| PXI technology summary |
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.
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