Industrial PCs, CompactPCI, and PXI – Selecting the Best Platform for Your Industrial Application

This white paper provides a detailed comparison of the features and benefits of PXI versus Industrial PCs

1. What Is PXI?
2. What are Industrial PCs?
3. Mechanical Comparison
4. Electrical Comparison
5. Software Comparison
6. Price Comparison
7. Summary
8. Glossary

What Is PXI?

PXI is a modular instrumentation platform that offers mechanical, electrical, and software features tailored to industrial data acquisition and automation applications. PXI (PCI eXtensions for Instrumentation) takes advantage of PCI, the standard high-speed architecture driving today’s most popular desktop computer software and hardware. The PXI specification is a superset of the CompactPCI specification, which defines an industrial version of the PCI bus architecture with rugged packaging for superior mechanical integrity, yet easy installation and removal of hardware modules.

PXI products thus offer the higher, more carefully defined levels of environmental conformance required by the vibration, shock, temperature, and humidity extremes of industrial settings. PXI adds mandatory environmental testing and active cooling to the CompactPCI mechanical specification to simplify system integration and ensure multivendor interoperability. PXI also complements the high-speed PCI bus with integrated timing and triggering designed specifically for measurement and automation applications.

As a result, PXI is a modular, rugged instrumentation platform that delivers PC-based, standardized, high-performance measurement and automation at an affordable price with significant performance improvements over older architectures. PXI users automatically benefit from the low cost, ease of use, and flexibility of PC technology; open industry standards; and complete interoperability with CompactPCI.

PXI Systems Alliance
The PXI platform is standardized around a specification maintained by the PXI Systems Alliance, www.pxisa.org Comprising more than 60 companies that specialize in PXI, this alliance has three main goals:

1. Promote PXI as an open standard and encourage multivendor support.
2. Ensure multivendor interoperability of PXI products at the mechanical, electrical, and software levels, while maintaining complete interoperability with CompactPCI.
3. Maintain the PXI specification, making revisions when necessary, while keeping backward compatibility with products built to earlier revisions of the PXI specification.

These goals are designed to simplify and improve the end user’s experience with PXI. Easy integration with both the PXI platform and existing platforms, such as desktop PCs, CompactPCI, and VXI, is a top priority of the alliance.


PCI
The PXI platform is based on the PCI bus, an open industry standard that offers many benefits for test and measurement engineers. Many devices created for conventional computers are built on the PCI bus. As a result, thousands of PCI devices have been developed and the bus has been optimized to take advantage of the newest computing technologies. What this means to PXI is the availability of component devices that are:

• Fully compatible with the most commonly used operating systems and software packages
• Optimized for the fastest processors and buses for increased data throughput.

A key component that determines system throughput is the interface bus between the measuring device and the computer. Although system performance has dramatically increased because of processor speed and improvements in system memory, the biggest limiting component of a system is the peripheral communication to instruments. The GPIB bus is the most common interface to instruments, but data transfer rates are much slower with GPIB than with VXI or PCI. Measurement systems based on VXI exhibit higher performance than GPIB, but users pay a premium for that performance. PCI, on the other hand, has been designed to work with newer processors and to transfer data at rates much faster than either GPIB or VXI.

Figure 1. Theoretical Bus Performance

Besides increased throughput, PCI offers compatibility with all major operating systems. PXI takes advantage of this to integrate GPIB and VXI into PXI systems. Because of its open architecture, many different vendors can create PXI devices that work in a single, integrated system. In order to maintain this interoperability between vendors, the PXI specification lays out software requirements. One of the most important requirements is that devices must work with Windows operating systems and must be shipped with driver software. This requirement not only maintains vendor compatibility, but also accelerates integration and application development on the end user’s side. Older architectures required that every end user develop his/her own driver using a set of device specific calls. The PXI specification has put this burden on the device manufacturer.

