The PXI StandardThe PXI standard is defined by the PXI Systems Alliance and was developed in response to the needs of test systems developers and users who required a new platform that is high-performance, functional, reliable, compact, yet easy to integrate and use. By leveraging off of the PCI, CompactPCI and VXI standards, PXI brings together the right technologies for PC-based test and measurement, instrumentation, and industrial automation. Further, since PXI is a PC-based platform, it maintains software compatibility with industry-standard personal computers, as well as all PC-based operating systems, software tools, and instrument drivers. Not only is PXI fully compatible with existing operating systems and software, it is fully compatible with the Virtual Instrument Software Architecture (VISA) standard that was created by the VXIplug&play Systems Alliance. VISA is used to locate PXI resources as well as communicate with PXI, serial interface, VXI, and GPIB peripheral modules and is supported by test development software packages such as ATEasy™, LabVIEW™, LabWindows/CVI™ and Agilent VEE™.
The PCI bus is a 33/66MHz 32/64bit bus that offers peak data transfer rates of 132 Mbytes/s (32-bit, 33MHz) and 528 Mbytes/s (64-bit, 66 MHz). PXI expands upon the PCI bus by using the rugged form factor of CPCI and then adding triggering, local buses and system clock capabilities. The result is a standard that incorporates all of the benefits of PCI and cPCI in an architecture that supports these standards’ mechanical, electrical and software features. Also because interoperability between CompactPCI and PXI is a feature of the PXI Standard, both instrument types can reside in the same PXI system without any conflict - offering users a broad selection of both 3U and 6U cPCI modules in addition to the PXI instrument offerings.
PXI Express leverages the electrical features defined by the widely adopted PCI Express specification for data movement. All PXI Express modules comply with the CompactPCI Express specification, which combines the PCI Express electrical specification with rugged euro card mechanical packaging and high performance differential connectors. This allows measurement and automation Systems based on PXI Express to have a data throughput of up to 4 GBytes/sec in each direction. PXI Express also offers two-way interoperability with CompactPCI Express products. For more information on PXI Express go to the following article on the PXI Express Standard.
PXI Instrument architecture:PXI instruments are available in two form factors: 3U (100 by 160 mm, or 3.94 by 6.3 in.) and 6U (233.35 by 160 mm, or 9.19 by 6.3 in.). These two form factors are shown below.
Figure 1: PXI 3U/6U Instrument Form Factors
3U PXI instruments have two rear connectors named J1 and J2. J1 is used to carry the 32-bit PCI local bus signals. J2 is used to carry the signals for 64-bit PCI transfers and the PXI signals for implementing the local bus, star trigger signals and trigger bus signals. These signals are described later in this article. Connectors J3/J4/and J5 are undefined for 6U PXI modules but may be used for special applications.
6U system controller modules and system slots may use P3/J3, P4/J4, P5/J5 for rear I/O purposes.
Note: 3U instruments can reside in 6U slots when used with a 3U-to-6U Panel adapter similar to the Geotest GX97005 panel adapter shown below.
Figure 2: Geotest GX97005 PXI 3U-to-6U Panel Adapter mounted on a 3U PXI/cPCI instrument
PXI Hardware Architecture OverviewA PXI system consists of three main components:
1) The chassis
2) The system controller
3) Peripheral instrumentation
Figure 3: GX7300 20-slot 3U PXI chassis
PXI Chassis and Backplane Information:The main components of a PXI chassis are:
1) The System Controller
2) The Star Trigger Controller
3) The Chassis Backplane
Per the PXI Standard, the System controller will always reside in Slot 1 and the Star Trigger Controller, if required, is always located in Slot 2. The chassis backplane contains the PCI Bus, the Star Trigger Bus, a PXI Trigger Bus, PCI-PCI Bridge devices and the 10MHz System Reference Clock.
In a typical 20-Slot PXI chassis layout, slot number 1 is dedicated for an embedded or remote controller. Slot 2 can be used by a PXI Star Trigger Controller or by a PXI/cPCI instrument. Slots 3 through 15 supports the PXI Star Trigger and Slots 16-20 accommodate PXI or cPCI instruments without the Star Trigger.
Figure 4: PXI Chassis architecture
Figure 5: GX7000A 20-slot 6U PXI chassis
System ControllersThe System Controller is always located in the first slot on the left-hand side of the chassis and is available in two configurations: Embedded controllers and Remote Controller Interface Kits:
Embedded controllers are basically a PC on a card and can run on a number of different Operating Systems, for example Windows and Linux. As they remove the need for any external control they are ideal for portable applications and applications that require “single box” solutions. In cases where the embedded controller requires more than one slot the PXI Standard allows for additional controller expansion slots to the left of the controller slot. In this way instrument slots are not used by the controller.
Embedded controllers come in two form factor, Single slot controllers and Multi-slot controllers:
- Single-slot embedded controllers are one PXI slot wide and do not require any controller expansion slots. They support multiple peripherals and I/O interfaces through the controller’s front panel and the chassis rear I/O panel. Connection to the DVD-RW drive, hard drive and chassis rear I/O connectors is via the controller’s internal I/O interface. As some of these controller cards have a flash drive port option, where the flash-drive port is built onto the controller card, they can be operated from a flash-drive without the need for a hard drive. All Geotest embedded system controllers adhere to the PXI and cPCI specifications and are PICMG 2.0 Rev. 3.0 compliant.
