PXI Instrumentation Systems
By Carsten Puls, <%=company%>
PXI (PCI eXtensions for Instrumentation) defines a rugged PC platform for measurement and automation applications. PXI modular instrumentation leverages off the high-speed PCI (Peripheral Component Interconnect) bus, which is the de facto standard driving today's desktop computer software and hardware designs. As a result, PXI users can enjoy all the benefits of PCI within an architecture that adds mechanical, electrical, and software features that make sense for test and measurement, data acquisition, and industrial computing applications.
PXI leverages off the CompactPCI specification, which defines a rugged package for PCI that offers superior mechanical integrity, as well as easy installation and removal of hardware components. PXI products offer higher and more carefully defined levels of environmental performance required by the vibration, shock, temperature, and humidity extremes of industrial environments. PXI also adds mandatory environmental testing, electromagnetic compliance (EMC) testing, and active cooling to the CompactPCI mechanical specification to simplify system integration and ensure multivendor interoperability.
PXI offers the same high-performance electrical features as PCI, such as 132 MB/sec data rates and Plug-and-Play functionality. The most compelling benefit for PXI, however, is PCI's dominance in the desktop PC marketplace, which is served by over 800 suppliers. The result is widespread availability of PCI-based silicon, firmware, drivers, operating systems, and software applications - all of which can be applied cost-effectively in PXI-based systems. As with CompactPCI, PXI offers nearly twice as many peripheral slots as desktop PCI systems per bus segment. Moreover, PXI meets the more specific needs of instrumentation users by adding an integrated trigger bus and reference clock for multiboard synchronization, a star trigger bus for very precise timing, and local buses for side-band communication between adjacent peripherals.
PXI defines system-level software requirements for standard frameworks such as Microsoft Windows NT and 95, which already have over 63 million users of more than 1,400 applications. This choice preserves multi-vendor compatibility and simplifies system integration tasks. Furthermore, all PXI peripherals must include appropriate device driver software, thus eliminating costly end-user development efforts.
As a result of the mechanical, electrical, and software features of PXI, instrumentation system developers acquainted with applications designed for desktop PCs can immediately apply these resources in the rugged PXI package at an incremental cost.
Mechanical Features
PXI modular instrumentation offers mechanical features that make PXI systems well-suited for industrial environments and make them easy to integrate. PXI uses the rugged Eurocard packaging system and high-performance IEC connectors called out by CompactPCI. But PXI adds specific cooling and environmental requirements. Finally, two-way interoperability with standard Compact-PCI systems is offered through the PXI specification. The following PXI mechanical features are shared with CompactPCI:
High-Performance Connector System: PXI employs the same advanced pin-in-socket connector system called out by CompactPCI. These highly dense (2 mm pitch) impedance-matched connectors are defined by the International Electrotechnical Commission (IEC-1076) and offer the best possible electrical performance under all conditions. These connectors have seen widespread use in high-performance applications, particularly in the telecommunications field.
Eurocard Mechanical Packaging: The mechanical aspects of PXI and CompactPCI are governed by Eurocard specifications (ANSI 310-C, IEC 297, and IEEE 1101.1), which have a long history of application in industrial environments. PXI specifies two module sizes - 3U (100 by 160 mm) and 6U (233.35 by 160 mm). Figure 1 shows the two primary sizes and the associated interface connectors for PXI peripheral modules. The most recent additions to the Eurocard specifications (IEEE 1101.10 and 1101.11) address electromagnetic compatibility, user-defined mechanical keying, and other packaging issues that apply to PXI systems. These electronics packaging standards define compact, rugged systems that can withstand harsh industrial environments in rack-mount installations.
Click here to see Figure 1.
All PXI features are implemented on the J2 connector of a 3U module and may selectively be used by peripheral modules. PXI-compatible backplanes must implement all PXI features. 6U PXI modules and PXI chassis need to implement only connectors J1 and J2. Future additions to the PXI specification may define the pinouts for connectors J3, J4, and J5 for additional functionality in 6U. Note that any 3U peripheral module can work in a 6U chassis by using a simple adapter panel.