Finally, in order to take full advantage of the PCI bus, the PXI specification states that all PXI devices shall be fully compatible with CompactPCI. CompactPCI modules use the same packaging as PXI and are also built on the PCI bus. PXI adds stricter environmental testing, software requirements, and some additional trigger features. This strict compatibility requirement means that more modules will be available to PXI users for applications that do not require PXI-specific features on all modules. It is also means engineers who may already be using CompactPCI modules can incorporate them into a PXI system.


Physical Features
PXI is a modular platform. The physical frame of the system is a chassis that can have from two to 31 slots; some chassis even come with built-in monitors and keyboards. The first slot (Slot 1) in the chassis is the controller slot. Many different controllers are available, but the two most common are embedded controllers and MXI-3 bridges. Embedded controllers are regular computers designed to fit in a PXI chassis. MXI-3 is an extender with which a desktop PC can control the chassis; it will be discussed in more depth later. The rest of the slots in the chassis are called peripheral slots and hold the functional modules. One can think of these slots as PCI slots in a computer.

The bracket for the PXI modules is different from the PCI slot in a desktop PC. Modules are held securely in place by two guard guides and a pin-and-socket connector. In fact, PXI employs the same advanced pin-in-socket connector system called out by CompactPCI. These highly dense (2 mm pitch) impedance-matched connectors, which are defined by the International Electrotechnical Commission (IEC-1076), offer the best possible electrical performance under all conditions. These connectors have seen widespread use in high-performance applications particularly in the telecommunications field.

Physically, a 3U PXI chassis is more compact than most desktop or industrial platforms, which is an important feature in many confined testing environments. Chassis are available with various numbers of slots. If space is at a premium, engineers can purchase the exact size they need. The chassis are designed for easy rack mounting in case it is desirable to integrate them into existing rack systems. The small footprint of a PXI chassis also makes it an ideal choice for portable or field applications. Besides built-in monitors and keyboards, some chassis can be powered from DC sources, with battery backup.

Finally, the PXI modular, front-loading architecture makes maintenance and upgrading very easy. If a module needs to be repaired, it can be removed from the system without affecting any of the other modules. Less system downtime due to this modularity helps to decrease maintenance costs over the life of the system.

Electrical Features
The PXI backplane has several unique features for test and measurement engineers. A dedicated system clock is available for synchronizing modules. Eight independent bused trigger lines can be used for synchronizing two or more modules precisely. Slot-to-slot local bus lines are provided to preserve the PCI bus bandwidth. Finally, a star trigger is an optional feature for extremely high-precision triggering. Comparable trigger lines, clocks, and local buses are not available in desktop PCs, industrial PCs, or CompactPCI chassis.

Integrated, Expandable Systems
Through the use of remote control interface technologies such as MXI-2 and MXI-3, PXI systems can be easily incorporated into existing test and measurement systems. Using these interfaces, engineers can take advantage of PXI technology while avoiding the high costs associated with replacing an entire test system.

A PXI system interfaced to a VXI mainframe with MXI-2 operates as though it were a VXI embedded controller plugged directly into a VXI backplane. From the PXI controller, an engineer can configure and talk to all of the system devices and thus incorporate an existing VXI system into a new PXI system. Engineers can gradually upgrade their systems from VXI to PXI as necessary.

With MXI-3, the CPU in a desktop PC transparently configures and controls the PXI/CompactPCI modules. To the BIOS and operating system, PXI modules look just like PCI boards plugged into the PC. Together, MXI-3 and a PXI chassis are great options for expanding system I/O. Architecturally, MXI-3 is a PCI-PCI bridge. A PCI MXI-3 board is plugged into the PC in a desktop and then cabled to a PXI MXI-3 module in the controller slot of the PXI chassis.

What are Industrial PCs?

Industrial PCs are typically a single-board computer and PCI backplane, complying with PICMG specifications, in a rugged case. Just like a desktop PC, they run standard operating systems. Measurement boards are installed in PCI slots. The number of PCI slots depends on the manufacturer of the computer. But an important thing to note is that the ratio of slots to PCI-PCI bridges is lower in an industrial PC than a PXI chassis. This is important because each PCI-PCI bridge can cause a significant loss in bandwidth. Maintaining an industrial PC can be more difficult than maintaining a PXI system. First the computer must be removed from the test rack. Because the PCI boards are cabled from the back, if the cables are not long enough they must be disconnected before the computer can be removed. Like a desktop PC, the external CPU cover must then be removed to access any of the PCI slots and later replaced, because it is critical to minimize dust and debris in the CPU.

