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.
We tend to specify PXI in a system for two key reasons:
We can use these in these systems because of some essential characteristics of PXI.
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.
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.
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.
As with all National Instruments equipment, it is a PC based technology. Similar to compactDAQ we have two options:
Sounds great, so why don’t we specify it everywhere?
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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