PXI defines the "system slot" location to be on the far left end of the bus segment, as shown in the system diagram of Figure 2. This arrangement is a subset of the numerous possible configurations allowed by CompactPCI (a CompactPCI system slot may be located in any single position on a backplane). Defining a single location for the system slot simplifies integration and increases the degree of compatibility between controllers and chassis from different vendors. Furthermore, the PXI specification stipulates that the system controller module should expand to the left into what are defined as controller expansion slots. These expansion slots do not have CompactPCI connectors associated with them on the backplane and are basically expansion space. Expanding to the left prevents the system controller from using up valuable peripheral slots.
Additional Electronic Packaging Specifications
All mechanical specifications defined in the CompactPCI specification apply directly to PXI systems. However, PXI includes additional requirements that simplify system integration. As previously discussed, the system slot in a PXI chassis must be located in the leftmost slot and controllers should be designed to expand to the left to avoid using up peripheral slots. The airflow direction for required forced-cooling of PXI modules is defined to flow from the bottom to the top of a module. The PXI specification recommends complete environmental testing, including temperature, humidity, vibration, and shock, for all PXI products and requires documentation of test results. Operating and storage temperature ratings are required for all PXI products. Electromagnetic emissions and susceptibility testing is also required by the PXI specification to ensure compliance with international standards.
Click here to see Figure 2.
Interoperability with CompactPCI
An important PXI feature is its interoperability with standard CompactPCI products. Many PXI-compatible systems may require components that do not implement PXI-specific features. For example, a user may want to use a standard CompactPCI net-work interface board in a PXI chassis. Likewise, some users may choose to use a PXI-compatible plug-in module in a standard CompactPCI chassis. In this case the user will not be able to implement PXI-specific functions but will still be able to use the basic functions of the module. Note that interoperability between PXI-compatible products and certain application-specific implementations of CompactPCI (other sub-buses) is not guaranteed. Of course, both CompactPCI and PXI use the PCI, which ensures software and electrical compatibility as depicted in Figure 3.
Click here to see Figure 3.
Electrical Features
Many instrumentation applications require system timing capabilities that cannot be implemented directly across standard ISA, PCI, or CompactPCI backplanes. PXI modular instrumentation adds a dedicated system reference clock, bused trigger lines, star triggers, and slot-to-slot local buses to address the need for advanced timing, synchronization, and side-band communication. PXI adds these instrumentation features while maintaining all of the advantages of the PCI bus. Finally, PXI offers three more peripheral slots per bus segment than desktop PCI, bringing the total to seven.
System Reference Clock
PXI defines the means to distribute a 10 MHz system reference clock to all peripheral devices in a system. This reference clock can be used for synchronization of multiple modules in a measurement or control system. Because the implementation of the reference clock on the backplane is strictly defined, its low skew qualities make it ideal for qualifying individual clock edges of trigger bus signals for sophisticated trigger protocols.
Trigger Bus
PXI defines eight highly flexible bused trigger lines that can be used in a variety of ways. For example, triggers can be used to synchronize the operation of several different PXI modules. In other applications, one module can control carefully timed sequences of operations performed on other modules in the system. Triggers can also be passed from one module to another to achieve deterministic responses to asynchronous external events being monitored or controlled. The number of triggers that a particular application requires varies with the complexity and number of events involved.
Star Trigger
The PXI star trigger bus offers ultrahigh performance synchronization features. The star trigger bus implements a dedicated trigger line between the first peripheral slot (adjacent to the system slot) and the other peripheral slots. An optional star trigger controller can be installed in this slot to provide very precise trigger signals to other peripheral modules with matched propagation times. Systems that don't require this advanced trigger can install any standard peripheral module in this slot.