Mechanical Comparison

PXI chassis and devices are designed to withstand industrial and remote environments. Modules have more robust connectors to protect against shock and vibration. Extra cooling capacity is built into the chassis in order to protect the circuitry. Additionally, devices are accessible from the front of the chassis so that they are easy to maintain and upgrade.

Designed to be Rugged
The PXI specification requires that all PXI equipment undergo environmental testing. Humidity, shock, and vibration testing are recommended from the manufacturer. These tests were suggested so that engineers could more knowledgeably select the right equipment for their applications. PXI systems are ideal for manufacturing or remote environments because they are designed to withstand the conditions.

One example of ruggedness is the module guides in a PXI chassis. Two guides, one on top and the other on the bottom, are used to securely hold the PXI modules in place. This mounting feature relieves the strain on the backplane as well as protecting modules from shock and vibration. Module front panels can also be screwed into the chassis to further prevent movement. In an Industrial PC, boards are usually held in place by a bracket and a card edge connector. Two points of contact, one of which is the backplane, makes the boards much more susceptible to shock in a rough environment. The PXI connector is also wider and longer then the PCI connector.

Finally, PXI chassis generally have little circuitry on the backplane, which means easier maintenance in the event that any circuitry needs to be replaced. Industrial PCs have two designs. First, some have an active backplane, meaning the motherboard is part of the backplane. In this case if something fails on the motherboard, the entire computer must be replaced. The second design has a passive backplane with only the PCI-PCI bridges. In this design a single-board computer (SBC) is used. The SBC plugs into the backplane with a connector similar to a PCI connector. This design makes the computer much more susceptible to vibration and shock than an embedded PXI controller.

Cooling
The PXI specification has strict requirements for cooling. At a minimum, the peripheral boards are cooled to the same level as in an industrial PC setting. PXI requires forced cooling with a defined airflow direction to cool all modules evenly. Most industrial PCs use a cooling mechanism with one or two fans; uneven cooling of the motherboard and the peripheral boards is common. Uniform cooling prolongs the life of the modules/boards, decreasing the maintenance costs over the lifetime of the system. Finally, all PXI equipment is tested and clearly documented with operating and storage temperatures. Thus, the user can develop his or her system more knowledgeably and know if extra measures must be taken to keep the system working in the environment.

Maintenance
The PXI platform was designed for easy maintenance. All of the modules and their connectors are accessible from the front of the chassis. There is a handle on the front of each module to help it be inserted correctly into the chassis and removed easily. Two guides ensure that the module is aligned properly, which prevents pins on the backplane from being damaged. One module can be removed from the chassis without affecting the rest of the system.

Maintenance on Industrial PCs is often much more time-consuming. First, the computer must be removed from the rack, which often involves unbolting it and sliding it out. And as stated previously, if the cables are not long enough, all of the PCI boards and cables must be disconnected. Then the outer cover can be removed so that the PCI boards can be removed.

Upgrading
Because of the modularity of a PXI system, it is easy to upgrade a single component. For example, if the user wants to upgrade to a faster processor, the controller unit can be removed and replaced. Because all of the devices are required to work with standard software, a new controller can be loaded with the software and device drivers and then put back into the chassis. The change should be almost transparent to the user.

Usually, in an Industrial PC setup, to upgrade the processor means the user would need to remove all boards from the PC and replace the entire PC or motherboard. This process adds time, expense, and complexity to an upgrade.

