PXI technology summary

- a summary, overview or tutorial about the PXI technology and the PXI standard for test equipment or instrumentation used for test, automation and data acquisition applications.

The PCI eXtensions for Instrumentation (PXI) specification defines a fast, cost effective instrumentation platform developed specifically for electronics test, measurement, automation and data acquisition applications. PXI is based around the modular Eurocard mechanical packaging system which enables it to be versatile and robust.

The PXI standard was born in 1997 when it was launched by National Instruments. Now it is managed by the PXI Systems Alliance (www.pxisa.org). This is a group that has more than 60 member companies. With the open architecture, and based around the PCI specification, this has enabled PXI technology to grow very rapidly. As a result a large number of products using PXI technology are available and the system has become an industry standard.

Major features of PXI technology

The PXI system boasts many features that make it a flexible platform for many applications:

• 33 MHz performance



• Up to seven peripheral slots available per bus segment



• 32 bit and 64 bit transfers



• Peak data rates of 132 Mbytes/sec (32 bit) and 264 Mbytes/sec (64 bit) attainable



• Plug and play capability



• Convenient Eurocard standard boards



• High performance connector specified

These features make PXI an ideal choice for many data acquisition, test and measurement applications.

PXI Electrical features

Although the PXI system is based around the PCI standard, this system cannot be used directly in this format. Many test equipment and data acquisition applications require accurate timing capabilities and clocks that cannot be implemented using the standard PC specifications including PCI and CompactPCI (cPCI) as well as ISA. The reason for this is that there is no reference clock. The PXI system builds on the basic PCI standard and implements these in the form of a dedicated PXI system clock and triggers. These triggers consist of the PXI trigger bus, PXI star trigger bus, and a slot to slot local bus that is available for use advanced timing and synchronisation requirements. In addition to this the specification defines a slot for the PXI system controller.

System slot

The position of what is termed a PXI system slot is defined. The location of this is on the left end of the PCI bus segment in a basic PXI system. This arrangement is a subset of the numerous possible configurations allowed by CompactPCI where it may be anywhere on the back plane. Defining a specific location for the system slot provides a number of advantages including a simplification of integration and an increase in the degree of compatibility between PXI controllers and chassis. Also the PXI specification requires that where necessary the system controller module is able to expand to the left into what are defined as controller expansion slots. By carrying this expansion to the left this prevents the system controllers from using up valuable peripheral slots.

PXI Reference Clock

In many applications it is necessary to accurately synchronise measurements or other actions to a single clock. The PXI standard allows for this by proving a 10 MHz TTL reference clock. The basic accuracy of the clock not mandated by the standard and is dependent upon the actual oscillator fitted to the chassis. Typically it will be better than 25 ppm. However the standard does specify that the track lengths from the oscillator shall be the same so that the skew between the edges reaching the different cards is less than 1 pS.

Trigger bus

There are many instances where a trigger is required, and the trigger bus can be used for many of these applications. The PXI standard defines a bus consisting of eight separate trigger lines. They enable synchronisation and timing signals to be passed from one module to another where one module may act as a master passing timing or synchronisation to the others that may act as slaves.

The trigger lines allow triggers, clocks, or handshaking signals to be transferred, although it is recommended that clock signals above 20 MHz are not transferred along this bus because of the signal degradation that may be suffered. Nevertheless the trigger bus is particularly useful for most applications.

Star trigger bus

The star trigger bus is a PXI bus that adopts a different approach to that of the ordinary trigger bus. It is used for applications where a high speed trigger with low levels of delay and skew are required. To achieve this, an independent line is routed from what is termed the star trigger slot (slot 2 in the PXI chassis) to each of the other slots in a star configuration. Again the line lengths are matched to ensure that the propagation delays are matched to within 1 pS.

PXI Local bus

A third form of PXI bus is known as the local bus. This receives its name because it is a daisy chain bus that connects one slot with the adjacent slots. The bus is 13 lines wide, and allows both digital and analogue (up to 42 volts) signals to be passed over it. In this way signals that may be required to be transferred within the chassis can be accommodated.

PXI software

An essential element of virtually every piece of electronics equipment these days is the software, and the PXI bus is no exception. By defining standards required, it ensures interoperability between all PXI modules, regardless of the manufacturer.

The PXI standard defines a common operating system framework and the relevant interfaces for software drivers for the peripherals. This is based around the requirements of Microsoft Windows. This approach not only does this allow for communication between the controller, and modules but also many industry standard software packages.

Most PXI instrument modules are register-based products. They use software drivers that are hosted on the central controller PC to configure in the way that they are needed for the particular application in question. By adopting this technique it enables them to provide considerably more flexibility as the controller computer is able access the instrument directly and this simplifies the embedded software in the modules while enabling a high level of flexibility to be obtained. The open architecture used for PXI technology allows hardware to be reconfigured to provide new facilities and features that are difficult to emulate in comparable bench instruments.

Mechanical aspects

The mechanical design of the PXI system is equally important as the electrical design and standards. PXI technology is based around the Eurocard packaging system. This provides a number of advantages including a system that is already established. Furthermore the connectors that are used are the IEC-1076 style. The pins are on a 2 mm pitch giving a very dense connection system. In addition to this they are impedance matched to provide the required performance at high frequencies.

The PXI system supports the two sizes. The 3U standard defines modules that are 100 by 160 mm (3.94 by 6.3 in.). These have two interface connectors. One carries the signals required for the 32-bit PCI local bus and the other carries the signals for 64-bit PCI transfers and the signals for implementing PXI electrical features. The 6U form factor defines modules that are 233.35 by 160 mm (9.19 by 6.3 in.). These may carry up to two additional connectors for future expansion of the PXI specification. The larger card size also allows for a additional circuitry that may be required for some instruments.

PXI summary

With these specifications set down, PXI technology is able to provide a resilient test and data acquisition approach that can meet the needs of a large number of applications within the electronics industry. PXI technology is widely used for general test equipment applications, as well as for test, automation and data acquisition. It is possibly for use within data acquisition systems that it has gained most of its use. For these data acquisition systems it enables a compact flexible system to be created at a reasonable cost. Accordingly PXI has become one of the leading standards for test, measurement and automation.