What Is PXI?

By James McNally | Technologies

PXI (and PXIe) are PC based technologies that instrument vendors have extended for use in instrumentation (PXI is an acronym for PCI eXtensions for Instrumentation).  The PXI Systems Alliance was formed in 1998 to manage the standard.

This article is designed to give you a feel for what the technology is and when it is used for data acquisition and test applications in the R&D process.

Why Use PXI?

We tend to specify PXI in a system for two key reasons:

  1. High Channel Count – Each card can support more channels than a compactDAQ card which makes them cheaper and easier to use in high channel count systems. An example for this is our case study with Lontra. We had a smaller system on compactDAQ and then went to PXI when we needed to create a new, larger test bed.
  2. High Speed/High Performance I/O – The PXI platform provides more power, bandwidth and physical space than compactDAQ or compactRIO. Signal conditioned I/O is limited to around 50kS/s on these platforms, but PXI provides the SC express family which doubles that. For higher performance, some digitisers have very high rates (up to 2 GS/s), and the FlexRIO FPGA cards offer high-throughput processing.

We can use these in these systems because of some essential characteristics of PXI.

Important Puzzle Pieces

1. Communications Bus

The PXIe platform provides the PCIe bus, commonly found in desktop computers, as a communications bus to the system computer. By leveraging this, PXIe delivers very high bandwidth and low latency communication to provide performance that is not achievable through external buses. This bandwidth is why, for a large channel count or performance level, this is the obvious choice in NI’s portfolio.

This graph isn’t the best comparison either as for compactDAQ and PXI that bandwidth is shared between all cards. For PXIe this is actually per card so the overall system bandwidth can be much higher.

2. Physical Packaging

The PXI platform provides standards around physical design based on the CompactPCI standard. This form-factor offers a more robust design than PCIe physical and for cooling and power.

It also has useful features for installation such as front-facing connectors and the option to install it on a desktop or 19″ rack. There are options for 4-18 slot chassis to fit these different use cases.

The most important physical feature of PXI over compactDAQ may just be that it is larger! This means that each card can use more power and there is more room for connectivity than on a compactDAQ system. Very high-density cards may still have a mass-interconnect input, but then there are 19″ rack accessories to break these out into individual channels.

3. Timing and Synchronisation

Timing and synchronisation are one of the most critical improvements over PCI. The extension to PCI technologies is that the connector includes dedicated lines for sharing clocks and trigger signals making it very easy to create synchronised acquisition systems with nanosecond precision.

The synchronisation is very important for the high channel count applications. We need to make the system look like a single acquisition which means ensuring every piece is sampling at the right time. The timing is achievable on compactDAQ but becomes complicated if you need to use multiple chassis.

Where is the Computer?

As with all National Instruments equipment, it is a PC based technology. Similar to compactDAQ we have two options:

  1. Integrated Controller – Slot 1 of a PXI chassis provides room for a controller. PXI controllers are  PCs compacted into the PXI form factor. The controllers have processor options from Intel Atom to Intel Xeon processors depending on your need.
  2. Remote Control – Slot 1 can also contain a remote control board. You can pair this with a controller card in a PC to allow for remote control through what is called MXI. The MXI link makes it appear to the PC that the cards are installed locally.
    This approach is popular when there is already a PC available that you want to use. The latest version offers a Thunderbolt 3.0 controller which means if your laptop supports it, this can also be a host device.

Why Wouldn’t You Use PXI?

Sounds great, so why don’t we specify it everywhere?

  1. Size – A PXI chassis is larger than the cDAQ chassis, so there are space savings to be had where the channel count isn’t required.
  2. Industrial Compatibility – PXI systems are more rugged than a PC, but they are still intended for relatively stable environments. compactDAQ systems have some specific advantages in industrial environments:
    1. Higher temperature and vibration specification.
    2. DC powered at typical industrial ranges.
    3. Fanless design for more comfortable integration and maintenance.
  3. Cost at Low Channel Counts – For high channel count systems PXI can be cheaper. If you only need 4 – 16 channels, then the high-channel cards cost more than having a couple of 4 channel C-Series modules.

Learn More

That should give a better feel for what PXI is all about. If you want to find out more you can:

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About the Author

I founded Wiresmith Technology to help engineers improve their systems and products with quality measurement systems. I'm a Certified LabVIEW Architect, Certified LabVIEW Embedded Developer and LabVIEW Champion.

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