PXI Express FAQ
Print
Overview
This document provides answers to frequently asked questions about PXI Express. It assumes a basic familiarity with the PXI platform.
Table of Contents
General
What is PXI Express?
PXI Express technology is the latest addition to the PXI platform. The PXI Express Specification integrates PCI Express signaling into the PXI standard, which increases backplane bandwidth from 132 MB/s to 6 GB/s, a 45 times improvement. It also enhances PXI timing and synchronization features by incorporating a 100 MHz differential reference clock and differential triggers. The PXI Express specification adds these features to PXI while maintaining backward compatibility.
How does PXI Express compare to other buses in bandwidth and latency?
Bandwidth measures the rate at which data is sent across a bus, typically in megabytes per second, while latency measures the delay in transmission of data across a bus. With PXI Express, users achieve a maximum 6 GB/s controller-to-backplane bandwidth, the highest bandwidth available in the test and measurement industry. Additionally, up to 2 GB/s of bandwidth can be dedicated to each peripheral slot, depending on the system specifics.
PXI Express offers the lowest (best) latency of all mainstream commercial test and measurement bus technologies, delivering latency comparable to—and in some cases, better than—PCI-based PXI. PXI Express latency is several orders of magnitude better than the latencies of external buses such as USB or Ethernet.
Does PXI Express replace PXI?
No. PXI Express is part of the PXI platform, and National Instruments and others will continue to invest in that platform by developing products that are based on both PCI signaling and PCI Express signaling. Many applications, including general data acquisition and motion control, do not require the increased bandwidth of PXI Express, so users need to choose which specification within the PXI platform is right for them. PXI systems already serve a large installed base of applications, and PXI systems based on PCI signaling will be deployed in large numbers for many years to come. Additionally, because PXI Express chassis from NI include both PXI peripheral slots and PXI Express hybrid peripheral slots, both existing PXI peripheral modules and PXI Express peripheral modules can be used together in the same chassis and systems.
See Also:
What Is PXI?
Compatibility
Is PXI Express backward compatible with PXI?
Yes. PXI Express maintains both software and hardware compatibility with PXI peripheral modules.
- Software: PCI Express uses the same operating system and driver model as PCI, resulting in complete software compatibility between PCI-based systems (such as PXI) and PCI Express-based systems (such as PXI Express). This software compatibility is ensured by the PCI Special Interest Group (PCI-SIG), a group composed of member companies, such as Intel, who are committed to the development and enhancement of the PCI and PCI Express standards.
- Hardware: PXI Express chassis provide hybrid peripheral slots that accept both PXI Express peripheral modules and hybrid slot-compatible PXI peripheral modules. These peripheral slots deliver signaling for both PCI and PCI Express.
Can I use my existing code written for previous PXI systems?
Yes. You can use code you have written for previous PXI systems with PXI Express systems because PXI Express maintains complete software compatibility with PXI. Software compatibility includes operating systems such as Windows XP and Linux®, application software such as Microsoft Office and NI LabVIEW, and user code such as LabVIEW VIs and C++ projects.
What are the different types of slots in a PXI Express chassis?
A PXI Express chassis can include:
- A system slot, which accepts an embedded or remote PXI Express controller
- PXI peripheral slots, which accept PXI modules
- PXI Express hybrid peripheral slots, which accept PXI Express peripheral modules, 32-bit CompactPCI peripheral modules, and hybrid-compatible PXI peripheral modules
- A system timing slot, which accepts both PXI Express peripheral modules and PXI Express system timing modules
What is a PXI Express hybrid slot?
PXI Express chassis have two kinds of peripheral slots: PXI peripheral slots and PXI Express hybrid peripheral slots. The PXI hybrid peripheral slots, shown below, can accept either PXI Express peripheral modules, 32-bit CompactPCI boards, or hybrid slot-compatible PXI modules.
[+] Enlarge Image
Figure 2. A PXI Express Hybrid Slot Accepts PXI Express Peripheral Modules, 32-Bit CompactPCI Boards, and Hybrid Slot-Compatible PXI Modules
What is a hybrid slot-compatible PXI module?
National Instruments PXI modules that do not include a J2 connector are already hybrid slot-compatible. For NI modules that do include the J2 connector, you must replace that physical connector to achieve compatibility with PXI Express hybrid peripheral slots. As shown below, the J2 connector is removed and replaced with the smaller eHM connector. You can still use the resulting hybrid slot-compatible module in existing PXI peripheral slots.
For NI modules that are modified, only the backplane connectors are replaced; the rest of the module remains unchanged. This modification takes advantage of the fact that the lower portion of the J2 connector is largely unused in 32-bit PXI modules, with the exception of its use in local bus.
