Even though the two platforms are software equivalent, there are substantial interface differences between them, and PXI is more widely used than PXI Express.

PXI

The majority of these modules run at 33MHz and 32 bits wide to provide a BW that is more than sufficient for their intended applications. PXI employs the PCI interface on a backplane. There are just three distinct slot types:

• A computer slot that can accommodate a controller or a controller’s remote interface
• A Star Trigger Controller that drives modules that employ the Star Trigger function, or a Star Trigger Slot that can receive a peripheral module. You can use the Star Trigger slot just like any other peripheral slot. Trigger systems on PXI switching modules are uncommon because the trigger model is often IVI (software) based.
• A slot for peripherals that will take any PXI module.

The backplane, which is shared by the modules, is represented by a series of Bus numbers (one for each PCI bus segment) and devices, typically numbered from 15 down. The 32 bit PCI bus segment is shared by all of the devices in a given Bus number; the standard caps the number of Buses at 256.

PXI Express

A PXI Express (PXIe) chassis connects to its peripherals from its system slot using the PCI Express serial interface. Due to the system slot’s incompatibility with PXI, a controller or PCIe interface with enough PCIe connections to enable the peripherals must be used. Since each peripheral receives one or more serial connections with a bit rate of 2.5Gb/s, the usage of the serial interface increases the amount of BW that is accessible to the peripherals. In theory, this BW is not shared (GEN1). Each connection in the point-to-point PCIe system is identified by a Bus number and Device 0. (no other devices appear on that slot). There is a 256 Bus limit, just like PCI. As a result, the maximum module count is smaller than with PXI. The most frequent bottlenecks in system performance have little to do with backplane speed, so using PXI Express does not ensure greater system speeds. Instead, speed advantages are found in modules that receive or transmit huge volumes of data. The mechanical and electrical interface of PXI Express is distinct from PXI. The chassis often includes a mix of PXI and PXI Express slots to make the most of the fast PCI Express lanes (a Hybrid Chassis). Therefore, a chassis may include many slot kinds. The slots are typically described as:

• Only controllers made expressly for PXIe and hybrid chassis will fit in the controller slot.
• Only PXI Express modules, of which there are comparatively few variants, can be used in PXI Express slots. Hybrid slots include both sets of connectors but the second PXI connector is reduced to accommodate the fitting of the PXIe connector.
• They accept EITHER PXI Express or PXI modules. The majority of our switching modules work with hybrid slots.
• Legacy slots accept PXI modules EXCLUSIVELY
• Trigger slot cannot be used by “regular” peripheral modules since it can only be filled by a module created specifically for that slot. For this reason, some chassis don’t have a timing slot.

Since high-speed applications are the primary reason for specifying a PCIe capable chassis, PXI Express chassis also differ in the number of PCIe lanes supported in each channel. This is transparent to the user, but there is considerable merit in ensuring that some of the PXIe slots have a high number of PCIe lanes. There are more electrical changes as well; the power sources, in example, differ. Supplies for 5V and -12V are absent from PXI Express. Since the majority of relays have 5V coils and are the parts that are stocked in distribution channels and are thus most easily accessible for service support, the 5V supply is a special issue. PXI rather than PXI Express is the standard for peripherals.

Bus Enumeration

In PCI, a bus can support up to 16 devices (in actuality, PXI supports 8 devices, including the bridges at each end), with the Devices referring to the individual modules on the bus segment and the Bus Number designating the bus segment. A bus number is assigned to each module in PCIe because it is a point-to-point system rather than a multi-drop system like PCI. In all situations, PCI has a 256 bus limit, which must include any bridges or virtual buses connected to the control’s internal buses. The theoretical maximum number of modules that may be connected via PCIe is thus a little less than that of PCI, but it is still a sizable number when compared to the number of buses in a typical single chassis. The PXIe chassis provider may strongly advise the usage of tested embedded or remote controllers and supply a list of these models because some PCs appear to have BIOS issues related to the enumeration of long chains of Bus connections. It is dangerous to believe that just because one controller model is available from a certain manufacturer, all controllers from that manufacturer will function; this is undoubtedly not the case. The problem seems to be caused by the fact that PCs often do not anticipate having many PCIe bus numbers, and therefore supporting many bus numbers contradicts with users’ desires for quick boot times.

Summery

In general the following is what we advise:

1. Use PXI Express if you require the faster backplane, such as to support continuous high-speed video streams or frequent massive file transfers. Avoid adopting PXI Express if your application primarily involves switching; you are unlikely to fill the PXI Express slots, and switching devices do not benefit from PXIe’s faster backplane speeds. A high-speed backplane does not modify the relay speeds.
2. Do not expect that switching to PXI Express would speed up your system. Data-intensive applications are frequently slowed down by the PC processor’s need to handle multiple activities at once as well as latency concerns related to the processing of the operating system and drivers. On the majority of PXI systems, the processor speed directly affects command execution times, and delays are also caused by operating system latency or timing indeterminism, particularly on Windows-based systems.
3. Keep in mind that PXI Express has a significantly smaller range of products and fewer vendors supporting it than PXI. Because there is less competition and it costs more to provide Hybrid slots, PXI Express is typically more expensive.
4. If you do require certain PXIe slots, you can choose a chassis that has mostly or all Hybrid PXI slots installed. Your chances of using the chassis effectively are maximized by avoiding chassis with PXIe-only slots. Choose the embedded system controller or remote interface carefully to ensure it has the bandwidth to meet the necessary operating speeds and correctly controls the bus enumeration. Ensure the chassis has an enough number of PCIe lanes to the critical slots. Keep in mind that you must select a PXIe controller for a PXIe or hybrid chassis. Additionally, it should be emphasized that hybrid chassis will cost more to produce than a PXI chassis of comparable quality due to its complexity.