Electrical Comparison

PXI chassis have many electrical features specifically for test and measurement engineers. One important feature is the easy I/O expansion, which is extremely important for large data acquisition systems that need as much bandwidth as possible. A second feature is the dedicated, 10 MHz system clock on the PXI backplane, which provides for increased accuracy and phase-lock looping. Chassis also have bused trigger lines for more accurate triggering. Slot-to-slot local buses have been implemented to preserve the PCI bandwidth. Finally, a very precise star trigger is available for applications requiring triggering above 10 MHz. Industrial PCs have no such built-in electrical features. The PCI bus can range anywhere from 0 to 66 MHz and is used for device communication. It cannot be relied on for accurate synchronization or precise triggering.

I/O Expansion
PXI systems are designed for expansion. The backplane integrates up to seven modules, versus four on an industrial PC, on one 33 MHz bus segment. Through the use of PCI-PCI bridges, a chassis can hold many more cards. Using MXI-3 interfaces, the user can easily expand into other chassis. The theoretical system slot limit is 256, with 33 MHz bus segments. As discussed previously, Industrial PCs have a greater number of PCI-PCI bridges. This fact means there can be a greater loss of bandwidth, which will significantly affect the performance of an application.

Dedicated System Clock
The PXI backplane has a built-in, dedicated 10 MHz system clock. This very high quality clock, with low skew signals between each slot, is used for synchronizing modules. When boards are synchronized in an industrial PC, or any other system for that matter, they must be cabled together with a proprietary trigger bus and timing sources on the boards are used to time and trigger the boards.
Because the quality and precision of the clock depends on the individual device, some are more or less precise than the industrial PC chassis clock. However, because the clock signals are bused, they have a higher skew – typically nanoseconds versus picoseconds – than the chassis clock and the signals are not shielded. The PXI system clock is able to maintain its quality by using a low-skew, fan-out buffer chip that essentially provides a unique clock for every slot. Furthermore, because the clock lines are built into the backplane, the lines are better shielded than external lines.

Bused Trigger Lines
If the user requires a trigger bus, PXI triggers can be routed over the backplane instead of having to cable the cards together as one must do in a PC. The PXI backplane has eight dedicated trigger lines that are bused to every slot, including the system controller slot. With these triggers, the user can achieve more sophisticated module synchronization, with features such as one module triggering another and asynchronous triggers. With asynchronous triggering, a module can be triggered by events occurring elsewhere. Finally, trigger signals can be routed into and out of the chassis with many of the newer controllers.


Slot-to-Slot Local Buses
The PXI bus also has the capability for modules in adjacent slots to communicate with each other over dedicated lines not on the actual bus. These lines make up the PXI local bus, which 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 slot connects to the left local bus of the next slot on the right, 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. This feature is very useful for data acquisition and instrumentation cards that use analog signals.

Local bus signals may range from high-speed TTL signals to analog signals as high as 42 V. These lines are very precise, exhibit low skew, and can be used for advanced operations. It is important to keep in mind that these lines are not available on the PCI bus or in an industrial PC.

The question then arises about what to do with the 13 left local bus lines on Slot 2. It does not make sense to route these lines to the system controller. Instead, these lines are used to implement the star trigger.

Star Trigger
The PXI backplane defines specific layout requirements so that the star trigger lines provide matched propagation time from the star trigger slot to each module for very precise trigger relationships between each module. The star trigger is a high-performance trigger signal, which can synchronize all of the modules in a chassis. You can also synchronize the modules using the normal PXI trigger bus, but the star trigger offers increased performance. Specifically, the star trigger provides a skew of less than 1 ns and a delay between the star trigger slot and each peripheral slot of no more than 5 ns.

To use the star trigger, a module with the ability to generate the star trigger must be placed in Slot 2 of the chassis. Slot 2 is dedicated for the star trigger controller (although any standard module placed in Slot 2 will function normally in an application where star triggering is not required). Receiving modules must also be designed to be able to accept the star trigger. The standard PXI trigger bus can perform the same functionality with a trigger signal from any chassis slot, but with less precise timing.

Software Comparison

Another feature of the PXI specification is the software requirement. All PXI hardware must work with at least one of the operating systems – Windows 95, Windows 98, Windows NT, or Windows 2000. The specification also states that, as other operating systems become widely accepted and offer the same degree of software coverage as the current frameworks, they may be added to the PXI frameworks.

