Comparing PXI and PXI Express

Most users are more familiar with PXI than PXI Express, although the two platforms are software compatible there are important interface differences between them.

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 slot:
  • 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 used 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 the that Bus, normally 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 requires the use of a controller or PCIe interface with an adequate number of PCIe connections to support the peripherals. The use of 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 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 can be seen on modules that receive or transmit large amounts of data.

The PXI Express mechanical and electrical interface is different to PXI. In order to make best use of the fast PCI Express lanes the chassis will typically contain a mix of PXI and PXI Express slots (a Hybrid Chassis). So a chassis may detail a number of slot types. Typically the slots will be defined as:

  • Controller slot. This will only accept controllers specifically designed for PXIe and Hybrid Chassis
  • PXI Express slots. These will ONLY accept PXI Express modules, of which only a relatively few types are available.
  • Hybrid slots. These will accept EITHER PXI Express or PXI modules, it has 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 which ONLY accept PXI modules
  • Trigger slot which can ONLY be occupied by a module designed to fill that slot so is not available for "normal" peripheral modules. Some chassis do not 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 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 particular problem since the most commonly used relays have 5V coils - and are the parts stocked in distribution channels and 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 a bus number and a device number. In PCI a bus can have up to 16 devices on it (in reality for PXI 8 devices including the bridges each end), the Bus number representing the bus segment and the Devices 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 its own Bus number. In both cases PCI limits to 256 buses, and this has to include 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 therefore rather lower than PCI but still a reasonably large number compared to the bus count in a typical single chassis.

Some PC's also seem to have BIOS issues associated with 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 will work, this is certainly not the case. The issue appears to be driven by the fact that PC's 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 really the 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 are not changed by having a high speed backplane.
  • Do not assume that conversion to PXI Express will speed 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 has a direct impact on command execution times and, especially for Windows based system, latency or timing indeterminism in the operating system creates delays
  • Remember the breadth of product and the number of vendors supporting PXI Express is much narrower than PXI. PXI Express is also usually more expensive, partly because of the the lack of competition but also because the provision of Hybrid slots is more expensive
  • If you do need some PXIe slots choose a chassis which is primarily or exclusively Hybrid PXI slots so either type can be fitted. Avoiding chassis with PXIe only slots will maximize your chances of efficiently using the chassis. Make sure the chassis has an appropriate number of PCIe lanes to the critical slots and chose 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 chose a PXIe controller. It should also be noted that because of their complexity Hybrid Chassis will be more expensive to manufacture than an equivalent PXI chassis.

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