Figure 6: GX7924 6U and GX7934 3U PXI Single-Slot Embedded Controllers
- Multi-slot embedded controllers operate the same as single-slot controllers but differ in that all the PC components, for example the hard-drive, are built onto the controller card and all the external connectors, for example Ethernet, USB, etc.., are available on the front faceplate. As these components and connectors require extra space, these controllers can occupy up to three slots (a system slot plus two controller expansion slots).
Figure 7: Typical Multi-Slot Embedded Controller
Remote Controller Interface Kits (also referred to as Bus Expansion Kits) are used in cases where the user wants to control a slave PXI chassis using an external PC, laptop or a Master PXI chassis. The interface kit consists of a pair of cards, one that plugs into the PC/Laptop or the Master PXI Chassis and another that goes in the system slot of the slave PXI chassis. The cards are then linked via copper or fiber-optic cable. For more information on the various PXI bus expansion options please see the following article on PXI Bus expander configurations
Figure 8: External PC controlling a slave PXI chassis with a MXI-4 Remote Controller Kit
Figure 9: Master PXI chassis controlling a Slave PXI chassis
Star Trigger ControllerThe Star Trigger (ST) Controller, if required, will always reside in Slot 2. This slot has dedicated trigger lines that can be used to synchronize up to 13 Peripheral PXI instruments that reside in the 13 consecutive slots to the right of the controller. It does this by utilizing backplane traces that are of equal length, providing for a skew of less than 1nSec between slots. If a Star Trigger Controller is not required, any PXI or cPCI instrument can be used in this slot.
For example, in the Geotest GX73xx and GX70xx 20-slot chassis’s Slots 2 through 15 support the Star Trigger while slots 16 through 20 accommodate PXI or cPCI instruments without the Star Trigger.
PXI Bus SegmentsThe PXI Standard allows for up to eight slots per 33MHz segment (one system slot and seven peripheral slots). Multiple segments can be connected together using a PCI-PCI bridge device. The PCI-PCI bridge presents one PCI load on each of the bus segments that it links together.
A typical 20-slot PXI chassis is divided to three bus segments (see Figure 8 below), each connected by a PCI-PCI Bridge device. The left bus segment supports the System Slot, the Star Trigger Slot and 5 more peripheral slots (Slot 3 to 7). The second segment supports slots 8 to 13. The third segment supports slots 14 to 20.
PXI Local BusThe PXI local bus is a daisy-chained bus connecting peripheral slots in the same bus segment. Each local bus consists of 13 user-defined lines that can be used to pass analog or digital signals between two adjacent modules or provide a high-speed side-band digital communication path that does not affect the PXI bandwidth. Local bus signals can support voltages from 0 to 42V DC and up to 200 mA DC current for any local bus line. The local bus lines for the leftmost peripheral slot of a PXI back plane (slot 2) are used for the star trigger facilities. Figure 8 schematically shows a complete PXI system including the local buses.
Figure 10: PXI Local Bus Segments
PXI Trigger BusThe eight PXI bused trigger lines can be used in a variety of ways. For example, triggers can be used to synchronize the operation of several different PXI peripheral modules. In other applications, one module can control carefully timed sequences of operations performed by other modules in the system. Triggers may be passed from one module to another, allowing precisely timed responses to asynchronous external events that are being monitored or controlled. The number of triggers that a particular application requires varies with the complexity and number of events involved.
The PXI trigger bus provides connectivity only within a single bus segment.which maintains the high performance characteristics of the trigger bus and allows the partitioning of instruments into logical groups as show in Figure 9. However, logical connections between bridge segment for the trigger bus are permitted, allowing communication between bridge segments. For example, all of Geotest’s PXI chassis can be configured programmatically to enable any of the trigger lines between adjacent segments as well as set direction.
Figure 11: PXI Trigger Bus Segments
System Reference ClockThe PXI 10 MHz system clock (PXI_CLK10) is also part of the PXI standard. It resides on the PXI backplane and is distributed to all peripheral slots. This common reference clock can be used for synchronization of multiple instruments or as a time base reference for measurement instrumentation. In cases where a more accurate 10MHz reference is required, the PXI clock can be sourced via the Star Trigger module. If the presence of this Star Trigger clock is detected, circuitry on the PXI backplane will automatically disconnect the backplane reference and connect the 10 MHz system clock lines to the Star Trigger module’s 10 MHz reference.
PXI Chassis Power Supply specificationsThe PXI Standard also specifies the minimum power supply requirements for each slot of the PXI chassis regardless of the number of slots or the instrument form factor (3U or 6U). The PXI chassis supplies +5VDC, +3.3VDC, +12VDC and -12VDC to each slot. Figure 12 below shows the minimum power requirements for each slot in the chassis.
Note: All Geotest PXI chassis form factors meet and/or exceed these specifications.
Figure 12: PXI Chassis Minimum Power Supply Specifications
SummaryThe PXI specification provides a very well defined and robust architecture for building high performance functional test systems. With over 1500 modules available today and over 60 PXI suppliers, the user can select from a broad range of products and vendors. The standard is over 10 years old which offers further proof to the longevity and acceptance of PXI as the primary card modular standard for the test and measurement industry.
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