[+] Enlarge Image
Figure 3. Connector Modification of a PCI-Based PXI Module for Hybrid Slot Compatibility
What is going to happen to my existing PXI equipment?
You can use existing PXI peripheral modules in both PXI Express chassis that have hybrid slots and in PXI chassis. Because the PXI Express Specification offers a feature set enhancement and not a replacement for PXI, National Instruments and others will continue to develop and sell PXI controllers, chassis, and modules based on PCI signaling.
To use existing PXI peripheral modules in a National Instruments PXI Express chassis, simply insert the module into one of the several PXI peripheral slots available. If the existing equipment does not have a J2 connector, you also can place the module in a hybrid slot. If there are more existing PXI modules that have J2 connectors than there are PXI slots available, you can send the remaining PXI modules to National Instruments to be made hybrid slot-compatible for a nominal fee.
Can I send in my National Instruments PXI modules to make them hybrid slot-compatible?
National Instruments modifies existing NI PXI modules for hybrid slot compatibility for a nominal fee, but, in most cases, this is not necessary. PXI Express chassis from NI include both PXI slots and PXI Express hybrid slots, so you can use your existing (unmodified) modules with the new chassis. For PXI modules sent to National Instruments, NI achieves compatibility by removing the J2 connector and replacing it with the smaller eHM connector. It should be noted that not all existing modules can be made hybrid slot-compatible. For example, legacy products such as E Series PXI data acquisition modules are not modified.
Will future PXI modules from NI be natively hybrid slot-compatible?
Yes. National Instruments now incorporates the smaller eHM connector on most of its PXI modules that previously used the J2 connector. As a result, you may use these modules in either existing PXI peripheral slots or PXI Express hybrid peripheral slots. Again, not all existing modules are made hybrid slot-compatible. For example, legacy products such as E Series data acquisition modules cannot transition to the new connector.
Technical
Is there a comparable specification for CompactPCI based on PCI Express signaling?
Yes. The CompactPCI Express specification, upon which PXI Express was built, was released in June 2005. The specification is owned and maintained by the PCI Industrial Computer Manufacturers Group (PICMG).
What does x1, x4, and x16 mean?
With PCI Express, data is sent serially through pairs of transmit and receive connections called lanes, which enable data transfer at 250 MB/s per direction. Multiple lanes can be grouped together into x1 (“by-one”), x2, x4, x8, x12, x16, and x32 links to increase bandwidth to the slot. For example, a x16 slot would have bandwidth of 4 GB/s per direction (250 MB/s * 16). Though different lane widths correspond to different physical slot sizes in PCI Express, PXI Express slots and connections do not differ physically based on the lane width of the link.
Does every PXI Express slot necessarily have its own dedicated bandwidth?
The answer depends on the implementation of the PCI Express bus in the chassis and controller. If the PCI Express lanes for the slot are directly linked to the controller chipset, then the slot has its own dedicated bandwidth. But if the controller and/or chassis use a PCI Express switch to split the link so that it fans out to several slots, those slots share the bandwidth. Controllers and chassis that fall into both the directly linked and switched categories will be available. The first 8-slot PXI Express chassis and the first PXI Express embedded controller will provide dedicated (x1) bandwidth to each PXI Express-enabled slot.
How is local bus affected?
The PXI local bus provides a daisy-chained connection between each PXI peripheral slot and the adjacent PXI peripheral slots to its left and right. With the exception of a single local bus pin, local bus is not available in the PXI Express hybrid slots or on PXI Express modules. The physical connector space previously used by local bus is occupied now by a connector for PCI Express signaling. Local bus is, however, still available between adjacent PXI slots in the PXI Express chassis, and will, of course, continue to be available in the future PCI-based PXI chassis.
What is the relationship between MXI Express and PXI Express?
With MXI Express technology, you can achieve transparent remote control of a PXI or PXI Express chassis from a PCI Express slot in a PC or an ExpressCard slot in a laptop. The "Express" in MXI Express refers to the interface on the PC/laptop side rather than the chassis to which the interface is connected.
Linux® is the registered trademark of Linus Torvalds in the U.S. and other countries.
Related Links:
PXI Specification Tutorial
Print
Reader Comments | Submit a comment »
Legal
This tutorial (this "tutorial") was developed by National Instruments ("NI"). Although technical support of this tutorial may be made available by National Instruments, the content in this tutorial may not be completely tested and verified, and NI does not guarantee its quality in any way or that NI will continue to support this content with each new revision of related products and drivers. THIS TUTORIAL IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND AND SUBJECT TO CERTAIN RESTRICTIONS AS MORE SPECIFICALLY SET FORTH IN NI.COM'S TERMS OF USE (http://ni.com/legal/termsofuse/unitedstates/us/).