external CPU – Both are supported

One of the new capabilities of PCI Express is the ability to run the bus at full performance with total software transparency over a cable as well as a backplane.  For CompactPCI and CompactPCI Express systems, this enables system configurations where the CPU element is external from the CompactPCI Express system.  Figure 4 shows a development system for CompactPCI Express that is based on an external PC attached via PCI Express over cable.

Deciding whether to have an attached PC versus an internal CPU involves several tradeoffs.  Attached PCs are often more robust, offering the latest CPU speeds, memory and peripherals.  Embedded PCs have the advantage of eliminating the second chassis, and providing a more consistent level of ruggedness.  Often, designers will choose the convenience of an attached PC for development, and then transition to an embedded CPU board for production.

Tree and Network Topologies

CompactPCI Express supports both tree and network topologies.  A tree architecture includes one CPU element connected to a number of I/O elements.  This is the most cost-effective topology and is suitable for many applications where a single CPU is needed. CompactPCI Express also supports multi-CPU applications. 

PICMG 2.16 Solution for Multiprocessing

Both CompactPCI and CompactPCI Express have built-in flexibility via their P3, P4 and P5 connectors that are available for user defined rear I/O and/or secondary buses or interconnects.  Several uses for these connectors have become standardized.  One of the most popular is PICMG 2.16 that defines how 1Gb Ethernet can be routed through the P3 connectors to a special 2.16 switch slot.  This mechanism allows multiple CPU boards to intercommunicate via the Ethernet in a network topology.  “Split backplane” solutions have extended this concept to allow multiple CPU domains (isolated CPU and I/O slots) to be integrated within a single system, with the CPU boards connected by Ethernet routed via 2.16.

Conclusions

CompactPCI Express provides the advanced features of PCI Express in the CompactPCI form factor.  In doing so, it extends the useable lifespan of the architecture for at least another decade.  The first products based on this new standard are now beginning to appear, and represent the first look into this powerful architecture of the future.

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