Controllers and devices must be VXIplug&play compliant and compatible with VISA software. In addition, driver software must be provided with modules, for standard communication between devices and decreased development time for engineers. Earlier platforms, such as VXI, had no such requirements, which meant that engineers had to spend several months writing instrument drivers for each device. PXI forces the manufacturer to expend that development time so the end user saves significant time.

Finally, PXI systems ship with standard .ini files to simplify system configuration. Although the full potential of these files has not yet been achieved, they do provide some extra features, such as slot recognition in configuration software.

Industrial PCs have no such software requirements. And although many PCI devices do work in most Windows environments, they may or may not be compatible with VISA and may or may not have driver software. So users may end up with code compatibility issues as well as having to develop their own drivers at the expense of significant development time.

Price Comparison

PXI systems are designed to have low set-up and maintenance costs. Because of the modularity of the system, the mean time to repair is reduced compared to other platforms. The rugged design of the system is also intended to prolong the life of the controllers and the modules.

Some industrial PC solutions are priced lower than PXI, but these are not apples-to-apples comparisons. PXI offers greater reliability, ruggedness and lower overall cost of ownership.

Summary

PXI was designed to take advantage of the PCI specification and to be rugged enough for industrial settings. PXI systems have many built-in features to decrease application development time and make life easier for test and measurement engineers. The most notable features are the modularity of the system, easy integration, easy access to the devices and their connectors, as well as the many electrical features that increase the accuracy and precision of board synchronization and triggering.

Key Advantages of PXI:

• Software requirements that decrease application development time
• Lower maintenance costs over the lifetime of the system
• Rugged design for industrial environments
• Modular design for easy system upgrades
• Increased number of high performance I/O slots
• 10 MHz dedicated system clock
• Bused trigger lines
• Slot-to-slot local buses
• Star trigger

Glossary

CompactPCI is a rugged, modular architecture that combines PCI electrical specifications with Eurocard packaging for industrial computing applications.

GPIB (general-purpose interface bus) is a digital 8-bit parallel communications interface with data transfer rates up to 1 Mbytes/s, in accordance with IEEE Standard 488-1987.

Industrial PC is a PC that has been fitted to withstand the rigors of an industrial environment.

MXI-2 is a bus extender between a PCI-based system and a VXI system. With it, the PCI system controls the VXI devices as if it were on the VXI backplane.

MXI-3 is a PCI-PCI bridge that is used to remotely control PXI chassis from a desktop PC.

PICMG (PCI Industrial Computer Manufacturers Group Specification 2.0) is the group of member companies that maintain current specifications for CompactPCI and PCI/ISA.

PXI is a rugged, modular architecture based on PCI with electrical and timing circuits specifically designed for test and measurement applications.

PXI Systems Alliance is a group of worldwide product manufacturers and systems integrators who control the PXI specification that defines system-level requirements for CompactPCI products used in test and measurement, industrial automation, and data acquisition applications.

RTSI Bus is a local bus used to share counter, clock, and trigger signals by direct connection between plug-in data acquisition and instrumentation boards

Star Trigger comprises the 13 left local bus lines in Slot 2, which are used to provide a very precise, low-skew trigger to Slots 3 to 15.

Trigger Lines are signal lines on the PXI bus that are dedicated for sending triggers between modules.

VISA provides the interface between programming environments such as Measurement Studio and languages such as LabVIEW, C, C++, and Visual Basic.

VXI is an industry-standard instrumentation bus, based on VMEbus, for modular, instrument-on-a-card systems.

VXIplug&play indicates the conformity of hardware and software to the VXIplug&play Systems Alliance specifications. VISA is the official software language of this alliance.

See Also:
PICMG
PXI Systems Alliance

21 Ratings | 3.90 out of 5

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add application examples

- Woody Reber,Lafarge-NA. woody.reber@lafarge-na.com - Mar 25, 2005