PCI/PCI Express/PXI Can Be Extended Anywhere with Bus Expansion Technology



Bus expansion technology is the simplest and most efficient way to provide more PCI slots for a host computer or to link PCI to other busses such as PCIe, PXI, or VME.
Bus expansion, or bus extension, is a technology that increases the number of bus slots of the same or different bus types for a host computer system. It is usually achieved by installing an extension card in the host computer and cabling it to a separate housing that contains a backplane and its own power supply to accommodate peripheral devices.

Typically, a bus expansion system is composed of three major parts, an extension card installed in the host computer, an extension chassis to provide more bus slots, and a specialized cable to connect them (Figure 1). The extension card works as a transparent bus bridge. Though the host computer and the extension chassis are physically separated, they are logically the same computer system. All cards installed in the extension chassis are recognized and operated as if they were installed in the host computer, without any additional drivers or software needed.

圖-PCI Express

Benefits of Bus Expansion Technology
The manifest benefit of a bus expansion system is increased bus slots. A modern desktop computer usually offers few PCI or PCI Express slots. If users need to plug multiple PCI cards into a single computer, bus expansion technology is the simplest and most efficient way to provide more PCI slots for a host computer.

Since the bus expansion system has a separate housing to accommodate peripheral devices, the interoperability of different buses also benefits users. For example, with a PCI-to-PXI extension solution, users can take advantage of the superb computing power of a server-grade computer and the accurate synchronization/triggering of PXI modules contained in the expansion chassis.

There are additional benefits provided by extension technology. For harsh environments, it’s a good idea to separate the computer and I/O cards so that users can place the computer in a safe place and place I/O cards near their sensors or actuators to be measured or controlled. The bus expansion system connects the host computer and extension chassis via a cable, which gives great system deployment flexibility.  And, by separating the computer and extension chassis, the measurement accuracy may be improved since all the I/O cards installed in extension chassis can be kept from high-frequency interference from the CPU, chipset, memory, and so on. The extension chassis contains I/O cards that can be modulized for easy replacement to reduce the mean time to replacement.

And remember, most bus expansion solutions provide complete hardware and software transparency. This means one can easily setup a bus expansion system without any additional drivers or software.

Types of Bus Expansion Solutions
Depending on the bus type in a host computer and an extension chassis, one can create many combinations of bus expansion solutions. The following are several existing bus expansion solutions on the market.

  • PCI-to-VME extension
    VME is a 32-bit bus developed in the early 80’s that has been widely used in the fields of instrumentation and military applications. The PCI-to-VME extension solution allows users to control thousands of VME products using a relatively powerful desktop computer in place of an expensive VME controller.
  • PCI-to-PCI extension
    PCI-to-PCI extension is a good solution to provide additional PCI slots to a desktop or industrial computer. It is usually bundled with an extension chassis with backplane and power supply installed. Users can choose extension chassis with different slot capacities according to their applications (Figure 2).

fig2

  • PCI-to-PXI extension
    PXI is a bus specification that combines the PCI bus and a special trigger/synchronization bus for instrumentation. Using the PCI-to-PXI extension solution with desktop or Industrial computers allows for interoperability with PXI modules or saves costs over relatively expensive PXI controllers (Figure 3).

fig3

  • PCI Express-to-PCI extension
    PCI Express is a new bus technology that delivers up to 4 GB/s of non-sharing bus bandwidth. Most modern desktop or server grade computers are equipped with PCI Express slots. With a PCI Express-to-PCI extension solution, users can adopt legacy PCI cards in new computers with PCI Express architecture.
  • PCI Express-to-PXI extension
    The same advantage of PCI Express can be also applied to the PCI Express-to-PXI extension solution, which allows computers with PCI Express slot to control PXI modules. Additionally, since the PCI Express bus features point-to-point connection, the extension system may have better latency and sustained data rates during data transmission (Figure 4).

fig4

Applied Bus Expansion Technology
PCI bus is a parallel bus which contains 32 data lines (32-bit PCI) or 64 data lines (64-bit PCI), while the PCI Express bus is a high-speed serialized bus which uses 2.5 Gbps LVDS signal pairs to transmit/receive data. Distinct extension technologies are used for different bus types. In this section, we’ll illustrate the technologies we use for ADLINK bus expansion solutions.

  • PCI-based extension technology (PCI-to-PCI, PCI-to-PXI, PXI-to-PXI)
    PCI bus is a 32-bit bus running at 33 MHz. By bridging the PCI bus signals directly to the extension chassis via a cable, the host computer can control an extended PCI bus as mentioned above. This solution, however, has some signification drawbacks. First, when transmitting 33 MHz clock signals over cables, the signals decay dramatically with an increase of cable length and the extension length is limited to 1.5 m. Second, the clock signals transmitted over the cable are highly susceptible to interference with no way to correct signal errors. This may cause system crashes due to severe interference.

Our company leverages a state-of-art StarFabric technology to design our PCI-based extension solution. StarFabric is a standard of data exchange using high-speed serialized signals and it works as a transparent PCI bridge and converts PCI bus signals to serialized StarFabric data format and transmits the data using LVDS signal pairs. Automatic error detection and correction are performed during the transmission to guarantee data accuracy, extending up to 10 meters with extremely reliable operation (Figure 5).

  • PCI Express-based extension technology (PCI Express-to-PCI, PCI Express-to-PXI)
    Different from the original PCI bus, PCI Express is inherently a serialized bus. Data between devices is transferred in the form of serialized data packets and over a 2.5 Gbps LVDS PHY. Designing PCI Express-based extension solutions is relatively easier than designing a PCI-based extension solution. A major concern is maintaining signal integrity while transmitting signals over cables and the key is to place a repeater, or equalizer, to enhance signal intensity. PCI Express-based solutions can provide up to 7 meters of extension distance, less than 0.5 µs latency and over 100 MB/s of sustained data transfer rate (Figure 6).

hsu_louis

Louis Hsu, manager of Measurement and IO Platform Verification and Application Product Center, ADLINK Technology

Louis Hsu joined ADLINK in 2001 as a hardware engineer. His work includes analog and digital circuit design. After six years as an electrical engineer, Hsu made the switch to product management in order to explore new markets and promote ADLINK’s digitizer and bus expansion product lines. Hsu is currently manager of the design verification and application team, aiming to enhance product quality. He has a Master’s Degree from National Taiwan University of Science and Technology. He can be reached at louis.hsu@adlinktech.com.

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