CompactPCI Serial Star Topologies, Part 1

Version 1

    Author: Manfred Schmitz, CTO MEN Mikro Elektronik


    The main motivation in making a computer system inexpensive and modular lies in a high reusability of standard components. With the parallel PCI bus disappearing, a standard PC is now based on a simple star topology. The CPU chipset at the system's center controls peripheral devices and components through point-to-point connections. This allows high data rates, while no device needs to concede bandwidth to another. This structure was used 100% for CompactPCI Serial – PICMG CPCI-S.0 –, so that there is no need for bridges and switches. In the end, the costs of a modular computer must stay competitive compared to non-modular computers. This does not mean that you could not integrate any switched fabrics into a CompactPCI Serial system, if you wanted to build up complex structures, but you simply don't have to. Simple systems do have a market share of around 90% of all CompactPCI systems.


    The engineering challenge now is to implement the central point of the star, i.e. the system slot. Even serial buses need many connection lines. For PCI Express, there is at least one line pair for clock, one pair to receive data, and one pair to transmit data. To increase the bandwidth, you use several lanes, so that one connection can easily count 9 line pairs or more. Because of the high data rate (currently up to 6 Gb/s) you also need to assure sufficient signal integrity, resulting in additional ground lines – at least one per signal pair.


    The star is supposed to be flexible and universal. This is why it must be able to control as many different devices as possible – but which? PCI Express as a replacement for PCI is especially used in a computer to communicate with further controller chips. It is the computer's classic "bus" extension and is software-compatible with its predecessor PCI. Mass storage devices today are controlled using SATA, especially hard disks that are directly built into a computer. A RAID system where the hard disks are included as individual plug-in cards is a typical example of this. USB is another feature that you can hardly imagine not to find in a state-of-the-art computer. There are three main fields of application for USB: removable storage (USB Flash drives, hard disks for data back-up etc.), wireless interfaces (GSM, UMTS, GPS, WIFI) and the replacement of the classic serial interface. FTDI has set standards here. Devices that were formerly controlled directly via an RS232/RS485 interface are now serviced through a USB-to-serial converter. USB 2.0 is an established standard. USB 3.0 will be integrated in the next-generation chipsets of Intel, AMD and so on. Computers are interconnected using Ethernet, so that Ethernet needs to be another point of the star. (Ethernet implemented exclusively in star topology is not sufficient – but this is a different story.)


    For a modular computer to be simple and transparent in use, every peripheral slot must have the same features, i.e. must support all interfaces. Then the user does not need to be concerned about which slot supports which board.