Local Bus
The PXI local bus is a daisy-chained bus that connects each peripheral slot with its adjacent peripheral slots to the left and right. Thus, the right local bus of a given peripheral slot connects to the left local bus of the adjacent slot, and so on. Each local bus is 13 lines wide and can be used to pass analog signals between modules or to provide a high-speed side-band communication path that does not affect the PCI bandwidth.
Local bus signals range from high-speed TTL signals to analog signals as high as 42 V. Keying of adjacent modules is implemented by initialization software that prohibits the use of incompatible modules. Modules are required to initialize their local bus pins in a high-impedance state and can activate local bus functionality only after configuration software has determined that adjacent modules are compatible. This method provides a flexible means for defining local bus functionality that is not limited by hardware keying.
The local bus lines for the leftmost peripheral slot on a PXI backplane are used for the star trigger. PXI electrical features are summarized in Figure 4.
Click here to see Figure 4.
Peripheral Component Interconnect (PCI) Features
PXI offers the same performance features defined by the desktop PCI specification with one notable exception. A PXI system can have up to eight slots per segment (one system slot + 7 peripheral slots), whereas most desktop PCI computers offer only three or four peripheral slots. This additional slot capability is a result of using the connectors defined in the CompactPCI specification, which PXI follows. Otherwise, all PCI features apply to PXI.
- 33 MHz performance
- 32 and 64-bit data transfers
- 132 Mbytes/s (32-bit) and 264 Mbytes/s (64-bit) peak data rates
- System expansion via PCI-PCI bridge chips
- 3.3 V migration o Plug-and-play capability
Software Features
Like other bus architectures, PXI defines standards under which products from different vendors work together at the hardware interface level. Unlike many other specifications, however, PXI defines software requirements in addition to hardware requirements to further simplify integration. PXI requires the use of standard operating system frame-works such as Windows NT and 95 (WIN32). PXI also requires configuration information and software drivers for all peripheral devices. Clearly, the PXI software specification is motivated by the benefits achieved through leveraging off existing desktop software technology.
Common Software Requirements
The PXI specification defines Windows NT and 95 software frameworks for PXI systems. A PXI controller operating in either framework must work with the currently available operating system as well as future upgrades. As a result, the controller will be able to use industry-standard application programming interfaces, such as LabVIEW, LabWindows/CVI, Visual Basic, Visual C/C++ and Borland Turbo C++.
PXI requires device driver software that runs in the appropriate framework for each peripheral module. Hardware vendors for other industrial buses that have no software standards often do not provide any software drivers for their devices. The customer is often given only a manual that describes how to write software to control the device. The cost to the customer, in terms of engineering effort, to use these devices can be enormous. PXI removes this burden by requiring that manufacturers, rather than customers, develop this software.
Other Software Requirements
PXI also requires other software components from peripheral module and chassis vendors. For example, initialization files that define system configuration and system capabilities are required with PXI components. This information is used by the operating software to ensure proper configuration of a system, such as whether adjacent peripheral modules have a compatible local bus. If any information is missing, the local bus functionality cannot be accessed. Finally, implementation of the Virtual Instrument Software Architecture (VISA), which has been widely adopted in the instrumentation field, is specified by PXI for configuration and control of VXI, GPIB, serial, and PXI instruments.
Conclusions
PXI modular instrumentation makes sense because it defines a measurement and automation platform for instrumentation users that clearly leverages off the technological advancements of the mainstream PC industry. By leveraging off the PCI Bus, PXI modular instrumentation systems can benefit from widely available software and hardware components. The software applications and operating systems that run on PXI systems are already familiar to end-users because they are already in use on common desktop PCI computers. PXI meets the needs of instrumentation users by adding rugged industrial packaging, plentiful slots for virtually unlimited I/O, and features that provide advanced triggering, timing, and side-band communication capabilities.
National Instruments, 6504 Bridge Point Parkway, Austin, TX 78730-5039. Phone: (512) 338-9119; Fax: (512) 794-8411.