Comparing PXI and PXI Express

Most users are more familiar with PXI than PXI Express; although the two platforms are software compatible, they have significant interface differences.

PXI

PXI uses the PCI interface on a backplane; most of these modules operate at 33MHz and 32 bits wide to provide a BW more than adequate for their intended applications. There are just three types of slots:

• A system slot that accepts a controller or remote interface to a controller
• A Star Trigger Slot that can accept a peripheral module or a Star Trigger controller that drives modules that use the Star Trigger feature. The Star Trigger slot can be used as an ordinary Peripheral Slot. It is not common to use trigger systems on PXI switching modules as the trigger model is typically IVI (software) based.
• A peripheral slot that accepts any peripheral PXI module.
  
  

The backplane is shared between the modules and appears as a set of Bus numbers (corresponding to each of the segments of the PCI bus) with Devices on that Bus, generally numbered from 15 down. All the devices in a particular Bus number share the same 32-bit PCI bus segment — the standard limits the number of Buses to 256.

PXI Express

A PXI Express (PXIe) chassis uses the PCI Express serial interface to connect from its system slot to the peripherals. The system slot is not compatible with PXI, so it requires a controller or PCIe interface with an adequate number of PCIe connections to support the peripherals. Using the serial interface improves the available BW to the peripherals since, in principle, it is not shared BW. Each peripheral gets one or more serial connections with a bit rate of 2.5Gb/s (GEN1). As PCIe is a point-to-point connection system, each connection is defined as a Bus number and Device 0 (no other devices appear on that slot). As with PCI, there is a 256 Bus limit. The maximum module count is, therefore, lower than with PXI.

Using PXI Express does not guarantee faster system speeds, the most common bottlenecks on system speed have little to do with backplane speed, but speed advantages are on modules that receive or transmit large amounts of data.

The PXI Express mechanical and electrical interface is different from PXI. To make the best use of the fast PCI Express lanes, the chassis typically contains a mix of PXI and PXI Express slots (a Hybrid Chassis). So a chassis may detail several slot types. Typically the slots are defined as:

• Controller slot only accepts controllers designed explicitly for PXIe and Hybrid Chassis

• PXI Express slots ONLY accept PXI Express modules, of which only a relatively few types are available.

• Hybrid slots accept EITHER PXI Express or PXI modules, they have both sets of connectors, but the second PXI connector is shortened to allow the fitting of the PXIe connector. All Pickering switching modules are compatible with Hybrid slots.

• Legacy slots ONLY accept PXI modules

• Trigger slot can ONLY be occupied by a module designed to fill that slot, so it is not available for "normal" peripheral modules. Some chassis don't include the timing slot for this reason.

PXI Express chassis also vary in the number of PCIe lanes supported in each channel; this is transparent to the user, but for high-speed applications, there is considerable merit in ensuring that some of the PXIe slots have a high number of PCIe lanes since that is the main reason for specifying a PCIe capable chassis.

There are other electrical differences as well; in particular, the power supplies are different. PXI Express lacks the 5V and -12 V supplies. The 5V supply is a problem since the most commonly used relays have 5V coils - the parts stocked in distribution channels and are, therefore, most readily available for service support.

The vast majority of peripherals are PXI rather than PXI Express.

Bus Enumeration

PCI and PCIe use the same basic process to "discover" PXI(e) modules based on the two standards. A PCI(e) module is identified by a bus number and a device number. In PCI, a bus can have up to 16 devices on it (in reality, for PXI, eight devices, including the bridges at each end), the Bus number representing the bus segment, and the Devices representing the individual modules on that bus segment. In PCIe, things are a little different, as it is a point-to-point system (and not a multi-drop system like PCI); each module gets a Bus number. In both cases, PCI limits to 256 buses, including the buses internal to the control and any bridges or virtual buses associated with them. With PCIe, the theoretical maximum number of modules that can be connected is somewhat lower than PCI but still a reasonably large number compared to the bus count in a typical single chassis.

Some PCs also seem to have BIOS issues associated with the enumeration of long chains of Bus connections; for that reason, the PXIe chassis vendor may strongly recommend the use of tested embedded or remote controllers and provide a list of these models. It is not safe to assume that because one controller model appears from a particular manufacturer, then all controllers from that manufacturer work; this is certainly not the case. The issue appears to be driven by the fact that PCs generally do not expect large numbers of PCIe bus numbers, and supporting large bus numbers conflicts with user desire for fast boot times.

Summary

In general, Pickering advises the following:

• Use PXI Express if you need the backplane speed advantage, for example, to support frequent large file transfers or continuous high-speed data streams such as video. If your application is mainly switching, avoid using PXI Express, you are unlikely to fill the PXI Express slots, and switching products do not benefit from the higher backplane speeds of PXIe. Relay speeds do not change by having a high-speed backplane.
• Do not assume that conversion to PXI Express speeds up your system; data-intensive applications are commonly limited in speed by the PC processor being asked to manage all its tasks and its latency issues associated with the operating system and driver processing. On most PXI systems, the processor capability directly impacts command execution times, and, especially for Windows-based systems, latency or timing indeterminism in the operating system creates delays.
• Remember that the breadth of product and the number of vendors supporting PXI Express is much narrower than that of PXI. PXI Express is usually more expensive, partly because of the lack of competition but also because the provision of Hybrid slots is more expensive.

• If you need some PXIe slots, choose a chassis that is primarily or exclusively Hybrid PXI slots; either type can be fitted. Avoiding chassis with PXIe-only slots maximizes your chances of efficiently using the chassis. Ensure the chassis has an appropriate number of PCIe lanes to the critical slots. Choose the embedded system controller or remote interface carefully to ensure it has adequate bandwidth to support the required operating speeds and correctly manages the bus enumeration. Remember, in a PXIe or Hybrid Chassis; you must choose a PXIe controller. It should also be noted that Hybrid Chassis will be more expensive to manufacture than an equivalent PXI chassis because of their complexity.

Pickering's PXI & PXIe Statement of Intent

Pickering has a continuous R&D program, introducing many new and innovative PXI products yearly (over 1,000 PXI products). Most recent designs are being released simultaneously in both PXI and PXIe formats. Because the software and hardware functionality between PXI and PXIe versions are identical, we have already ported several hundred of our current PXI modules to also be available in PXIe, and we will continue to work our way thru our extensive PXI module range to add identical functionality PXIe versions. 

PXI will continue to be an essential platform for products that do not require the bandwidth that PXIe offers, both now and well into the future. Pickering will stay committed to both formats with our typical 15 to 20-year product support. If there is a specific Pickering PXI product that you need to be available in PXIe, please contact your local Pickering sales office for further assistance.