CompactPCI Serial Star Topologies, Part 2

Version 1

    Author: Manfred Schmitz, CTO MEN Mikro Elektronik


    CompactPCI Serial – PICMG CPCI-S.0 – supports 8 peripheral cards, each with PCI Express, SATA, USB and Ethernet for 3U and 6U boards. Every slot with 1 PCIe x4, 1 SATA, 1 USB 2.0, 1 USB 3.0 and 1 Ethernet is controlled by the system slot. All peripheral slots are equal, i.e. every board can be plugged into every slot.


    This makes the system slot a star with 8 points. All points are identical – almost, because 2 slots support an additional PCIe x8 each. There are three main applications for this addition.


    One classic application is graphics. For PCs and modular computers, the graphics functions integrated in the chipset are sufficient in most cases. Some applications require a higher graphics output performance. Graphics cards are usually accessed via PCIe x16. For two graphics cards, PCIe x16 is split into 2 PCIe x8 ports. CompactPCI Serial also supports this architecture. There are already graphics chips that can drive 4 screens. This means that two graphics cards like these can support 8 screens. Such boards may be used, for instance, in control rooms for rail signaling at a station or in other critical applications. At the same time, these two graphics cards offer the highest performance, because they are driven directly by the chipset, which is optimized for doing this. As PCI Express adapts itself to the available bandwidth, you can plug a graphics card into any slot of a CompactPCI Serial system. (A theoretical 8 graphics cards for control of 128 screens is feasible – it remains to be checked if this makes sense or is supported by software.)


    The second classic use case is in networking. Ethernet controllers and switches in high-end telecommunications need high bandwidth. In many cases, it is DSP or FPGA based boards that benefit from PCIe x8. For these applications, it makes sense to use 6U boards, which also benefit from the unique rear I/O functionality of CompactPCI Serial. 6U boards can communicate with their I/O via any type of connector – even optical – in the rear area of the system.


    The third application is CompactPCI Serial systems with more than 8 peripheral slots. For high-end systems, you may not integrate a classic CPU in the system slot but a switched fabric board. The CPU is located in a peripheral slot and communicates with the switched fabric via PCIe x8. The latter forms the interface to several CompactPCI Serial buses, in order to implement complex systems with 9+8 or with 9+16 slots. (This will be dealt with in another article.)


    The architecture of CompactPCI Serial is simple and clear. It allows to set up inexpensive modular computers, yet is also flexible enough to implement high-end computers. Based on PCI Express 3.0, theoretical data rates of 8 GB/s between boards and 80 GB/s inside the system are possible.