Extension to the standard
07 March 2007
David Owen, Business Development Manager at Pickering Interfaces, looks in-depth at PXI Standard and PXI Express.
PXI Standard-
PXI, or PCI Extensions for Instrumentation, is a test and measurement standard developed by the PXI Systems Alliance, of which Pickering Interfaces is a Board Level member.
The standard describes a modular platform which is based on cPCI specification, but adds additional hardware trigger and local bus interconnect systems. Modules inserted in the chassis are controlled through a standard PCI electrical interface that enables each of the devices to be a direct extension of a PC¡¦s internal PCI Bus. Typically the modules are directly controlled at the register level, relying on drivers and software tools running on a central computer to present the instrument functionality. The standard is PC centric relying on the controller to process raw information from measuring instruments to present results in meaningful form. Most modules use a 33 MHz 32-bit PCI interface to allow the exchange of high-speed data essential for the controller to provide instrumentation emulation for the PXI modules.
Controllers can either be embedded in the chassis or connected to the chassis via a control interface in Slot 1 of the chassis.
Chassis and modules have to conform to specifications that control the form factor, power supply requirements, and cooling aspects of the PXI system. Two mechanical formats are specified, 3U and the less ubiquitous 6U, each with the same depth of module. Power for modules is provided from a chassis via the backplane connectors. Triggers include a PXI trigger bus that provides a backplane based trigger bus with point to multipoint capability and a Star Trigger that provides point to point operation.
The PXI standard ensures:
„h PXI modules designed within the standard can be operated in any vendor¡¦s chassis
„h New designs can be created quickly through the use of the PCI interface
PXI solutions are ideal where a high diversity of different sorts of devices are required in a small footprint, particularly where the functionality and parametric performance required of the module are easily addressed in this form factor.
PXI Express-
This is a new extension to the PXI standard, based on the cPCI Express specification that uses a PCI Express control interface. The format is similar to PXI but it uses different backplane and module connectors. The increased BW of the Express interface makes the new standard ideal for applications such as high-definition streaming video where the exchange of large amounts of data in real time is required. Since these applications are relatively small parts of the overall market for modular instrumentation, the earlier version PXI products are expected to dominate the market for many years.
PXI Express chassis are expected to contain both PXI Express slots, where high data applications are concentrated through PCI Express lanes, and PXI slots where standard PXI modules are to be supported through PCI interfaces. Chassis will have Hybrid Slots which will support both the PCI and the PCI Express interface on the backplane, but only one interface is fitted to the module. PXI Express modules use just +3.3V and the +12V power supplies whereas PXI modules have four supplies (additional +5V and ¡V12V).
PXI Express modules cannot be inserted in PXI slots; they can be used in Hybrid Slots or PXI Express slots. PXI Modules cannot be inserted in PXI Express slots; they can be inserted into Hybrid Slots provided they do not use the Local Bus and that if they use the PXI Trigger system they use a shortened J2 connector (referred to as eHM).
Since applications that require high BW to exchange data will often want to use as many PXI Express lanes as possible to the critical modules, and the number of PXI Express lanes available is finite, it is likely that PXI Express chassis (particularly the larger ones) will have a mix of PXI Express or Hybrid slots combined with PXI slots. The PXI Express chassis will need to be selected to provide the required configuration that supports the high-speed applications in PXI Express and other functions in PXI. This integration of the original PXI standard with newer higher speed applications means PXI Express will keep PXI in the forefront of many test applications for years to come.
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