Comparing PXI and SCXI

Pickering Interfaces have been asked about the relative merits of PXI and SCXI systems. This application note is intended to give users of modular test systems guidance on the two systems. The information supplied is believed to be correct at the time of writing, March 2009.
PXI and SCXI are both modular systems intended for use in test and measurement systems.

PXI SYSTEM

The PXI standard is an open standard controlled by the PXI Strategic Alliance (PXISA). The specification for the PXI standard is published on the PXISA web site and on the web sites of its members, including Pickering Interfaces.You can also get our free PXImate book for a more simple explanation of PXI.

PXI is based on the PCI and cPCI standards derived from industry standard computing systems. A chassis accepts modules with front connectors to provide functional access and a PCI based backplane system at the rear controls the functions within the modules.

Measurement results can be extracted over the PXI backplane directly into a computer's PCI backplane for capture, analysis and display of results in a PC based software environment.

The standard specifies physical and electrical properties of the modules and the chassis. The standard also defines the software framework for the operation of the modules.

PXI provides system support for switching, data acquisition, signal conditioning and instrumentation modules in a common chassis.

There are a large number of manufacturers and suppliers of PXI products who can be located from the PXISA website.


SCXI SYSTEM

SCXI is a proprietary standard developed by National Instruments (NI) and is primarily used for data conditioning applications. Some switching functions are also available in SCXI. Data acquisition is typically performed by a PCI card in an external PC.

NI does not openly publish the standard, but there are a few small third party companies that manufacture SCXI compatible modules who can be found from the NI web site.

Rather confusingly NI usually include SCXI products in their PXI catalogue, although mechanically and electrically the standards are substantially different.

In a typical SCXI system a proprietary chassis is used which can support SCXI modules. Input signals are provided to the front panel connectors of the modules, often via a terminal block system. The chassis provides backplane interconnection (analogue and digital) to exchange signals between modules. The digital backplane controls the function of the modules. The analogue backplane routes the signals between modules and enables one module to route the signals to one Data Acquisition Card (DAQ).

Each SCXI module has a rear connector that can be used to connect to the analogue output from the chassis to the DAQ. In most applications a single module is used as the "master" and is connected by a proprietary cable to the DAQ card. The DAQ card is typically based on a PCI card inside an external PC.


MIXING PXI AND SCXI SYSTEMS

It is practical to mix SCXI and PXI. However, the two standards are mechanically different (SCXI cards are larger than 3U PXI) and electrically very different.

To use a mixed system for Signal Conditioning the user can either purchase a mixed SCXI/PXI single source chassis from NI or use two different chassis, typically controlled from an external PC. Since both systems are (at this level) then controlled via the PCI bus they can operate together in a common software environment.

This forces the user to make some crucial decisions, often at a stage where some uncertainty in the system design may remain:

Use of two chassis systems, one of which is proprietary, with the cost and system size penalties that this incurs

Use of a mixed chassis from a single source supplier, forcing parts of the system to be from a single source and fixing the proportion of PXI and SCXI at an early stage in the project.

A more effective solution is to adopt PXI, giving the user freedom of choice of supplier for every part of the system and not forcing early partitioning of the system into two different chassis systems.

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