PXI Bus and Backplane

A summary and notes of the PXI bus and PXI backplane used within the PXI test system used for test instrumentation.

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The PXI bus system enables the overall PXI technology to offer a great amount of flexibility in terms of controlling the different instrumentation cards that are available. In view of this, the PXI test instrumentation system is used in a wide variety of applications from automatic test equipment to data acquisition.

The bus is carried along the PXI backplane. This not only links the PXI bus to the individual cards, but also supplies other essentials such as power to the cards.

PXI bus basics

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 bus system builds on the basic PCI standard and implements these in the form of a dedicated system clock and triggers. These features include the following:

  • PXI bus reference clock
  • PXI Local bus
  • PXI trigger bus
  • PXI star trigger bus
  • Slot to slot local bus

In addition to this the specification defines a slot for the PXI system controller.

PXI bus 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 bus 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 reference 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. This is important to ensure that triggering relative to the operation of the overall system remains constant.

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 bus triggers

There are three triggers that are defined within the PXI standard. Each type defines a different form of trigger capability, enabling the standard to offer a variety of different trigger functions dependent upon the requirements of the measurement.

  • PXI 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 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 bus power supplies

The PXI backplane carries the power for the various cards within the chassis. The power supplies must naturally be standardised to enable a variety of cards to operate satisfactorily within their allocated slots.

Most VXI chassis support a variety of different supply voltages including +5V, +3.3V as well as +12v and -12V. Supply capabilities vary, but the PXI standard version 2.1 provides for the following capabilities:

Chassis backplane power supply capabilities
Figures for Version 2.1 of the PXI standard.
  System slot Peripheral slot System slot Peripheral slot All All
Nominal supply voltage (V) +5 +5 +3.3 +3.3 +12 -12
Max recommended current (A) 6 2 6 2 0.5 0.25

In order to remain within the maximum current capability of the pins of 1 amp on the backplane, the high current capacity supplies require multiple pins. For example the 5V system slot +5V supply requires six pins.

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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